[Radiance-general] Photosphere

[Gregory J. Ward]

Hi Martin,

> matte saturated colored samples

Don't expect good results, but this is definitely worthwhile.

> matte Munsell gray cards
> variation of f-stop from 2.8 to 8 (min and max on my digital camera)

You should combine this with the off-axis tests, since vignetting is 
known to increase at larger apertures.  You can actually plot the 
fall-off as a function of angle from the image center and fit a curve 
to it for each f-stop.  This can then be applied as a correction using 
pcomb.

> incandescent vs. fluorescent vs. daylight (daylight is tough as it 
> varies)

You can use different color-balance fluorescents instead of relying on 
daylight.

> on-axis vs. 40 degrees off axis
> colored and gray cards on black vs. white background
> sample size around 4 X 4 cm^2 photographed at a distance of 5 meters 
> or so
>
> tested against spot luminance meter 1 degree

The flare measurement we just performed on our camera is one I 
recommend.  Start with a uniform light source, such as a fluorescent 
fixture with a good diffuser.  Capture an HDR image of it, and take a 
luminance reading at its center.  Then, cover it with a black 
posterboard with a small hole cut in it and capture an image of the 
hole using exactly the same settings.  If the camera lens is perfect, 
the measurement of the hole should match the original uniform field.  
It will be lower, and the difference is due to scattering in the lens.

-Greg