VISUAL PHOTOMETRY: The branch of science which deals with measurement of brightness or luminous intensities of light emitted by different sources using certain standards and techniques is called visual photometry.

SOLID ANGLE : The ratio of the area of the surface of the portion of a sphere enclosed by the conical surface forming the angle to the square of the radius of the sphere is called solid angle.

LUMEN: Lumen is the amount of light energy emitted per second by a uniform (point) source of one candela with in a cone of unit solid angle.

CANDELA: Candela is defined as the luminous flux emitted per unit solid angle along normal to the surface by one sixtieth (1/60) square cm area of a black body radiator (source) kept at the temperature of solidifying platinum (at 2046 K).