radiation over a wide spectral range, e.g. 200–800 nm. An array of several hundred photodiodes on one
or two chips, positioned to receive dispersed radiation from a monochromator, enables a wide range of
wavelengths to be monitored simultaneously without the need for mechanical scanning. The current
generated by each photodiode is proportional to the intensity of radiation it receives and it can be
measured sequentially many times a second under the control of a microprocessor. The digitized signals
can be stored by a micro- or minicomputer for immediate electronic processing and visual display on a
cathode ray screen (VDU) in the form of a spectrum. Diode arrays are also utilized in vidicon tube
detectors which are similar in construction to a small television tube. Such tubes can also be used in
nonscanning spectrometers but generally resolution is limited if detection over a wide wavelength range
is required. More complex instruments incorporate an integration facility enabling the total signal
accumulated over a period of time to be measured. This facility is commonly used in emission
spectrometry. In the infrared region, thermal or photon detectors are used. Thermal detectors depend on
the heating effect of infrared radiation producing a temperature sensitive response. They include
thermocouples (voltage changes), thermistors (resistance changes), pyroelectric (electric polarization
changes) and Golay cells (changes in the pressure of an enclosed gas). Photon detectors are more
sensitive and have a more rapid response. They are semiconductor devices made of such materials as
mercury cadmium telluride (MCT) which at liquid nitrogen or helium temperatures generate a variable
voltage according to the energies of incident photons of infrared radiation.
Radiations outside the ultraviolet, visible and infrared regions cannot be detected by conventional
photoelectric devices. X-rays and γ-rays are detected by gas ionization, solid-state ionization, or
scintillation effects in crystals. Non-dispersive scintillation or solid-state detectors combine the
functions of monochromator and detector by generating signals which are proportional in size to the
energy of the incident radiation. These signals are converted into electrical pulses of directly
proportional sizes and thence processed to produce a spectrum. For radiowaves and microwaves, the
radiation is essentially monochromatic, and detection is by a radio receiver tuned to the source
frequency or by a crystal detector.
Further Reading
Denney, R. C., A Dictionary of Spectroscopy (2nd edn), Macmillan, London, 1982.