Preconditions

When measuring lighting, it is important to observe that the following conditions impact results:

• Assessment grid of photometric parameters for design and measurement

• Arrangement of luminaires and location of the assessment grid for design and measurement

• Luminous flux of lamps: in case of gas discharge lamps consideration of control gear in use; in case of low-voltage tungsten-halogen lamps consideration of transformers in use

• Ambient temperature: for luminaires with lamps where luminous flux depends on temperature

• Other conditions such as furniture, reflectivity

This necessitates uniform measurement and assessment regulations. The measuring grid to be applied, meaning the array of measuring points in the room in question, is defined on a European level for work place lighting (in standard EN 12464-1), sports lighting (EN 12193 Light and lighting – Sports lighting) and for operating room lighting (Particular requirements for basic safety and essential performance of surgical luminaires and luminaires for diagnosis). In Germany, however, there is an additional specialised standard DIN 5035-6 Artificial lighting – Part 6: Measurement and evaluation, which also defines the selection of measuring instruments to be used and further basic parameters for measurement and recording.

For the measurement of artificial lighting, the following conditions among others should be met:

• Interfering light in the form of daylight or other light sources must be avoided.

• For new installations, use clean and possibly cleaned luminaires as well as new ones; when using fluorescent lamps, use lamps with 100 hours of wear.

• Stationary operation of the lighting installation must be ensured; in case of discharge lamps, the installation must be switched on at least 30 minutes prior to measurement.

In some cases, an elimination of daylight is not feasible without issues. For orienting measurements, it is also possible to conduct differential measurements. This means that in step 1 the sum of daylight and artificial lighting is measured, and in step 2 the daylight component is measured upon switching off the artificial lighting, and the result is then subtracted from the initial value. However, it is important to realise that the daylight component can change rapidly and noticeably and thus significantly impact the measurement result.

Furthermore, data on the room and the lighting installation must be recorded before measuring:

• Geometric dimensions of the room

• Furnishings and equipment (e.g. furniture, reflectance values) of the room

• Type of room use

• Arrangement of workstations

• Maintenance state (e.g. State of soiling) of room and lighting installation

• Luminaire arrangement

• Luminaire and lamp type

• Luminaire manufacturer

• Special properties of the luminaires, e.g. type of control gear

• Control state of the luminaires (measurements are usually performed at 100% luminous flux)

• Mains voltage and ambient temperature

• State of air conditioning or ventilation installations, as applicable

• Location of measuring points

• Time of measurement

• Lighting installation age

Lighting measurements should be executed with illuminance or luminance measuring instruments of at least class B according to table. Calibration of measuring instruments is recommended at least every two years.

Information on measuring instruments can be found in CIE-publication 69/1987 "Methods of characterizing illuminance meters and luminance meters; performance, characteristics and specification". They have also been adopted into the German standard DIN 5032-7:1985-12 "Photometry; classification of illuminance meters and luminance meters".

In order to assess compliance with workstation lighting quality criteria required by standards, an orienting measurement using a class C photometer according to the aforementioned standard is generally sufficient. These are significantly less costly than the technically highly sophisticated precision measuring instruments of classes A and B.

However, these instruments also fulfil rigorous demands:

• the overall measuring error must not exceed 20%, in relation to the displayed, for the entire measuring range,

• the illuminance (or luminance) measurements result from the gathered radiometric radiation intensity (or radiometric radiation density) and their weighting using the spectral sensitivity of the human eye for photopic vision according to the V(λ) curve (see figure),

• the error of V(λ) adjustment must not exceed 10% for any wavelength of the incident radiation.

This in particular ensures that the maximum measuring error to occur is always minimal due to the spectral composition of the light, compared to the permissible overall error. The measurement result is therefore independent of the light source (e.g. daylight, incandescent lamp light, fluorescent lamp light, LED light etc.).

Simple "measuring instruments" such as the light sensor of a smartphone usually do not satisfy this requirement. Erroneous weighting of visible – and partially also invisible – radiation components lead to a display of false illuminance results, which means that the identified value often does not correspond to the perception of brightness.