Page 9 - Spec Tech Vol 1 Issue 09
P. 9
SPACE SENSORS
Infrared spectrometers come in a variety of and pyroelectric detectors. Detectors that
types but have many common features. All utilize photon energy to free bound
have a source that emits all the IR radiation electrons in the detector material are
of interest. These are usually various solid called photodetectors. Photodetectors,
materials heated to incandescence by unlike thermal detectors, do not respond to
an electric current. The radiation energy all wavelengths but have a long-wavelength
distribution as a function of wavelength limit where the photon has insufficient
approaches that of a theoretical black energy to excite the electrons. One example
body where the energy reaches a maximum is the photoconductive detector, in which the
at a wavelength (μm) equal to 2897 / T, absorption of photon energy promotes
where T is the absolute temperature (K). bound electrons to free states. This results
The operational temperature is such that the in increased electrical conduction.
radiation energy is usually at a maximum In between the source and detector,
near the short-wavelength limit of the the spectrometer must have some means of
spectrum (usually ÿ2 μm) and decreases as analyzing the radiation so that an intensity
the wavelength gets longer. In the far-IR can be deduced for each wavelength
region, source energy is very low.
resolution element. Two completely different
All spectrometers must have some kind of types of devices are used,
detector. These are devices that, in one way monochromators and interferometers.
or another, change radiation energy into an Monochromators with gratings or prisms are
electrical signal that can be amplified and used in dispersive instruments, and
processed to yield a spectrum. Thermal interferometers are used in Fourier
detectors measure the heating effect of the transform instruments.
radiation and respond equally to all
wavelengths. Examples
include thermocouples, bolometers,
Space Explorer 2022 9
