Am 03.04.2017 um 16:14 schrieb muyu:

> The incident solar radiation is parallel but not always nadir. For instance, I
> set the light source (zenith anglen 20 degree)
> light_source{<0,36.397,100> color rgb<0,1,0> parallel}

That definitely explains the slight difference in brightness when the
light source is close.

> Then for a planar surface with optical properties `finish { diffuse 1 specular 0
> ambient 0 }`, I got reflectance 239 (or 240 when the light source is moved
> further away). This value was considered as the calibration factor (f) for
> reflectance 1.0.
> 
> Then for given camera configuration, I render the image of the targetting canopy
> with the same light source. Then the encoded pixel value of the image was
> averaged (color) and divided by the calibration factor instead of 255 to get the
> canopy reflectance (Ref), i.e. Ref = color/f.
> 
> Or it should be Ref = color/255 + (1 - f/255) ?

It depends on what exactly you actually want to measure. I have a hunch
that what you actually need is color/255, i.e. the using zenith sun as
the reference, regardless of the sun angle you want to take a
measurement for; but that ultimately depends on the specifications of
the task you have been given.


Another potential source for error would be the averaging of the image;
depending on the software you are using, and the transfer function used
in the image encoding; it might be done on non-linear values, in which
case the result will under-estimate the value.

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