How do you focus a light?

LED Flashlights – General Info 18650 Flashlights
1

I have often heard of folks here talk about focusing a light for greater output and throw but few described what they did to focus the light. I have heard of people looking at the front end of the light to see how much of the yellow LED phosphor is visible in the reflection - if it fills the reflection it means it is focused? However my experience is that how much the phosphor fills the reflection is also dependent on how far away you view the light from.

I don’t know how you guys do your focusing. Is it by trial and error, ie move the light cup nearer or farther away from the LED until you get the highest candela readings? How do you determine whether to shift the light cup nearer or farther from the LED? Really wonder if there are some practical guidelines that newbies like me can learn from.

Anyone care to share how you focus your lights? TIA!

2

I’m no expert on this. But if you think of focusing how a Maglite works, buy moving the LED further in or out of the reflector. There will be an optimum for the tights focus and most throw.

With other lights you can try and do this manually. But you’ll need some way to secure the reflector in relation to the LED and then hope it still all fits back together ok (the bezel may not want to screw all the way down, or the reflector rattle).

However the exact point that you’ll get optimum focus will depend on:

-The LED you are using. As LED’s are of different size and shape and have different angles they emit light at.

-The reflector. Not all reflectors are equal or even the same shape. And this is especially true around the opening at the bottom which can vary a lot.

Usually when people re-focus is due to some other mod, such as replacing an XR-E LED with an XP-G2 or XP-E2, or converting an XM-L light to a smaller LED.

To aid this you can sometimes get adapter rings, I have some little plastic rings that fit over an XP-G sized LED on one side, but match an XM-L opening in a reflector on the other. I’ve used these in my XP-G2 and Nichia219 C8’s to good affect.

3

For a thrower I look at the beam, the hotspot needs to be small and round, even slightly out of focus and the beam will look distorted. Focussing a dedomed led involved in most cases removal of all centering gaskets, the reflector sits right on the ledboard (with all sorts of electrical shorting trouble emerging :-( )

4

How to focus an emitter?

Let me count the ways.

Move reflector up or down. Remove bottom of reflector. Shim the reflector. Endlessly center the reflector. There is a trick for every light but basically it’s trial and error. I have spent more time properly aligning a reflector than I have on the rest of a mod combined.

The way to start is to get everything but the bezel in place and shine the light at the ceiling. If it’s not a nice tight beam then move the reflector until it is. After that a good trick is to push down hard on the lens while screwing on the bezel. If you allow the lens to move around while screwing the bezel on 9 times out of 10 it will change the focus and you will need to start all over.

5

Remember this is BLF and we all love to share (have you seen how much open source stuff is here?!) and your already getting good advice but proper focus (especially on super high performance throwers) is somewhat of a secret among modders, I know many of us take great pride in how well we can focus a light and the lux we obtain, not that we don’t want to help others but its a skill we all take a long time to learn and perfect. Again not that we dont want to see everyone be able to obtain the best levels of performance possible, not saying that at all, but there also isnt only one way to do it, so there isnt really a specific answer we can tell you really quick and have you be able to go and do it every time.

6

Since the phosphor is the source of all the light we have, that’s a decent First-Order Approximation. For me, that’s enough, as I’m not that concerned with Throw, but YMMV.

The dirty secret is, with an LED you really CAN’T achieve “perfect” focus, since the Parabola shape of the reflector only has a single point of focus and the LED is a relatively huge planar surface.

Not that this helps any…

7

This is how I do it, a little OCD here he he he :stuck_out_tongue:

I tape a meter stick to a wall and measure the hotspot size of the original setting for future comparison

Then I remove shims and measure hotspot with the reflector sitting bare at the MPCB (You might want to use Kapton tape to avoid shorting the reflector to the wires of the led. Measure hotspot on the meter stick, Compare Did it get bigger, smaller?

Then add shims of several thickness and measure hotspot on wall, until I find the best one.

Then comes the centering, sometimes I have a centering disk shim and just grind down to the desired thickness using a micrometer and sandpaper, install and done!

Sometimes I have to center it by eye, a little tedious but worth it, look directly to the led die (Hopefully you turned the light off first :bigsmile: ) and eyeball the distance from each corner of the die to the reflector wall make sure all are about the same distance.

After you got as centered as possible, turn light on on lowest mode and go to a wall get really close, like 30cm from the wall or less and you will notice a clover leaf light pattern on the wall (Hope this makes sense) look at each leaves they should be the same size like a + sign, simmetrical if you see all leaves the same size then you are at the exact center, sometimes you have to open the light and do very very small adjustments to the led, I small screwdrivers to gently and carefully push the led or mpcb to the side, until I have perfect focus.

I sometimes tailstand the light and rotate it looking at the corona, it should look simetrical, if it has more corona spill to one side you need to push the led to the other side just a hair or less, take the sides of the led die as reference for adjusting.

Hope this helps and makes sense.
AlexGT

8

Thank you all for the valuable advice, I learnt something new from your collective experience today :slight_smile:

To give a background on why I am asking the question….Most of my lights have a screw-in pill. I usually would screw the pill all the down to maximize contact area with the light body and increase heat transfer. I would then tinker with the reflector to get a good fit.

What I don’t quite know is how LED reflectors and their focus points are designed. For those old filament/krypton lights that we used to have, the bulbs would usually protrude into the reflector and the focus point is thus designed to be quite a distance from the flat base of the reflector. I would think that the reflectors for LED flashlights would be designed differently because of the different position of the light source….but I am not that sure. Is the focus point usually just immediately above (<~0.5mm) the flat base of the reflector or is it further in, at around 1 - 2 mm above the reflector base?

Therefore, to achieve the tightest beam, should the phosphor (or the dome) of the LED just clear the inner flat base of the reflector or should I try to “push” the LED in as far as I can (possibly by sanding the reflector base down)….or none of these really matter at all? I understand that reflectors can be designed differently and there may not be a universal answer to my question. However I would like to hear from the collective wisdom here, which configuration tend to work best for you?

9

I just tried this. Great piece of useful advice! :beer: :party:

Do you know what does it mean if one of the “leaves” seems to be darker than the rest, even though the arrangement looks symmetrical?

10

This is definitely one of those trial and error kind of things, since every light is different. On a thrower, when the emitter is focused correctly (assuming you have a good reflector) the hotspot will be tight and sharply defined at a distance. I can't say I have any secrets, it's just all trial and error. Once you get a light figured out you can usually repeat it. It often requires sanding spacers, reflectors, and getting flat joints. Dedomed emitters are visually much more sensitive to reflector position than domed emitters.

11

Slowly.

By eye.