Light measurement

The luminous flux designates that portion of the radiation sent by a light source, which is visible for the human eye. Thereby the varying spectral sensitivity for different colors is taken into account. Its unit is Lumen (lm).

Spectral sensitivity

The human eye has a maximum sensitivity for green light at a wavelength of 555 Nm. The value of 100% is assigned to this maximum ([3], part 3). The sensitivity decreases quickly towards smaller or larger wavelengths. (Figure 1).

Those values are defined for the so-called photopic vision. This means luminances that allow the cognition of colors.

Figure 1: The eye's spectral sensitivity as a function of wavelength.

The illuminance marks the luminous flux received by a lit surface, divided by its area. Its unit is Lux (lx) resp. lm/m².

The luminance designates the luminous flux sent by a luminescent or lit surface divided by the solid angle, into which the radiation is emitted. The unit of luminance is Candela per square meter (cd/m²).

Contrast is the difference in luminances of two areas. As the photometric measurement noticed by the human eye actually is the luminance, recognizing outlines depends strongly on the magnitude of contrast.

Correlated color temperature: Even white radiating lamps can appear colored with direct view. For the characterisation of this 'color' the light is compared visually with that of a temperature emitter. The temperature, at which both sources of light seem to differ least, is called the correlated color temperature.

The daylight factor (defined for a random point of a horizontal plane) is the ratio of the illuminance indoors, usually on the working plane, expressed as a percentage of the illuminance outdoors for a cloudy sky (no distinct solar radiation). ([1], part 1).


The natural skylight is best suitable for most purposes, because of the human eye's optimal physiological adaption to this light supply. Therefore, attention should be paid to the use of daylight in buildings wherever this is possible. Working places in the vicinity of windows are generally to be preferred over such in larger depth of a room.

In addition to their permeability for daylight, windows have an important psychological function: they expose the users to the environment. Windows should allow a view to all of the three ranges: foreground (direct environment, vegetation, roads), center (building, landscape, horizon) and the sky [5]. To meet these requirements a reasonable arrangement of window openings is necessary. The balustrade must not be chosen too high and the width of the window not too small ([1], part 4):

The skylights in the ceiling cannot establish this contact to the environment. But they are excellently suited to realise high levels of illumination by daylight with very good spatial uniformity.

The daylight factor is one possible measure to characterize a room's level of illumination by daylight. According to [1] its value must reach 0.75% at the most adverse point for living and working rooms. If overhead lights are used, a percentage of 4% must be reached to prevent the room from being sensed as dim.

On the one hand, natural light supply of buildings depends on the size of glazed areas in outer walls and of their orientation. On the other hand, daily and annual variations have to be taken into account. Direct sun penetration can cause significant heating of rooms which - dependent on the ambient temperature - can be desired or must be avoided. Devices for shadowing can affect these conditions in a suitable way. Furthermore, they protect users from glare on low sun elevation.

Light conducting systems can avoid glare as well as a room's illumination uniformity can be improved.

Figure 2: Example of a light conducting system.

Artificial light

To guarantee a steady minimum light supply for the users of a building, electric light must supplement daylight. For certain purposes the demand for light can be very high, therefore artificial light is indispensable. Examples are working places of precision engineering and medical examination and surgery rooms.

Other buildings that keep light sensitive objects may need to be fully protected from daylight. This applies for some museums and for the area of photographic image processing. Such rooms have to be illuminated by special lamps.

Purpose Nominal illumination
in Lux
Halls and floors of buildings 50
Service rooms, lavatory / restrooms 100
Reading rooms, canteen 200
Heavy industry 50 - 500
Offices 300 - 500
Open-plan offices 750 - 1000
Precision engineering 1000
Jeweller, watchmaker, artistic work 1500 - 2000
Table 1: Nominal illumination of working places according to [2], part 2.

