LIGHTING TERMINOLOGIESCandlepower (cp):

unit of luminous intensitycandela [cd] (SI units)Candela and candlepower has the same magnitude

Lumen (lm)

quantitative unit for measuring flow of light energy (luminous flux) emanating from 1 ft2 of a 1 ft2 surface of 1 candlepower

[SI units] – 1 lumen is the luminous flux from 1m2 of a 1 m2 surface of 1 candela

Illumination – density of the luminous flux (lumens per unit area)

Footcandle

1 lumen of luminous flux spread uniformly over an area of 1ft2 produces an illumination of 1 footcandle

[SI units] – 1 lumen spread over 1m2, the illumination is expressed in lux (lx)

Footcandles = lumens ft2

lux = lumens m2

Absorptancedarker objects absorb more than lighter colored objects

Reflectance, Reflection Factor, Reflectance Coefficient

Specular Reflection – “regular reflection” (observed in mirror)angle of reflection = angle of incident light

Diffuse Reflection – reflected light is scattered in all directions

Transmittance, Luminous Transmittance, Transmission Factor, Coefficient of Transmissionratio of light *transmitted through a material transmission of light - direct, diffuse or combination

Direct Transmission – light passes through clear, transparent materials; angle at which the light leaves is the same as that at which it enters

Diffuse Transmission – transmitted light is scattered evenly Materials that transmit light in a diffuse pattern are known as “translucent”

Refractionbending of a ray of light as it passes obliquely through a materialFootlambert (fL)Quantitative unit for measuring brightnessValue – 1 lumen per square foot• When the illumination is on a surface, the lumens per square

foot are measured as footcandles • When brightness is from a surface, the lumens per square foot

are measured as footlambertsBrightness/Luminance – index of intensity of light being emitted, transmitted or reflected from a surface

Brightness – perceived lightLuminance – measured quantity

FormulasLuminance = illumination x reflectanceLuminance = illumination x transmittance

Footlamberts = footcandles x reflectance factorFootlamberts = footcandles x transmission factor

(SI units) – the unit of brightness is lambert (L): luminance of a surface reflecting, transmitting, or emitting 1 lumen per square centimeter

1 lambert = 1,076 footlamberts1 millilambert = 1.076 footlamberts

Glareeffect of excessive brightness in the field of view, causing

annoyance, discomfort and interfering with vision (direct from light/reflected from a shiny surface)

Work Stationimmediate contiguous area in which a worker performs visual

task (50 ft2/4.6 m2 – allotted for work station)Work PlaneSurface on which visual task is performed (assumed to be a

horizontal plane 30” (0.76 m ) above floor

Primary SourceLuminous source where light energy is generated and transmitted directly

to a task

Secondary SourceSurfaces that derive their brightness from reflected incident illumination

LampGeneric term for artificial light

Incandescent filament lamps – “bulbs”fluorescent lamps – “tubes”HID light sources – “lamps”

Reflector

device for redirecting radiant energy of a lamp by reflecting it in the desired direction

Refractordevice for redirecting radiant energy of a lamp in the desired

direction by refraction through a lense

Luminairecomplete lighting unit/ “fixture”