The Welcome Mat - a FAQ

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May 26, 2002
Missouri, U.S.A.
The Welcome Mat thread, with the thrust of it being the opening post, written and maintained by TigerhawkT3, and reflecting suggestions from the member input of the replies contained in the thread, was inadvertently lost.

This archival copy of that important post, is reprinted below:

Table of Contents
General FAQs
-Q: What's the difference between "lumens" and "lux"?
-Q: What are "CCT" and "CRI"?
-Q: What's "anodizing"?
-Q: What are bezel-up and bezel-down carry?
-Q: What methods are there for holding a flashlight?
-Q: Where does a classic 2AA Minimag fit in?
-Q: What is Fenix's naming scheme?
Flashlight Beam Anatomy
Terminology FAQs
-Q: What's a "drop-in"?
-Q: What's tactical?
-Q: What's "candle mode"?
-Q: What's "UI"?
-Q: What's "EDC"?
Parts FAQs
-Q: Lens? Window?
-Q: What goes into a flashlight besides the essentials?
-Q: What do filters do?
-Q: How do I choose a switch?
Battery FAQs
-Q: Cell? Battery?
-Q: What do protected/unprotected mean in regards to Lithium-Ion?
-Q: What are the different types of Lithium-Ion?
-Q: What's up with these five-digit batteries?
-Q: Why can't I use alkalines in more lights?
Electronics/Electrical FAQs
-Q: Please explain volts, amps, watts, and C.
-Q: What are series and parallel?
-Q: What are "direct drive" and "regulated"?
-Q: What are "boost" and "buck"?
-Q: What is "linear regulation"?
-Q: Can I wire driver boards in series?
-Q: Can I wire driver boards in parallel?
Incan FAQs
-Q: What does "overdriving" mean?
-Q: What is "double-tapping"?
-Q: What is "instaflash"?
-Q: What's "magic smoke"?
-Q: Is turning an LED on and off bad for it? What's "PWM"?
-Q: How can an LED be more efficiently driven?
-Q: What's the "Luxeon Lottery"?
-Q: What's "Vf"?
-Q : Why is Vf important, and how do I measure it?
-Q: What's "bin"?
-Q: What's the deal with LEDs and heat?
-Q: What's the deal with LEDs wired in series or parallel? What about "current hogging" and "thermal runaway"?
-Q: LEDs are all the same, right?
LED Types
-Luxeon I/III
-Luxeon V
-Luxeon K2
-Cree 7090 XR-E
-Edison Opto KLC8
-Luxeon Rebel
-Luxeon K2 TFFC
-Practical considerations
-Q: What are "HID" and "metal halide"?
Laser Basics
-Q: Do I need some sort of safety equipment?


The following is a general introduction to the world of flashlights. Feel free to add links or other FAQs/info that you think should be included. Feel free to use or hotlink "Scorpion_HDR.png", but be sure to give me credit for it.

Common questions about the forum:

How to post pics:

Threads of Interest:

General Flashlight threads of interest (TOI):
Incan TOI:
Batts/Electronics TOI:
Spotlights TOI:
Reviews TOI:
Materials/M/M TOI:
Homemade and modified lights:
Laser TOI:

Choosing a flashlight:

Self-defense light:
Poor Flashaholic's Top 10:
Not So Poor Flashaholic's Top 10:
Does anyone DISlike the ProPoly Luxeon?:
SF clickies:
500+ light choices... which actually get used? Your input?:
Maglite and Minimag upgrading and modding:


Lube, 1:
Lube, 2:
Bike light mount, helmet:
Bike light mount:
Bike light topics by category:
Flashlight Stories - Chicken Soup for the Obsessed Flashaholic's Soul:
CPF Specials:
Why do I need a flashlight?:


Li-Ion Dangers:
Laser Dangers:

General Info:

eBay lasers:
Vf, Color/Flux bins, and more:
Visually identifying LEDs:


General flashlight info:
The CPF Wiki:
Philips Lumileds:
Seoul Semiconductor:
Edison Opto:
Formulae (Duracell):
Battery Characteristics (Energizer):
Batteries and Temperature (Energizer):

General FAQs:

Q: What's the difference between "lumens" and "lux"?
A: Lumens measure the total amount of light output from a particular source. Lux measures the intensity of the light hitting a specified area. For example, an ordinary household lightbulb generates about 1000 lumens, but the intensity of its light at a particular point, such as on a book you're reading, will be comfortably low. Almost all flashlights rely on an optical device such as a reflector or lens to squeeze most of their output into a small area, which allows you to illuminate a point of interest with enough intensity, but without requiring a lot of power. To illustrate this concept, try the following: First, look at your room's ceiling light. In all likelihood, you can stare at it without much discomfort. Now, try looking into a weak flashlight like a traditional incandescent Mini-Maglite. You'll notice that it seems very bright. This is lux. Now, remove your Minimag's head to put it into candle mode. Try switching between its output and your ceiling light's output. Since they're now illuminating approximately the same area, the much higher lumen value of the ceiling light will provide much higher lux values at a chosen point. An extreme example of high lux and low lumens is a laser, which doesn't really create that much light, but focuses it into a tiny, brilliant point.

Q: What are "CCT" and "CRI"?
A: CCT stands for "Correlated Color Temperature" and CRI stands for "Color Rendering Index." CCT is expressed in terms of degrees Kelvin, corresponding to the temperature of a black-body radiator (such as the Sun) at that color temperature. For example, a black-body radiator heated to about 8,000 degrees Kelvin would appear slightly bluish, so a light (an MH HID, for example) with a CCT of 8000K would have a bit of a bluish tint to it. CRI is expressed as a number from 1-100 and refers to how well a light source reveals colors regardless of its CCT. For example, a power LED that creates blue light that is filtered through a yellow phosphor to end up with white has no red component, leaving red and yellow objects slightly faded. This will lead to a low CRI rating. Incans, on the other hand, generally have a CRI of around 100, as they emit all spectra of visible light (as well as IR and UV light).

Q: What's "anodizing"?
A: Anodizing is a process that oxidizes a surface (usually aluminum) and gives it a hard, wear-resistant coating. Many lights are Type II anodized, which means that they're reasonably resistant to wear. Type III anodizing, also known as Hard Anodizing, or HA, is very wear-resistant. Some lights are hard-anodized to military specifications, which is then called Mil-Spec HA.

