Researching this topic, I came across two methods that seem to contradict each other. Need to decide which to use.
I am a novice in this field so please be gentle. Also, I would have loved to use the fancy math signs in this post, but have no idea how that works. My apologies. As a filmmaker, I am trying to build a simple photometric calculator to calculate the output of different light sources under different circumstances. Now, I'm confronted with two different methods of calculating Lux (that give very different results) and I wonder which to choose.
- $Ev(lx) = Iv(cd) / (d(m))^2$ (see link): This one seems to be used in most of the Photometrics calculators I've seen.
However, Isn't this too simple? For one thing it doesn't take into account the apex angle of the light source. Many of the lights used in filmmaking have the ability to adjust the angle of the light coming out. And in my personal experience this has really changed the intensity of the light.
I've started to do some of my own calculations, before I found the formula mentioned above.
2a. First, I convert the Candela to lumen as follows:
2a. $cd \times sr$ | $\Phi v(lm) = Iv (cd) \times 2 \pi(1- cos(\thetaº/2))$
2b. Then I calculate the surface area of the base of the light beam.
2b. $r = distance \times \tan(\theta/2)$
2b. $\pi \times r^2$
2c. Then I simply divide the lumen by the surface area of the (base of the) beam.
2c. $\Phi v(lm)/m^2$
The 2 methods give very different results. Example Light A, has 9500 cd, a variable beam between 60º and 80º, and a distance of 10 meter.
Method 1 results in 95 Lux no matter the beam angle.
Method2 results in 76.36 Lux with a 60º angle and 63.13 Lux with an 80º angle.
Can anybody tell me what method is more accurate in my situation? All help is greatly appreciated!
Kind regards, Remco