The purpose of this glossary is to offer an overview of terms used to describe CCD and PDA based systems. In order to keep the data as practical as possible, some approximations and simplifications have been made.
Analog to Digital Converter
The signal charge from the detector is an analog signal, which ranges in value from zero to the saturation charge. The Analog to Digital Converter (A/D) converts this analog signal to a (binary) number, thus allowing it to be manipulated by the computer.
The Background is simply a scan or trace recorded in the absence of the signal. It is the fixed pattern noise plus the charge due to dark current. For measuring a sample signal, normally one takes a “background scan” with the same exposure time as for the signal scan. This is subtracted from subsequent scans, and can be performed by the 78877 LineSpec™ system [https://www.newport.com/p/78877] or through the TracQ BASIC software application.
A charge is generated by each pixel in a diode array or CCD, due to the signal photocurrent as well as dark current. Incident light on the pixels generates photocurrent, while the dark current is caused by charge leakage within the pixel. In the dark, charge leakage occurs at an approximately constant rate. In the process of light detection, each pixel (element) is read in sequence (not simultaneously), and the electronic charges measured by that pixel are recorded. This is why detection limits, signal levels, noise, etc. are usually quoted as charge, in units of electrons. Pixels are always generating charge, even while other pixels in the array are actively being read.
PDAs, CCDs and InGaAs detectors have dark current, which is also referred to as leakage current; this results in a charge build-up in each pixel, even if there is no light falling on the detector. Dark current is exponentially dependent on temperature, and is halved for approximately every 7 °C drop in temperature. The dark current adds to the signal and has important consequences:
The detection limit is a measure of the smallest signal that can be measured in a single readout. The smallest detectable signal is equivalent to the noise accompanying that signal, i.e. a Signal to Noise (S/N) ratio of one. This signal works out to be about equal to the readout noise, once a suitable temperature and exposure time have been selected, which ensures that the noise contribution of the dark current is minimized.
The dynamic range is the ratio of the largest signal to the smallest detectable signal i.e. the ratio of the saturation charge (well capacity) to the detection limit (approximately the readout noise). It serves as an indication of the range of signals the detector can measure.
Due to the very low read out noise of a CCD, we expect this type of device to have a slightly better dynamic range than a PDA.
There are two distinct categories of noise: Pixel Noise and Fixed Pattern Noise (FPN). Pixel Noise is the “real” noise, i.e. uncertainty in measurement results. FPN is not noise in this classical sense. When people speak of system/measurement noise they are usually referring to Pixel Noise.
Noise (Fixed Pattern)
If the entire linear or two-dimensional array is read out, there will be small differences in the values between neighboring pixels, even if there is no light falling on the detector. This variation from pixel to pixel does not change from one complete frame readout to another, and is referred to as the Fixed Pattern Noise (FPN).
To define this term let us assume that light is falling on a pixel (element) of the array (either PDA or CCD). It is assumed that this light signal has no fluctuation apart from its shot noise, which, from fundamental considerations, cannot be removed.
The noise has three main components:
- Readout noise, due to the amplifier and electronics. This noise component is independent of dark current and signal levels, is mildly dependent on temperature, and is present every time the array is read
- Shot noise from the light signal itself. A detailed description of shot noise is beyond the scope of this document, but for our purposes we will state that if the signal (or dark current) is N electrons, then the shot noise will be the square root of N
- Shot noise from the dark current. This is dependent on the exposure time and is very dependent on the temperature
This is the largest signal that can be measured, and is due to the limited well depth capacity of the pixel. It is quoted in electrons or coulombs. (one coulomb = 1.6 x 10-19 electrons). The electron storage capacity per unit area is similar for PDAs and CCDs.
Signal to Noise Ratio
The Signal to Noise (S/N) ratio is the ratio between the signal and the noise associated with that signal. This noise is the fixed readout noise and shot noise components, which are dependent on the dark current and signal. Since shot noise is dependent on the root of the signal, the S/N will increase as the signal increases as seen in Figure 1.
Figure 1. Signal to noise ratios for InstaSpec™ CCDs and Silicon PDAs
Signal to Noise Ratio (Maximum)
The Maximum S/N is the ratio between the maximum signal (saturation) and the noise associated with that signal (as distinct from the noise associated with the smallest detectable signal).
Quantum Efficiency (QE) and Spectral Response
Photons of different wavelengths have different probabilities of producing a photoelectron; this probability is called QE. QE is a measure of the probability of a single photon producing a photoelectron. Spectral Responsivity is the amount of current that will be produced per unit power.
About Oriel Instruments
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