Q: What are bezel-up and bezel-down carry?
A: When a flashlight has a pocket clip attached at the tail end, it is carried with the head facing down. When it is attached near the head, the flashlight is carried with the head pointing upwards. Holsters can also be bezel-up or bezel-down. Some holsters depend on holding a flashlight bezel-up so that the large head prevents the flashlight from falling. One common example of this is the simple Maglite ring holster, in which the light's body goes through a ring that the head is too large to fit through, providing a simple bezel-up retention method. Other holsters depend on bezel-down carry, and still others can hold a flashlight bezel-up or bezel-down. This is similar to the point-up or point-down debate with knife carry.

Q: What methods are there for holding a flashlight?
A: First is the overhand or "icepick" method, so named because it looks like the user is holding an icepick. The light is held somewhere above the user's shoulder. It's most useful for lights with a switch in the tailcap and is slightly more "tactical" than the underhand method, which is held near the waist and is useful for lights with side-mounted switches. Some lights, like Maglites, can be comfortably held either overhand or underhand. The Rogers-Surefire technique is a very specialized grip where a small flashlight's body is lodged between two fingers and the tailswitch (usually some sort of momentary one) rests in the palm. For users with a handgun in their other hand, both hands can be held side by side in a strong, stable manner.

Q: Where does a classic 2AA Minimag fit in?
A: A Minimag uses alkaline cells to drive an incandescent in an unregulated fashion. It has candle mode and tailstand capabilities. It produces about ten lumens, with output falling sharply and turning orange-yellow within about half an hour. The bulb and battery are both old technology, and both runtime and output suffer as a result, with most modern lights (in its class) delivering much greater runtime, output, or both (not to mention any other improvements).

Q: What is Fenix's naming scheme?
A: [The following info was contributed by CPF's Gunner12.]

Here's how it works. All are Type 3/HA unless otherwise stated.

E=AAA powered
P=CR123 powered
PD=Essentially improved designs of the P2D and P3D series. Better tailcap knURLing, improved tailcap design with better laynard ring, easier to turn head which is also an anti-roll hexagon shape, lower low, medium, and high, smoother talicap switch. Longer runtime on low, medium, and high.
L=AA or AAA powered
LD=Essentially improved designs of the L2D. Better tailcap knURLing, improved tailcap design with better laynard ring, easier to turn head which is also an anti-roll hexagon shape, lower low, medium, and high, smoother tailcap switch. Longer runtime on low, medium, and high.
T=CR123 powered with a forward clickie, two mode, tighten for high, loosen for low, uses a Cree XR-E Q5 LED (for now; when they add more models, this might change).

0=1 AAA powered, for the E series, 1 5mm LED, currently the Nichia CS, all twisties.
01=1 AAA powered, E series, multicolor anodizing, 1 5mm Nichia GS. 1 AAA powered LD series. Cree XR-E Q5 and three modes only, medium-low-high. Reflector is also improved and PWM is faster.
1=1 battery, for the E series, 1 Nichia Power LED, for the P series, a twistie. For the T series (T1) a 2 stage forward clickie light powered by 2 CR123 or RCR123 batteries with a clip. Uses a Cree XR-E Q5 LED in a textured reflector. Built to have thicker walls then the E and L series lights.
2=2 battery powered or for the P series, 1 CR123 with a clickie
3=For the P series 2 CR123 powered
K10=TK10, an "improved" version of the T1 with a removable grip ring, aluminum bezel, and removable clip. Performance is not changed. --- LD10 "improved" version of the L1D. Look at LD to see what improved over the L1D.
K11=TK11, an "improved" version of the TK10 and T1 with a smooth reflector and the ability to accept 18650 batteries.
K30: Part of the T series, multimode controlled by a ring that is twisted to adjust output (low, medium, high, strobe), 3 CR123, crenellated bezel, smooth reflector, clickie.
K40: Part of the T series, Fenix's first multidie LED flashlight, has a Cree MC-E in a textured reflector powered by 8 AA batteries but can use 4 in an emergency, 2 modes of output, first mode is turbo, low, medium, and high, second mode is strobe, slow flash, SOS, fast flash, clickie.
20=T20 or LD20 or PD20 --- T20 is a 2AA-powered light similar to the other T series but has a neutral white LED instead of a cool white one. Has a smooth reflector, a rubber sleeve on the body tube for grip, and a removable clip. --- LD20, "improved" version of the L2D, adding body knURLing. Look at LD to see what changes were made over the L2D. --- PD20, "improved" version of the P2D. Look at PD to see what changed.
30=PD30, "improved" version of the P3D. Look at PD to see what changed.

+=Special stainless steel run, a small run prototype that Fenix sold
P="Premium" Luxeon I, V2.0 means 2 modes, Luxeon
S=Two mode (tighten for high, loosen for low) Luxeon I, Type II andodizing
T=Two mode (tighten for high, loosen for low) Luxeon III, V2.0 means Rebel 80, twice the output
D=Digital, Multimode
CE=Cree XR-E LED, newer and at least twice as efficient as the Luxeon I/III LEDs. For the same power, it puts out twice or more output
Rebel 80/100=Luxeon Rebel 80/100 used, usually has a warmer tint
No designation=Luxeon I or III

If there is a thing after the CE, that is to designate the Bin of the Cree XR-E LED used. Q5 means it uses the Cree XR-E Q5 LED. No designation after the CE means it uses the P3 or P4, depending on date of manufacturing.

The Civictor V1 is a 1 AA twistie with Type II anodizing. There was a multimode stainless steel model made.

There was a special Titanium version of the L0D made. It was called the L0D Ti. There was also a Christmas version(red type II anodizing with Christmas decorations) called the L0D SE. There is also a Q4 version.

Flashlight Beam Anatomy:

The first terms you should know when discussing beams are "throw," "flood," and "artifact."

-Throw is a flashlight's ability to project light over a distance. In flashlights with a lens, parabolic reflector, or other optical system attempting to get parallel rays of light, a beam with higher point brightness in the center will throw better than others. Remember that this isn't universal: For example, a flashlight with an ellipsoidal reflector would create a very tight, bright spot at a certain distance away (the ellipse's other focus), but almost no beam at all at other distances.

