| 1. What are
pyroelectric detectors anyway?
Pyroelectric detectors are thermal detectors. That is, they
produce a signal in response to a change in their temperature.
Below a temperature Tc known as the Curie point, ferroelectric
materials such as TGS or Lithium Tantalate, exhibit a large spontaneous
electrical polarisation. If the temperature of such a material is altered, for
example, by incident radiation, the polarisation changes. This change in
polarisation may be observed as an electrical signal if electrodes are placed
on opposite faces of a thin slice of the material to form a capacitor. When the
polarisation changes, the charges induced in the electrodes can be made to
produce a voltage across the slice if a the external impedance is comparatively
high. The sensor will only produce an electrical output signal when the
temperature changes; that is, when the level of incident radiation changes.
This process is independent of the wavelength of the incident
radiation and hence pyroelectric sensors have a flat response over a very wide
spectral range. The limiting feature on the spectral range is the window
material used in the manufacture of the sensor housing. By using different
windows materials it is possible to detect radiation at different frequencies.
2. Can you please
define the terms D* and Noise Equivalent Power?
D* is a figure of merit value which is defined as the rms
signal to noise ratio in a 1 Hz bandwidth per unit rms incident radiant power
per square root of the sensor area. D* may be defined in response to a black
body source as D*(t,f,1) where t is the temperature of the reference black
body, f is the modulation frequency in Hz and 1 represents unity bandwidth. The
units for D* are cmHz1/2W-1.
Noise Equivalent
Power (NEP) is the rms value of the incident chopped radiant power necessary to
produce an rms electrical signal equal to the rms electrical noise. The rms
electrical noise refers to the value calculated for unit square root bandwidth,
VHz-1/2. The units for NEP are WHz-1/2. |