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Lights are essential for providing comfortable, productive and safe working environment. Lighting system comprises of luminaires, some form of control, and services. Basic terms involved in lighting system are discussed below:
Lamp is equipment which produces light from electricity. The most commonly used lamps are described briefly as follows:
Luminaire or a light fixture is a device that filters, distributes or transforms the light emitted from one or more lamps. The luminaire is a complete lighting unit which includes one or more lamps, all the parts necessary for fixing and connecting the lamps to electric power supply, protecting equipments like lamp socket, ballasts, optical devices designed to distribute the light like reflective material, lenses, refractors or louvers, housings etc. The mechanical components required to support the luminaire is also included in the luminaire. The basic physical principles used in optical luminaire are absorption, refraction, reflection and transmission. Luminaires are designed and manufactured for all common types of electric lamps. Luminaires are commonly available for lamps such as Incandescent filament including tungsten halogen and infrared lamps, Fluorescent lamps, Compact fluorescent lamps, Induction or electrode less lamps, including fluorescent and sulfur lamps, High-intensity discharge lamps such as metal halide lamps, high pressure sodium lamps and mercury vapour lamps, Low-pressure sodium lamps etc.
Luminaires are less common for xenon arc and carbon arc lamps. The size, materials, thermal properties, photometric performance, and power requirements of a luminaire depend on the type of lamp used. For example, lamps that emits a large amount of infrared radiation (heat) require luminaires that are provided with vents for convection, and fluorescent lamps that are sensitive to environmental temperature must be protected from low air temperatures. Various types of luminaires are available in the market and they can be classified based on the lamp source, mounting type, construction, application, and photometric characteristics.
As the name indicates lighting control gears are basically used to control light. Apart from that control gears performs various functions like stabilizing as well as limiting the current in an electrical circuit, guarantying continuous operation of the lamp, by providing ignition voltage, reducing energy consumption, eliminating flicker and noise and increasing efficiency etc. The control gear used varies depends on the type of lamps. For example Incandescent lamps can operate directly from mains electricity whereas discharge lamps require control gear, because for initiating the discharge and to sustain it various electrical conditions are required. The most common gears used in the lighting equipments are as follows:
Luminous intensity( I)
Luminous intensity is the light flux of a light bulb or a lamp emitted in a specific direction. Its unit is Candela (cd). The light intensity distribution of a lamp or luminaire can be determined by the type of reflector and is normally shown as a light intensity distribution curve.
Footcandle(fc)
The measurement of the intensity of illumination is called foot candle and is denoted by fc. A footcandle is the illumination produced by 1 lumen distributed over a one square foot area. About 30 to 50 footcandles of illumination is sufficient for most home and office work. For works which require more light, 200 footcandles of illumination or more provides more accuracy and reduces eyestrain. The definition of footcandle expresses the candela in terms of the watt and steradian. The steradian is defined as the solid angle subtending an area on the surface of a sphere equal to the square of the sphere's radius. The steradian is an SI supplementary unit.
Candela (cd)
Candela is used instead of candle power. It is defined as the light emitted by a lamp, radiating in all directions equally, to produce 1 lumen per steradian. As an equation, 1 cd=1 lm/sr.
Light Measurement
Illuminance(E)
Illuminance is the quotient or concentration of the luminous flux incident on an element of the surface at a point of surface containing the point, by the area of that element. The unit of illuminance is Lux. The lighting level formed by a lighting installation is usually qualified by the illuminance formed on a specified plane. In the majority cases, this plane is the key plane of the tasks in the interior and is commonly called as working plane. The performance of the tasks and the appearance of the space are affected by the illuminance provided by an installation.
Luminance(L)
Luminance is sort of a combination of luminous intensity and illuminance. It is how much light is striking (or coming out of) a surface per unit area (like illuminance) per solid angle (like luminous intensity). Its units are thus candela/m2. Fundamentally, luminance is a measure of how bright a surface appears to a human eye at a specified angle to the surface. Its main present day use is in characterizing the brightness of computer monitors.
Lumen(lm)
Lumen is a unit of luminous flux or light flow. The lumen rating of a lamp is a measure of the total light output of the lamp. It is the quantity used to describe the brightness of a light source. Light sources are labeled with an output rating in lumens. A 5W LED bulb is able to produce more than 500 lumens.
Lux(lx)
It’s the unit of Iluminance. Lux is defined as the illuminance produced by a luminous flux of one lumen, uniformly distributed over a surface area of one square metre. One lux is equal to one lumen per square meter.