-Flood describes a light's ability to illuminate a large area, especially at close distances. It's especially useful for when you're indoors or working with something in your immediate surroundings. Some flashlights "focus" (get it? Focus? :) ) entirely on flood, such as lights with little or no reflector and lanterns. These are very useful for general illumination during power outages or on camping trips, but their lack of distance lighting capability makes them less popular than throwers.

-An artifact in a flashlight's beam refers to a flaw or imperfection that interrupts the beam's geometric shape. The beam from a traditional incandescent Maglite or Minimag is chock-full of artifacts, as the reflectors are smooth and project a more or less direct image of the filament, producing strange shapes. Artifacts can be decreased or even eliminated by using stippled reflectors or diffusing filters. Spill describes the light that did not hit a flashlight's reflector and so is not part of the bright central spot. The angle of this spill can be up to 90 degrees or more, which is actually quite useful for illuminating the objects surrounding the well-lit point of interest. Some flashlights, such as those with lenses and especially aspheric lenses, produce little or no spill, creating a light with lots of throw but poor flood capability.

Furthermore, there are several components to a classic flashlight's beam. The following image and description are for a flashlight with a power LED or incandescent bulb and a parabolic reflector. The image is a beamshot of a Scorpion LED modded with a Seoul P4 USW0I.


The hotspot is the brightest, central area in the beam. It is the portion of the beam that provides throw. If you were to illuminate someone's face with the hotspot (not recommended) and they were to look directly at it (not recommended), they would see the reflector's entire surface area illuminated (assuming an ideal, artifact-free hotspot). If they are being illuminated by the flashlight but not by the hotspot, they will see the light source plus a partially illuminated reflector, with the illumination level being determined by how close to the target the hotspot is.

Surrounding the hotspot is the corona. It is less intense than the hospot, but more intense than the light surrounding it. It is important to note that many flashlights do not have a corona to speak of. These lights are usually optimized for throw, as the more light that falls into the corona, the less available for the hotspot. However, the corona is an excellent attribute for a general use (indoor, navigation, etc.) light, as it provides a wider area of illumination around the hotspot while being brighter and more useful than the surrounding light, known as sidespill.

Sidespill, or spill, is the very wide, dim circle of light between the rest of the beam and the surrounding darkness. This is the light that is going directly from the light source to the target without hitting the reflector. A person whose face was illuminated by spill (not recommended, but not as bad as the hotspot) would actually see the light source, but the reflector would be reflecting very little light toward them. Spill allows people to use a "throwy" flashlight for general purpose tasks, as, although most of the beam would be in the hotspot and possibly the corona, there would be a generous wash of light over a wide area.

Lights that use TIR optics or aspherics have different beams altogether. TIR lights have a hotspot and possibly a corona, but no sidespill. Aspherics create only a hotspot, with the tightest and best-throwing spot achieved when the light source (usually a Cree XR-E) is at the lens's focal point. The lack of spill in these lights significantly reduces their popularity, with aspherics being especially optimized for throw and little else.

Lanterns are even more unique. They produce a wide flood, either in a 180- or 360-degree pattern, and artifacts are very undesirable. Lanterns that simply use bare emitters are technically all spill, but the intense emitters can be uncomfortable to even glance at. Many lanterns employ some sort of integral diffuser to soften the light as well as reduce artifacts.

Terminology FAQs:

Q: What's a "drop-in"?
A: A drop-in is a unit that can easily replace a stock lamp assembly or light engine. A classic example is the MagLED module, which looks very similar to a stock incan Mag bulb. Users can simply remove the old bulb and "drop in" the drop-in for an instant upgrade. Popular drop-in hosts are Mags and various Surefire models, such as the G2, G3, 6P, 9P, etc.

Q: What's tactical?
A: The general idea of a tactical light is a simple, robust, high-output light that can be easily used in difficult situations. Such a flashlight usually has only one or two output levels, or stages, and does not use a clicking switch. These flashlights are very popular even with non-tactical users, as they are reliable and effective.

Q: What's "candle mode"?
A: This is simply the removal of a flashlight's head in order to expose its light-emitting portion and provide a smooth, lantern-like area light. The Mini-Maglite is well-known for this heavily advertised capability.

Q: What's "UI"?
A: UI stands for User Interface. You may have heard of the computer term "GUI," which is simply Graphical User Interface. A UI has the same purpose: it provides a way for the user to get their flashlight (or other device) to do what they want it to. As an example, some people value a simple on-off UI for its ease of use and perceived potential reliability. The other extreme is lights that require pages of spreadsheets and instructions that explain how to program the light to perform many functions. These complicated UIs are not necessarily difficult to use, but it is more of a challenge to keep them user-friendly.

Q: What's "EDC"?
A: EDC is more than just an acronym meaning Every Day Carry. It's a way of thinking that values preparedness for a particular situation anywhere and anytime. This way, if an EDCer suddenly needs to light something up, start a campfire, cut open a box, and so on, they will have what they need on their person. A tool that's too inconvenient to use when it's needed is worse than useless, so lights (and other items) that can be conveniently carried around with little bother are highly regarded by many people.

Parts FAQs:

Q: Lens? Window?
A: The clear material in front of a flashlight's light source and optical system, intended to protect them, is usually called a lens. However, it is more accurate to call it a "window," as a lens could also refer to some sort of optical system where light goes through a piece of clear glass or plastic and is distorted in some way (e.g. collimated or dispersed). Technically, a window is nothing but a plano-plano lens (flat on both sides), but it's easiest and clearest to call it a window and save "lens" for the above-mentioned optical system.

Q: What goes into a flashlight besides the essentials?
A: Flashlights aren't just a battery, body, switch, and light source (and sometimes electronics).

One of the most important additions is some sort of optical system, such as a lens or reflector. These focus the light from a source into a narrower beam that can more easily light up distant objects. Reflectors can be made out of plastic, aluminum, steel, or other materials. They must be given a mirror-like surface somehow. Plastic reflectors are very cheap, but can melt in flashlights with an incandescent bulb of more than 15W. Some reflectors are given a slightly textured surface in order to smooth out flaws in the beam, reducing unwanted artifacts. These are known as "stippled," "orange peel," "textured," or "stochastic" reflectors. Optics, or lenses, can take several forms. There is the popular TIR, or Total Internal Reflection, optic, which sits over a light source (usually an LED) and collimates most of its output into its intended beam pattern with little sidespill. A more extreme optic is the aspheric lens, or asphere. Such a lens is simply a plano-convex piece of glass or plastic, with the convex side not quite the shape of a sphere. A spherical convex portion creates chromatic aberration, which basically means a fuzzy image. An asphere can produce precise collimation with an intense hotspot, little corona (if any), and no sidespill. Some lights based on 5mm LEDs will use a basic plano-convex lens to provide a rudimentary level of collimation.

