How Do You Measure The Brightness of An LED?

[TheGeekInTroy]

What measurement is used to determine the brightness of a single led?

 

[WadeF]

lumens, and you need something like an integrating sphere to accuractly measure the total output.

 

[TheGeekInTroy]

Interesting terminology. Are there specific ratings for specific types of LEDs. For example Brand A LED is rated at so many lumens.

 

[Marduke]

It depends partly on how much electricity (current) you decide to feed the LED. You can adjust the brightness that way. LED's are like computer chips. After manufacture, they are graded based off how efficient they are. Those gradings (called "bins" here) are compiled for most common LED's here:

http://www.candlepowerforums.com/vb/showthread.php?t=156772

Another useful thread is:
http://www.candlepowerforums.com/vb/showthread.php?t=172991


:welcome:

 

[Jarl]

an LED is rated by lumens- the amount of light is produces. Most modern LED's can get ~100 lumens/watt at 350ma, with more at lower powers and less at higher powers.

 

[TheGeekInTroy]

What is the electrical limit of an LED as far as lumens go. For example, if you pump so much ma's into and set of LEDs, will they "explode"? In other words, does the amount of lumens have some bearing on wear and tear. Can you wear down an LED faster by pumping more electrical current into it. The answer I would assume would obviously be yes, but how much?

 

[Marduke]

What is the electrical limit of an LED as far as lumens go. For example, if you pump so much ma's into and set of LEDs, will they "explode"? In other words, does the amount of lumens have some bearing on wear and tear. Can you wear down an LED faster by pumping more electrical current into it. The answer I would assume would obviously be yes, but how much?

Heat buildup is actually what degrades an LED, and it is very complicated to say by how much. It's specific to each LED, depends on how the LED is mounted, etc. If you are really interested, the data is published in the manufacturer spec sheets, which can be found in the above linked thread.

 

[Yoda4561]

There is no theoretical limit to max output as far as I know, there is one of course but it's probably upwards of 10,000 lumens. As far as white phosphor leds there is a ~250 lumen/watt theoretial maximum (per watt) and multi-die RGB emitters should be able to achieve a ~350 lumen true white output. With current technology we're sitting at just over 100 Lumens/watt which decreases as you pump more power into an LED, which is why a 5 watt emitter with 100L/w will only put out 250ish at 5 watts instead of 500.

As far as waste heat goes, 100l/w is around 33% efficiency, so 66% gets turned into heat which adds up fast as you increase power, and heat kills LEDs. Luxeon V emitters (with a 10,000+ hour lifetime rating) apparently only last around 500 hours in a real flashlight with gradually decreasing output due to the heat wearing out the emitter. Shove too much current into an LED and it will just die.

 

[LEDninja]

:welcome:
There are 2 ways to measure the light from an LED.
The light directly in front of the LED, usually in Lux or MCD.
The total light in all directions, in Lumens.
Here are Quickbeam's explanations at Flashlight Reviews:
http://flashlightreviews.com/features/lux.htm
http://flashlightreviews.com/features/lightbox_output.htm
http://flashlightreviews.com/features/output_vs_throw.htm#lumens

Heat and high voltage kills LEDs.
Size of the LED and the speed of the removal of heat determines how much current an LED can take before going

5mm LEDs usually have 1 to 5 0.25x0.25mm die. There is no easy way to get rid of heat. So at 20mA they dim significantly in 1/2 year and practically useless after 1 year of continuous use. At 100mA lifetime can be down to 50 hours.

When Lumileds make the Luxeon series they are rated
1 watt (max 350mA @ 3.6V)
3 watt (max 700mA @ 3.6V)
5 watt (max 700mA @ 7.2V) This is just 4 1 watt jammed into a single case.
K2 (max 1500mA @ 3.6V)

The Cree XRE was originally designed for 350mA but has now been certified to 1000mA @3.6V.

The SSC-P7 put 4 XRE LEDs in a case and can take 2.8A @ 3.6V.

The upcoming Cree MCE also have 4 XRE LEDs in its case but can be wired from 4x1A @ 3.6V to 1A @ 14.4V.

All the power LEDs require good heatsinking to get rid of heat if you want to run them above 20mA or so.

 

[TheGeekInTroy]

Marduke » Thank you for your straight-forward response to my question.

Yoda4561 » Your post was very informative. In response to your well written post I have an additional question. Taking that practical limits of the LED (not theoretical) how does that stand up to the incandescent light-bulb. Maybe I can phrase my question this way. If you built an LED and an incandescent light-bulb with the same size "filimant" so-to-speak and arranged both to take the same amount of wattage, which one would last longer. Which would burn longer regardless of temperature. Do you see my point?

LEDninja » Thanks for the links. I enjoy pages with diagrams very much. Words are sometimes very limited. I will review those pages later in more detail. I appreciate the links. You said "5mm LEDs usually have 1 to 5 0.25x0.25mm die." Could you explain this statement to a beginner like myself. When you say "5mm" what are you referring to. What is being measured?

 

[Marduke]

For the same amount of power used, LED's are now up to 10 times more efficient. LED's in general far outlast incan bulbs, and almost never actually "burn out".

I highly recommend reading the Welcome Mat, as it will answer many of your beginner questions.

 

[Yoda4561]

Well let's look at that from the surefire P60 perspective since it's very easy to do so. Instead of looking directly at wattages we'll use light output (lumens), since I'm too lazy to figure out just how much wattage a P60 draws. With the latest emitters and an efficient driver circuit, a 60 lumen P60 LED can be run in regulation (non dimming) for about 8 hours. A 60 lumen P60 incandecent will run approximately 1 hour, half of which is spent below 60 lumens.

