Camera Biases, Binning, and Cosmic Ray Corrections
The next level of radiometric image quality includes compensating for camera biases, binning and cosmic ray corrections. “Cosmic rays” are a CCD camera industry catch all term for large background signals measured by a CCD camera unrelated to light exposure. But generally these signals are not only high energy particles from the cosmos but terrestrial radiation events from the environment or the components of the camera itself (almost all materials are at least slightly radioactive). The cosmic ray events must be detected and corrected in the final image. Binning is accomplished by grouping together pixels to form a superpixel. For example, 2×2 binning would be grouping 4 pixels together into one superpixel which would add the values of all the pixels into the one superpixel. The effective superpixel increases the signal to noise ratio but at the expense of resolution (knowing that in live animals resolution will rarely be better than 1mm due to scattering in tissue). The camera bias is a positive value added to the signal of each pixel by the electronics readout system because it cannot report negative values. The bias is a fairly large quantity that is mostly stable but as higher sensitivity is required, it becomes much more important. Bias is usually on the order of 500-1000 counts. In systems where the radiance is very low, a small fluctuation in the bias background level can have a large impact on the final result.
For a more detailed discussion of the concepts described here see the paper originally published in the Journal of Biomedical Optics in 2001, “In vivo Imaging of light-emitting probes”.