Optical films are applied to surfaces and have the property of modifying the reflection (R) or transmission (T) of light at those surfaces. We observe a range of electromagnetic energy called visible light, but the full electromagnetic spectrum extends below to the ultraviolet spectrum, above to the infrared spectrum, and also beyond in both directions. Rules developed for optical filters work in these three spectral areas. Maxwell developed equations that explain the field theory for this branch of physics.

Places along the electromagnetic spectrum are identified in terms of wavelengths. The visible spectrum (the range of light that we see) extends from 380-780 nm (for color calculations). Higher-number wavelengths are in the infrared spectrum; lower-number wavelengths are in the ultraviolet spectrum (Fig. 1.1). The most prominent colors associated with these wavelengths are defined as

Violet 410 nm

Blue 460 nm

Green 515 nm

Yellow 570 nm

Orange 595 nm

Red 620 nm

Each color exists in a range (for example, red extends to about 750 nm), and as these ranges reach each other, what we think of as basic colors combine.