Another important component is the window. Plastic windows are more easily scratched up than glass ones and don't let as much light through, but they are cheaper, lighter, and less likely to break when impacted. Windows can be given anti-reflective coatings which reduce the amount of light reflected back into the flashlight, providing a slight increase in output. There are several glass window types available: mineral glass, sapphire, Borofloat, and UCL are the most common. Mineral glass is simply ordinary glass. Sapphire is literally the mineral (not to be confused with "mineral glass") sapphire, which is very hard and tough, but more expensive than glass. Borofloat (Boro) is a type of glass made by Schott. It is more resistant to thermal shock (repeated heating and cooling), and is thus ideal for high-powered incan hotwires. UCL stands for "Ultra Clear Lens," and is indeed clearer than other types of glass windows, with anti-reflective coatings on both sides. It is not as tough as sapphire or Boro windows, but has the best light transmission properties. According to testing by (who sells UCL and Boro lenses), polycarbonate transmits 91% of light directed at it, Boro transmits 94%, and UCL transmits 99%.

A light's O-rings are simple rubber rings which are squeezed between two of the flashlight's components in order to prevent water and other environmental hazards from entering the light. Usually, there is an O-ring just above any threads in the head or tail. When the head or tail are screwed on, they press down on the O-rings to form a seal. Some manufacturers use double O-rings, which just means having two O-rings one after another in a particular part of the light. O-rings work best when they are lubed (see links above).

Q: What do filters do?
A: A filter is a semi-transparent covering over a flashlight's window that changes the beam in some way. There is a wide variety of filters available for various tasks. A red filter, for example, will turn the flashlight's beam red, which is useful in low-powered flashlights for preserving human night-adapted vision. Remember, however, that traditional white power LEDs don't have much of a red component at all, so a red filter on such a light will significantly reduce output. There are also IR and UV filters, as well as diffusers which soften a flashlight's central spot and provide a wider, more close-range beam. Surefire manufactures many kinds of filters and diffusers for use with their lights, and some can be used on flashlights from other manufacturers.

Q: How do I choose a switch?
A: The "reverse clickie" is very commonly available in inexpensive and general use household and outdoor flashlights. It has a button which must be fully pressed until it clicks and then fully released in order to get light. Once the light is on, a soft press will momentarily turn the light off, and once the button is released, the light will reactivate. This is a very inexpensive (and hence popular) switch type, but it is not suited for signaling or momentary operation.

A "forward clickie," however, can be gently pressed from the Off position to momentarily activate the light, and it will turn off once the button is released. The user can also push the button until it clicks to keep the light on. This is available in some Maglite, Streamlight, Inova, and Surefire models.

Another type of switch is the Lock-Out Tailcap, or LOTC. Such a switch has no clicking mechanism. It can be "locked out" (prevented from turning on, useful for storage and transport) by unscrewing the tailcap to some degree. If the tailcap is tightened enough, pressing down on it turns the light on until it is released. If it is tightened further, the light turns on and stays on. This type of switch is most common in tactical Surefire models. Some lights have a switch similar to this that cannot be locked out, such as Streamlight's TL-2 and TL-3. They are still suited to tactical use.

The simplest switch is the twisty. In such a flashlight, two components (usually the body and head) are tightened together to close a circuit and turn the light on. These are highly valued for their perceived reliability, but thread quality and frequency of use are a large factor in the system's longevity.

There are several other types of specialty switches, including magnetic reed switches, Hall effect switches, membrane-covered electronic contact switches, and more, but they are best discussed in conjunction with the lights that use them.

Battery FAQs:

Q: Cell? Battery?
A: A cell is a single "piece" of power. For the alkaline chemistry, for example, a single cell has a voltage of 1.5V. For NiMH, it's 1.2V, and so on. Cells can be combined in series or parallel to form a "battery" of cells.

Q: What do protected/unprotected mean in regards to Lithium-Ion?
A: As mentioned in the "what is double-tapping" question, a "protected" Li-Ion has a small electronic circuit integrated into the cell packaging. It protects against common dangers, such as overcharge, overdischarge, short-circuit (overcurrent), and temperature. These cells are safer to use, both individually and in batteries. They are less likely to ignite and cause personal or property damage, a phenomenon known as "venting with flame." Unprotected cells do not have this protection circuit, so they can have more capacity and current capability than protected cells. Some cells, such as 10440 (AAA), are too small to accomodate a protection circuit and are only available in unprotected form. Users must decide whether they prefer to stick with protected cells or accept the bigger responsibility and danger associated with unprotected cells.

Q: What are the different types of Lithium-Ion?
A: Lithium-Ion is a rechargeable family of cell chemistries. Ordinary Li-Ion types are referred to simply as Li-Ion, RCR, or by their metric size designations (see the question "what's up with these five-digit batteries?"). When they have just been charged, they have an open-circuit voltage of about 4.2V (make sure your charger terminates at 4.20V at the most). They have a nominal voltage of about 3.7V, after a bit of use and under a moderate load. You should recharge them once they reach about 3.4V-3.5V. To make Li-Ion a more suitable replacement for primary (nonrechargeable) CR123As, with their 3V nominal voltage, some manufacturers add some sort of circuit or dropping resistor into their 16340-sized cells, giving them a lower voltage intended to mimic that of primaries. These have reduced capacity compared to primaries or even ordinary 3.7V Li-Ion, but they serve their purpose. A relatively new type of Li-Ion is LiFePO4, or Lithium Iron Phosphate. With a resting voltage of about 3.2V-3.3V, it has lower voltage (and slightly reduced capacities) as compared to the classic Li-Ion discussed above, can handle significantly more current, and is safer. For example, CR123A-sized LiFePO4 cells can handle about 5A, whereas the same current would require an 18650-sized Li-Ion cell or greater. LiFePO4 is available in 16340 (CR123A), 14500 (AA), and 18650 sizes.

