Temperature Measurement Range

The temperature measurement range specifies the minimum and maximum temperatures between which the device can measure temperatures. These temperatures can be shown on a digital display directly on the device, transferred to software and displayed there, or output as current or voltage proportional values via an analog interface. A distinction is sometimes made between a temperature display range and the specified measuring range. While the temperatures within the specified measuring range must meet the accuracy specifications in the technical documentation, the device may also be able to evaluate and display radiation signals outside this range, but without any claim to accuracy. Infrared cameras often have several temperature measurement ranges that can be selected manually. Each pixel can display temperatures in the specified interval. However, there are also cameras with relatively narrow measuring ranges that can automatically switch between these ranges in adaptation to a statistical image evaluation.

This results in a low NETD but prevents the highest and lowest specified measured values from being displayed in the scene at the same time. Infrared cameras with logarithmic signal processing are an alternative. These allow the simultaneous measurement of minimum and maximum temperature values distributed in the image scene but at the expense of a higher NETD. For infrared imagers and pyrometers, the temperature measurement range depends on the effective measuring wavelength, the detector technology, and the dynamics of the processing electronics. If the detector technology converts the radiation energy linearly into an electronic signal, the longer the spectral range of the detector, the wider the temperature measurement range can be. For example, the temperature measuring range of a detector with an effective spectral response of 2.4 µm can be more than twice as wide as that of a detector with 1.6 µm at the same initial temperature.

Two minimum measuring temperatures are often specified in the documentation for ratio pyrometers. On the one hand, these pyrometers can display the single-channel temperature. This corresponds exactly to the measuring principle of standard pyrometers. On the other hand, they can determine a ratio temperature, which is calculated from the quotient of the signals from two detectors. The additional uncertainties associated with this method require an increased initial temperature of several dozen Kelvin.

It should also be noted that the temperature measuring ranges usually apply to the condition that the measured object is a black body, i.e., a body with an emissivity of 1.000. Particularly at the start of the measuring range, i.e., when the thermal radiation from the measurement object only causes very low signal strengths at the detector, low emissivities mean further signal attenuation, which may exceed the sensitivity limit.