When 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 which has
mechanical blades driven by a motor which periodically interrupts the incident radiation from the
measured target to the detector. During each interruption the detector is exposed to an internal reference
source having a defined and measured temperature.

Why is the Chopped Radiation Method used?

Chopping is a requirement when using a Pyroelectric detector.
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.

Thermal Drift is eliminated when properly chopping the signal of a Pyroelectric detector. Detectors in all infrared pyrometers simultaneously intercept infrared radiation emitted by the measured target and the radiation emitted by the pyrometer’s forward internal surfaces. For un-chopped pyrometers that use a thermopile, the radiation from the pyrometer's housing temperature and other surfaces within view of the detector’s wide acceptance angle gives rise to a bias on the output signal of the detector and subsequently to thermal drift whenever the housing temperature changes. Although thermopile based pyrometers will employ internal temperature compensation, the result is inferior to a chopped pyroelectric detector based pyrometer.

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Recommended Procedure for Cleaning Infrared Pyrometer Lenses

Procedure for cleaning infrared pyrometer lenses, including items needed for the cleaning process, and care instructions for both uncoated and coated lenses.

Lens Cleaning Technote (17k PDF)