What is SNR?
The signal-to-noise ratio (SNR) defines the measurement’s quality. SNR in CCD imaging refers to the signal’s relative magnitude compared to the signal’s uncertainty on a per-pixel basis. It’s the ratio of the measured signal to the overall measured noise at that pixel (frame-to-frame). In applications requiring precise light measurement, a high SNR is especially critical.
Within the silicon layer, photons incident on the CCD transform to photoelectrons. These photoelectrons not only carry the signal, but also a statistical variation of photon arrival rate variations at a specific site. This phenomenon is referred to as “photon noise,” and it is based on Poisson statistics. Furthermore, CCD noise sources produce electrons that are indistinguishable from photoelectrons. All noise sources must be taken into account when computing total SNR:
Photon noise: The intrinsic natural variation of the incident photon flux is referred to as photon noise. Photoelectrons acquired by a CCD have a Poisson distribution and a square root signal-to-noise relationship.
Read noise: The uncertainty generated during the process of quantifying the electronic signal on the CCD is referred to as read noise. The on-chip preamplifier is the source of the majority of readout noise.
The stochastic variance of thermally produced electrons within the silicon layers that make up the CCD causes dark noise. The rate of creation of thermal electrons at a certain CCD temperature is referred to as dark current. The square root of the number of thermal electrons generated within a given exposure is dark noise, which similarly follows a Poisson relationship. Dark current is reduced by more than 100 times when the CCD is cooled from ambient temperature to -25°C. Furthermore, many scientific-grade CCDs use MPP technology to reduce dark current even more.
Dark noise: Signal-to-noise ratio (abbreviated SNR) is a measure used to compare the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in decibels. A ratio higher than 1:1 (greater than 0 dB) indicates more signal than noise.
When read noise exceeds photon noise in low-light settings, the image data is said to be “read-noise limited.” Increase the integration time until photon noise outnumbers both read and dark noise. The image data is said to be “photon restricted” at this point.
The following equation can be solved for the minimal photon flux required once you’ve identified acceptable values for SNR, integration time, and the degree to which you’re willing to bin pixels. As a result, for the given experimental settings and camera specifications, this is the lowest light level that can be detected.
