Lighting systems (Units of Light)

Color Rendering Index

CRI

Transparent

Capable of transmitting light so that bodies situated beyond or behind can be distinctly seen

Color Rendering Index

Difference between natural & artificial light/color; Lower CRI, bigger difference

Luminous intensity

Intensity of a light source that can be estimated by human eye; Also called as illuminating power; Luminous flux emitted from the source per unit solid angle; expressed in *candela* (Cd)

Lux

SI unit of illumination; lumen per *sq. meter*

Lumen

SI unit of luminous flux (lm)

Refraction

The change of direction of a ray of light as it passes obliquely from one medium into another in which its velocity is different

Law of reflection

The principle that when light or sound is reflected from a smooth surface, the *angle of incidence is equal to the angle of reflection and the incident ray*, the reflected ray, and the normal to the surface all lie in the same plane

Luminance

quantitative measure of brightness of light source

Illumination

*intensity of light* falling at any given place on a lighted surface; light flux density Also called as *Illuminance*; expressed in lumens per unit of area

Candela

basic SI unit of *luminous intensity* Also called, *standard candle* (Cd)

Specular

directed from a smooth, polished surface

Foot-candle

Unit of illumination on a surface equal to *lumen per sq. foot* (FC)

Opaque

impenetrable by light; not reflecting light

Incandescence

light from heat; emission of visible light by a body when heated to high temperature

Candlepower

luminous intensity expressed in candelas

Luminous flux

the *rate of flow of visible light* per unit time, expressed in lumens

Diffraction

the modulation of light or sound waves as they bend around the edges of an obstacle in their path

Reflectance

the ratio of the radiation *reflected* by a surface to the total incident on the surface

Reflection

the return of light rays from a surface

Incidence

the striking of a ray of light or sound wave on a surface

Transluscent

transmitting and diffusing light so that bodies on the opposite side are not clearly visible.

Foot-lambert

unit of luminance *per sq ft*; unit of brightness (fL)

Lambert

unit of luminance *per sq. centimeter* (L)

Candle

unit of luminous intensity used prior t 1948

Efficacy

*Lumen per watt* How well light source produces visible light