Two Color Measurement

A two-color pyrometer is an advanced pyrometer for non-contact temperature measurement using a two-wavelength measurement.

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

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 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 measure reliably even 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, and if changing transmissions are present in the optical path of the pyrometer—such as dust, steam, dirt, and windows. These devices are used in challenging industrial applications, such as some metal processing applications, where the emissivity is unknown and changing but alters equally at both wavelengths.

Most ratio pyrometers use semiconductor detectors. Therefore, time constants of 1 ms to 20 ms are common for temperature measurements today, while the start of the temperature measuring range is usually well above 100 °C.

Most suppliers offer a parallel representation of the temperature measured using the single-wavelength measurement. If the slope and emissivity are set correctly, many quotient pyrometers can also calculate and display the percentage signal loss due to shading or window contamination. While a classic pyrometer with one-color measurement cannot distinguish between a drop in temperature of the measurement object and contamination of the optics, a ratio pyrometer has this ability.