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THz Intensity Measurement …Choose A Golay Cell Or A LiTaO3 Pyroelectric Detector?
April 30, 2008
•Application Note: THz Intensity Measurement …Choose A Golay Cell Or A LiTaO3 Pyroelectric Detector?
Before we start to describe the particular attributes of one detector versus the other it makes sense to spend a little time giving some general technical information about what each is and how they work.
What is a Golay Cell and how does it function?
It is a "photo-acoustic" device that is sensitive, works at ambient temperatures and has
broad spectral response. The basic elements that make up a Golay Cell are the 6 mm
HDPE input window, a small fragile gas chamber that includes a thin, partially absorbing
film and what is called an "optical microphone section". When IR or THz radiation is
absorbed by the thin film in the gas cell, the gas is heated, expands and distorts the
mirrored back wall of the cell. This distortion (or movement) is monitored and measured
by the combination of an LED, optics, grating and photodiode. The output of the
photodiode is proportional to the displacement of the mirrored wall of the gas cell. It's
output is calibrated against a source of known power output in Volts/Watt.
What is a LiTaO3 Pyroelectric Detector and how does it function?
It is an AC Thermal detector that is sensitive, typically used at room temperature and has
broad, flat spectral response across most of the electromagnetic spectrum. The Pyro is
based on a thin permanently poled, ferroelectric crystal (i.e. LiTaO3) that exhibits a
pronounced thermal effect (the pyroelectric effect) where it's instantaneous polarization
is a function of the rate of temperature change of the crystal. By applying conductive
electrodes to the top and bottom surfaces of the crystal the resulting charge can be
coupled out of the device and calibrated in terms of µA/W. The Pyroelectric Detectors
normally include a voltage or current mode circuit for optimum performance and are
ultimately calibrated in Volts/Watt or Volts/Joule.
How does the performance of a Golay Cell compare to that of a LiTaO3 Pyroelectric
Detector?
Now lets take a look at the performance specifications that are typical for the Golay Cell
versus a Pyroelectric Detector. Note that there is only one type of Golay Cell at present,
where a Pyroelectric device can take many forms …i.e. Hybrid Detector/Amp and
Analog or Digital Radiometer (i.e. detector, discrete electronics and microprocessor).
We'll choose the Pyroelectric Hybrid Detector for this comparison.
SOURCE: Spectrum Detector has become Gentec-eo USA, Inc.
Click Here To Download:•Application Note: THz Intensity Measurement …Choose A Golay Cell Or A LiTaO3 Pyroelectric Detector?