Figure above shows the relationship between candelas, lumens, lux, and footcandles. In the figure, a point source with luminous intensity 1 cd is shown at the center of a sphere of unit radius whose surface has a reflectance of zero. The illuminance value at any point on the sphere is one Iux if the radius is one m, or one fc if the radius is one ft. The solid angle subtended by the area ABCD is one sr. The flux density is therefore one lm /sr, which corresponds to a luminous intensity of one cd as originally assumed. The sphere has a total area of 4π m2 or ft2, and there is a luminous flux of one lumen falling on each unit area. The source thus provides a total of 4π lumen.
Radiant Intensity(Ie)
Radiant power emitted by a point source in a given direction. Its unit is Watts per steradian (W/sr).
Radiance
Radiance is the radiant intensity of a source in a given direction, divided by its orthogonally projected area in that direction. Its unit is Watts per steradian per square meter (W/(sr.m2)).
Radiant Flux(Φe)
Radiant Flux is the radiant power of a source at all wavelengths. Its unit is Watts (W).
Irradiance
Irradiance is the radiant power incident on a surface. It’s unit is Watts per square meter (W/m2).
Luminous Efficacy (lm/W)
This is the ratio of luminous flux emitted by a lamp to the power consumed by the lamp. It is a term used to indicate the efficiency of energy conversion from electricity to light form. The luminous efficacy is calculated by diving the total lumens by total lamp wattage. The more light (lumen) obtained for the amount of power input, the costs will be less. Efficacy of LED lamps is above 100.
Colour Rendering Index (CRI)
CRI is a measure of the degree to which the colours of surfaces illuminated by a given light source confirm to those of the same surfaces under a reference illuminant; suitable allowance having been made for the state of chromatic adaptation. Usually sun is considered as reference illuminant. Therefore CRI can also be defined as light source's ability to render colors the same way as sunlight does. CRI value ranges from 1 to 100. The top value of the CRI scale is 100 and is based on illumination by a 100-watt incandescent light bulb. To find out the CRI of a lamp, the colour appearances of a set of standard colour chips were measured with particular equipment in a reference light source with the same correlated colour temperature as the lamp under evaluation. If the lamp renders the colour of the chips matching to the reference light source, its CRI is taken as 100. If the colour rendering varies from that of the reference light source, the CRI is taken as less than 100. A low CRI value indicates that some colours will appear unnatural when illuminated by the lamp with low CRI. A light source with a CRI of 80 or higher is considered acceptable for most indoor residential applications.
Colour Corelated Temperature (CRT)
The luminous colour of a light source is described by the colour temperature. Color temperature is measured in Kelvin (K) temperature. With filament lamps this is about 2,700 Kelvin (K) and for fluorescent lamps CRT is above 6,000 K. A light source with colour temperature 3,000 K has a larger part of red colour whereas a light source with 6,000 K has a larger part of blue colour. Lights with higher Kelvin temperatures, between 3600K–5500 K are considered as cool lights and lower color temperatures, between 2700K–3000 K are considered as warm lights. Usually cool light is preferred for visual tasks as it produces higher contrast than that of warm light whereas warm light is preferred for living spaces since it is more suitable to skin tones and clothing.
Flicker
Flicker refers to what happens when a light turns off every time the AC line goes through zero volts. Incandescent lamps are subject to flickering, but it’s not noticeable because the filament takes so long to cool down whereas fluorescent tubes, including CFLs extinguish their plasma arc within about 100 μ sec. Thus they will turn on and off 50 to 60 times per second. This produces an annoying effect called flicker in the light. The chance of flickering is more in case of Light Emitting Diodes as they turn off faster than fluorescent lamps. A general lighting installation that produces visible flicker not preferred universally unless it is being used for entertainment. The magnitude of individual differences can be detected in the retina even when there is no visible flicker, which implies that a clear safety margin is necessary to make flickering not perceivable anyone.
Glare
Glare is defined as the too much brightness from a direct light source that makes it hard to see what one wishes to see. Glare occurs in many ways. One of the possibility is producing too much light. Too much light creates a simple photophobic response, in which the observer strain to see, blinks, or looks away. Too much light is common in full sunlight. A bright object placed on a dark background usually causes glare. Bright lights reflecting from a television or computer screen or even from a printed page produces glare. Bright incandescent lamps produces more direct glare than large fluorescent lamps as they are intense light sources than fluorescent lamps. However, glare is primarily caused due to the improper placement of light sources and the corresponding objects being viewed.
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