IR window lenses do not allow all infrared light to pass through, just as sunglasses or dirty windows don't allow all visual light through. Most modern IR cameras have a menu setting where the user can enter the external optic transmission rate to account for this. A common myth is that there is one rate for each lens material that comes from factory calibration or lab testing. Actually, the transmission rate for any optic material can change from location to location, camera to camera, window to window, or day to day!
A transmission rate can be thought of as an average of many factors. In addition to optic material, some of these factors are listed below:
- optic temperature
- target temperature (of the object you're measuring)
- camera response curve (can vary from one camera to the next)
- debris on the optic
- scratches on the optic
- impurities in the optic material
- ambient temperature
- reflected temperature from the surroundings
- presence of protective grills over the optic
- lifetime exposure to humidity
- distance and angle of the camera relative to the optic
- thickness of the optic
Users are often not aware of all these factors because the effect from many of them is rather small and can be neglected without introducing unreasonable errors to temperature measurements. However, it is important to note that every IR material available today is affected significantly by several of these factors in any given installation. This is true for coated crystals, uncoated crystals, and polymer optics. The rule of thumb is that transmission is not a constant value and it should be measured periodically for every IR window in the field if accurate quantitative measurements are to be expected.
Download the attached Technical Publication "Easy IR Window Transmission Measurement" for more in-depth information.