Light collimation with an LED flashlight

Curiosity: in (castillian) spanish the word “led” is an acronym, and of course we pronounce it as a word in its own right. Odd it is to see a certain subset of english speakers pronouncing it as a sequence of independent character initials: I wholeheartedly support the ;-) “jamaican” way (listen to the speechs in wordreference).

Regarding this thread's title, thus saying “a led flashlight” is equally correct.

Cheers :-)

Thank you for this thread Enderman. I tried your method above with the 2 pieces of paper. It verified that the GT was, at a minimum, very close to focused. So much better approach then the way I have been doing it (i.e. eyeballing and repeatedly measuring throw).

My hat felt , by itself , in regard of your work , Enderman ! As always , science at the highest level ! Thanks , and my humble respects !...

Thanks great to hear!
The GT does look very well focused from the beamshots.

Thanks :slight_smile:
I wanted to find a way to make sure I was getting maximum throw without having to take lux measurements 1+km away while focusing the flashlight at the same time.
So I figured this out :smiley:

A few questions since I plan on trying this:

  • I should be able to just resize your image to whatever size I need right? No need to make my own, just keep the proportions and resize to whatever light I want?
  • The second sheet is just a copy right? Your photo is zoomed in so it almost appears like the remote paper is larger but I suspect it’s just an illusion and they are the same size since we want parallel lines.
  • In the below image you say it’s “overfocused”. In the case of a normal reflector would this image mean the LED is too far IN the reflector or vice versa? And just to be very explicit the solution would be to make the centering ring thicker or thinner?

I wonder why I’m just now seeing this thread. Thanks for linking back to it from the Osram “White Flat” thread, Enderman. And thanks for making this thread a while back.

contactcr, the second sheet is a copy, the same size as the first. You were right in thinking they need to be parallel. I don’t know the answer to your second question, but I would guess overfocused means the LED is too far IN. Let’s see if I’m right or wrong when somebody who knows comes back around.

Great article, thanks for the repost!

I’m also guessing the LED is too far_ in _[the reflector] as well, since if you reversed the square centers at the target, through the pinhole and reflector, they would converge closer to the lens side?

Yes, it can just be resized as needed.
You want both sheets to be the same, you’re exactly right we want parallel lines from the holes in one sheet to the center of the squares in the other sheet.

In a reflector, the LED would be too far forward.

Your threads are allways intresting,
thanks for doing the great work and explantations

Regards Xandre

Thanks :slight_smile:

Lovely guide, thank you!

The issue in achieving highest possible intensity is multi-faceted. An LED emits a broadband light range. If you notice that your “squares” aren’t actually squares but are rather skewed out rainbows, due to chromatic aberration, you’ve arrived at an important conclusion. The lens setup isn’t sharply focusing all color rays the image is made of to approximately the same point. You are essentially choosing the wavelength which is in focus. Focus the blue end of the spectrum and the colors furthest away in wavelength will be out of focus (red rays will blur). Focus the red end and the opposite occurs, and blue end will be blurred. Blur equates to rays that are not as focused as they could be, which means they are occupying a larger space than they would when in optimal focus. A larger ray distribution lowers image intensity. Ideally, all ray colors would be in focus, which places all rays onto the smallest possible separation distance, creating the sharpest possible image that can be made with the range of wavelengths emitted. This sharpness factor is defined by lambda (wavelength). With a perfect lens, the image becomes “diffraction-limited”, or as sharp as it can be due to the wave size distribution. The shorter the wavelength, the smaller the minimal diversion can become. If one had an infinitely short wave, a laser beam that stays parallel forever would be possible, though that would require infinite energy. (Darn it)

What you want is minimal chromatic and spherical aberration, with maximum light transmission. That will provide highest image intensity at infinite conjugate focus with a given light source.

1 Thank

Not to eh, reply to myself, or unbury the dead (I had realized later how old this OP was), but I suppose I should have added a couple points which can still be used.

Generally among flashlight enthusiasts who want to maximize throw, the consensus is usually to locate “the best” aspheric lens that one can find, at the largest diameter to fit into something as a host. Aspheric lenses that can accomplish excellent focus across the wide band of colors in an LED isn’t always easy or practical to find. Just because they are glass, or coated, or cost a million bucks each won’t mean they will focus an LED’s spectrum very well to a sharp image. That’s because it’s a single lens, and initially it had a design wavelength it was made for, while each LED has a different spectrum of available light for which to create intensity with.

Chromatic aberration, caused by the varying refractive indices of different colors of light, is a challenge in lens design. Using multiple lenses with different characteristics allows for the tuning of individual light sources. This is why a camera lens doesn’t use 1 aspheric lens, but rather a combination of negative, positive, spherical, and aspherical lenses, to achieve excellent focus. The difference is that a camera sensor can increase sensitivity (through ISO, or by lengthening the exposure time). With an LED, you can increase power to overcome surface reflection losses of multiple lenses, but the degree as to which device is more effective at changing it’s own conditions to deal with those losses would have to go to the camera. However, a 2-lens system, using coated lenses that can even each be of the spherical surface type, when placed both at the correct distance from the LED and from each other, can surpass easily the intensity maximum that could be produced from a single aspheric lens at same useable optic diameter. Many achromatic pairs solve for chromatic aberration quite well and are just a negative glued to a positive lens, both with spherical surfaces. It’s much easier to acquire large lenses more cheaply that way, if you seek high throw on a budget.