ILLUMINATION ENGINEERING - PART - 01 - TERMS AND DEFINITIONS

ILLUMINATION ENGINEERING PART – 01 - IMPORTANT DEFINITIONS In general, ‘Illumination’ means ‘light’. But technically, the two terms are different. LIGHT is the cause and ILLUMINATION is the effect. The MOON illuminates the earth by the light obtained from the SUN. SUN is the greatest source of energy. By passing current through a resistor R heat is produced, and the temperature of the resistor increases, at high temperature, the resistor becomes RED HOT and emits infrared (heat) rays. On further heating, the resistor becomes WHITE HOT; then, its starts emits ultra violet rays. The rays now have a smaller wavelength. LIGHT is a radiant energy which is assumed to be propagated in the form OF TRANSVERSE waves through an invisible medium known as ETHER. Velocity of light is 2.9976 x 10^6 m/sec. approximately or 3 x 10 ^8 m/sec. Energy is radiated by the sun as electromagnetic waves of which of 90% have wave lengths in the ranges of 0.2 to 0.4 micro metre. SOLAR ENERGY REACHING THE TOP OF THE ATMOSPHERE CONSISTS OF 8% ULTRA VIOLET radiation [Short wave length less than 0.39 micro metre] 46% VISIBLE LIGHT and [0.39 micro metre to 0.78 micro metre] 46% INFRA-RED [Long wave length more than 0.78 micro metre] WAVE LENGTH (λ) is defined as the difference between two successive points of same phase. Its practical unit is represented in Micron or Angstrom. One Micron = 10-6m or 10-4 cm One Angstrom = 10-10m or 10-8cm or 0.1 nanometre. Wavelength (λ) = V/f = Speed of light / frequency Frequency of X – ray is 3 x 1020 Hz and its wavelength is 10-10 cm All types of radiation do not produce sensation of light in human eyes. The radiation between wave lengths of 4000 Å to 7000 Å only produces sensation. The human eye is the most sensitive to 5500 Å. A wavelength through it differs from man to man and age to age. Human eye is sensitive to 5500 Å this is taken as unity. The colour corresponding to this wave length is YELLOWISH GREEN which is the most suitable colour for all purposes. 01. PLANE ANGLE (θ) when two lines, lying in the same plane, meet at a point, the angle formed is called a plane angle. Plane angle = Arc / Radius, which is measured in radians or degrees. 02. SOLID ANGLE (ω) – The angle subtended by an area of a point is called solid angle. It is measured in steradians. Solid angle = Area enclosed /Radius^2 Solid angle subtended by a sphere at its center = Area of sphere/Radius^2 Solid angle subtended by a sphere at its centre = 4∏ steradians 03. ILLUMINOUS FLUX (ɸ) – It is the rate of energy radiation in the form of light waves and is denoted by ɸ = Q/t where Q is the radiant energy. Its unit is lumen. 04. LUMEN (lm) – One lumen is defined as the luminous flux emitted by a source of one candle power in a unit solid angle. Lumen = Candle power x Solid angle = C.P. x ω One lumen (lm) = 0.0016 watts (approximately) 05. CANDLE POWER (ω) – It is defined as the number of lumens emitted by a source per unit solid angle in a given direction. It represents the light emitting capacity of a source. 06. ILLUMINANCE [OR] ILLUMINOUS [OR] DEGREE OF ILLUMINATION (E) – It is defined as the luminous flux received by the surface per unit area. Its unit is lux or foot-candle. 07. METRE CANDLE OR LUX (M.C.) – It is the unit of illumination received inside a sphere of radius 1 metre with a source of 1 C.P. at its centre. 08. FOOT CANDLE (Ft.C) - It is the illumination of the inside of a sphere of radius 1ft. with a source of 1 C.P. at its centre. One foot candle = 10.76 lux. 09. LUMINOUS INTENSITY (I) – It is defined as the illuminous flux emitted by a light source per unit solid angle. I = ɸ/ω. Its unit is candela. 10. LUMINOUS OR RADIANT EFFICIENCY – The ratio between the energy radiated in the form of light to the total energy radiated (in all other forms, i.e. heat, etc.) is known as radiant energy. RADIANT EFFICIENCY= Energy radiated in the form light / total energy radiated by the body. 11. BRIGHTNESS (B or L) – It is defined as the luminous intensity per unit projected area of the surface in the given direction. B or L = Luminous intensity / Apparent area.