The There Are No Stupid Questions Thread

Thanks figured it out from another blf member

Guys, I can’t find the answer although I’m pretty sure there is one within BLF “borders”. Hence my post here - I feel stupid asking you for a shortcut instead of making full research.

The question is: how to calculate the size of hotspot at a particular distance?

Let’s say I want a hotspot of diameter =3 meters at a distance of 150 meters. (numbers are taken out from nowhere, so might be odd). Which LED should I choose (de-doming/slicing is within scope of my poor abilities)?
Or the other way round. Let’s say I have Samsung LH351D, how can I know what would be the hotspot size at distance of 150 meters?

Your hints will make my research way easier.

I am by no means an expert but I’d say it’s all up to the reflector or optics. Too many variables to be able to use a simple formula.

I found something relevant here
But my geuess is that’s not the only one. Anyway, thank you Enderman

I’m sure you’re right but since I’m interested only in hotspot I think the reflector has not that much impact?

But it’s the reflector that focuses the light into that hotspot. Just see what happens when you don’t have any reflector at all, it turns into a mule light with no beam at all. With any given LED you can have widely different beams just by switching reflectors.

I didn’t know about Enderman’s calculations. At least with that you can enter some reflector data.

Yes, you’re right Mike, w/o ref it goes insanly wide. So if won’t have any other option I’ll use Enderman’s method.
However, the LED area are must have a meaning, since by just replacing the led you get more/less focus as well.

Newbe question. Difference between Candelas/Lumen?

There is only trial and error. Luckily there are already many different lights on the market so we kind of know how they scale up and down.

Basically it’s the ratio of die size to reflector size, assuming we are talking about basic flashlight design using a reflector. For a fixed reflector size, a smaller die creates a smaller hot spot and a bigger die makes it bigger.

For a fixed die size, a smaller reflector makes a bigger hot spot and a bigger reflector makes a smaller hot spot.

It’s easiest to just sample flashlights to see how they perform and decide if the hot spot is large enough and bright enough at the distance you want. If the hot spot is too small, you need to go with a bigger die or a larger reflector. If it’s too big, go with a smaller die or bigger reflector.

Typically a person will have a distance requirement, let’s say to easily see out to 200 meters. You can find lights that meet this requirement by looking for ANSI FL1 distance ratings of at least 600 meters. (Typically 1/3 the rating will be bright enough to see really well). Then it is a matter of how big of a hot spot do you want at that distance. A tiny 500 lumen black flat led in a Convoy L6 could easily do 600 meters, but it’s hot spot would be tiny. On the other extreme, a 6000 lumen xhp70.2 in the same L6 would also do 600 meters but make a huge hot spot. By keeping the L6 as an example host you have a full range of led die sizes to play with. Now you have to find the right compromise of flashlight size, weight, battery longevity, thermal control, etc… that best fits your needs.

Also keep in mind there will be a corona around the hotspot which will make the hot spot seem a bit larger. This means your “needed hot spot size at X meters” can actually be a bit smaller than you thought.

So based on what you need, folks here can get you in the ballpark, but it’s not an exact science. Reflector geometries and surface smoothness do tend to vary a little which throws off the calculations a bit.

Candela is a measure of light intensity in a tiny portion of a fully formed beam. This tells you how far the light will travel.

Lumens is a measure of all the light being produced captured and combined back together, integrated, and then measured. Lumens are much harder to measure.

So candela varies greatly based on the reflector and beam angles.

Lumens are not effected by the reflector* or beam angle.

*Reflectors due vary in the amount of light they absorb and reflect, efficiency, which does effect lumens.

Reflector makes a huge difference. It’s crucial.

Thanks Jason, after all I find your clues very useful. If I need to do it by iterations - fine.

Anyway, maybe somebody has already gone through the problem I’m analyzing now. To be more substantial: I need 3 meters hotsport at a distance of 70meters. I’m aiming to use Convoy L2, but then I’m confused if I should go for:

- Samsung LH351D (very welcome due to hight CRI) / “” De-domed?

- XHP50.2 De-Domed (since I’d rather throw over spill)

- Luxeon V

- SST-40 De-domed (less preffered due to greens and CRI)

  • else?

Living in central Europe I don’t have that easy access to some less popular LEDs, so Arrows and other serious providers are out of my capability. Just common KD, BG, Ali, L4P, INTL… eventually MTN but very expensive and lenghty delivery.

Do you have the L2 or a light with similar sized reflector right now?

Keep in mind the Luxeon V, SST-40 and LH351D are all about the same die size, about 2mm x 2mm. No big difference there. Slicing the dome off will make the big difference. Xpl-hi is like a factory dedome and also 2mm x 2mm.

Then the 50.2 die is 3mm x 3mm. I would estimate the other 3 leds with dome on would make about the same hot spot size as the 50.2 with dome sliced off.

So all these leds and dome on or off will only give you roughly 3 different hot spot sizes.

Are you going to use the stock L2 driver or swap a new one?

User DTA has made many beam shot videos of lights at fixed distances. We can probably find the right led once we know the distances. I’m waiting on his response.

Here is his video of the Convoy L2 with the xhp50.2.

I don’t know the distances in the video, but maybe it gives you a good visual idea of this reflector/led combo.


Jason, once again thank you for involvement. Yes, I have the L2 right now. I’m thinking of L4P 20mm driver since the flashlight is supposed to have just one mode (light for machine gun fixed on the car roof).
The bad news is L4P driver is not buck, so hard to use 2x26650 which I would wish to. Only 6V led would allow to do it without any further adjustments.

Such deep thinking going on! Thought I’d “lighten” the subject a bit.

My idea of a good companion for my D4S—a Nite ) Ize Innovation holster

Building on left is 125 meters and building on right is 30 meters.

Hi did not think that holster would but had long lost that holster may pick one up

Hi. A noob/stew-pid question.

What do the acronyms XHP-50.2, and SST-20 stand for? “Extra High Performance/Power?
Super Standard Transmittance?”

Also, what significance are the numerical designators. Especially the .2?

Thought I’d ask. :student: not