Q: What's up with these five-digit batteries?
A: Cells of various types can be referred to by their standardized size codes. The first two digits are the cell's diameter in mm, the second two digits are the cell's length in mm, and the "0" usually found at the end indicates a cylindrical cell. An ordinary AA cell, for example, is 14mm in diameter, 50mm long, and cylindrical, so it would be a 14500. Other common sizes are 10440 (AAA), 26500 (C), 16340 (CR123A), 17500, 18500, 14670, 17670, 18650, and more.

Q: Why can't I use alkalines in more lights?
A: Alkalines can't deliver current nearly as well as other chemistries, such as NiCad, NiMH, lithium, or Li-Ion. A AA cell, for example, can't really do better than 4-500mA. Any more, and it will only provide a tiny fraction of its advertised capacity. The other chemistries mentioned above can provide high current quite reliably throughout their advertised capacity.

Electronics/Electrical FAQs:

Q: Please explain volts, amps, watts, and C.
A: That's not a question, but okay. :) Volts are electrical potential, amps are electrical current, watts are total power equal to volts*amps, and C is electrical current as a function of battery capacity. Think of volts as the width of a pipe: In general, a wider pipe has more punch than a narrower one. Think of amps as the water flowing through a pipe: Some pipes can only handle little trickles of water, and others can handle lots of water pushing through with great force. Think of watts as a combination of volts (pipe width) and amps (flow of water): A large pipe with water flowing through really slowly has the same output as a small pipe with water blasting through it. This is why high-voltage applications are preferred over high-current applications, as a stream of water zooming at 200mph through a 1"-diameter pipe is much more dangerous and difficult to maintain than a calm, 3mph flow of water through a 4'-diameter pipe. As for C rates, that's just a function of current draw and battery capacity. Any power source discharged at a 1C rate will be depleted in 1 hour, any power source discharged at a .25C (or C/4) rate will be depleted in 4 hours, and so on. As an example, a 1.8Ah AA NiMH capable of an excellent 10C discharge rate can manage 1.8*10=18 amps.

Q: What are series and parallel?
A: Series connections have a device's positive terminal connected to the next device's negative terminal. This is what you get when you line up some ordinary C-cell alkalines (for example) end-to-end, like in a Maglite or other flashlight. This arrangment adds up the voltages of the cells. Such a battery neither handles more current nor contains more mAh capacity than a single cell. This is the opposite of a parallel configuration, which has positive terminals joining together and negative terminals joining together. An example is those 3AA>1D adapters where all three AA cells' positive terminals meet at the top, and all their negative terminals meet at the bottom. Such a configuration has the same voltage as a single cell, but can handle more current draw (or contains more capacity). For example, 1AA alk can push about 500mA at around 1.5V for about four hours. 2AA alks in series can push 500mA at around 3V for about four hours. 2AA alks in parallel can push 1000mA at around 1.5V for about four hours (or 500mA for eight hours, and so on).

Q: What are "direct drive" and "regulated"?
A: A direct drive (DD) light is one that has the battery directly connected to the bulb or LED. A regulated light has some sort of driver circuitry between the two. A DD setup is heavily affected by the battery size and type. In a regulated light, the circuitry will try to minimize the effects of the battery. The huge majority of incandescent lights are DD. They start out bright, then fade over time. The effect is greatest with alkalines, which don't do well in many situations. The effect is least noticable with Lithium-Ions, which maintain a steady voltage under relatively heavy loads. This is why traditional Maglites, which are DD by alkalines, start out bright for about half an hour, then quickly fade out and become dim for the next few hours until the battery gives up. One example of a regulated incan, which provides rock-steady output for the majority of the battery life, is Surefire's A2. In order to drive mostly similar LEDs with wildly different battery solutions, a regulation circuit allows steady output for as long as the battery has power. As an example, the Fenix E0 runs on a single AAA alkaline for eight hours with no decrease in output. If it were DD, it wouldn't light up at all, much less provide constant output. An appropriately DD LED flashlight would be one driven by button or coin cells at somewhere above the LED's Vf. This results in a long runtime with slowly decreasing output, determined by the battery's remaining power.

Q: What are "boost" and "buck"?

A: Boost and buck circuits increase and decrease, respectively, the output voltage of a battery. This is used because of Vf requirements (discussed elsewhere in the Welcome Mat). Such a circuit will usually have battery + and - inputs as well as LED + and - outputs. The interesting thing about these circuits is that they can also be used to tweak battery current consumption, as a boost circuit will draw more current from the battery than is flowing at the output, and a buck circuit will draw less current from the battery than is flowing at the output. This generally means that boost circuits are hard on cells, while buck circuits are easier on them.

For example, 2AA NiMH powering an XR-E with a Vf of 3.7V at 700mA would require a boost circuit. If the circuit was 100% efficient (not actually achievable), the following equation would apply:

3.7V/2.4V*700mA= ~1080mA

This means that we can use a lower voltage source like 2.4V, but we will have to draw over 1A to produce the desired 700mA at the emitter.

For real-life circuits with efficiencies under 100%, simply divide the required battery current by the efficiency (expressed as a number between 0 and 1). For example, an 85% efficient boost circuit applied to the above situation would result in the following equation:

1080mA/0.9= ~1270mA

For buck circuits, the opposite situation applies. For example, powering a 5mm LED with a Vf of 3.4V at 20mA with a 90% efficient buck circuit on a 9V battery would result in the following equation:

3.4V/9V*20mA/.9= ~8.4mA

Keep in mind that these are simplified situations, with real flashlights being influenced by a number of limiting factors.

Q: What is "linear regulation"?
A: LDOs (Low Dropout) and linear regulators, like the AMC7135, have Iin=Iout. The energy loss comes from reducing the Vin down to Vout without reducing Iin below Iout (which is what a buck converter would do). So, a Vin of Vf+Vdropout+Vdrop, with Vf being the LED's Vf at whatever current you chose, Vdropout being the regulator's minimum voltage drop (which is very small for LDOs like the 7135), and Vdrop being any additional dropped voltage, would waste (Vdropout+Vdrop)*I watts for an efficiency of Vf/Vin. As Vin falls due to sagging battery voltage, Vdrop will decrease, resulting in a more efficient regulator but, of course, reduced cell capacity. If Vin falls below Vf+Vdropout, the light will basically DD with as much voltage as it can muster, with Vout (no longer a fixed value, but determining the emitter's Vf, and therefore its If, as well as Iin from the battery) being equal to Vin-Vdropout. At that point, efficiency would be Vout/Vin or, equivalently, (Vin-Vdropout)/Vin.

