(μm). The IR spectral range is an invisible (to us) portion of the electromagnetic spectrum from 0.7–1,000 microns. Basically, anything warmer than absolute zero (0 °K) emits ener- gy somewhere within this range. An IR ther- mometer aimed at an object has this energy passing through the unit’s optical system, which is then converted to an electrical signal at the detector. The converted signal is dis- played as a temperature (°F or °C). The prin- ciple of IR thermometers resembles that of the human eye. In general, a wavelength as it passes through the eye will be interpreted as a color. With IR thermometers, color is used to determine temperature. IR energy is also transmitted through ob- jects from other sources and reflected o object surfaces. IR thermometers not only sense the emitted energy of an object but also reflected and transmitted energy. Because of these interferences, the IR thermometer can only be used as a screening tool. The accuracy of IR thermometers used in food safety is largely determined by two im- portant concepts: 1) emissivity and 2) dis- tance-to-spot size ratio. Emissivity is the ratio of radiation emitted by an object’s surface to radiation emitted by a perfect black body at the same temperature, which is another way to describe an object’s ability to emit or ab- sorb IR radiation. A perfect black body nei- ther reflects nor transmits light energy and has an emissivity of 1.0. Water and most or- ganic materials such as foods have an emis- sivity of approximately 0.95. Therefore, most IR thermometers used in our industry have a fixed preset emissivity of 0.95. While the temperature taken of a food might be accurate, temperature readings might vary when using a preset unit on stainless steel or any shiny surface such as glass, plastic, or ceramic. In other words, even though the temperature of food stored in a hotel pan has come to equilibrium, the temperature detected on the food surface versus the hotel pan sur- face might dier considerably. To compensate, the area being measured can be covered with masking tape, a self-adhering black spot, or flat black paint. After the temperature of the tape, spot, or paint has come to equilibrium with that of the surface, the area temperature can be accurately measured. Distance-to-spot size ratio is the relation- ship between the distance of the measuring device from the surface as well as the area of
with laser or light aiming devices that denote the exact area being measured have com- pletely taken the guesswork out of the dis- tance-to-spot size ratio. Laser or light aiming is not a part of IR technology. It is only there to identify the object where the temperature is measured. The beauty of these devices is that what you see is what you measure. At this point, it is appropriate to remind ourselves that every IR thermometer has limi- tations. For instance, our IR thermometer also measures reflected and transmitted scat- tered light energy from sources near the target. Therefore, once we get beyond a 3–4 ft distance from the target, the IR thermometer significant- ly loses accuracy from measurements taken of walls, ceilings, and other large surfaces. There is a good rule of thumb to follow: For the best results, the distance to the object should not be greater than the size of the object. •Each manufacturer of these devices is required to provide specifics of their application and operation. Each model is unique. Always read the instructional manual before using the instrument. •Do not attempt to measure temperature through glass or plastic film. IR thermom- eters will measure the temperature of the glass or plastic, not the object beyond. Other Limitations and Rules of Use • Do not attempt to measure temperatures through dust, smoke, fog, or steam. The temperature might be from particles in the air rather than from the object you wish to measure. Keep the lens of the IR thermom- eter clean and free of condensation for the same reason. • IR thermometers will only measure surface temperatures and not internal tempera- tures. Normally, foods heat and cool from the outer surface to the interior. Therefore, a surface temperature reading could give a false indication of the interior temperature. • Be cautious in measuring the tempera- tures of foods, particularly where the background area is hotter than the food being measured. IR thermometers tend to measure reflected background energy and might interfere with accurate readings. This tendency is why a thermocouple or thermistor thermometer is essential to ver- ify temperature findings that might have regulatory applications.
the surface being measured. The target area di- ameter increases proportionally as the distance from the thermometer to the surface increases. Briefly, this ratio is the distance-to-spot size. The unit takes the average temperature of the target area. If the area being scanned is larger than the target area, it might result in less ac- curate measurements, particularly if tempera- tures vary across a given surface. Distance-to-spot ratios are posted on all IR thermometers. The most common ratio for our use in retail food inspections is 12:1. These units measure a spot of 1 in. at a distance of 12 in., or a spot of 2 in. at a distance of 2 ft. Depending on personal preference and need, units with a 20:1 ratio and other ratios are also available. For institutional use, we found that a 9:1 ratio, with an expanded temperature range, is the most versatile since we need to evaluate laundry operations, and comfort ventilation with eects of radiant energy. Most recently, we found a small inexpensive pocket device with a 1:1 ratio. This device comes in handy for spot checks in most situations where the distance to the target is not an issue. The good news is that most popular IR thermometers come with a laser or lighted “circle” to help make that estimation. Units Infrared thermometer validation is conducted using an aluminum tin with a black spot that is filled with hot or cold water and is measured using a temperature standard thermometer. Photo courtesy of Dr. Robert Powitz.
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