Fresnel lens thermometers, like the RayTemp 4 and RayTemp 8 are the most common type used in the food industry. Unlike the mica lens, the fresnel thermometer lens is typically plastic, which offers several key advantages.
Fresnel lens thermometers often come with laser guides to help you aim your measurement. However, the plastic fresnel lens has a narrower temperature range than the more versatile mica lens. It is also more susceptible to inaccuracies due to sudden swings in ambient temperature, called thermal shock, than the other types of infrared thermometers.
If, for example, you carry your fresnel lens thermometer from room temperature into a walk-in freezer to take frozen food measurements, the sudden drop in temperature can actually change the shape of the lens as the plastic contracts with cold. Most fresnel lens thermometers will display error alerts when this happens and will give faulty readings until the lens has had a chance to acclimate to the new environment. Similar distortions occur at the upper range of temperatures in a fresnel lens thermometer's specification.
The good news is that allowing your fresnel lens thermometer to rest in the new ambient temperature for 20 minutes or longer before taking your measurements can dramatically reduce the distortions due to thermal shock.