Guiding & Principles

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1Principle
1.1The efficiency of a light source is indicated by luminous efficacy, lm/Watt. Manufacturers usually give this value after testing the lamps at laboratories. It is difficult to establish the luminous efficacy value of lamps at site conditions.
1.2All the light emitted by the lamp does not reach the work area. Some light is absorbed by the luminaire, walls, floors & roof etc. The illuminance measured, in lumens/m2 i.e. lux, indicates how much light i.e. lumens is available per sq. metre of the measurement plane.
1.3Target luminous efficacy (lm/Watts) of the light source is the ratio of lumens that can be made available at the work plane under best luminous efficacy of source, room reflectance, mounting height and the power consumption of the lamp circuit. Ideally, we would expect the target luminous efficacy to be available on the work plane.
1.4However, over a period of time the light output from the lamp gets reduced, room surfaces becomes dull, luminaires becomes dirty and hence the light available on the work plane deviates from the target value. The ratio of the actual luminous efficacy on the work plane and the target luminous efficacy at the work plane is the Installed Load Efficacy Ratio (ILER).
1.5A second aspect of efficiency of utilisation is to take into account, the light available at task and non-task areas. Usually for commercial areas, the recommended illuminance at the non-task areas is at least one-third of the average task illuminance, while keeping a minimum illuminance required at the horizontal plane to be 20 lux. From illuminance measurements the ratio of illuminance at non task areas and task areas can be estimated to understand whether the non-task illuminance level is more than required or not.
1.6Illuminance levels recommended at various work spaces are given in Annexure-1.
2Pre-test Requirements
2.1Measurement of illuminance in an electrical lighting system should be done after dark. This is essential especially in outdoor installations. For indoor lighting, measurements with lights ON and Lights OFF technique can be followed provided the daylight variation is not too much and the survey time is not too long.
2.2In an installation of fluorescent discharge lamps, the lamps must be switched on at least 30 minutes before the measurement to allow for the lamps to be completely warmed up.
2.3In many situations, the measuring plane may not be specified or even non-existent. Hence it is necessary to define measurement height, typically 0.8 to 1 meter from the ground or floor level.
2.4Stray light from surrounding rooms, spaces and through external windows should be minimised by use of blinds, curtains, etc.
2.5Any automatic lighting control or daylight linked controls should be set such that the output of the lamps is at full power and will not vary during the tests. All lighting in the area that would normally illuminate the area test grid should be operating.
2.6It is convenient to have a second person recording the readings called out by the person moving the photocell.
3Precautions
3.1Care must be taken not to shadow the photocell when making measurements.
3.2In single-phase supply of power for lighting in an area, when measuring lamp circuit power using a clamp on type meter, measure the power preferably on the phase conductor. If current/power is measured on the main cable, which encloses both phase and neutral conductors, the current and power will indicate zero.

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