Lighting terminology

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Some of the terminology used in lighting practice and analysis, and in studying the environmental effects of light, are not commonly used elsewhere, or have different meanings when used in this context. Some terms are commonly used when talking about lighting practice, but have different meanings to different people; this is a frequent source of talking cross-purposes in our field.

Luminous Flux (Ø lm)

The total light output of a lamp measured in lumen (lm), the lamp manufacture specifies the nominal luminous flux of this lamp.

Luminous intensity (I = Ø / sr (cd))

 The luminous flux radiated at a certain solid angle divided by that solid angle, measured in candela (cd). The luminous intensity always relates to a certain direction and is connected with reflectors and diffusers. The distribution reproduced in a polar curve.

Lighting level (E = Ø/S (lx))

Lighting level refer to the incidence of the light flux on the surface, per unit of area. Lighting level is measured in lux (lx). There is a difference between vertical and horizontal lighting level.

Luminance (L = E / Ss (cd/m²))

Expression of the (mean) degree of brilliance with which the human eye perceives an illuminated surface from a certain direction. The luminous intensity per unit of visible surface of light source (direct) or an illuminated surface (reflection).  Luminance is indicated in candelas per square meter (cd/m²)

Color temperature (Tc (K))

Temperature of a black body whose radiator has the same type of color as that a given light source GLS lamps have a color temperature of 2700 K, but halogen lamps have a color temperature which is perceived as "cooler" light. The color temperature is indicated in Kelvin.

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Color rendering index (Ra (%))

A lamp doesn't reproduce each color true to nature. This is called illuminant (perceived) color shift. The color rendering index is a measure of the degrees to which the color rendition of a light source deviates on a average that of the lamp. At Ra (8) the illuminant (perceived) color shift of eight reference color is measured (as a percentage) and arithmetically averaged to give one number. Values 100 to 90 are good, 90 to 80 less good and lamps with Ra's below 80 must not be used where color rendition is an important factor.

Roomindex (K)

This indicates whether a room is narrow and high or low and wide. It influences the amount of light which is emitted from the fitting on to the working plane. The room index is defined on the basis of the dimensions of the room according to the following formula:

                    axb

       K= ـــــــــــــــــــــــــــ

                 hx(a+b)   

a = length of the room

b = width of the room

h = hight  between the luminaire and the working plan

Maintenance factor (light loss factor) (L.L.F) 

   

The lighting level decreases in time due to soling and deterioration of the lamp itself. Allowance must be made for this when designing the lighting. Mean values are 0.7 for a normal interior, and 0.8 for a very clean interior.

Luminance efficiency (lm/W)

 The luminous efficiency indicates how efficiently a lamp concerts electrical energy to light. This is indicates as lumens per watt (lm/w).

Luminaire efficiency (ɳ (%))

 The luminaire efficiency indicates how well a luminaire uses the luminous flux of the lamp. It is indicated as the ratio of the luminous flux of a luminaire divided by the luminous flux of the "bare" lamp (s) and is expressed as a percentage. The luminaire efficiency is indicated in the lighting graphs by LOR (Light output ratio)