Thank you for your welcome and also clear answers! It was a big help.
In response to your explanation for my first question, I’ve devised an algorithm to calculate the surface area corresponding to each pixel so that I could calculate luminous intensity by multiplying luminance by surface area of each pixel. And in the multiplication when I used surface area normal to the ray like the one in the picture below, I obtained more plausible values compared to using the the surface area of the geometry model (the real surface in the picture). Therefore, I inferred the luminance is not the luminance of the actual plane but a virtual normal plane at the distance. Is it correct?
(source: http://ecoursesonline.iasri.res.in/mod/page/view.php?id=124398)
And in regard to the last answer, in some previous studies to assess veiling luminance, the authors assumed (solar illuminance at the observer’s eye) = (direct normal solar illuminance) * (reflectance of the material) under specular reflection. And when tested in Radiance, that relation is reproduced in luminance but not in illuminance. In the case of illuminance, it often appears to be higher than the value of direct normal illuminance multiplied by reflectance. By the way I used ‘rtrace -I’ command to compare the illuminance on the plane. Could this discrepancy be because, physically, that relation doesn’t make sense, or did I overlook something?
Many thanks,
Inwoo