I am using a fluorescent microscope with a CCD attached so that I can take images. What I'm wondering is how the scale between the pixel values works? is it different for different CCDs?
Digital imaging sensors are linear in their response to light. If you expose one to twice as much light, either by making the light twice as bright or by exposing for twice as long, the amount of voltage produced by each sensel will double until full well capacity is reached.
Different sensors will be more efficient or less efficient than other sensors, but they are all linear in their response. Simple amplification, that is, multiplying all measured voltages by the same amount of gain, is all that is needed to make a less efficient sensor output the same strength of signal as a more efficient sensor when both are exposed to the same amount of light.
Gamma correction is an operation by which the linear response of imaging sensors is converted to a logarithmic response that mimics the response of the human vision system. We are more sensitive to minor differences in brightness in a moderately lit scene than we are to differences in brightness of very bright or very dark scenes.
(Note that gamma correction in the processing of a digital image is not the same thing as what we mean when we say gamma correction with regard to the output of a video signal when it is send to a display device. Although the concepts are related, they are two different things done at different points in the processing pipeline between capturing an image with a digital sensor and viewing an image on an emitted light device.)