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Of Light, of Falling O N I Square Foot

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Of Light, of Falling O N I Square Foot surface measured in candelas per square tion of that surface, termed illu­ Jore technical information, metre. minance. The illuminance caused by I belp and advice. That was The light that a source emits is lumen uniform ly on 1 square metre is tbe clear message sent back termed its luminous flux and is defined as I Lux in the SI system *, alter­ by our readers in response measured in Lumens. A lumen is defin­ natively 1 Foot-candles for I lumen to our survey. But how to ed such that a point source of light, of falling o n I square foot. accomplish this when the on e candela intensity, radiating If we refer to the luminaire sectio ns knowledge and experience uniformly in all directions emits 4n of The Strandbook we find figures of il­ of our readers ranges from lumens. Section 6 of The Strandbook luminance quoted at selected distances s tudent to experienced reveals a staggering range of lamp fro m the luminaire. We w ill also note lumen 'packages' from a modest 5000 that illuminance reduces in proportion professional designers, lumens, for a 300W M38, to a massive to the square of the distance from the engineers and technicians? 1,100,000 lumens, for a 12kW HMI! source to the surface; generally referred And where interests vary This figure, for an individual lamp is the to as the inverse sq uare law. from Theatre to Television, rota! output measured by integrating or For fixed lighting installatio ns recom­ J-fotion Picture and summing the luminous flux output over mended illuminances for a wide varie­ Architectural Lighting. Our all directions emitted. ty of applicatio ns are published by solution is· to include a Lamps are used in housings referred professio nal bodies such as the CIBSE regular technical to as luminaires. A luminaire is an in Britain or the IES in America, and are background feature to assembly providing mechanical support appropriate to particular visual tasks or augment the reports of and electrical connection to the lamp work place situatio ns being lit. lighting applications and and incorporates a means of controlling Whilst a certain am o unt of light may news of new equipment - the light emitted by the lamp. It w ill be incident on a surface its brightness, also comply with appropriate standards known as luminance, is dependent on and where better to begin, for electrical safety. Whilst the output the proportion of light reflected , than with light itself. of a luminaire may have any area, governed by that surface's reflectance. simplifications become available if it is Reflectance can range from O (perfect small eno ugh such that it can be treated black) to 1 (perfect w hite). as a point source and thus it may be A Reflectance Factor is commonly considered to have a single luminous in­ employed when evaluating contribu­ THE MEANING tensity for each direction . tio ns of light reflected from walls, ceil­ A luminaire at a distance of several ing and fl oors in addition to direct light, metres can usually be considered a w hen calculating illuminance at the p oint so urce of light. Thus calculations working plane . Most surfaces may be OF LIGHT on luminaire output may be simply con sidered matt such th at their made (using only distance and intensi­ luminance is independent of the angle ty) provided that the distance is at least of viewing. Surfaces w ith specular pro­ THE LANGUAGE OF LIGHT ten times the source size. Calibration of perties w ill p referentially refl ect light. ighting, regarded by some as an luminaire intensity is normally mad e at a phenomenon utilised in reflector L intriguing blend of art and a distance of 10 metres. d esign in luminaires. science, p ossesses its own If we now direct our source towards •SI is the International Systems of Un it of ,·ocabulary, used to describe the a surface, then the luminous flux (light) Measurement. lighting system shown in Fig. l. causes a certain amount of illumina- ' The Strandbook of/er - see page 15. Figure I. Surface receiving illuminancc (incidence) TERMS, DEFINITIONS AND CONVERSIONS The lamp is a source of light w hose 'brightness' is know n as luminous intensity. once more \ familiarly measured in candle power, but now in Cande las (eel). Wh ilst an ideal light source would Light flow radiate evenly in all directions, when the light source cannot be considered as an homogeneous ha,•ing whole, then its incensity is described appropriate tO the area under consideration. T hus the term Luminance is introduced as the intensit y per unit area o f a bod y's surface and is measured in luminous Candelas p e r square metre (cdlm' ) or old units such :1s Foot-Lambe rts. flux Luminous Intens ity : The unit of measurement is given by the luminous intensity of a plane, black bod y surface o f 1 square ce mimem: area heated to 2045°K - the solidification point of platinum Surface having - wh ich is defined to have an intensity o f 60 candelas. lumi nance (re flectance) Lumino us Flux : A Lumen is the lumino us fl ux emitted by :1. uniform point source of 1 candcla intensity in I steradian. A stcradian is the solid angle subtcmkd at the ce ntre of a sphere of unit radius by one square unit area on the surface. The t01al lumino us flux thus emitted by rhe source is The terms most commonly employed 417 lumens. can be summarised as follows. Illumina n ce: The illuminance caused by I lumen on I square metre is defined as I Lu x. • Luminous Flux is th e output of a Luminan ce: Luminance (cdlm ') = llluminance (lux) x Reflectance. =>are lamp, measured in lumens. 17 • Luminous Intensity is the output Lux, = Candelas Foor Candles = Candelas =- the luminaire measured in candelas. Distance' (m) Distance' (ft) Inverse Square Law: If a luminaire gives an illuminance o f Ll Lux at distance DI metres. then ir • Di,-iding the candela o utput by the will gi,·e illuminance L2 at distance D2, (assuming linear beam d ivergence). ~ua re of the distance in metres gives Thus 1.2 = Ll x DJ' -'le ill uminance. D2' llluminance is the amount of light Conversion Factors (SI Units to I mpe rial). 1 candcla per sq. metre (cd/m ') = 0.2919 Foot-La mbercs (fl) o.:li.ng on a subject measured in Lux. I candela per sq. metre (cdlm ' ) = 17 Apostilb (asb). • Luminance is the amount of light I Lux = 0 .0929 Foot candles (fc). --:::-:ccred o r brightness of the subject Thus Lux = I0.76 x Foot-candles and Foot-candles = 0.0929 x Lux. tJ .
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