- Optical Pyrometer Definition: An optical pyrometer is a device that measures the temperature of glowing objects by comparing their brightness to a reference light.
- Construction: It is a simple device with a lens, lamp, colored glass, eyepiece, battery, ammeter, and rheostat.
- Working Principle: It operates by matching the brightness of the lamp filament to the brightness of the hot object.
- Calibration: The temperature is determined by the ammeter reading when the filament and the hot object’s brightness match.
- Measurement Range: This pyrometer measures temperatures from 1400°C to 3500°C and is limited to glowing objects.
The working principle and construction of an Optical Pyrometer shown here describe a visual disappearing-filament instrument. This type of temperature sensors is a non-contact measuring instrument for targets that emit enough visible radiation.
The instrument focuses the target image onto a calibrated lamp filament and limits the viewed radiation to a spectral band. The operator adjusts the lamp’s electric current until its brightness matches the target image.
At the match, the filament appears to disappear into the target image. The calibrated current indicates spectral radiance temperature at the instrument’s wavelength. Converting that reading to true target temperature requires the correct spectral emissivity unless the target behaves like the calibration blackbody.
Construction of Optical Pyrometer
The illustrated housing has an objective lens at one end, an eyepiece at the other and a calibrated filament lamp in the optical path. Apertures define the viewed target area. A coloured filter, often red, restricts the wavelength band. The lamp connects to a battery through an ammeter and rheostat as shown.
The optical pyrometer uses the rheostat to adjust lamp current and therefore filament radiance. Its calibration relates current to brightness temperature for the specified wavelength band.
The match occurs when the filament blends into the target image through the eyepiece. The calibrated ammeter reading then gives brightness temperature. Accuracy depends on lamp stability, focus, target size, filter response, sight-path transmission, target emissivity and the observer’s judgement.
Limitations of Optical Pyrometer
A visual disappearing-filament pyrometer has these main limitations:
- The target must emit enough radiation in the instrument’s visible wavelength band and fill the required field of view. Smoke, dirty windows, reflections or an obstructed sight path can bias the match.
- The usable range of an optical pyrometer is instrument-specific rather than universally 1400oC to 3500oC. Lamp calibration, filters, attenuation and target radiance set the lower and upper limits.
