Illuminance:  The strength of light falling on a given area of surface.  Measurable by an incident-light (illuminance) meter.

Illuminance meter:  See Incident light meter.

Incandescent light:  Light emitted when a substance is heated by electricity:  for example, the tungsten filament in an ordinary light bulb.

Incident light meter An exposure meter that measures the amount of light incident to (falling on) a subject. 

Indoor film:  See Type A film; tungsten film.

Infinity:  The farthest position of on the distance scale of a lens.  It includes all objects at the infinity distance (about 50 feet) from the lens or farther.  When the infinity distance is within the depth of field, all objects at that distance or farther will be sharp.

Infrared:  The band of invisible rays just beyond red, which people perceive to some extent as heat.  Some photographic materials are sensitized to record infrared. 

Instant film:  A film such as Polaroid Time-Zero that contain the chemicals needed to develop an image automatically after exposure without the need for darkroom development.

Intensification:  A process increasing the darkness of an already developed image.  Used to improve negatives that have too little silver density to make a good print or to increase the density of an existing print.

Interchangeable lens:  A lens that can be removed from a camera and replaced with another lens, usually of a different focal length.

Inverse square law A law of physics stating that the intensity of illumination is inversely proportional to the square of the distance between light and subject.  This means that if the distance between light is doubled, the light reaching the subject will be one-quarter of the original.

ISO:  A numerical rating that describes the sensitivity of a film to light.  The ISO rating doubles as the sensitivity of the film doubles.