As far as time to failure/burnout, a P60 incan will last between 25 and 50 hours on average, the hottest overdriven LEDs can last at least 500, and most are a bit better than that. As leds increase in efficiency towards 100% electricity>photon conversion the reduction in heat will allow them to last much longer, perhaps even as long as the old "100,000 hour" manufacturer claim when LEDs were becoming popular. If both are driven to say, 10 watts in a P60 type assembly, assuming the LED is heatsinked properly and a xenon lamp incan. you run into problems for both of them.

A 10 watt LED in a small handheld light is putting out ALOT of heat, if it doesn't go somewhere then it will severely shorten the life of the LED, by how much depends on how bad the overheating is and how well that particular LED handles temperatures. If you can take care of the heat by using a larger flashlight body or more efficient LED you're looking at 1000's of hours before failure. The incandecent will still have a roughly 30 hour bulb life but due to its construction is more prone to failure via shocks or vibration.

 

[LukeA]

Well let's look at that from the surefire P60 perspective since it's very easy to do so. Instead of looking directly at wattages we'll use light output (lumens), since I'm too lazy to figure out just how much wattage a P60 draws. With the latest emitters and an efficient driver circuit, a 60 lumen P60 LED can be run in regulation (non dimming) for about 8 hours. A 60 lumen P60 incandecent will run approximately 1 hour, half of which is spent below 60 lumens.

As far as time to failure/burnout, a P60 incan will last between 25 and 50 hours on average, the hottest overdriven LEDs can last at least 500, and most are a bit better than that. As leds increase in efficiency towards 100% electricity>photon conversion the reduction in heat will allow them to last much longer, perhaps even as long as the old "100,000 hour" manufacturer claim when LEDs were becoming popular. If both are driven to say, 10 watts in a P60 type assembly, assuming the LED is heatsinked properly and a xenon lamp incan. you run into problems for both of them.

A 10 watt LED in a small handheld light is putting out ALOT of heat, if it doesn't go somewhere then it will severely shorten the life of the LED, by how much depends on how bad the overheating is and how well that particular LED handles temperatures. If you can take care of the heat by using a larger flashlight body or more efficient LED you're looking at 1000's of hours before failure. The incandecent will still have a roughly 30 hour bulb life but due to its construction is more prone to failure via shocks or vibration.

I feel I should add that hot/overdriven LEDs are not subject to shock/vibration problems like incandescent bulbs.

 

[Smile]

:twothumbsGreat discussion,but it really past my understanding.:thinking:

 

[LEDninja]

5mm LED
picture here
http://ledsupply.com/5mm-leds.php

I do not have a picture of the internals of a 5mm LED.
Originally the 5mm LEDs have only 1 die (the part that emit light).
As it is more difficult and expensive to make bigger dies, some 5mm LED manufacturers put more dies inside the case to get more light. (The technique was originally developed to make multi-colour LEDs - green for working, yellow for standby, red for off on a DVD player for example.)

This is also used in bigger LEDs.
SSC makes a P4 LED with 1 XRE die.
To get a brighter LED they simply put 4 XRE dies in a single case. (The flashlight manufacturers only need to wire up 1 LED instead of 4, buy 1 reflector instead of 4)
Notice how the P7 has 4 dies (those yellow squares in the middle) and the P4 has 1.

In addition to the Welcome mat there is also a CPF Wiki for basic info. 2nd row in the CPF menus.

 

[Gunner12]

How do LEDs compare to incans? Well Yoda has explained a lot about how long they last compare to incans, now lets talk power.

Using the Surefire P60 as an example, it takes almost 8 watts of power(6v x 1.3 amp) to produce 100 lumen of light at the bulb (Surefire rates conservatively, I also added the light that was lost within the reflector and going through the window of the Flashlight). That makes the P60 almost 13 lumen per watt efficiency wise.

Now lets look at a Cree XR-E P4 bin driven at 700 mA. The normal voltage for the LED at this current can be from almost 4v at the extreme and 3v at the other extreme, normal being somewhere around 3.5-3.6v. Lets use 4v. So the LED is taking 0.7 amp x 4v so 2.8 watt. At this current, the LED should be producing around 140 lumen. More then the Incan at 7.8 watt. If you use a higher bin LED, like the Cree XR-E Q5, you'll be getting around 180 lumen at 700 mA current. So the LED can produce much more light then the Incan bulb can for the same power. The problem is LEDs above 10 watt aren't that efficient. Above 10 watt, it might be better to use HID lamps, another technology.

5mm LED usually run at 20 mA

As for die size, it seems like the larger the Die is, the more current it can handle, presuming enough heatsinking. Even one of the most efficient commercial LED is only 50-60% efficient at a maximum. So there is still a good bit of heat produced. With heat comes a lower efficiency(and quicker degration), so as you get into higher currents, the LED get less and less efficient.

Die is the heart of the LED. It produces the light you see when you power up the LED. For a white LED, this die produces blue light(royal blue?). The yellow you see is the phospor which changes some of the blue light into yellow and other colors of the spectrum. The problem is most white LEDs peak in output at the Bule and Yellow parts of the spectrum, Red and other colors are produces but not much compared to the yellow and blue. That is why things can appear to be "flat" with LED lighting.

Binning is how LED companies spurt their LED. At a minimal, the bin tells the color, tint, and efficiency of the LED. Charts here.

Sorry if my post was confusing.

 

[Windscale]

I do it the old fashion way by using a handheld incident lightmeter and reading the EV values from 1 meter away.