Gain 101

[kirkster501]

Hi, is there a 101 or beginners guide document or using gain and offset on CMOS cameras please for soemone coming over from the dark side of CCD?  I'm a bit confused on the subject.  For instance, what is "unity gain" ???

Many thanks, Steve

[vlaiv]

Not that I'm aware of, but topic is really not complicated at all.

CCDs also have gain, but it was for the most part fixed and you could not change it.

Gain is simply conversion factor between electron count and ADU values. That is it.

With CCD camera, you could have 15K FW for example and you still read 64K (16 bit) ADU value. Gain is set to something like 0.25 e/ADU in this case.

This simply means that if you have ADU value of for example 60000ADU and want to see how much electrons were counted in that pixel - you need to multiply by e/ADU gain value, in this case it will be 60000 ADU * 0.25 e/ADU = 15000e.

Unity gain just means gain at which this conversion factor is 1 so 1 e/ADU gain.

Gain can be thought of as linear coefficient. In linear equation we have multiplier and additive constant Y = a * X + b, right?

b is Offset ...

It is there because of the way A/D conversion works. We don't really count electrons, but rather we try to measure voltage those electrons produce. Voltage is not absolute thing - it is rather potential difference between two points. For this reason, once we see electrons as voltage over certain point - we need to "set" that point to some value - this is offset.

Main reason offset is used is to allow for noise to have proper distribution.  Measured value will be +/- true value and if you don't detect any electrons - you still need to allow for negative noise value as on average it needs to be zero. Sometimes it will be positive and you need negative values to balance it out.

ADU value is always positive value, or rather zero or larger. You can't represent negative values in ADUs. What you can do instead is say - every read number has some offset added to it. Let's say 40 ADU.

This means that if you read 30ADU, you actually read (30-40) ADU = -10ADU. Handy way to read negative values and allow for noise to have proper distribution. It is just consequence of the fact that voltage is relative (or difference between two points).

Again - CCDs have both offset and gain, but they are set and you can find gain in specs. Offset can be measured (via bias average value).