LIGHT TERMINOLOGY

Glossary

Luminous flux Ф(lm)

The luminous flux describes the quantity of light emitted by a light source per second.
The luminous efficacy is the ratio of the luminous flux to the electrical power consumed (lm/W). It is a measure of economic efficacy of a light source.

where:
Φ- luminous flux
lm- Lumen

Illuminance L (lx)

Illuminance describes the amount of luminous flux falling on a surface. Relevant standards specify the required illuminance according to location and activity (EN 12464-1 “Lighting of indoor workplaces”, EN 12464-2 “Lighting of outdoor workplaces”, EN 12193 “Sports facilitylighting”)

E= Φ/A     lx=lm/m2

where:
E- illuminance
Φ- luminous flux
A- area, m2
lx- lux

Typical recommended levels of maintained illuminance

Location/Task

Typical recommended level of maintained illuminance, lx

UGR

Uo

Ra

Offices

 

 

 

 

Writing, typing, reading, data processing

500

19

0.6

80

Computer work stations

500

19

0.6

80

Technical drawing

750

16

0.7

80

Conference and meeting rooms

500

19

0.6

80

Reception desks

300

22

0.6

80

Filing, copying, etc.

300

19

0.4

80

Archives

200

25

0.4

80

Circulation areas within buildings

 

 

 

 

Circulation areas and corridors

100

28

0.4

40

Stairs, escalators, moving walkways

100

25

0.4

40

Elevators, lifts

100

25

0.4

40

Loading ramps/bays

150

25

0.4

40

Rest, sanitation and first aid rooms

 

 

 

 

Canteens, pantries

200

22

0.4

80

Rest rooms

100

22

0.4

80

Rooms for physical exercise

300

22

0.4

80

Cloakrooms, washrooms, bathrooms, toilets

200

25

0.4

80

Sick bays

500

19

0.6

80

Rooms for medical attention

500

16

0.6

90

Educational buildings

 

 

 

 

Classrooms, tutorial rooms

300

19

0.6

80

Classrooms for evening classes and adults education

500

19

0.6

80

Black, green wallboards and whiteboards

500

19

0.6

80

Entrance halls

200

22

0.4

80

Stairs

150

25

0.4

80

Circulation areas, corridors

100

25

0.4

80

Sports halls, gymnasiums, swimming pools (general use)

300

22

0.6

80

Store rooms, frozen food stores:

 

 

 

 

Store and stockrooms

100

25

0.4

60

Dispatch packing handling areas

300

25

0.6

60

Storage rack areas:

 

 

 

 

Gangways: unmanned

20

-

0.4

40

Gangways: manned

150

22

0.4

60

Control stations

150

22

0.6

80

Front of (high-bay) racks

200

-

0.4

60

Factory assembly areas:

 

 

 

Rough work

300

25

0.6

80

Medium work

500

22

0.6

80

Fine work

750

19

0.7

80

Very fine work:

 

 

 

Instrument assembly

1,000

19

0.7

80

Jewellery assembly/repairs

1,500

16

0.7

90

Hospital autopsy tables and dissecting tables

5,000

-

-

90

Sports facilities:

 

 

 

 

Aerobics, athletics, dancing, gymnastics, swimming

500

-

-

60

Basketball, football, handball, table tennis, tennis, volleyball, weight lifting

750

-

-

60

School sports

750

-

-

60

where:
UGR- Unified glare rating serves to define the psychological glare (visual discomfort) of lighting installations in indoor spaces
Uo- Uniformity is the ratio of minimum illuminance to average illuminance on a surface
Ra- Colour rendering index

Luminous intensity I (cd)

The luminous intensity describes the quantity of light emitted in a particular direction per unit solid angle. This is a useful measurement for directive lighting elements such as reflectors. It is represented by the luminous intensity distribution curve.

