Multi Color Measurement

A multi-color measurement is an infrared temperature measurement principle that analyzes the infrared radiation emitted by an object at multiple wavelengths, at least at two wavelengths. Some manufacturers refer to an instrument that uses two-wavelength and additional input information as emissivity characteristics as a multi-color instrument, although only two wavelengths are measured. Additionally, an instrument that measures a high quantity of different distinguishable wavelengths is often referred to as a spectrometer.

Unlike single-color pyrometers, which measure radiation at a single wavelength, multi-color pyrometers can improve accuracy and ensure reliability under conditions like varying emissivity or the presence of contaminants such as dust or smoke.

These instruments are often called ratio, two-color, or dual-wavelength pyrometers and function differently than single-color pyrometers.

Wien’s displacement law states that the peak wavelength of radiation emitted by a black body is inversely proportional to its temperature. As an object’s temperature rises, its emitted radiation increases within the overall wavelength range, and additionally, the maximum of the spectral-specific radiation shifts to shorter wavelengths.

There is a significant difference between intensity-based single-color pyrometers and ratio-based pyrometers. Single-color pyrometers measure the power of the emitted infrared radiation within a specific bandwidth. In contrast, ratio-based pyrometers measure the infrared radiation at two closely spaced wavelengths and evaluate the relationship between the two intensities. The bandwidth of these two colors can be partly overlapping or completely separated. This unique approach of ratio pyrometers allows for more accurate and reliable temperature measurements.

Unlike single-color devices, a ratio pyrometer can even measure reliably when the emissivity is unknown or changing with the temperature, as long as both wavelength signals are affected proportionally by emissivity or process changes. Therefore, one of the standout features of the two-color pyrometer is its ability to provide repeatable and accurate measurements even in adverse conditions.

Ratio pyrometers are often used if the measuring spot is not fully placed on the object or the object is smaller than the pyrometer spot size if changing transmissions are present in the optical path of the pyrometer—like dust, steam, dirt, and windows. These devices are used in challenging industrial applications, such as some metal processing applications, when emissivity is unknown and changing but alters equally at both wavelengths.