How to measure illuminance?

I am trying to figure out illuminance of an image. A more direct way to do that is by using an illuminance meter. I was wondering if there is an indirect way to achieve that using image processing tools in Matlab. Please assist.

Thanks.

[Hirak]

You better work with iraf . its an easier way to find things from basic image data files

[Hunny Kanwar]

Thanks Hirak. In case you have experience using Iraf, can you suggest how I can use it to measure Illuminance or any photometric property that could be a measure of brightness perceived by human eye?

Answer 1

[Image Analyst]

What is iraf?

The gray level of an image is a unit of luminous energy. You can get the illuminance by dividing the gray level by the area of the pixel. But these will have units of gray levels per square pixel, not Lux like real SI units of illuminance. To get that you'd need to do an intensity calibration or a whole bunch of math to estimate it.