Why do throwers have to utilize Asperics? (why not just plain convex lenses?)

[GarageBoy]

Last time i checked, Aspeherics were to counter act the affects of chromatic abberation and to cut down on coma.
can we use a simple convex lens instead?

 

[TorchBoy]

Respectively: Who said they have to? Have you tried? You'll find a magnifying glass throws a very nice and extremely narrow beam.

 

[GarageBoy]

I just assumed because every mod utilizes an aspheric

 

[TorchBoy]

Not every mod. Most do, certainly, as they have a much shorter focal length.

 

[yellow]

is there a possibility any high magnification lense gets called "Aspheric" in common speech?
:thinking:


PS: as aspherics, high mag lenses, fresnels, ..., all do the same with the led beam ... does it count what is used? If the outcome is useful?

 

[TorchBoy]

is there a possibility any high magnification lense gets called "Aspheric" in common speech?
:thinking:

Very probably. But it would be a little unusual for a high magnification, short focusing lens to have spherical surfaces wouldn't it?

 

[kengps]

The problem with another lens type is the focal length. If you move the LED 6" away from the lens you're only gathering a small amount of the total spread of light. LED's have a wide-angle of output. You want to capture as much as you can, thus be close to the lens. What DEFT does now is use a pre-lens that is right next to the LED. It collects the LED light and then decreases the angle to bring it down to the diameter of the Aspherical lens.

 

[Walterk]

Whats named here Aspheric lens is usually planoconvex btw.
What people here call an aspheric lens, is that it is a fat plano convex lens.
With fat I mean it has a big thickness relative to the diameter.
More shape gives more transformation of the lightrays, and therefore shorter focal length.
In effect a plano-convex could work, even an almost-plano convex lens.
Plano convex lenses are also used for high quality optics, such as camera's and projectors. More extreme lenses are manuafactured mostly to be used as so called 'condensor lens', suitable to direct and guide light, but have not enough quality for imaging purposes.

Asphericals are the more extreem, affordable plano convex lenses that give us the short focal length we want to catch all the light emmitted from the Led's viewingangle.

So; yes, simple convex lenses can be used, preferable short focal length, and the other focus length on a distance that is in your interest. Just try, when it works for you it works.

 

[ma_sha1]

The Aspheric used in flashlight is a subtype of plano convex lens.

Plano convex lens get lots of distortion when it get to short EFL
type, thus they use Aspheric surface to reduce distortion.

Since Flashlight always use short ELF lens, non Aspheric type Plano convex would give too much distortion. In fact, most mfg. won't even make short EFL Plano convex lens with regular spherical surface, when it gets to a short EFL, they'll almost always make it aspherical to reduce distortion.

 

[Walterk]

This picture might clear things up:

The aspheric is more a curve than a part of a circle, shapes may differ.

The curved material is effective of influence on the beam. The thickness of the rim is for easy manufacturing and mounting only.
EFL is the theoretically focuslength, but for us what counts is the so called 'back focal length'.
You can find out the back focal length by measuring the distance between led and the plane surface at which it gives the best projected beam.

I have looked into it somewhat, but can not define what parameters have what result. I made the following comparison:

As you can see the focal length determines what angle the lens covers from the light that is emitted from the Led. As you can see the green 75mm lens catches the most light.

Actually I am in the blind for most of the rest :thinking:.
I dont know how to read the diagram!
What does the crosshatching tell me?

The 75mm seems better then the red 76mm lens, but why?
More emitted light is catched and spread, but over greater diameter of the lens....How to reason and calculate with lumen/mm2 over the aspheric?
What is the optimum? How to figure this out?

Hope some usefull AND some theoretical post will follow! Then this thread might grow into a guide for lenses!!

(I have no data to add a HID-Headlight lens to the comparison. If anyone could give me measurements and findings?! )

 

[TorchBoy]

Re: Why do throwers have to utilize aspherics?

The 75mm seems better then the red 76mm lens, but why?

What red 76 mm lens?

 

[COAST]

.. I used my telescope and that throws SERIOUSLY far .

 

[Walterk]

Makes sense, I've read using the telescope reversed is a common way to change the beam. Found in every day's objects like microscopes, telelens, telescopes,..
It expands efficiently the beam to a wider diameter.

The funny thing is that although I discovered this effect with a scope already, It took me months of reading CPF before I by accident discovered this...

See this thread at CPF : LED long-throw design suggestion

Hint for 'those interested' to save you hours to spend really working on flaslights; Google 'Huygens telescope' and 'Keplerian' and 'Galilean beam expander', to see how the precollimating Deft, the finest example of Led-applied beam expander, works.

 

[Apollo Cree]

An aspherical lens can produce a better beam than a spherical lens.

Shine it on a distant wall. The best aspherical lens can put more of the light into a small spot on the wall than the best spherical lens.

For a spherical lens, the light hitting the edges of the lens won't end up going to the same spot as the light hitting the center of the lens. If you want the light hitting the edge of the lens to hit the same spot as the light hitting the center of the lens, you have to use an aspherical lens.

The aspherical advantage becomes more significant as you move the lens closer to the light source, and as the diameter of the lens gets higher. (This equates to focal length.) You want the lens to be close to the LED and wide so that more of the light from the LED goes through the lens.

In practice, it gets more complicated as you consider problems with the size of the die vs. the size of the lens, reflectors, etc.

 

[gcbryan]

I asked this in another thread but it wasn't answered so I'll ask it here.

With everything else being equal, what magnitude of increased throw would I notice with aspherical lenses if I increased the diameter from 30mm to 40mm and then to 50mm?

I'm asking for a practical reason and keep in mind I'm using cheap aspherical lenses and don't always know the specs other than the diameter. The rest is just experimentation.

