Optical lenses usually have low transmittance after precision polishing, and can only be used after coating. Coating is a process of overlaying the film material on the lens surface layer by layer by evaporation. The thickness of each layer of optical film needs to be strictly controlled. Generally, there are two ways to monitor film thickness: light control and crystal control. The film thickness is usually measured in nanometers.
The visible light band range is 400-700nm, which is the color that the human eye can distinguish. For example, the green light is 532 nm and the red light is 633 nm, with a difference of 100 nm. The lens used in this range has high requirements for the peak value of the coating center, and the offset is usually controlled within 5 nm. The shorter the wavelength is, the higher the accuracy of the center peak position is required. For example, the UV 355 nm and 266 nm, and the offset is less than 5 nm. This is a very harsh test for coating equipment and process.
In contrast, the infrared optical film is of long wavelength um level, such as 10.6 um (10600 nm) for CO2 laser, 3-5 um and 8-14 um for infrared thermal image film. The film is also very thick, usually of um level, so the position accuracy of the center peak is not very high. The same peak shift is 0.1um (100nm), which is equivalent to red changing to green in the visible light field, and the film is definitely unqualified. However, the same situation occurs in the infrared light field, which is equivalent to 10.6um to 10.5um. The maximum value of transmittance does not change (usually flat curve), only the appearance color changes.
Therefore, in the infrared film system, because the film layer is very thick, the accumulated thickness difference may become larger, resulting in color difference. However, in the infrared light, it does not affect the use. Usually this color difference occurs in different batches of coating, and the color is almost the same in the same batch of coating!
If customers need to have consistent requirements for the color of different batches of infrared film, it will be a challenge for manufacturers. This requires additional control precision, which increases the cost of the product virtually, resulting in an increase in the price of the product. This pursuit of color on the surface will not help the actual performance.
