The There Are No Stupid Questions Thread

I’ve got a couple potentially stupid questions.

1) I’ve got a utorch lantern/bug zapper that takes a 18650. It has a sleeve inside that when removed it looks like it can fit a 21700. Will this break the light? isn’t the voltage the same?

2) Simon already said no, but if I had a convoy m3 that takes one 26500 battery and found a long enough tube to fit two 26500’s would it break the light? why?


1/ 21700 batteries are fairly new, and predominantly used in high-end flashlights. That’s why I think this sleeve is meant to fill the space needed for a 3*AAA battery carrier. Those have a diameter of a bit over 22.6mm.
Edit: a 21700 has the same voltage as an 18650, that does not hurt a bit. But I may be too long to fit.

2/ 2 batteries “in a row” have a voltage of 2*4.2V = 8.4V. You fry the driver when that is built for 1*4.2V.

I bet a 26650 battery may fit

And yes two batteries stacked together results in 8 volts instead of 4 volts

I found the perfect holster for the emisar d4s, I just received it today. It is a pistol magazine/flashlight holster I found at my local academy

Any way I can upload a pic on here to share? I may find the link on academy’s website and copy and paste it but I’d rather show you guys how perfect of a fit it is!!!

thanks everyone! there will be more.

You can’t directly upload photos to BLF. You’ll need to upload them to a photo-sharing site and post a link.

It’s not complicated. Here’s a how-to guide put together by BLF member ‘raccoon city.’

I recommend using as the host site. It’s easy and you don’t need to create an account.

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.