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5.4.5 Detectors
A radiation detector converts the radiant power in the measurement to an electrical signal
that can be processed, recorded, and displayed. Detectors have the widest range of performance and cost of all the subcomponents of an optical spectroscopy system.
There are three main detector categories: photon, thermal, and multichannel. Photon
detectors respond to the arrival rates of the photons and have a spectral response that
changes with wavelength rather than responding to photon energies such as thermal detectors, which exhibit a near-uniform wavelength response. The performance criteria for all
detectors include the spectral range, arrangements as single elements or arrays, the required
detector area, acceptable signal-to-noise ratio (SNR), and dynamic range.
The largest item driving cost is the SNR requirement. Wide selections of detectors are
available from a host of companies. There are literally thousands of photomultiplier tube
types, array detectors (line and 2-D array), and semiconductor light sensors and thermal
infrared detectors covering a spectral range from 150 nm to more than 40 μm. The wavelength range often determines the choice of detector type.
The responsivity of a detector is a function of the ratio of the output signal level to the
incident radiant power (e.g., amps/watts), while the sensitivity of a detector can be characterized from the rate of change of signal output with respect to changes in the incident
radiant power. This subtle difference is important when the response exhibits nonlinear
properties. For all photon detectors, and many thermal detectors, both the responsivity
and sensitivity of the detector are wavelength dependent properties and, hence, define the
spectral response of the detector. The sensitivity of the detector may also change with other
variables such as temperature, applied bias voltage, and other components in the signal
processing circuit. Some detectors, for example, photomultiplier tubes, can have different
output modes – as a charge or current (analog mode) or as a pulse rate (photon counting).
Thus, care must be taken not only in the detector choice, but also in how it is going to be
used in the data acquisition scheme.
Optical detectors can vary widely in their speed of response and in their ability to record
quickly changing signals. The rise time of the detector can be important for many applications and can vary from <1 ns to seconds, depending on detector type. In the absence
of any input optical signal, a detector will still produce an output, which is related to the
inherent dark signal and dark noise of the detector or system. The dark signal is caused by
a relatively small electric current that flows through all photosensitive devices even when
no photons are entering the device. The dark noise relates to the fluctuations in the number
of photons contributing to the current inherent within the detector, especially because these
photons are all independent to each other (random). As a result the characteristics of these
signals vary greatly with detector type and mode of operation. An understanding of the
noise and possible drifts associated with the dark signal is important before this inherent
“system noise” can be subtracted from the optical signal of interest.
The main photon detectors used in commercial fluorimeter systems are based on either
photomultiplier or photodiode devices. A photomultiplier is a vacuum device with a
168
5.4.5 Detectors
A radiation detector converts the radiant power in the measurement to an electrical signal
that can be processed, recorded, and displayed. Detectors have the widest range of performance and cost of all the subcomponents of an optical spectroscopy system.
There are three main detector categories: photon, thermal, and multichannel. Photon
detectors respond to the arrival rates of the photons and have a spectral response that
changes with wavelength rather than responding to photon energies such as thermal detectors, which exhibit a near-uniform wavelength response. The performance criteria for all
detectors include the spectral range, arrangements as single elements or arrays, the required
detector area, acceptable signal-to-noise ratio (SNR), and dynamic range.
The largest item driving cost is the SNR requirement. Wide selections of detectors are
available from a host of companies. There are literally thousands of photomultiplier tube
types, array detectors (line and 2-D array), and semiconductor light sensors and thermal
infrared detectors covering a spectral range from 150 nm to more than 40 μm. The wavelength range often determines the choice of detector type.
The responsivity of a detector is a function of the ratio of the output signal level to the
incident radiant power (e.g., amps/watts), while the sensitivity of a detector can be characterized from the rate of change of signal output with respect to changes in the incident
radiant power. This subtle difference is important when the response exhibits nonlinear
properties. For all photon detectors, and many thermal detectors, both the responsivity
and sensitivity of the detector are wavelength dependent properties and, hence, define the
spectral response of the detector. The sensitivity of the detector may also change with other
variables such as temperature, applied bias voltage, and other components in the signal
processing circuit. Some detectors, for example, photomultiplier tubes, can have different
output modes – as a charge or current (analog mode) or as a pulse rate (photon counting).
Thus, care must be taken not only in the detector choice, but also in how it is going to be
used in the data acquisition scheme.
Optical detectors can vary widely in their speed of response and in their ability to record
quickly changing signals. The rise time of the detector can be important for many applications and can vary from <1 ns to seconds, depending on detector type. In the absence
of any input optical signal, a detector will still produce an output, which is related to the
inherent dark signal and dark noise of the detector or system. The dark signal is caused by
a relatively small electric current that flows through all photosensitive devices even when
no photons are entering the device. The dark noise relates to the fluctuations in the number
of photons contributing to the current inherent within the detector, especially because these
photons are all independent to each other (random). As a result the characteristics of these
signals vary greatly with detector type and mode of operation. An understanding of the
noise and possible drifts associated with the dark signal is important before this inherent
“system noise” can be subtracted from the optical signal of interest.
The main photon detectors used in commercial fluorimeter systems are based on either
photomultiplier or photodiode devices. A photomultiplier is a vacuum device with a