Planning and calculation tips

A lighting installation necessitates that careful consideration is paid towards the selection of the appropriate lighting system, equipment, control systems and the use of daylight. Here are some tips about energy efficiency and evaluations.

Energy efficiency in lighting installations

A lighting installation needs to confirm to the various requirements stipulated for its specific area, without compromising on energy efficiency or visual comfort. This necessitates that careful consideration is paid towards the selection of the appropriate lighting system, equipment, control systems and the use of daylight.

A measurement of efficiency in a lighting installation is the installed output, in W/m², that is required to meet defined demands.

In addition to a low installed output, energy usage should be limited in an installation with the help of different control systems so the lighting can be adapted to requirements and used as efficiently as possible.

A better method of assessing the energy efficiency of a lighting system is to evaluate the annual energy consumption. This method is described in the EN 15193 standard, which is linked to the Energy Directive, see separate section. More precise calculations can also be performed in DIALux, or similar calculation programs. Here, you can take into account the reduction factors for control depending on presence/absence – light and constant light control.

General advice

The following points should be considered in order to create low energy usage in a lighting installation:

  • Selection of light sources with optimal luminous efficacy for the required colour reproduction.
  • Energy efficient lighting system with installed lighting output suitable for the required task.
  • Efficient luminaires with the correct light distribution and good cut-off properties.
  • Efficient utilisation of daylight.
  • Efficient utilisation of artificial and natural light through the selection of a light interior colour scheme.
  • Control of lighting through presence detection.
  • Possibility of localised control by the individual.
  • High frequency operation with dimming.
  • Well planned maintenance of the lighting installation to obtain a high maintenance factor.


Installation typeInstalled lighting outputRequired illuminance while operational (lx)Notes.
Corridors 5–10 W/m² 100 lx  
Corridors 10 W/m² 200 lx  
General public areas 10–12 W/m² 300 lx  
Workplaces 10–12 W/m² 300 lx *)
Workplaces 10–15 W/m² 500 lx *)
Workplaces 15–30 W/m² 1000 lx *)
*) Required illuminance within the working zone according to EN 12464-1. The lower value normally presupposes a localised lighting system adapted to the working area in the workplace.

Evaluation of a lighting system

Visual ergonomic aspects of the workplace’s design are important for a stimulating working environment. There is an excellent method called “visual evaluation” available to review and assess a room with an installed lighting system, which is simply based on describing what you see in the room.

Evaluate the room visually through its lighting system, colours and design. These factors affect each other and are difficult to assess individually. The room colours should not be distorted, and visual work should be possible without the discomfort of glare or reflections.

The room’s visual quality governs, to a large extent, your health and your performance capacity. It is therefore important that you do not completely rely on computer calculated results. Try to make a visual evaluation of your own workplace using the adjacent table.

Rank the opposite pairs on a scale of 1–5.

Light level – is it dark or light in the room? dark – light
Light distribution – how is the light distributed in the room? varied – equally
Light colour – is the light colour experienced as warm or cold? warm – cold
Colour – how are the colours and objects viewed? distorted – natural
Glare – does unpleasant glare occur? troublesome – not noticeable
Shadows – whether they are hard or soft? hard – soft
Reflections – whether they are intense or diffuse? intense – diffuse



The purpose of the maintenance factor is to take into account the light depreciation

According to the new standard, designers now have a greater planning responsibility for the installation. One aspect, the maintenance factor, has a direct effect on the installations energy consumption.

In order to enable the choice of a high maintenance factor, careful consideration must be exercised when choosing light sources, luminaires and overall lighting systems. 

Regardless of the type of light source, the luminous flux will always decrease over time. The degradation of luminous flux is reported as an L-value for each type of LED luminaire. L90 at 50,000 h means that the luminaire generates 90% of its original output after 50,000 h. In a light calculation, the function of the maintenance factor is to make adjustments for the installation’s light depreciation over time. In other words, the maintenance factor ensures that the lighting system fulfils lighting requirements across its stated operational life, taking into account both light depreciation and contamination with dirt.

The maintenance factor forms the foundation for establishing the maintenance plan for the lighting system.

Table 1. Proportion of the maintenance factor formed by the light source's light depreciation 

Examples of light sourcesLight depreciation factor
Straight fluorescent lamps with barrier layer for low light depreciation and high pressure sodium lamps 0.90
Other fluorescent lamps, compact fluorescent lamps and mercury lamps 0.85
Metal halide lamps 0.75
LEDs Depending on the L-value


Table 3. Proportion of the maintenance factor corresponding to the contamination of the luminaire, taking into account the luminaire type, surroundings and cleaning interval 

Number of years between group cleanings2.
Luminaire type Surroundings Surroundings Surroundings Surroundings
  Clean Dirty Clean Dirty Clean Dirty Clean Dirty
Open luminaire 0.96 0.85 0.94 0.77 0.92 0.72 0.9 0.66
Closed luminaire 0.98 0.87 0.96 0.84 0.94 0.78 0.92 0.71
Indirect uplight luminaire 0.91 0.68 0.84 0.54 0.77 0.40 0.71 0.29
Table 3 is an adaptation of CIE 97:2005 2nd Edition. 
Open luminaire refers to both direct and direct/indirect distribution, while uplight luminaire is 100% indirect.


