Photometry

Photometry Points : Photometry, what is Photometry Before photometry we must know some important terms. i. Light It is defined as the radiant energy from a hot body, which produces the visual sensation upon the human eye. It is usually denoted by Q, expressed in lumen-hours and is analogous to Watt-hour. ii. Luminous Flux It is defined as the total quantity of light energy emitted per second from a luminous body. It is represented by symbol F and is measured in lumens (or cdsr). The conception of luminous flux helps us to specify the output and efficiency of a given light source. iii. Luminous Intensity Luminous intensity in any given direction is the luminous flux emitted by the source per unit solid angle, measured in the direction in which the intensity is required. It is denoted by symbol “I” and is measured in candela (cd) or lumens/steradian. iv. Lumen The lumen is the unit of luminous flux and is defined as the amount of luminous flux given out in a space represented by one unit of solid angle by a source having an intensity of one candle power in all directions. v. Candle Power Candle power is the light radiating capacity of a source in a given direction and is defined as the number of lumens given out by the source in a unit solid angle in given direction. It is denoted by symbol C.P. vi. Illumination When the light falls upon any surface, the phenomenon is called the illumination. It is defined as the number of lumens, falling on the surface, per unit area. It is denoted by the symbol “E” and is measured in lumens per square meter or lux or metre candle. vii. Lux or Meter Candle It is the unit of illumination and is defined as the luminous flux falling per square metre on the surface which is every where perpendicular to the rays of light from a source of one candle power and meter away from it. viii. Foot Candle It is also unit of illumination. ix. Candela It is unit of luminous intensity. Since the square of distances are involved in measurement, therefore, distances should be measured accurately. In order to obtain distance exactly two points are determined at which there is a perceptible difference in illumination from the two sides and the point half way between them is then taken as the position of equal illumination.

Most of the photometer heads consists of some device by means of which the illumination of two surfaces, side by side — one illuminated by standard source and other by the source whose candle power is to be determined may be compared under exactly similar conditions without movement of eye. The photometers which Brodhum photometer. These photometers are best for use if the sources to be compared give light of same or approximately similar colours.