Teach me about pre-collimators

I mostly understand the basic concept/theory, I need some help with the practical application. I think I’d like to put a pre-collimator into a Haikelite HT35 Aspheric.

  • Do you bother with trying to calculate (design) on paper what the ideal pre-collimator lens would be, or is it easier in practice to just buy a few lenses and see what works best?
  • Do you do optimize the setup on the bench and then try and recreate that in the light?
  • Where do you get lenses?
  • Do you have any good links or build threads I can read through?

I’ll probably think of more questions as (if) I get responses, so I’ll just leave it at that for now.

Pre-Collimators are small lenses which are placed above a de-domed LED to reduce the angle of the emitted light so that more of it hits the backside of the apheric main lens used in some flashlights. They increase the otf lumens by making the hotspot (when the light is fully focussed) larger. When defocussed the beam is generally a bit tighter, but the luminous intensity is higher. Also, when defocussed, there will be a donut hole in the beam. The size of this donut hole can be influenced, but nobody has studied this yet. I think a larger pre-collimator causes a smaller donut-hole. This is actually bad because you can make the donut hole so big that it basically covers the entire beam and thus you don’t notice it.

You need to use spherical (not aspherical) lenses with a suitable (small) diameter and a very short focal length. I would not go above F1.0 (Fnumber = focal length / diameter). Below is probably better. Generally there should be no loss in luminus intensity (cd, “lux@1m”) when adding the pre-collimator even when the pre-collimator doesn’t have an ar-coating. You can influence this by changing the distance between the pre-collimator and the LED (which obviously can’t have a dome). The distance should probably be less than 1mm (Vinz says that 0.3mm is good).

I have never seen any calculations concerning this type of lens configuration. Only that you can generally expect around double the lumens when the light is focussed to a spot compared to before if it has a standard lens with a common F-number.

You can get these types of lenses from places like optolife (good quality and cheap), edmund optics and thorlabs (both high quality but expensive).

Here is a nice thread where some experimenting was done. A 15mm diameter lens with 15mm focal length (so F1.0) was successfully used. He used a very expensive one, but the same type of lens even with ar-coating can be had from Optolife for a fraction of the cost (BTW: the focal lengh I talk about is the so called EFL (effective focal length), not the BFL (back focal length)). I tried to do just that here (use Google translate, it’s in German) and it worked. There are some caveats though. The pre-colimator is very large and the donut hole when defocussed is very noticeable (see pic here).

I would recommend trying using one of the smaller pre-collimators form optolife.

One thing to note is that you will never get all of the light from the LED into spot by just adding a pre-collimator and using a standard aspheric lens. For maximum efficiency a low F-number main lens is also needed (so one with a really short focal length which is really “thick”).

Probably the most optimized light ever made in this regard is the Mjölnir from well known modder Vinz. He actually combined specific lenses to get all of the light from the LED into the main beam. He even tried to minimize spheric abberation by not letting any light hit the outer 5% of the main aspheric lens. He also also polished the cover plate above the LED to reflect any small of amounts of remaining stray light inside the light to also be reflected outside (but as spill around the spot).

In my opinion it probably wouldn’t work because the top side of TIR lens is made for a parallel exit of the light. So when you invert it the angles of the TIRs sides (where the light is reflected internally) don’t match the angle of the light hitting it from the LED.

Thanks for the info, The_Driver. For some reason google translate isn’t working on my computer browser at work, so I’ll read those TLF threads tonight.