Table 4. Proportion of the maintenance factor corresponding to the contamination of the surfaces in the room, taking into account the luminaire type, surroundings and cleaning interval. A 3 year interval is recommended for comparison.

Number of years between group cleanings2.
Luminaire type Surroundings Surroundings Surroundings Surroundings
  Clean Dirty Clean Dirty Clean Dirty Clean Dirty
Direct 0.97 0.95 0.97 0.95 0.97 0.95 0.97 0.95
Direct/Indirect 50/50 0.95 0.90 0.95 0.90 0.95 0.90 0.95 0.90
Indirect 0.92 0.77 0.92 0.77 0.92 0.77 0.92 0.77

Reflection factors 70/50/20 and 50/30/20 dirty.

Tables 1, 3 and 4 must be used together to calculate a maintenance factor.

Standard formula for maintenance factor = LLMF x LSF x LMF x RSMF (Lamp Lumen Maintenance Factor x Lamp Survival Factor x Luminaire Maintenance Factor x Room Surface Maintenance Factor = maintenance factor) (see explanation under Quantities, units and their significance).


For example:

LED luminaire with L90, open luminaire in clean surroundings, 3-year cleaning interval and direct-indirect luminaire, exchange immediately in the event of failure

0.9x1x0.94x0.95 = 0.8

Selecting a luminaire with L90 is an advantage.
  • The L-value after a specified number of burning hours affects the maintenance factor.
  • A higher L-value gives a higher maintenance factor.
  • Luminaires with higher maintenance factor meet the operating values with fewer luminaires, which can positively impact on energy consumption.

For example:

Classroom with L80 or L90 luminaires.

Calculation of maintenance factor with pendant luminaires in a clean environment with 3-year cleaning interval.

Values for lamp survival factor, luminaire contamination and room contamination are reported in the Lighting template.

The difference in a light calculation:

An operating value of 445 lux is obtained for an installation with L80 luminaires, which is not sufficient to meet the standard requirement of 500 lux. For the system to comply with the requirement, 12 luminaires would have to be installed. L90 luminaires, that have a higher maintenance factor, meet the standard requirement of 500 lux with only nine luminaires. Threfore using the L90 luminaire the requirement is  met with fewer luminaires.




  Classrooms with pendant
L80 luminaires
Classrooms with pendant
L90 luminaires
Requirement according to EN 12464-1 500 lux  500 lux
Maintenance factor 0.71  0.8
Value when new 625 lux  625 lux
Operating value Em 445 lux (does not meet requirement)  501 lux  


Light measurement

The luminaire’s light distribution is measured on several C-planes around the luminaire, at intervals of at least 15°. First measurement plane (C=0°) is across the lamps’ longitudinal axis. γ-angles – several angles are measured, at least every 5 degrees (see figure).

Light distribution curve

The light distribution curve drawn in a polar diagram denotes the luminaire’s luminous intensity in different directions as a function of viewing angle in one or more planes. It is usually represented by an unbroken line that indicates the light distribution perpendicular to the light source’s longitudinal axis and with a dashed line that indicates the light distribution in direction of the longitudinal axis.

The values of the light distribution curves are scaled to correspond to 1000 lm from the light source (cd/1000 lm, cd/ klm). This is why it is often possible to show luminaires with different outputs on a common polar diagram.

Normalized luminous intensity
Normalised luminous intensity data of a luminaire (equipped with a lamp or lamps of specified type and power) shall be given as a table, normalized to a luminous flux of 1,000lm and provided in candela per kilolumen (cd • klm -1).


530cd/klm in 0 degrees

Absolute luminous intensity 
Absolute luminous intensity data (in candela(cd)) of a lamp or luminaire (equipped with a lamp or lamps of specified type and power) shall be given as a table. Lamp type and rated power should be declared.


In this case the lumen output is 3300lm.
530cd/klm multiplied by 3300lm gives you 1760 cd.

Isolux diagram


The diagram shows, using curves (or scales) a predefined area, inside which the horizontal illuminance exceeds the curve’s lux value. The position of the luminaire is usually indicated on the diagram. Alternatively, the isolux diagram can be represented by a 3D diagram, which is best suited for showing the uniformity of the lighting installation.

The calculation points’ illuminance values can be introduced onto the premises’ layout drawing and the results given in table form. The results are available in all the above mentioned forms in DIALux.

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 L=I/A
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 L=I/A
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 steradian
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.