[HDRI] converting RGB value to illuminance

[Mahlab, Seyedeh Farzaneh]

Hello
I am using a 4.5 mm fisheye lens to take HDR image. I was wondering if there is a way to convert RGB value to illuminance (lux). I would say I need to have the reflectance of the material but I don't know how to apply it to these per pixel value. Any help is appreciated. Thanks. Farzaneh

[Lars O. Grobe]

Hi Farzaneh,

the integral of the entire image, if it covers one hemisphere, would
equal to the illuminance at the entrance pupil of your lens. However, I
guess that you are more interested in relating pixel values to the
illuminance at the corresponding locations on the reflective surfaces.
For that, you could approximate the illuminance from luminance (what is
what you camera measures, if it is properly calibrated), as long as you
can assume the surface to be perfectly diffuse. This introduces errors,
but may give some reasonable idea at least if you pick only appropriate
surfaces without strong specular / glossy reflection.

You may want to go through John's slides from last year's Radiance
workshop:
https://www.radiance-online.org/community/workshops/2016-padua/presentations/202-Mardaljevic-DaylightInHeritageSpaces.pdf

We also presented a method employing markers some time ago:
https://zenodo.org/record/545809#.WV03j4qkJUM

Cheers,
Lars.

Post by Mahlab, Seyedeh Farzaneh
Hello
I am using a 4.5 mm fisheye lens to take HDR image. I was wondering if
there is a way to convert RGB value to illuminance (lux). I would say
I need to have the reflectance of the material but I don’t know how to
apply it to these per pixel value. Any help is appreciated. Thanks.
Farzaneh