Q: Can I wire driver boards in series?
A: Except in certain cases, no. If, for example, you have a driver with a max speced voltage of xV, putting two of these drivers in series with a 2xV battery may fry your drivers.

Q: Can I wire driver boards in parallel?
A: Yes, that's fine.

Incan FAQs:

Q: What does "overdriving" mean?
A: Every incandescent bulb has a rated nominal voltage, which is basically the manufacturer recommended driving voltage. A bulb connected to a lower-voltage source than is recommended with have weaker and more orange-tinted output, but it will have increased durability and lifetime. A bulb driven above spec will have whiter output, but decreased durability and lifetime. A given bulb's efficiency actually increases along with drive level.

Q: What is "double-tapping"?
A: This refers to a phenomenon involving high-powered bulbs and a protected Li-Ion battery. Such a battery contains one or more Li-Ion cells, each with a protection circuit that cuts power when it detects overcharging, overdischarging, overheating, or short-circuit. The protection circuit determines a short-circuit by checking the current draw at a preset drive level. A common 18650 cell, for example, may have its protection set at around five amps (5A). This comes into play when the battery runs a high-powered incandescent bulb. Incans have lower resistance when they are cold, and the resistance increases with temperature. As such, a bulb with a cold filament may draw too much current for a protected Li-Ion battery, triggering its short-circuit protection. Once this has occurred, however, the filament will be slightly warmer, increasing resistance and decreasing current. This may allow the battery to power the bulb without triggering the protection circuit.

Q: What is "instaflash"?
A: Since an incan bulb is whiter, brighter, and more power-efficient when it is overdriven, it is common practice to push bulbs almost to their bursting point. If a particular flashlight drives its bulb right at the edge, it may sometimes provide too much voltage and current (when the battery is fresh, for example), which will cause the bulb to burn out.


Q: What's "magic smoke"?
A: When an LED is blown, it emits a small wisp of smoke. The smoke is then jokingly said to have been the magical element that made the LED work, and its release renders the device nonfunctional.

Q: Is turning an LED on and off bad for it? What's "PWM"?
A: No. LEDs are semiconductors, which are quite happy with transitioning between an "on" and an "off" state. In fact, most LED flashlights provide extra output levels by flickering the LED at a faster rate than the human eye can usually detect. When the output is not filtered and smoothed out, the flicker can be seen when the light source, subject, or viewer moves. This flicker is commonly known as "PWM," or "Pulse Width Modulation." Unfiltered PWM must be driven to frequencies of at least 50Hz, or else the flickering will be too pronounced and will irritate users.

Q: How can an LED be more efficiently driven?
A: LEDs are more efficient at lower drive levels. This means that slow, unfiltered PWM is less efficient than filtered and smoothed output, known as "current regulation." This is because an ordinary PWM light is active part of the time and completely off at other times, but during the active portion, the LED is being driven at full blast, which is comparatively inefficient. Therefore, a multi-emitter, current-regulated light is more efficient than a single-emitter, PWM light, as the LEDs in a multi-emitter light will operate more efficiently (and use less power) than the LED in a single-emitter light when pushed to the same amount of output.

Q: What's the "Luxeon Lottery"?
A: Different samples of "white" Luxeon emitters, even ones of the exact same classifcation (known as "bin"), can vary in their light output and tint at a particular drive level. Some are slightly green, or purple, or other colors, while others have less noticable coloring. This term can apply to any high-powered LED with natural variations in tint and output.

Q: What's "Vf"?
A: Vf is the term for an LED's forward voltage. Forward voltage is the voltage required to activate an LED. For example, if you want to power a Cree XR-E at 150mA, it needs about 3V. If you want to power it at 700mA or thereabouts, the Vf will rise to about 3.8V. It's like a minimum activation voltage for particular currents.

Q: Why is Vf important, and how do I measure it?
A: To determine a particular LED's Vf, you'll have to look at its datasheet. For example, Cree's XR-E datasheet is found at In page 6 of that PDF, you'll see a graph of Vf. This means that the Vf of a particular emitter is not fixed, but varies, with the available voltage determining the emitter's "natural" drive level. For example, if you look at where the graph reads "3V," you'll see that it corresponds to a "Forward Current" drive level of about 160mA. This means that if you solder an XR-E to the appropriate terminals in an ordinary Minimag, which operates at 3V, you'll get about 30 lumens of floody, white output.

The downside to this is that small changes in voltage can have a huge effect on forward current. For this reason, most drivers supply a constant current instead of just a constant voltage, as current is really what determines an emitter's output. Therefore, if you want to power an XR-E at 700mA from a pair of AA NiMHs (a common task), you'll need a driver that can boost the voltage from 2.4V to something closer to 3.6V as well as supply a constant 700mA. If you tried direct drive with this setup, simply connecting the emitter to the battery, the emitter would barely light up at all.

It is important to note that even emitters of the same type and "bin" (sorting system based on several characteristics) can have small variations of Vf. For example, you may have an emitter that has a Vf of 3.7V when driven at 1000mA (or 1A), while someone else has an emitter of the exact same type and bin with a Vf of only 3.5V at the same 1A current. This means that their flashlight will consume only 3.5W of power while yours consumes 3.7W, although both would have the same output given the identical drive level.

You can find the Vf of your emitters at particular drive levels with a basic DMM. If you are using a driver and you don't know the current it provides, or if you're not using a driver (a DD setup), put the DMM in the circuit next to the emitter and check the current. Next, complete the circuit without the DMM in it, and put the DMM's + and - probes on the emitter's + and - contacts, respectively. This is the Vf at the current level you measured for this particular emitter.

