All of the current throwiest LEDs–Osrams, SFT25R/42R/60, etc., seem to have an effective luminance in the vicinity of 180cd/mm^2, after optical losses. (I think this is for good reason–larger, less intense emitters suffer lower losses from imperfections in the optic, and these effects cancel out.) For this reason, if you know the reflector/optic’s inner diameter D in mm, you can simply estimate throw via scaling luminance by frontal area:
180 * 3.14 * (D/2)^2 = 140D^2 cd, or equivalently, approximately 24D meters of ANSI throw.
Examples:
- Convoy L21B: reflector inner diameter around 58mm, so throw is around 58*24=1392m.
- Convoy C8/M21A: inner diameter around 40mm, so throw is around 40*24=960m.
- Convoy M21B: inner diameter around 29mm, so throw is around 29*24=696m.
- Convoy S6/S21B: inner diameter around 20mm, throw around 20*24=480m.
All of these estimates seem consistent with known measurements, so this formula should serve well as a quick sanity check for the next thrower you get!
For SFT/SBT90 the luminance is usually lower if driven by single cell, which usually fails to provide full current. Scale intensity by 63%, and distance by 80%.
If your thrower has an LED that is not one of the above, then 180cd/mm^2 is no longer a good estimate. Instead of 180, look up koef3’s test of the LED, find the luminance, and then take -25% off for optical losses; call this new luminance L. You will need to scale the above formula for candela back by a factor of L/180, and ANSI m by a factor of sqrt(L/180).