According ISO 10110 standard optical coatings is divided on two types: protective coatings and functional coatings.
Protective coatings are coated on optical elements when safety of optical surfaces and functional coatings is needed. For example, if using of optical device in aggressive environment is planned.
Functional coatings are coated on optical elements to provide necessary refractive or reflective coefficient.
Let’s look an example of the lens optical production drawing, which we showed here, and find cell which defines parameters to optical coating in surface requirements column. This cell looks like image below.
First of all, you will see Greek letter with circle around it. This mark indicates that functional coating needed. Then ARC abbreviation follows. Decryption of this abbreviation is “Anti Reflection Coating”. This is purpose of the optical coating. In this case coating should decrease reflection coefficient. The next inscription (R<0,5%) is requirement for maximal reflection coefficient. At the end you can see wavelength range. Optical coating requirements are defined exactly for these wavelengths. Instead wavelength range, sometimes are defined discreet wavelengths or sets of wavelengths. In this case its values are separated by commas.
An optical engineer defines optical coating requirements not only from point of view optical system efficiency. They also assess influence of light reflected from lens surface onto image or sensor. Reflected or scattered light can destroy image quality or signal to noise ratios, so defining precisely the coatings needed is an important design task.