In all systems there are two noise components to consider, dark current noise and Shot noise. Additionally, sub-optimally designed systems may introduce noise in many different areas including the analog-to-digital conversion process or the amplification of the signal.
Dark Current noise: Even in the absence of light input, the thermal emissions from any photon-detecting device can produce a low amount of noise (measured in electrons per second) that has been termed Dark Current noise. In the case of PMTs, Dark Current noise can originate from the photocathode and/or from current leaking through the dynodes of the PMT. The GenePix 4000B has been designed to reduce dark current noise to negligible levels through two approaches: 1) the PMTs chosen for use in the GenePix 4000B show very low dark current levels, and 2) the dwell time of the lasers on each pixel is very short resulting in a negligible number of dark current electrons generated for each pixel.
Shot noise: Shot or statistical noise is the only significant type of noise that needs to be considered in the GenePix 4000B. It is generated by the input of light and results from the variable nature of photons. With shot noise, it is important to realize that as the signal intensity increases the shot noise increases as the square root of the signal collected, so the SNR actually decreases as signal intensity increases.
Signal-to-noise ratio is used in many signal-detection disciplines (radio, electronics, imaging, etc.) as a quantitative measure of