Quantifying light intensity from an image

[cghw01]

Hi,
I would like to get a measure of the absolute light intensity striking a detector, but I do not have access to the voltage output of that detector, only the output in the form of an image. I can read this output as the value of a pixel as a number between 1 and 2^16 (i.e. 65536). There are two settings that I can adjust when acquiring the image - brightness and contrast. I am presuming that in order to create the image from the acquired light intensity, the output is a straight line, ie y = mx + c, where x is the voltage from the sensor, c is related to the brightness setting, m is related to the contrast setting and y is the number in the image for a particular pixel. I am thinking that if I put filters over the sensor so that I can cut the amount of light by one half or one quarter, I could determine m and c for a particular brightness and contrast setting of the detector and thus I could determine the light intensity (or at least a value proportional to the light intensity) for any future measurement using any pixel in the image.
I suspect things cannot be as simple as this, but has anyone tried to determine what the light intensity actually is directly from an image or series of images?

Thanks
Chris

[snibgo]

I don't know what you mean by "absolute light intensity".

Cameras, and light meters pointed at objects, measure the luminance of the object. The unit of luminance is "nit".

A light meter placed at an object, pointing back to the light source, measures the illuminance of that light source. (Well, it measures the total illuminance of all the light sources.) The unit of illuminance is "lux".

If you take a photo of an object with a digital camera, convert the image to RGB space, the value at each pixel will be proportional to the luminance of the corresponding point on the object, more or less.