Most emitters can handle a bit of current at reverse voltage, so if you wire one backwards but don't activate it for more than a second or two (at most), it should be okay. Just don't make a habit of it. :)

Q: What's "bin"?
A: Bin codes are used to sort LEDs by luminous flux (lumen output at a specified drive level), color, tint, and Vf. For example, a U-bin Lux III will have more output than a T-bin Lux III at a set drive level. Commonly desirable flux bins (at the time of writing) are:
-Lux I: R or S
-Lux III: T or U
-Lux V: W or X
-XR-E: Q2 through R2
-SSC P4: U or V
-Rebel: 0080 or 0100

Q: What's the deal with LEDs and heat?
A: Forget anything you learned about LEDs from the National Geographic Channel's "Manmade" show focusing on flashlights, especially from the interview with the Philips employee. LEDs are quite efficient relative to other light sources, but they do produce heat. In fact, most aren't even 30% efficient! This means that the more power you pump through them, the more light and heat they will produce. Unlike incandescent bulbs, however, LEDs are actually damaged by heat. It's common for a well-driven power LED to exceed 120F (quite hot to the touch). Too much heat for prolonged periods can decrease the life of an LED or even kill it. The efficiency (and therefore output) of an LED suffers with heat as well, meaning that with most lights, there is a certain drive level above which the increased heat will actually result in less output than a more moderate drive level. To combat this, well-designed flashlights provide a method to get the heat away from their LED. The most basic (and by far the most common) method is the heatsink. This is nothing but a chunk of metal that contacts the LED and is heated by it, leeching the damaging heat away from it. The next step is to somehow transfer that heat to the environment, where it can dissipate. This means that the heat must have a "thermal path" which leads from the LED to the heatsink to the surrounding flashlight to the environment. Some flashlights benefit from being held by someone's hand so that their bloodstream can act as a heat pump (the blood near the flashlight is heated, moves away, and cools, and the cycle continues). Other lights have fins that increase the surface area which contacts the outside air. LED dive lights don't have much of a problem here, since the surrounding water is like an enormous heatsink. Bike lights benefit from the cool night air rushing past them.

Q: What's the deal with LEDs wired in series or parallel? What about "current hogging" and "thermal runaway"?
A: If your input voltage and current are acceptable, LEDs can be run in series or parallel. The issue is how well the setup will work, and how reliably. LEDs experience "Vf shift" when they heat up, with Vf dropping as temperature rises. With a constant-current (CC) source, this is no problem, as the only effects will be less light (because of the heat) and less power consumed (a lower Vf affects the V*A=W formula). Otherwise, there won't be much to worry about. With a constant-voltage (CV) source, however, the Vf shift comes into play. If Vf for a particular current goes down, but you're only keeping the VOLTAGE constant, the current at that voltage will go up. This is a problem because it will cause more heat and further Vf shift, drawing more current, leading into a feedback loop where the LED(s) eventually pop from the stress. This is what is called "thermal runaway." As you might guess, this is an issue when deciding between a series or parallel LED setup. Recall that devices in series will all draw the same current (at whatever voltage each device needs for it), while devices in parallel all see the same voltage while having potentially different current levels going through each one. If you have a CC driver running a series LED string (a "string," or "leg," is a group of devices, like LEDs, connected in series), you won't have thermal runaway problems. However, LEDs in parallel, even driven by a CC source, can experience "current hogging." As the devices are in parallel, they won't necessarily have the same current. Each string in a parallel array driven by a CV source can experience thermal runaway. If driven by a CC source, one string can still undergo thermal runaway, but another string sharing the CC source will simply get less of the total current - the current was "hogged' by the other string. If your CC source in such a setup is set to drive LEDs near their limits, then having one string bear the load intended for two or more strings can be problematic. Remember that when LEDs pop, the circuit breaks, leaving the other strings in a parallel setup with a kind of surplus of current, which leads to even more rapid thermal runaway, which will eventually leave you in the dark. This is why it's generally recommended to wire LEDs in series, as well as using a CC source if possible. If you don't have a CC source, use an adequate resistor to limit current.

Q: LEDs are all the same, right?
A: Wrong. See below.

LED Types:

5mm/Nichia: This is the ultra-common low-power LED that gives off a wide spot of directed light. They come in many colors, and the "white" version is usually bluish, to some degree. They cost about $0.15 each. The Vf of these LEDs varies with color, with red 5mm LEDs having a Vf of a little over 2V and white 5mm LEDs needing about 3.5V. An ordinary white 5mm LED will produce from 5-10 lumens at a safe drive level of about 20mA. Because of this very low current requirement, these LEDs are ideally suited to keychain flashlights that run on small button or coin cells, as these cells can't handle much current anyway.

Luxeon I/III: These have been the standard range of high-power LEDs for years. They cost between $8 and $15, depending on bin. Prices dropped along with demand upon the release of the new high-efficiency power LEDs (explained below). The only difference between Luxeon I and Luxeon III is their speced drive currents. Lux Is are rated for a particular light output level at 350mA, and Lux IIIs are rated for a particular light output level at 700mA. This has led to the common marketing practice of calling Lux Is "1-watt" LEDs and Lux IIIs "3-watt" LEDs. This is misleading, because these wattages only describe the speced capabilities of these emitters (Vf of 3.4V * 350mA=1.2W, and Vf of 3.7V * 700mA=2.6W) and not the actual power consumed by the LED in any particular flashlight. Thus, a "3W LED flashlight" could be running at any actual drive level.*

A common Luxeon I driven at a decent 1W will provide between 30 and 60 lumens, depending on bin. A common Luxeon III driven at a decent 3W will provide between 60 and 90 lumens, depending on bin. They will emit those lumens in some sort of pattern: Lambertians emit more of their light straight forward, creating a nice 180-degree flood. Batwings are similar, but emit slightly less light in the center. Side-emitters emit most of their light to the sides.

* Some misleading sellers advertise LED flashlights as "10W," "15W," and so on. Even if these figures were accurate, they would only relate to power delivered by the batteries, as most of those dozen watts would have to be burned up by a resistor if such a flashlight were to last more than a few minutes. The LEDs used in these flashlights could never actually take 10-15W of power.

