Illuminating engineering values and their measuring units

Luminous flux of LED lamps

Characterises the emission power of optical radiation visible to human eye expressed in its special measuring unit: lumen [lm]. The luminous flux is the most important characteristic of a lamp. A conventional 100 Watt incandescent lamp has a luminous flux of 1,300 lumens whereas the luminous flux of a 70 Watt metalohalogen lamp is 6,000 lumens.


This term stands for the density of a luminous flux falling on a surface of a specified area. The measuring unit of illuminance is lux [lx] and it is one of the main values used in lighting regulations. Usually, regulations are applied to horizontal luminance (i.e., measured in the horizontal plane). Illuminance of artificial lighting is within the range of 1 lux to 20 lux outdoors and within the range  of 20 lux to 5,000 lux indoors.  Natural illuminance amounts to E = 0.2 lux in full moon, within the range of 5,000 lux to 10,000 lux in daytime with heavily clouded sky, and up to 100,000 lux on a bright sunny day.

Luminous intensity

This term denotes spatial density of a luminous flux restricted by a solid angle. Its measuring unit is candela [cd], expressed in terms of one of the standards of the International System of Units (SI)..

Spatial distribution of luminous intensity (light intensity curves, LIC) is one of the most important characteristics of illumination devices that are needed for illumination measurements and calculations. The LIC of illuminators is usually expressed in polar coordinates for a generic lamp with a luminous flux of 1,000 lumens – that is, in cd/klm (candelas / kilolumens).


For matt (diffusive or equally luminous) surfaces, this value is proportional to the area density of a luminous flux reflected by a certain surface or of a luminous flux emitted by the surface in question. Expressed in a more general fashion, it equals the ratio of the luminous intensity focused on the observation point to the illuminating surface (projection) that is visible from the subject point. The measuring unit used to express luminance is cd/m² (candelas per square metre). Luminance is directly related to the level of visual perception whereas distribution of luminance within the field of vision (for example, in the interior) defines the quality of illumination In complete darkness, a human being reacts to luminance in the amount a 1/1,000,000cd/m². A ceiling fully covered with light emitting sources with a luminance over 500cd/m² creates discomfort. Luminance of the Sun is in the vicinity of 1,000,000,000cd/m² whereas a luminescent lamp’s luminance is within the range of 5,000cd/m² to 11,000cd/m².


Reflection coefficient [ρ] and transmittance coefficient [τ]

Defined as a ratio of luminous flux reflected [ρ] or transmitted [τ] by the material to the luminous flux that has dropped. Below are the coefficients of reflection for certain finishing materials:

— white paint (0.7–0.8)

— bright wallpaper (0.5–0.7)

— white marble – 0.45

—red brick – 0.3

— dark wood (0.1–0.25)

— asphalt – 0.07

If an indoor area is finished in bright colours (particularly if its size is small when it comes to height), the illuminance level significantly increases. The coefficient of reflection of a background against which an object is being examined is one of the indicators characterising the conditions of visual performance in the workplace Background is defined as ‘bright’ if the reflection coefficient exceeds 0.4; ‘average’ if it is within the range of 0.2 to 0.4; and ‘dark’ if it is below 0.2. An increase in the coefficient of background reflection improves the visibility of the object..

Luminous efficiency

This is the primary characteristic of energy efficiency of lamps and it equals the ratio of the luminous flux of a lamp to its wattage. Application of lamps with high luminous efficiency is the primary way to save power in the illuminating installations. For instance, by replacing incandescent lamps which have a luminous efficiency of 7 to 22 lm/W (lumens/Watts) with compact luminescent lamps that have a luminous efficiency within the range of 50lm/W to 90lm/W, power consumption may be cut by approximately 5-6 times whereby there is no reduction in the illuminance level.

Glare and discomfort indices

These indices specify direct blinding impact of luminous sources or illuminators. A glare index can be used to estimate the degree of visibility deterioration under impact of glaring luminous sources. For instance, if this value equals 100, the visibility is down by 10%. The requirements pertaining to precision workmanship area stipulate that the value of this index must not exceed 20. The discomfort index (M) denotes a degree of inconvenience or strain if exceedingly glaring light sources are located within the field of vision.