L=I/(A*cosϵ)       L=(E*ρ)/π

where:
Ω- solid angle into which luminous flux is emitted, sr
A- area hit by luminous flux, m2
A*cosϵ- visible area from direction ε
ρ- reflectance of area (for diffuse surface areas)
π- 3.14

Luminance (cd/m2)

The intensity and brilliance of light emitted from an item in a given direction. Luminance is often used to characterize emission or reflection from flat, diffuse surfaces. The luminance indicates how much luminous power will be detected by an eye looking at the surface from a particular angle of view. Luminance is thus an indicator of how bright the surface will appear.

L=I/(A*cosϵ)       L=(E*ρ)/π

where:
Ω- solid angle into which luminous flux is emitted, sr
A- area hit by luminous flux, m2
A*cosϵ- visible area from direction ε
ρ- reflectance of area (for diffuse surface areas)
π- 3.14

Beam angle (degree, °)

The Beam Angle is the angle between the two directions opposed to each other over the beam axis for which the luminous intensity is half that of the maximum luminous intensity.

Power consumption (W)

The electrical power used by a luminaire or light source. Usually expressed in watt-hours (power consumed per hour).

Energy efficacy (lm/W)

The figure gained by dividing the total luminous flux by the power consumed. Used to express the luminous flux (amount of light) gained per watt, the higher the figure the more efficient is the luminaire.

LED lifetime (h)

The lifetime of a LED luminaire illustrates the target life expectancy (for example 50 000 hour life expectancy) which is a combination of light output degradation (L80 when the light output has reduced to 80% of its initial level) and source performance expectation (B10 when 10% of the LEDs lumen outputs falls below 80% of the nominal initial value) under specific conditions.
L- Lumen maintenance. L value tells that how many percent of luminous output is still left from the original.

B- Failure fraction. The value B10 means that minimum 90 % of the LED modules will meet the declared L-value and only 10% will have a lower flux level.

Colour temperature (K)

Used to numerically express the colour of light sources, the redder the colour the lower the number and the bluer the colour the higher the number
where: K- Kelvin

2700K- extra warm white
3000K- warm white

4000K- natural white
5000K- cool white
6500K- day light

Colour rendering index (CRI)

Colour rendering describes how a light source makes the colour of an object appear to human eyes and how well subtle variations in colour shades are revealed. A colour rendering index is a quantitative measure of the ability of a light source to reveal the colours of various objects faithfully in comparison with an ideal or natural light source. The ability of the light source to render colours naturally, without distorting the hues seen under a black full spectrum radiator, like daylight or incandescent lamps. A minimum of 80 CRI is recommended for working areas.

Light sources are divided up into colour rendering levels:
CRI > 90 very good colour rendering

CRI > 80 good colour rendering

Unified Glare Rating (UGR)

UGR is a method of calculating glare from luminaires. The UGR value is calculated using a complex equation that includes the luminance value of the luminaire, the value of background luminance, the solid angle of the luminaire as seen by the viewer and several other values. Within an office setting, for the luminaire to be classified as “low glare” it must have a UGR below 19 at desk level. Anything above this may cause discomfort – this further enforces the need for high quality interior lighting that is rated UGR<19.


UGR limits (UGRL) that must not be exceeded:
≤ 16 Technical drawing
≤ 19 Reading, writing, training, meetings, computer-based work
≤ 22 Craft and light industries
≤ 25 Heavy industry
≤ 28 Railway platforms, foyers

Recognition distance (m)

Signs placed at all emergency exits along escape routes should be so illuminated as to indicate the direction to a place of safety in an unambiguous manner (EN1838Lighting applications. Emergency lighting). Exit and direction signs should be clearly visible from anywhere along the escape route. All signs indicating emergency exits and escape routes should be in the same colour and format, whereas their minimum illuminance should be 2 cd/m2.
As people in the building may not be familiar with its layout, internally illuminated, permanently powered safety signs are recommended.

Note that internally illuminated safety signs are visible from a greater distance than the same size signs illuminated from the outside.

d=s*p

where:
d- viewing distance (maximum distance at which a sign is visible and recognizable), m
p- height of sign, m
s- constant value: 100 for externally illuminated signs and 200 for internally illuminated signs

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