I have modified a flashlight with a 28 mm aspherical lens (using a XR-E R2) and it effectively throws 200 feet. It's more but it dims out after that so I'm considering that this set-up throws 200 feet.

If I changed nothing other than the head/aspheric diameter what distance could I consider the effective distance with a 40mm diameter aspheric and then again what distance if I increased that to 50mm?

Any ballpark figures that I could use?

 

[Walterk]

Still looking for the ballpark figures. Here's what I found so far:

Formula for throw:
Found here , including useful notes and diagrams on Led and aspherics.
Ibeam = ((G x πR2) / ds2) x Isource
G is the correction factor for the form of the lens. (Look at diagram at linked site.)
Pixr2 is the diameter of the lens
ds2 is the area of the led-die
Isource is the Lux of the led at a given Amp.

For optics:
Throw is determined by the area (diameter=3,14xradius2)) of the lens.
Theoretically: 4 x the surface area, gives 2 x throw. (something with inversed square law?)
The longer the focal length, (or the higher the F-number, as it is related) the narrower the beam. At the same distance the spot or projection is smaller with a wider lens. As you can not make more light then there is from the Led, this means with a more narrow beam you have the same light but more concentrated beam (or spot at same distance ) hence more throw.

Diameter, F-number and focal length are related.
The focal length (back focal length as it is named for plano convex ) is the distance between Led and lens at which it projects a sharp image of the die.
F-number is focal length divided by the effective lens diameter (or 'clear window without rim/edge'). This F-number (see wiki) determines how many light of the Led can be effectively used from the led, when compared to the Flux-vs-Angle diagram in the datasheet of the given Led.
In practice the distance between Led and lens is limited to a few inches, as hardly any light will enter the lens otherwise.

There must be an optimum in each set up, but I dont know yet how to calculate this. Also I dont know yet how to compare above formula to real world lux-measurements. Better experiment if you have some lenses at hand.

 

[gcbryan]

Still looking for the ballpark figures. Here's what I found so far:

Formula for throw:
Found here , including useful notes and diagrams on Led and aspherics.
Ibeam = ((G x πR2) / ds2) x Isource
G is the correction factor for the form of the lens. (Look at diagram at linked site.)
Pixr2 is the diameter of the lens
ds2 is the area of the led-die
Isource is the Lux of the led at a given Amp.

For optics:
Throw is determined by the area (diameter=3,14xradius2)) of the lens.
Theoretically: 4 x the surface area, gives 2 x throw. (something with inversed square law?)
The longer the focal length, (or the higher the F-number, as it is related) the narrower the beam. At the same distance the spot or projection is smaller with a wider lens. As you can not make more light then there is from the Led, this means with a more narrow beam you have the same light but more concentrated beam (or spot at same distance ) hence more throw.

Diameter, F-number and focal length are related.
The focal length (back focal length as it is named for plano convex ) is the distance between Led and lens at which it projects a sharp image of the die.
F-number is focal length divided by the effective lens diameter (or 'clear window without rim/edge'). This F-number (see wiki) determines how many light of the Led can be effectively used from the led, when compared to the Flux-vs-Angle diagram in the datasheet of the given Led.
In practice the distance between Led and lens is limited to a few inches, as hardly any light will enter the lens otherwise.

There must be an optimum in each set up, but I dont know yet how to calculate this. Also I dont know yet how to compare above formula to real world lux-measurements. Better experiment if you have some lenses at hand.

Does this make sense? Am I interpreting it correctly? It would appear (to me) that you're saying that increasing the area of an aspheric optic increasing throw by 50 percent of the gain.

So if a 30 mm optic outputs 10,000 lux and throws 100 meters then a 60 mm optic would output 15,000 lux and throw 150 meters. Is that it?

 

[Walterk]

Throw is determined by surface brightness (lux/mm2) x diameter (mm2) x efficiency.

When efficiency and surface brightness is the same, then:
Diameter lens 30mm, then surface is 706mm2.
Diameter lens 60mm, then surface area is 2827mm2.
So, two times the diameter is 4 times the surface area.

30mm Lens then 1 x 15.000Lux. For 15.000Lux inverse square gives 1 Lux reading at 122 meter.
60mm Lens then 4 x 15.000 => 60.000Lux. For 60.000Lux inverse square gives 1 Lux reading at 244 meter.
So, two times bigger lens gives four times the Lux, and two times more effective throw-length.

 

[monkeyboy]

Lenses in general are manufactured with spherical cross sections as this is much easier to manufacture and therefore cheaper. This is spherical approximation is perfectly good for many purposes, but a short focal length, wide aperture, single element spherical lens would have too much spherical aberration to be used for imaging. Traditionally aspherics had to be ground to that exact shape which is an extremely expensive process but these days the cheaper ones (and some expensive ones too) are moulded. Sharp optics are not essential for the purposes of CPFer's so a spherical lens would also serve the purpose. It's just that such a lens would be useless for imaging which is why no one makes them. I guess the aspherical lens would give a little more throw too due to focusing better.

In this sense, the word "aspheric" is misused and misunderstood here on CPF in that people are just referring to a refracting lens. The fact that it is aspherical just happens to be a property of that lens. For example, you wouldn't call a flashlight an "aluminium" so why call a refracting lens an "aspheric"?

 

[PCC]

There's a local plastics store that sell 1.5" diameter acrylic half spheres called "cabochons" for less than $3 each. I keep forgetting to get one when I go there but I might try to get one some time soon. I have a smaller 1" cabochon and place it in front of bare emitters and reflectored lights to see what it does with the light and the results aren't bad, they're just not great. Keep in mind, though, that these are made for decorative purposes so the optical quality probably isn't up to par with true aspherics or other lenses.