Quantities, units and their significance

For standardised definitions and further clarification of magnitudes, units and concepts, see EN 12 665 – Basic terms and criteria for specifying lighting requirements. (EN 12 665 – Basic terms and criteria for specifying lighting requirements).
Luminous intensity
(of a light source, in a given direction)
I candela (cd) I=dΦ/d
cd = lm • sr-¹
The ratio between the luminous flux dΦ that leaves the light source within the solid angle element dΩ containing the given direction, by the solid angle element (unit: cd = lm • sr-¹).


Note: luminous intensity is the intensity of the light in a given direction – luminous flow per unit solid angle (ω).

(at one point on a surface) (E)
E Lx E= Φ/A The ratio between the luminous flux dΦ incident on an element of the surface, containing the point, and the surface dA of that element (unit: lx = lm/m²).


Note: Illuminance refers to the luminous flux that hits a given area – luminous flux per unit area (m²).

Cylindrical illuminance Ez Lx
Ez = (1/π)
L sin εdω
4 πsr
The total luminous flux on the curved surface of a very small cylinder placed at a specified point, divided by the surface area of the cylinder (unit: lx).


Note: cylindrical illuminance (at one point, for one direction) (Ez) quantity defined by the formula

Ez = (1/π) L sin εdω
4 πsr

dω is the solid angle of each elementary beam passing through the given point

L is its luminance at that point

ε is the angle between it and the given direction – unless otherwise stated, the direction is vertical.

Modulation Ez / Eh Ez / Eh The ratio between the cylindrical and horizontal illumination at a point.


Note: the balance between diffuse and directional light. A value between 0.3 and 0.6 is usually an indication of good modelling.

(light density)
L (cd/m²)
L = I/A
(L = I/Acosα)
The luminance in a given direction, at a given point on a real or imaginary surface


Note: luminance is also known as light density, and is defined as the light density in a specific direction on a predetermined point/surface on a light source/luminaire or illuminated surface.

Luminous flux Φ lumen (lm) Φ=I/ω The total luminous energy emitted from a light source, defined as the luminous energy obtained when the radiant luminous flux of the light source is evaluated against the eye’s sensitivity in daytime (photoptic) vision (ISO/CIE 10527).
Light output ratio (of a luminaire)
(Light Output Ratio – LOR)
ηA     Ratio between the total flux from the luminaire, measured under specified practical conditions with its own light sources and equipment, and the total individual luminous flux from the same light
sources operating outside the luminaire using the same equipment, under specified conditions.
Ballast Lumen Factor BLF Defines the ratio in luminous flux from a reference light source measured using a commercial ballast or a reference ballast at an ambient temperature of 25 °C.
Colour temperature Tc kelvin (K) CIE 17.4 The temperature of a Planckian (black body) radiator whose radiation has the same chromaticity as the given stimulus. (unit: K)


Note: Colour temperature describes the colour impression of a light source, which is normally perceived as hot at < 4000 K and cold at > 4000 K. Colour temperature is expressed as an absolute temperature or in respect of absolute zero, which is defined as 0 K = –273.17 °C or 0 °C= +273.17 K

Correlated colour
Tcp kelvin (K) CIE 17.4 The temperature of the Planckian (black body) radiator whose perceived colour most closely resembles that of a given stimulus at the same brightness and under specified observation conditions. (Unit: K).
Colour rendering index CRI Ra CIE 17.4 CIE 1974 general colour rendering index for a specified set of 8 colour samples.


Note: is a measurement of a light source’s ability to render colour compared to a reference light source at a predetermined colour temperature. An Ra index is used for graduation which, according to CIE, can be at most 100 and which should, for lighting work premises, be a minimum of 80.

Luminous efficacy of a light source η (lm/W) η=Φ/P The ratio between the emitted luminous flux and the power consumed by the light source.


Note: luminous efficacy can be described as a measurement of the efficiency of the light source.

Luminous efficacy – systems
(light source + ballast)
η c (lm/W) η=Φ/P The ratio between the luminous flux emitted by a light source and the electrical power that it consumes, incl. ballast losses.
Luminous efficacy –
(light source + ballast)
l /LLE (lm/W) η=Φ/P The ratio between the luminous flux emitted from a luminaire and the electric power that it consumes with the light source, incl. ballast losses.
Glare     CIE- 31, 112, 117 Visual conditions where discomfort or a reduced ability to see details or objects occurs, caused by inappropriate distribution or levels of luminance, or by extreme contrasts.


Note: glare is normally subdivided into discomfort glare (UGR/NB) and disability glare (TI/GR).

Discomfort glare UGRL   CIE- 117 Discomfort glare can be expressed with the help of a “psychometric scale”derived from psychophysical experiments. If it is expressed by means of a”unified glare rating”, the following UGR values should be used (see CIE 117): 10; 13; 16; 19; 22; 25; 28.


Note: verified UGR data reported in accordance with the tabular method described in CIE publication 117 are available for an array of luminaire manufacturers. Manufacturers who publish UGR tables calculated using a different distance-to-height ratio than that which is described in CIE publication 117 must state this fact.

