Need help with some math..

Hi.

Im looking at two videolights with different beam angel and lumen.

Light 1:
http://www.bigbluedivelights.com/products-detail.asp?Product=157
5800lumen
120degree angel

Light 2:
http://www.bigbluedivelights.com/products-detail.asp?Product=146
15000lumen
160degree angel

What im looking for is how much light intensity will be difference between thoes models. Lumen is ~260% more on Light 2 but as angel is larger the lux should not be 260% more. I dont know if it is as simple as 160/120= 33% larger area and 33% less lux? The angel is not really an advantage for me so i want to know how much intensity the brighter light will give me. Price is about twice as much.

Thanks in advance.

Ok, this is only an approximation, I assume that the luminous flux over the opening angle is constant, without hotspots, artifacts etc. (Thats a very daring assumption!)
Im also assuming that the lights throw a perfect cone of light.

Then we have lux = flux/area (lumen per m^2)

At 1m distance, the radius of the light circle would be r=tan(angle/2)

The correspondig lit area would be A=Pi*r^2

At 1m, the first light will light up an area of 9.4m^2, that would be 617lux.

The second light will light up an area of 101m^2!! and will give only 150lux!

These numbers are NOT exact, because the flux is NOT constant over the angle, far from it (hotspots and stuff), but there is a high probability that the first light will have much better throw.

Hope this helps!

Edit: Corrected an emarassing mistake...

Thanks for the reply Cordyceps.

But 15lux copared to 617lux? Are you sure you got that right?

Under the assumptions ive made, yes. It sounds farfetched, but 160deg is one hell of an opening angle.
The real numbers ARE higher for both lights, because the flux is usually more concetrated in the middle (hotspot), while the far greater sidespill gets less of it.

If I had the flux over angle functions of the lights and their optics, i could give you exact numbers.

Still, imho, the second light will have less throw. But youll need to see it in person to be 100% sure. There is not enough info for exact math

Oh, im sorry, its 150lux of course. Forgot a zero

Oh, and I also assumed zero absorption in the transfer medium, which is ok for air, but water will lower the numbers significantly, but equally for both lights.

Ah ok. I understand now. seems hard to do math on this kind of stuff, guess i just have to see them in reality and even use a lux-meter to get some real answers. Anyway, i think i will go for the 5800lumen version for now.

Again, thanks for your time!

edit, 150lux makes more sense, but still surprising. I thought the 15000lumen light would be brighter. Yes, i understand it might still be, in reality.

I looked up the lights on the website and i strongly assume that the advertised lumens are LED lumens, because there seems to be a linear progression direcly corresponding with the number of leds. 10 XMLs - 5800lm, 25 XMLs - 15000lm. You wont see the same linear progression in the important OTF lumens, because of phenomena like interference.

Comparing them in a real underwater situation is the only way to be sure.

Glad i could help!

Cordyceps said:

I looked up the lights on the website and i strongly assume that the advertised lumens are LED lumens, because there seems to be a linear progression direcly corresponding with the number of leds. 10 XMLs - 5800lm, 25 XMLs - 15000lm. You wont see the same linear progression in the important OTF lumens, because of phenomena like interference.

Comparing them in a real underwater situation is the only way to be sure.

Glad i could help!

Click to expand...

Yes and the battery time with lumen output is not realistic either. But they have great reputation and i have never read anything bad about them so i think i will get pretty good lights for the price.