Chopped Radiation Method
What is the chopped radiation method?
In describing the working principle of infrared radiation
pyrometers, the term "chopped radiation" is used to characterize
the mechanical or optical modulation of radiation, primarily
thermal infrared radiation, intercepted by the infrared
detector. In general, this is accomplished by an optical
chopper, basically mechanical blades driven by a suitable
electromagnetic device, such as an electric motor, which
periodically interrupts the incident radiation from the
measured target to the detector. During each interruption
the detector is exposed to a reference radiation, generally
an internal blackbody reference source having a defined
Why is the chopped radiation method used?
to Operate Pyroelectric Detectors
High quality, high performance infrared detectors of the
pyroelectric type must be operated in the chopped radiation
method, because they respond to radiation differences only,
not to absolute radiation intensities. Pyroelectrics are
the best uncooled detectors available, in terms of detectivity,
fast response, reliability and stability.
to Eliminate Thermal Drift
Detectors in all infrared pyrometers intercept infrared
radiation emitted by the measured target and, at the same
time, radiation emitted by the detector enclosure. For unchopped
pyrometers, the radiation from the detector enclosure, which
corresponds to the pyrometer's housing temperature, gives
rise to a bias on the output signal of the detector and
subsequently to thermal drift, whenever the housing temperature
Also, for unchopped pyrometers measuring low temperatures
at or below the ambient or when focussing on small targets,
the bias exceeds the measured signal by two orders of magnitude.
In a typical design with a 20 degree detector field-of-view
the radiation bias is approximately 100 times higher than
the measured signal from a target at ambient temperature.
Correct compensation of this bias for a specified accuracy
of 1°F would require a stabilization of the housing
temperature for better than 1°F/100 = 0.01°F, a
virtually impossible specification for any affordable industrial/commercial
The chopped radiation method eliminates this problem completely.
A pyrometer with this method evaluates two subsequent signals:
S1 = Starget + Bias
S2 = Sreference + Bias
Sdelta = Sl - S2 = Starget
During the short chopping cycles, which are normally in
the millisecond range, the temperature of the pyrometer's
housing and the bias do not change. The bias is thus eliminated
completely and substituted by the reference signal, which
can be easily measured or controlled within the specified
reference accuracy of < 1°F over the permissible
ambient temperature range.
To Reduce Signal Noise
The chopped radiation method automatically provides a modulated
signal with a precisely defined frequency. Such a signal
lends itself to a narrow band selective filtering by phase
sensitive demodulation or digital signal processing with
unsurpassed noise suppression and signal stability.
Chopped radiation method benefits
- Virtually no thermal drift
- Excellent dynamic compensation of thermal shock
- Unequaled noise filtering
- High temperature resolution
- High spatial resolution (small target detection)
- Fast response
- Long-term stability
It is often argued that the chopped radiation method has
its weak link in the optical chopper, as it tends to make
pyrometers more expensive and unreliable. This may have
been or possibly still is the case for obsolete or badly
designed pyrometers. Of course, an optical chopper always
adds to the manufacturing costs, but it is generally less
than 5% in comparison to the total cost of the pyrometer.
Today, reliability and life time of advanced mechanical
choppers are fully consistent with overall system performances.
MTBF of the latest Heimann chopper, manufactured since 1986,
is specified at 72,000 hours (9 years) of continuous operation
under specified environmental operating conditions.