An infrared thermometer is a thermometer which infers temperature from a portion of the thermal radiation sometimes called blackbody radiation emitted by the object being measured. They are sometimes called laser thermometers if a laser is used to help aim the thermometer, or non-contact thermometers or temperature guns, to describe the device's ability to measure temperature from a distance. By knowing the amount of infrared energy emitted by the object and its emissivity, the object's temperature can often be determined. Infrared thermometers are a subset of devices known as "thermal radiation thermometers".
Sometimes, especially near ambient temperatures, false readings will be obtained indicating incorrect temperature. This is most often due to other thermal radiation reflected from the object being measured, but having its source elsewhere, like a hotter wall or other object nearby - even the person holding the thermometer can be an error source in some cases. It can also be due to an incorrect emissivity on the emissivity control or a combination of the two possibilities.
The most basic design consists of a lens to focus the infrared thermal radiation on to a detector, which converts the radiant power to an electrical signal that can be displayed in units of temperature after being compensated for ambient temperature. This configuration facilitates temperature measurement from a distance without contact with the object to be measured. As such, the infrared thermometer is useful for measuring temperature under circumstances where thermocouples or other probe type sensors cannot be used or do not produce accurate data for a variety of reasons.
Some typical circumstances are where the object to be measured is moving; where the object is surrounded by an electromagnetic field, as in induction heating; where the object is contained in a vacuum or other controlled atmosphere; or in applications where a fast response is required, an accurate surface temperature is desired or the object temperature is above the recommended use point for contact sensors, or contact with a sensor would mar the object or the sensor, or introduce a significant temperature gradient on the object's surface.
Infrared thermometers can be used to serve a wide variety of temperature monitoring functions. A few examples provided include:
- Detecting clouds for remote telescope operation
- Checking mechanical equipment or electrical circuit breaker boxes or outlets for hot spots
- Checking heater or oven temperature, for calibration and control purposes
- Detecting hot spots / performing diagnostics in electrical circuit board manufacturing
- Checking for hot spots in fire fighting situations
- Monitoring materials in process of heating and cooling, for research and development or manufacturing quality control situations
There are many varieties of infrared temperature sensing devices available today, including configurations designed for flexible and portable handheld use, as well many designed for mounting in a fixed position to serve a dedicated purpose for long periods.
Specifications of portable handheld sensors available to the home user will include ratings of temperature accuracy (usually with measurement uncertainty of ±2 °C/±4 °F) and other parameters.
The distance-to-spot ratio (D:S) is the ratio of the distance to the object and the diameter of the temperature measurement area. For instance if the D:S ratio is 12:1, measurement of an object 12 inches (30 cm) away will average the temperature over a 1-inch-diameter (25 mm) diameter spot. The sensor may have an adjustable emissivity setting, which can be set to measure the temperature of reflective (shiny) and non-reflective surfaces.
A non-adjustable thermometer sometimes can be used to measure the temperature of a shiny surface by applying a non-shiny paint or tape to the surface, if the allowed measurement error is acceptable.
The most common infrared thermometers are the:
- Spot Infrared Thermometer or Infrared Pyrometer, which measures the temperature at a spot on a surface (actually a relatively small area determined by the D:S ratio).
Related equipment, although not strictly thermometers, includes:
- Infrared Scanning Systems scan a larger area, typically by using what is essentially a spot thermometer pointed at a rotating mirror. These devices are widely used in manufacturing involving conveyors or "web" processes, such as large sheets of glass or metal exiting an oven, fabric and paper, or continuous piles of material along a conveyor belt.
- Infrared Thermal Imaging Cameras or Infrared Cameras are essentially infrared radiation thermometers that measure the temperature at many points over a relatively large area to generate a two-dimensional image, called a thermogram, with each pixel representing a temperature. This technology is more processor- and software-intensive than spot or scanning thermometers, and is used for monitoring large areas. Typical applications include perimeter monitoring used by military or security personnel, inspection / process quality monitoring of manufacturing processes, and equipment or enclosed space hot or cold spot monitoring for safety and efficiency maintenance purposes.
A photographic camera using infrared film and suitable lens, etc., is also called an infrared camera. This only captures the short wave and is not sensitive to the thermal radiation from room temperature objects.
An early model of the Thermofocus IR thermometer
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- Introduction to Infrared Thermometers - Omega