The decrease in illumination away from the optical axis in an optical system caused by clipping of off-axis rays by apertures in the system.

Virtual Image

A virtual image does not represent an actual convergence of light rays. A virtual image can be viewed only by looking back through the optical system, such as in the case of a magnifying glass.

Visible Radiation (light)

Electromagnetic radiation which can be detected by the human eye. It is commonly used to describe wavelengths which lie in the range between 400 nm and 700-780 nm. The peak of the human spectral response is about 555 nm.

Visible Spectrum

The region of the electromagnetic spectrum ranging from 400 nm to approximately 700 nm.