Electric Lighting PDF

1. BASIC LIGHTING TERMINOLOGIES 1.0 BASIC LIGHTING TERMINOLOGIES * Radiation : Is the emission or transfer of energy in the form of electromagnetic waves. * Light : Is any radiation capable of causing a visual...

1. BASIC LIGHTING TERMINOLOGIES 1.0 BASIC LIGHTING TERMINOLOGIES * Radiation : Is the emission or transfer of energy in the form of electromagnetic waves. * Light : Is any radiation capable of causing a visual sensation on the eye directly. The band of wave length that corresponds to visible light lies between 380 nm and 760 nm (1 nm = 10 -9 m) * Visual spectrum : The band of wave length that causes a visual sensation. The various color bands 380 nm - 435 nm : violet 435 nm - 500 nm : blue 500 nm - 566 nm : green 566 nm - 600 nm : yellow 600 nm - 630 nm : orange 630 nm - 760 nm : red * Vision : The eye is capable of picking up visual information due to the two nerve fibres present in the retina i.e rods & cones. * Rods : Responsible for black/white vision, called scotopic vision and acts at intensities = 0.035 cd/m2 * Cones : Responsible for color vision, called photopic vision & acts at higher values of intensities =>3.5 cd/m2 * Eye sensitivity : The human eye is sensitive to different wave lengths (color) with varied efficiency. It is different for scotopic and photopic vision. 4 * Luminous flux : Quantity of radiant flux (light) emitted by a lamp. The unit is 'lumen'. * Luminous efficacy : Quantity of light (lumen) emitted for each unit of electrical power (watt) consumed. The unit is 'lumen/watt' (lm/W). * Illuminance (E) : Quantity of light (lumens) incident on a surface per unit area (m2). The unit is 'lux' (lumen/ m2). * Luminance (L) : Intensities (candela) of light reflected back from a surface per unit area (m2) in a given direction. The unit is candela/ m2 * Color rendering : Expression for the effect of an illuminant on the colour of an object in conscious comparison with their color as seen under a reference illuminant (daylight). * Color rendering index (Ra): Measure of the degree to which the colors of objects illuminated by a source conform to that when illuminated by a reference source (daylight). * Color temperature : Temperature of the black body that emits radiation identical as the radiation under study. The unit is °K * Black body : A thermal radiator that absorbs all incident radiations. The emitted radiation corresponds to specific color appearances at a given temperature. * Glare : A condition which leads to discomfort or a reduction in the ability to see objects, or both, due to distribution of intensities/ extreme contrasts in the field of vision. * Light output ratio (LOR) : The ratio of the total flux from the luminaire, to the sum of the luminous fluxes of lamps. * Downward light output : The ratio of the flux emitted below the horizontal ratio (DLOR) plane passing through the luminaire to the total bare lamp flux. 5 * Color appearance : There are 3 broad categories as :- i) 5300°K : cool (bluish white) ii) 3300 - 5300°K : intermediate(white) iii) 3300°K : warm(reddish white) * Coefficient of utilization : It is the ratio of the lumens reaching the working (C.O.U.) plane to the total lumens produced. It is also called "Utilisation Factor". C.O.U. depends on, light distribution of the luminaire; light output ratio of the luminaire; reflectance of the ceiling, walls & working plane; room index and arrangement of the luminaires in the room. C.O.U. is based on new, clean equipment and in practice a maintenance factor is introduced to convert initial to in- service illumination. * Maintenance Factor : The maintenance factors is the ratio of the lumens produced by the system while in service to the lumens produced by system when newly installed * Room Index (K) : The room index K is a function of the room luminaire and is given as follows LxW K = ————— Hm x (L + W) L = Length of room W = Width of Room Hm = Mounting height of the luminaires * Uniformity Ratio (UR) : It is the ratio of minimum lux in a given plane to the average lux. * Lamp lumen : The lumen output of all lamps decrease with use, Depreciation (LLD) but the rate of decrease varies widely between lamp types and between manufactures. Lighting calculation must take into account specific depreciation in lumen output of the light source. -------------------------------------- 6