I don't think it was much of a coincidence. Jones was chairman of the Z38 Section Committee, Photography of the American Standards Association from 1940 to 1950. I've also noticed a number of standards or revision of standards occurred shortly after a new paper was published. But I don't care what people say, I don't think Jones was trying to push through an agenda.
Originally Posted by Bill Burk
The proof for illuminance comes from the incident meter calibration equation.
A^2 / t = (I * S) / C
Using Sunny 16 and solving for I, it becomes I = 16^2 * C
According to the exposure meter standards, C = 30
256 * 30 = 7680 footcandles
The average reflectance between the illuminance of the incident meter and the luminance of the reflection meter is:
(297 * pi) / 7680 = .12
You can also calculate the average reflectance using the constants:
(1.16 * pi) / 30 = .12
According to the ANSI photographic exposure guide, "Daylight reaches a maximum illuminance of approximately 11,000 footcandles at a solar altitude of 90 degrees." The solar angle used for the measure is approximately 40 degrees. According to the standard, this is approximately 2/3 of a stop less light than the maximum.
I've done a little calculating and 11,000 footcandles at 90 degrees will yield 8426 footcandles at 40 degrees and 7778 footcandles at 45 degrees. For the illuminance to equal the incident exposure meter value of 7680 at 40 degrees, the maximum illuminance would have to be around 10,200 footcandles. I think that's only a tenth of a stop difference.