Illuminance - Density of luminous flux incident on a surface. Unit is footcandle or lux.
Illuminating Engineering Society of North America (I,S or IESNA) - The professional society of lighting engineers, including those from manufacturing companies, and others professionally involved in lighting.
Incandescent lamp - Light is produced by a filament heated to a high temperature by electric current.
Indirect lighting - Indirect lighting is where light from luminaires is directed exclusively onto a ceiling or walls and reflected back into the room. Lamps are screened from view and light emission above eye level prevents glare.
Indirect lighting is generally used in combination with direct lighting. Used on its own, it can create a diffuse atmosphere with pale shadows. Where indirect lighting is used, ceiling and walls should be light-colored for high reflectance; in dark rooms, indirect lighting would require high-wattage lamps, which would result in high energy consumption.
Infrared radiation - Electromagnetic radiation with longer wavelengths than those of visible light, extending from the nominal red edge of the visible spectrum at 700 nanometers to 1 mm.
Intensity - The degree or amount of energy or light.
International Dark-Sky Association (IDA, Inc.) - A non-profit organization whose goals are to build awareness of the value of dark skies, and of the need for quality outdoor lighting.
Inverse-square law - Illuminance at a point varies directly with the intensity, I, of a point source and inversely as the square of the distance, d, to the source. E = I / d2
IES file - An electronics format of photometric data of an individual luminaire created according to the standard no. IES LM-63-02 - Standard File Format for Electronic Transfer of Photometric Data published by Illuminating Engineering Society of North America. This IES file is widely used by lighting professionals who use lighting design software as part of their lighting specification process.