The optical reproduction of an object, produced by a lens or mirror. A typical positive lens converges rays to form a "real" image which can be photographed. A negative lens spreads rays to form a "virtual" image which can't be projected.

Incident Light

A ray of light that falls on the surface of a lens or any other object. The "angle of incidence" is the angle made by the ray with a perpendicular (normal) to the surface.

Index of Refraction

For a specific material, the ratio of the speed of light in a vacuum (c) to the speed of light in the material. Because the speed of light in vacuum is faster than in any material, the index of refraction is always greater than 1.

Infrared Radiation (IR)

Invisible electromagnetic radiation with wavelengths which lie within the range of 0.70 to 1000 micrometers. This region is often broken up into IR-A, IR-B, and IR-C.

Integrated Radiance

Product of the exposure duration times the radiance. Also known as pulsed radiance.


The magnitude of radiant energy.


An instrument that utilizes the interference of waves of light for precise determinations of , wavelength, flatness, or other attributes.

Intrabeam Viewing

The viewing condition whereby the eye is exposed to all or part of a direct laser beam or a specular reflection.

Ion Laser

A type of laser employing a very high discharge current, passing down a small bore to ionize a noble gas such as argon or krypton.

Ionizing Radiation

Radiation commonly associated with X-ray or other high energy electromagnetic radiation which will cause DNA damage with no direct, immediate thermal effect. Contrasts with non-ionizing radiation of lasers.

Irradiance (E)

Radiant flux (radiant power) per unit area incident upon a given surface. Units: Watts per square centimeter. (Sometimes referred to as power density.)


Exposure to radiant energy, such as heat, X rays, or light.