Besides of a sufficient level of illumination, the optimal lighting of rooms must take further important aspects into account:

Table 2 can serve for a rough estimation of the necessary light power [4]. For other than neon lamps the correction factors of table 3 must be applied.

Nominal illumination in Lux 1000 750 500 300 200 100 50
Lamp appr. 2 m above working plane 50 38 25 15 10 5 3
Lamp appr. 3 m above working plane 60 45 30 17 11 6 3
Lamp appr. 4 m above working plane 64 48 32 19 13 6 4
Table 2: Estimation of required number of (neon) lamps for a given illumination.


Type of lamp Factor
Lightbulb 4.0
Halogen lamp 1.6
Neon lamp 1.0
Mercury vapor high pressure lamp 0.8
Indium amalgam lamp 0.6
Sodium vapor high pressure lamp 0.5
Halogen metal vapor lamp 0.5
Table 3: Correction factors for different kinds of lamps.

Neon lamps are often used in buildings because they have a lot of advantages: relatively good efficiency, this means low loss and a long lifetime at long burning periods. This kind of lamp uses vaporized mercury which is activated to emission of light between two electrodes in a closed tube. A luminescent material on the inside of the tubes converts invisible radiation to visible light. Disadvatages of these lamps are the shortening of lifetime for short burning periods, high power consuption during warm up and only moderate color reproduction.

The so-called compact luminescent lamps ('energy saving lamps') use the same principle as the above mentioned neon lamps but have a clearly longer lifecycle than those. Furthermore they do not need more energy on start up than for burning. Compared to bulbs they are energetically clearly superior. A technically similar device is available for upgrading already mounted neon lamps: it replaces the conventional glow lamp starter. The lifecycle of a neon lamp is extended threefold.

Lightbulbs are filled with inert gas which protects the glowing filament. This kind of lamp posesses a good color reproduction but a very bad efficiency in turning electric energy into light. This produces high heat loss and the lifecycle also is very short. Bulbs have a significantly higher power consumption on start up compared to burning time.

The halogen lamp is a further development of the classic lightbulb. A special gas within the bulb allows much smaller dimensions. In addition, the lifecycle is also extended but this holds only true for burning at maximum power. By dimming, the burning temperature and lifetime are reduced, possibly beyond that of a normal bulb. Halogen lamps are advantageous from a designer's point of view because of their compactness and the almost spot like light source, but they are no energy saving lamps.

Type of lamp Efficiency
in Lumen per Watt
Energy class** Lifetime
in hours
Color temperature
in Kelvin
Neon lamps 70 - 100 A 7500 - 20000*  3300 warm white
 4200 neutral white
 5000 daylight-white
Compact neon lamps 'energy saving lamps' 45 - 60 A, B 10000
Low and high voltage halogen lamps 15 - 20 C, D 2000 2800 - 3200
Bulbs 5 - 15 D, E, F 1000 2600 - 3000
* with electronic starting device instead of conventional glow lamp starter
** The EU energy saving label shows the energy efficiency class: Letters A and B mean low energy consumption, C and D mean medium and E and F mean high energy consumption.
Table 4: Important characteristics of different kinds of lamps.

Literature / references

[1] DIN 5034 - Tageslicht in Innenräumen; Deutsches Institut für Normung e.V. Berlin; Beuth Verlag, 10772 Berlin

[2] DIN 5035 - Beleuchtung mit künstlichem Licht; Deutsches Institut für Normung e.V. Berlin; Beuth Verlag, 10772 Berlin

[3] DIN 5031 - Strahlungsphysik im optischen Bereich und Lichttechnik; Deutsches Institut für Normung e.V. Berlin; Beuth Verlag, 10772 Berlin

[4] Verordnung über Arbeitsstätten, Verlag W. Kohlhammer, ISBN 3 17 0053809

[5] Bell, James and Burt, William; Designing Buildings for Daylight; Construction Research Ltd.; Watford; ISBN 1 86081 026 8