Luxeon V: Starting at around $10 each, these are LEDs with four "dice," or light-emitting chips. They still fit into a small package, but the larger emitting surface makes for focus and throw challenges. It's all too easy to get a "donut hole," which is a beam that has a darker spot in the center (like a donut; get it? :) ), and it's much more difficult to throw a small, tight spot with these than with the Luxeon I and III explained above. The benefit of these LEDs is that they have more output, with most samples reaching 100-140 lumens at proper drive currents (around 700-900mA), depending on bin. The downside with this package is the higher Vf of 6-8V, which necessitates specialized driver circuits and consumes more battery power. Luxeon V LEDs are available in Lambertian and Side-Emitting packages (explained above).

Luxeon K2: These came out a couple years ago, and they bore the hopes of flashaholics everywhere. Functionally, they were like Lux IIIs speced for drive currents in the neighborhood of 1.5A, with increased lumen output and heat tolerance. However, multiple delays, limited availability, and competition with the new high-efficiency power LEDs (explained below) made it a disappointment. It wasn't a leap forward so much as a shuffle in a general direction. For these reasons, they never met with much success.

Cree 7090 XR-E: These came out in the fall of '06, generating a flashaholic frenzy. They could be driven at voltages and currents simliar to Luxeon IIIs, but they were twice as efficient, meaning that in two flashlights identical except for the emitter, the XR-E flashlight would have twice the output! LED flashlights producing 150 lumens became a reality. Better bins released over the coming months provided even more efficiency and output. Unfortunately, they are not direct replacements for Luxeon lights due to a unique package (explained below). They cost between $6 and $15, depending on bin and seller.

SSC P4: Released a few months after the XR-E, these use the XR-E's EZ1000 die, giving them the same high efficiency and output as the XR-E. However, they have a significantly different package, bearing more resemblance to the old Luxeons. They are approximately $7 to $15, depending on bin and seller.

Edison Opto KLC8: These are high-efficiency power LEDs also using the EZ1000 chip in a package similar to the Lux III, but with some differences (explained below). They are around $4-$6.

Luxeon Rebel: These came out in mid '07. They have the same efficiency characteristics as the XR-E and Seoul, but the package (and intended application) is very different from the other high-power LEDs. They are ideally suited to surface-mount applications assembled with reflow soldering.

Luxeon K2 TFFC: This is an updated version of the K2, with new "Thin Film, Flip Chip" technology. It can compete with dice like Cree's EZ1000 (used in the XR-E and SSC P4).

SSC P7: Released in early '08, this is basically Seoul's version of a Luxeon V, using their P4 as a base. It has four dice all wired in parallel (compared to the LuxV's 2S2P), with the expected output of four SSC P4s.

Practical considerations: Lux Is and IIIs have a hard acrylic dome, smooth 180-degree radiation pattern, negative slug (underside), and easily soldered "legs." Lux Vs are similar, but with higher Vf and larger emitting surface. Luxeon K2s are also similar, but can be driven at higher power levels. XR-Es have a narrow radiation pattern (about 120 degrees) that projects a very faint gridlike pattern (bare, without optics) which lends itself well to collimation by aspheric lenses, a floating acrylic dome, and tiny pads that are very difficult to solder. The SSC P4 has a soft gummy dome to which dust sticks, very similar appearance to a Lux III except for tiny metallic portions visible surrounding the dome, smooth 180-degree radiation pattern, positive slug, easily soldered legs, and the tendency to tint-shift toward a cool blue when it heats up (if improperly heatsinked). The KLC8 has a positive slug, 180-degree radiation pattern, acrylic dome, legs, slightly less luminous flux when compared to the other EZ1000 packages, and collimation by a reflector results in a yellow-green ring around the hotspot. The Luxeon Rebel has a rectangular board with the emitter surface located off to one side, a neutral slug, and tiny pads that are very difficult to solder. The Luxeon K2 TFFC is similar to the ordinary K2. The Seoul P7 has quite a large overall package, and the parallel dice make the quest for suitable drivers a major challenge, but it has excellent output, especially for its size compared to single-die emitters.


Q: What are "HID" and "metal halide"?
A: "HID" stands for High Intensity Discharge. It refers to a family of bulb technologies, such as low-pressure sodium, metal halide (MH), xenon short arc (XSA), and others. This family produces light neither by exciting electrons (LED) nor by heating a tungsten filament (incan), but by various other means. XSA, for example, contains two metal surfaces between which travels a spark of electricity, creating a tiny, intense ball of plasma. MH lamps operate in a mostly similar fashion, but require a ballast to regulate the conditions inside the lamp. Most HID lights used in flashlights (and by CPFers) are of the metal halide type, so the terms are often used interchangeably. MH lights are quite efficient, and their efficiency increases at higher power levels. At around 15-20W, they are about as efficient as modern LEDs. This creates huge output with very reasonable runtimes. MH lamps' CCTs vary from 3000K up to 20,000K.

Laser Basics:

Q: Do I need some sort of safety equipment?
A: Yes. You should use appropriately-speced eye protection when using lasers over 5mW, of any color.

Lasers come in various colors. The most common and inexpensive color is red, with a wavelength of around 650nm. A popular, relatively new color is green, usually at 532nm. Because the human eye is more sensitive to green light than to red, a green laser will look significantly brighter than a red laser of equal power. There are also lasers of other colors, such as blue, violet, yellow, and more, but they are much rarer and more expensive. Red lasers are pretty simple and robust, but green lasers have a number of delicate internal components and are thus susceptible to damage from bumps and falls.

In the U.S., a Class II laser radiates less than 1mW (one-thousandth of one watt) of power, a Class IIIa laser radiates between 1 and 5mW of power, a Class IIIb laser radiates between 5 and 500mW of power, and anything more than 500mW is considered Class IV. The system has recently changed, with Class II now being called Class 2, Class IIIa being called Class 3R, Class IIIb being called Class 3B, and Class IV being called Class 4. More detailed class ratings and other safety information can be found at

You can find lots of cheap green lasers on eBay, but as a rule of thumb, these lasers will be cheaply made, with inferior parts or dangerous attributes. One such characteristic of a common eBay laser is the lack of an infrared, or IR, filter. Simply put, most green lasers create green light by first creating infrared light, and then converting it into green. Even after this conversion, however, there can still be IR radiation in the beam, so the better lasers have an IR filter. This is what makes unfiltered eBay lasers both disappointing and dangerous: a laser can truthfully be advertised as having 100mW of output, but without an IR filter, it could be radiating only 10-15% of that as green light, with the rest leaving the unit as invisible but still hazardous IR radiation.
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