Quote Originally Posted by ic-racer View Post
Good question. The two measurements with the dome can then be averaged or weighted to give a value you can use to make an EXPOSURE. Whereas the two values obtained with the flat disk, pointed right at the light source, will only give you a RATIO. The absolute numbers won't easily compute to an exposure value you can use.
That's a very good point, the flat plane receptor is an ideal for calculating the contrast ratio when pointed from the subject position at each light not the exposure, to estimate the exposure use the normal hemispherical dome pointed at the camera.