LEDs & Other Stuff - (Reference Guide)

Diameter is not a factor for spot size when comparing fully focussed lenses. Only LED die size and the focal length of the lens determine it. Generally there are limits concerning the focal length though, meaning that it does increase with the diameter at some point.

As I said - with constant lens shape (and therefore constant F number), larger diameter makes the spot size smaller.

One can explain the same effects on thow and spot size by talking about shape and diameter like I did.
One can talk about focal length and diameter instead.
These approaches (as well as some others) are different but both are correct.

i will working on these ways after fully realizing F number and diameter effect on throw and select the correct lens.
this is what i realize :for smaller spot diameter i need a lens with higher F number and if i choose a lens with higher F number(more distance between lens and LED)then lens’s diameter unfortunately should be higher to prevent light losses; is that true?

One correction:
You use the “F number” where you really mean either “Back Focal Length” or “Focal Length / Effective Focal Length”.
See wiki to understand the difference between Back Focal Length (BFL) and Effective Focal Length (EFL or simply FL).

F number (F#) = EFL / diameter

F number is dimmensionless and depends on lens shape and material (but not size).
Focal length depends on both shape and size.

I explain that because the misunderstanding may lead to confusion at some point.
But you seem to understand this correctly now - a lens with longer focal length will produce a smaller spot, regardless of diameter. To concentrate more light on that spot (and therefore make it brighter) you can increase lens diameter.

thank you
but another question.
is that possible two lens with equal focal length and diameter but different build quality have different spot diameter?if yes which parameter affect this?

Precision can somewhat affect this. Though not much. Sometimes it may not be clear what to call “spot”.

Extreme case:

But beam artifacts aside - no, same focal length = same spot size.

it seems there is no way to have a small spot(my goal) with a 76 milimeter lens and collect whole light!
so is a reflector better than lens for this purpose?i think a reflector can collect more light(mean less losses) and can concentrate while light in one point and here there is no diameter’s problem of lenses.
is that true?

A reflector won’t help.
There are pretty much the same rules, throw is proportional to optics working area (note: reflectors lose a bit due to cutout in the middle which doesn’t contribute to throw).
A deep reflector can concentrate a lot of light which gives it a pretty large (and still intense) spot. But not more intense than that of a lens of equivalent size.

If possible for you, dedoming is probably the best thing you can do.
A Wavien Collar is probably the second.

for example BLF GT has about 2000meter throw.at 2000meter how much is spot diameter do you think?can i achive that with my lens and setup?
i have a XHP70.2 70CRI 6500K (6V)and a DIY driver that made by myself with 12A maximum current and a good 95watt cpu cooler.

There is. You need to use a pre-collimator (small spheric lens very close to the LED, LED needs to be de-domed). This is the only way to collect as much light as possible with an aspheric lens. If you then want a small hotspot, you just need to use a suitably small LED.

Here is a formula you can us to calculate the size of the hotspot:

luminous_intensity_in_specific_distance [lux @ xx m] = luminous_flux [lumens] / area [m2]

Generally you need to know how many lumens are actually in the beam. For reflector lights you can assume 75% of the total lumens.

Here you can find some nice calculators to help you.

this way is just for collect whole light.but thas is bad for spot diameter because it become bigger!
i was test it with a small lens(focal lenght=1cm and diameter=1.5cm) and not de-domed XHP70.2 and resault was a bigger spot and ofcourse more collected light.but i want a narrow beam with atleast 1.5KM of throw

You can use my calculation for any distance. You just not to calculate the lux value in that specific distance (or the Candela value, which is basically lux @ 1m). This always works!

Also see here. There is another formula for spot size using the focal length.

but my problem was another thing.not calculation of hotspot diameter.
main question is how can i have a small hotspot with my XHP70.2 and 76 milimeter spheric lens for atleast 1.5KM of throw.what is the solution?!

First you need to calculate if this is actually possible. What is the maximum possible luminance [cd/mm2] of the XHP-70.2? Can you achieve this in your light?

The thread I linked to shows you how to calculate throw based on luminance. You can calulate the maximum possible throw with the XHP-70.2 and your lens this way.

are these formules right and enough?
according to these formules luminance at 907m^2 with a 2 degree lens at 1KM distance is about 0.007cd/mm^2.are these calculation ok?

For reference, the GT70 has a FET driven 70.2 which is close to max output and the GT reflector is 118mm and it gets about 1600 meters. I think a shaved dome bumps it up to about 2000 meters.

My SP70 with 70.2 at about 12A and 71mm diameter reflector gets about 860 meters. Shaving the dome should bump up the throw to over 1000 meters, maybe 1100m.

The 70.2 is not known for its long throw. It’s a big led.

Here are luminance measurements for XHP70.2.

it says page not found!
but here i found luminance measurments.at 8.6 amperes it is 56.9 cd/mm2

Fixed the link, please click again.

i read this topic : Project Excalibur - Next Generation LED Thrower (many pics) - UPDATE 2018-01-24
that flashlight has a good throw and spot size near my goal.but i have a question.if in that project the led was XHP70.2 then what would happen?how was throw distance and spot size(at 1000meter)?