The readout electronics of the CCD camera report the camera values for each pixel in analog to digital units (ADU) which are commonly referred to as ‘counts’. Most high end cameras use electronics that can measure 16 bits which can measure up to 2^16 counts or 65,535. Pixels containing values above this number are over-exposed or saturated. Converting these counts into radiance units is what makes an absolutely calibrated imaging system measuring the amount of light leaving the object being imaged. Such a system is making a quantitative radiometric measurement vs. a system reporting only counts which is merely providing a picture.
It’s important to realize that making an absolutely calibrated imaging system depends on many imaging components, not just the CCD camera. To convert CCD camera counts to absolutely calibrated radiance units, the system must compensate for exposure time, lens factors (efficiency and f/stop), binning and field of view. Using an absolutely calibrated light source as a reference, images must be taken at every possible f/stop, FOV and binning to determine the number of photons per electronic count. Of course this requires the components of the system to remain consistent but depends mainly on the stability and linearity of the camera. This is yet another reason we use Spectral Instruments cameras.