Cylindrical illuminance [Ec]

This value characterises the degree to which an indoor area is saturated with light and is defined in terms of luxes as average vertical illuminance created in a specified observation point. This value is defined in regulations applied to indoor areas such as lobbies, entrance halls, anterooms, lounges, auditoria, showrooms, study chambers, reading retail floors, reception halls etc. Light saturation is increased if the Ec value is at least 100 luxens.

Colour and colourfulness

The notion of colour is defined as a capability of visible illumination to evoke a visual perception of colourfulness (colour tone + chromaticity) and luminance of objects. The colour tone (red, orange etc) is characterised by the wavelength of visible radiation whereas colour saturation (chromaticity) is defined by colour purity related to the degree of proximity to the spectrally pure colour from the white point. For instance, colour tones that are low in saturation are obtained by ample dilution of colourant with white paint. The colour of the same object may significantly differ depending upon the spectral composition of illumination.

Colour temperature [Tc]

This is one of the major characteristics of the light sources; it determines the chromaticity of lamps and the colour values (warm, neutral, or cool) of spaces illuminated by the said lamps. It roughly equals the temperature of heated body that is of the same colour as the specified light source. It is expressed in Kelvin Temperature Scale: t° = (Centigrade/°C +273) °K.

Tc values for certain selected light sources:

— candle flame – 1,900ºK;

— incandescent lamps – 2,500ºK to 3,000ºK;

— luminescent lamps – 2,700ºK to 6,500ºK;

— the Sun – 5,000ºK to 6,000ºK;

— clouded sky – 6,000ºK to 7,000ºK;

— cloudless sky – 10,000ºK to 20,000ºK..

As per GOST R 54350-2011 (ГОСТ Р 54350-2011) requirements, there are the following values in a standard range of colour temperatures: 2,700ºK; 3,000ºK; 3,500ºK; 4,000ºK; 4,500ºK; 5,000ºK; 5,700ºK; 6,500ºK.

Colour rendering index [Ra]

This is one of the primary colour characteristics defining the quality of lamp produced. It denotes the degree of colour rendering of various materials illuminated by a lamp, as compared to a benchmark light source. The highest value of CRI is Ra=100. High pressure sodium vapour lamps that are the worst in terms of colour rendering have CRI value of Ra=25. Current German requirements stipulate that Grade 1 colour rendering (“Excellent”) is assigned to devices showing values of Ra≥80; Grade 2 colour rendering (“Good”) is for devices having Ra values within the range of 60 to 79; Grade 3 colour rendering (“Satisfactory”) stands for Ra from 40 to 59; and Grade 4 colour rendering (“Insufficient”) is for the value range of 20 to 39.

[/vc_column_text][vc_tta_section title=”Comparison of wattage and luminous flux of various light sources” tab_id=”1455542212466-7bd1d0c2-df18″][vc_column_text]

Incandescent lamp wattage, WLuminescent lamp wattage,
LED lamp wattage,
Illuminator with a mercury luminescent
arc lamp
Illuminator with a sodium tubular arc lampLED illuminator, WLuminous
[/vc_column_text][/vc_tta_section][vc_tta_section title=”Illuminance requirements for various objects” tab_id=”1455543927151-1c1cf3bc-730d”][vc_column_text]
ObjectsRequired horizontal illuminance value, lx (luxen)
Highways/motorways and public urban roads with traffic intensity (both ways) of 1,000 vehicles / hour20

Highways/motorways and raion (county) roads with traffic intensity (both ways) of up to 1,000 vehicles / hour

Highways/motorways and public urban roads with traffic intensity (both ways) of 500 to 1,000 vehicles / hour
Roads and local streets with traffic intensity (both ways) of over 500 vehicles / hour6

Highways/motorways and raion (county) roads with traffic intensity (both ways) of 500 to 1,000 vehicles / hour

Roads and local streets with traffic intensity below 500 vehicles / hour
Designation (Name)
Symbol used
Aspect ratioFocus area of maximum luminous intensityLight intensity curve
ConcentratedK≥30-15° К
Semi-ExpandedS1,335-55º Л


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