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Illuminance: definition, measurement and calculation

Graphical depiction of illuminance.

Illuminance E as a measure of luminous flux incident per surface unit A.

Illuminance (E) describes the quantity of light incident on a surface. It is the ratio of the incident luminous flux (Φ) to the size of the surface (A). The unit of measure of illuminance is lux (lx). This is defined as lumens per square metre (1lx = 1lm/m2). Illuminance recedes with the square of the distance from the light source (photometric law of distance).

Overview of illuminance

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How is illuminance measured?

A luxmeter or photometer is used to measure illuminance directly. A light sensor with a defined diameter measures the incident light and displays the corresponding values in lux on the device. There are different quality classes for photometers. These provide information about the tolerance of the determined values:
Class A: tolerance of +/- 5%
Class B: tolerance of +/- 10%
Class C: tolerance of +/- 20%
Therefore, only calibrated photometers of high quality are used in professional lighting technology. There are various apps available for smartphone devices that also enable illuminance to be measured via a light sensor. However, these are usually not calibrated and have strong deviations compared to professional luxmeters.

Good to know:
Calibrated lux meters should be used to correctly measure illuminance. The measuring head must be aligned parallel to the measuring surface and ideally rest on it. Because the photometer measures all the light incident at this point, care must be taken that the person taking the measurement does not block any luminaire in the measuring field!

How can illuminance be calculated?

The mean horizontal illuminance Em is calculated from the luminous flux Φ that falls on the observed surface A:

illuminance (lx) = luminous flux (lm) / surface A (m2)


For practical calculation with a calculator, the illuminance at a point can also be calculated via the luminous intensity. The luminous intensity can be taken from the luminous intensity distribution on the data sheet of the luminaire. The formula to be applied is:

illuminance (lx) = luminous intensity (cd) / distance (m)2


This formula describes the photometric law of distance: the luminous intensity in candelas (cd) is divided by the square of the distance (m) from the light emission surface to the measuring surface. It is important to note here that the light hits the target surface at right angles.

What value must illuminance at the workplace have?

Good lighting is important at the workplace in order to concentrate over longer periods, but also to work safely. The European standard EN 12464 specifies the lighting requirements that must be complied with: the main parameters are, in addition to illuminance,

  • luminance distribution,

  • danger of glare,

  • light direction,

  • light colour and colour rendering,

  • flicker, and

  • daylight.


The importance of daylight was specifically emphasised in the updated DIN EN 12464-1 from 2011. In addition, it is repeatedly pointed out that lighting should be controllable or adjustable.
The Committee on Workplaces supervised by the German Ministry of Labour and Social Affairs also reflects the state of scientific knowledge for occupational safety and health in the setting up and operating of workplaces:

  • The mean illuminance in the surrounding area of a workstation with 300lx illuminance must be at least 200lx.

  • For workstations illuminated with 500lx or more, the mean illuminance in the surrounding area must be at least 300lx.

  • Illuminances above 500lx in workstation areas may require a higher mean illuminance in the surrounding areas.

  • The minimum illuminance in the surrounding area must not fall below 0.5 times the mean illuminance of the surrounding area.


For light visual tasks, sufficient visual performance is achieved with low illuminance levels, whereas complex visual tasks need high illuminance levels. For example, 20 lux is a bottom limit at which people's facial features can only just be distinguished.

Good to know:
The illuminance generated by two or more light sources can be added together. If one luminaire generates illuminance of 500lx on a desk and a second one does the same, then the illuminance on the desk is 1000lx if both luminaires are operated.

What is the difference between illuminance and luminance?

Figure showing the difference between illuminance E and luminance L.
Illuminance and luminance are two important photometric parameters in lighting design. Both take into account the brightness sensitivity curve of the human eye. Illuminance is a measure of the light incident on a surface, whereas luminance describes the light reflected from it. If a white and a black sheet of paper are illuminated with the same intensity, the same illuminance will be measured on both sheets. However, the white sheet reflects significantly more light than the black sheet and thus has a higher luminance.

What is point illuminance?

Illuminance
In contrast to the nominal illuminance (Em), which gives an indication of the average illuminance of a surface, point illuminance (Ep) describes the precise illuminance at a certain point.

Using the photometric law of distance, it is also possible to calculate the illuminance at individual points in space. Indirect lighting components are not taken into account in the calculation, but can be included via an additional calculation. Calculating point illuminances mainly plays a role in lighting design for narrowly defined areas illuminated by individual luminaires, e.g. with exhibits in an exhibition.

ERCO Light Knowledge is also available as a whitepaper:

Compact Light Knowledge: Photometry

Download the whitepaper

Handbook of Lighting Design

Download brochure

Further topics on photometry

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