 – illuminance
 – luminance
Uo   Emin/Emean
The ratio between the lowest value and the average (mean) value over a specific surface, unless otherwise stated.
Daylight factor D     The ratio between the illuminance at a point on a given plane, caused by direct or indirect light from a sky of assumed or known luminance distribution, and the illuminance on a horizontal plane caused by an unshielded hemisphere of the same sky.


The contribution from direct sunlight is excluded from both illuminances.

Luminaire luminance Lave cd/m2
The average (mean) luminance of the illuminating parts of the luminaire or solid angle.
Luminaire luminance – limit values (for working at a monitor) Lave cd/m2
The average (mean) luminance of a luminaire’s illuminating parts must be measured and/or calculated in the C-plane at 15° intervals, starting at 0°, and the elevation at γ angles 65°, 75° and 85°.


Note: should normally be provided by the luminaire manufacturer based on the maximum (light source/luminaire) efficacy. The values should not exceed the limit values specified in Table 4 (see also EN 13032-1 and EN 13032-2).

Shielding angle for luminaire’s light source   degrees The angle between the horizontal plane and the first line of sight, at which parts of the light sources in the luminaire become directly visible.
Optical cut-off angle;
Cut-off angle for luminaires
  degrees The angle, measured upwards from the nadir, between the vertical axis and the first visual direction at which the light sources and surfaces with high luminance are not visible.
Solid angle ω
ω=A/r² The ratio between the cut out area A on the sphere caused by a beam of light on the surface and the square of the sphere’s radius r.
Median life
 – light sources
hours (h) The median life is defined as the time after which 50 % of a sizeable sample of light sources have discharged (normally indicated for incandescent lamps, halogen lamps and fluorescent lamps).
Service life
 – light sources
hours (h) The service mortality rate is defined as the point at which 80 % of the lighting installation’s original luminous flux remains. The depreciation in luminous flux depends on the reduced luminous flux and spent light sources.
Economic life
 – light sources
hours (h) The economic mortality rate is defined as the point at which 70 % of the lighting installation’s original luminous flux remains. The depreciation in luminous flux depends on the reduced luminous flux and spent light sources.
Calculation Points p   p = 0,2 x 5log d A grid with a defined set of calculation and data points in each direction on the measurement plane.


Note: note that the distance and location of the calculation points should not coincide with the distance between luminaires.

A grid that approaches the shape of a box or square is to be preferred, and the ratio between the distance of the length and width of the grid should be kept within the range 0.5–2 (see also EN 12193).

The maximum distance between the calculation points in the grid will be: p = 0,2×5 log d where;

p ≤ 10
d is the longer length of the surface
p is the maximum distance between the calculation points (m).

The number of points for the longer distance is determined to the nearest odd integer of d/p.

LENI   kWh/m², year   Lighting Energy Numeric Indicator: The numeric indicator of the lighting’s annual energy consumption within a building or a specific area in accordance with EN 15193 (see instructions under separate section).


Note: the LENI number can be used as a comparison of lighting energy efficiency between different buildings and places with the same function and activities.

Working area       The area in which the job is carried out.


Note: refers to the lighting requirements of EN 12464-1 and EN 12464-2.

Immediate surroundings       A band/area around the working area within the field of view, with a width of at least 0.5 m.


Note: refers to EN 12464-1 and EN 12464-2.

Peripheral surroundings       The peripheral surroundings refer to a band/area of at least 3 m around the immediate surroundings. Where the peripheral surroundings touch a wall, the area of the peripheral surroundings is restricted by a zone 0.5 m from the walls of the room. The illuminance within the peripheral surroundings must be at least 1/3 of the illumination within the immediate surroundings.


Note: refers to the next edition of EN 12464-1.

Maintenance factors       Formula LLMFxLSFxLMFxRSMF. See the tables in the section dealing with lighting planning.
Lamp Lumen Maintenance Factor LLMF     The ratio between the luminous flux from a light source at a given time during its life, and the initial luminous flux.
Lamp Survival Factor LSF     The proportion of the total number of light sources that still work at a given time under defined conditions and ignition frequency.
Luminaire Maintenance Factor LMF     The ratio between operational efficiency of a luminaire at a given time and the initial operational efficiency.
Room Surface Maintenance Factor RSMF     The ratio between the reflectance of the room surfaces at a given time and their initial reflectance.


Note: maintenance factor for the room surfaces, dependent on room contamination.

Emergency Ballast Lumen Factor EBLF     The ratio between the light source’s luminous flux, measured with a ballast during testing, at the lowest voltage that can occur during emergency lighting operation following a power failure (at the recommended start-up time for the application’s demands) and in the case of continued emergency lighting for a specified time, and the luminous flux from the same light source in operation with a reference ballast at the rated voltage and frequency.




EBLF is the luminous flux factor of the emergency ballast;
BLF is the luminous flux factor for the ballast;
Fmin /is the emergency lighting factor/worst case for the reduction factors during emergency lighting operation.

Note: the light source is operated during emergency lighting by an emergency lighting unit instead of the regular ballast. Emergency lighting operation works at a reduced light source output, normally between 5 and 30 % of the normal output.