Broadly problems usually fall into one or more of five areas: emissivity related, environment related, reflected radiation problems, incorrect signal processing, and design/selection of equipment. In brief, these are:

1. Emissivity related problems. There are many types of emissivity related problems. Emissivity can vary with wavelength, product curvature, product surface roughness, viewing angle, temperature, surface film effects, and a few more obscure items. And you can't measure an accurate temperature without knowing the emissivity value to use. For more on this, click here on EMISSIVITY

2. Environment related problems: Certain atmospheric gases, water vapor or steam or particles can absorb or scatter radiation from the sight path between the radiation thermometer and surface to be viewed. This causes an erratic low temperature reading for brightness units and can cause either a high or low reading for ratio units. It is sometimes also forgotten that power line fluctuations or noise, and too high or low of an ambient temperature can cause a radiation thermometer to read incorrectly. Excessive vibration can actually shake units apart. For more on this, click here ENVIRONMENT.

3. Reflected radiation problems: For opaque materials, reflectivity is equal to 1.0 minus emissivity. If there is any source of external radiation that could reflect off the area being viewed, it could reflect into the radiation thermometer, adding to the radiation from the surface. This will cause the indicated temperature to be incorrectly high because of the additional radiation received. Two common problems are that the area viewed may be lit by sodium vapor lamps or illuminated through an overhead window. Both contain infrared radiation. Another problem is attempting to view material being heated in a furnace. Yet another problem (and a big one!) is attempting to measure the temperature of reflective product that may only be one or two hundred degrees hotter than room temperature. It is often forgotten that at these temperatures, everything around will emit infrared radiation and you will have a problem with reflected radiation, guaranteed! For more on this, click on REFLECTIONS

4. Signal processing.  In many applications, the radiation received by the radiation thermometer may fluctuate, most commonly because of sight path interference or absorption. There is a general rule: anything interfering with the sight path will cause a reduction in radiation received. I've seen people try to use a lower emissivity setting combined with signal averaging, but that's the wrong approach as the amount of interference is not constant. A better solution (if the interference cannot be avoided) is to use the smallest spot size possible combined with "peak picking" methods. Other problems can include variations of emissivity with product shape, processing  or product roughness, which may be solved using imbedded computers combined with other measurements of the product characteristics. For more on this, click on SIGNAL PROCESSING

5. Equipment selection & design: Each application requires careful consideration as to what equipment and overall design will work best considering all the possible problems listed above. A large number of choices exist in the basic radiation thermometer type (fiber optic versus refractive versus reflective optics, wavelength, spot size, single or multiwavelength, computer adjusted) which must all be carefully considered. For more on this, click on EQUIPMENT/DESIGN