From colour temperature to light colour

For humans, the light colour given by the sun and the sky is always ideal. Special cells on our retina in the eye register the intensity and colour temperature and set the internal clock accordingly. Daylight dynamics are also key to an active and fulfilled life. The further north our location is from the equator, the greater the seasonal variation in daylight duration. At the equator, for example, there is only a variation of a few minutes, whereas north of the Arctic Circle the seasonal variation between polar night and midnight sun is 24 hours. The human body faces a great challenge dealing with this. After all, a person can only be successful if a nocturnal sleep phase alternates with an activity phase during the day. This synchronisation to the natural light conditions takes place continuously and results in corresponding hormone releases in the body.

The cradle of mankind is Africa, so there are natural limits to seasonal adaptability. The human body cannot adapt to the polar night and midnight sun. We need artificial lighting during the polar night, and dimming during the midnight sun. Even in central Europe, especially in spring, the rapid increase in day length is exhausting and is known to us as spring fever. Darkening in summer, during the night, and artificial lighting during the day should be selected such that our synchronisation system can adapt well to it. A moderate day length of 10 hours in winter and 14 hours in summer, plus about 40 minutes of twilight in each case, does not pose our synchronisation system a major challenge. This moderate seasonal variation in day length is recommended for people who spend most of their time indoors. In hospitals, a daylight pattern set in this way helps patients recover quickly. It enables residents of care homes to organise their day as actively as possible. The light colour also changes continuously with the length of the day. The range extends during twilight from supercold white at 30000 Kelvin to warm white 3000 Kelvin at sunset. Campfires, candles, and the incandescent lamps we still know well have even warmer colour temperatures.


All the light sources mentioned so far have colour points that are on the Black Body Line (BBL). These cool white to warm white colour temperatures are pure white and have no colour admixture such as greenish, yellowish, or pink. Colour admixtures are known from artificial light sources such as fluorescent lamps, metal-halide lamps, and LEDs. They have an unattractive effect and should be avoided if possible.

Even when selecting LEDs, care should be taken to ensure that they are sorted as close to the BBL as possible. Furthermore, the LEDs should have colour points that are as close to each other as possible, so that no colour admixing can be detected. For panel luminaires, XAL uses a three SDCM (Standard Deviation of Colour Matching) sorting. The LEDs are within a three-step ellipse. The many LEDs mix to form a colour point, such that almost no differences are discernible between individual panel luminaires. For single spots, the colour points must be even closer together to avoid colour differences. Two SDCM sorting is used here.

Colour temperatures up to 3300 K are described as warm white. From 3300 to 5300 K one speaks of neutral white; colour temperatures above 5300 K are called cold white.

Light colour for offices

To support the inner clock's synchronisation, we need a cylindrical illuminance at eye level of 250 MEDI lux (Melanopic Equivalent Daylight Intensity) for > 4 hours in the morning. This corresponds to ~ 370 lux, with a melanopic effect factor MR=0.75. The colder the colour temperature, the higher the melanopic effect factor. This means the required 250 MEDI can be achieved more energy-efficiently with a cool white light colour. However, an investigation by the Technical University of Darmstadt has shown a preference indoors for under 5500 K. Despite the higher melanopic effect factor, colder colour temperatures than 5500 K should not be offered for extended periods. The DIN T/S 67600 standard for biological lighting effects states that before and after this 4-hour activation phase, the colour temperature should drop to 2700 K at the end of the day and should not exceed 50 lux at night.

Although a daylight dynamic has clear advantages for the performance of users and thus also for employees, the constant light colour 4000 K is usually used in offices. For rigid working hours, like in government or administration, 4000 K can be a moderately good solution. If the working hours are very flexible, as in graphic design or architectural offices, dynamic daylight is preferable.

Light colour for retail

Shop operators want to use light to present their goods such that the customer has the feeling of getting good value for the advertised price. A specific light colour is therefore necessary, depending on the product group. The degree of specialisation is greatest in food retailing. Separate light colours for sausages, fresh meat, cheese, and pastries are available. For fresh fish, a distinction is made between white and red fish.

Spectrum of light colours and their area of application
Spectrum of light colours and their area of application

In supermarkets, 3000 K is usually chosen for packaged goods of all kinds. A good mixture of blue and red wavelengths makes the packaging look colourful and the goods in the display window look fresh. Technical goods, for example in electronics retailers, are usually illuminated at 4000 K.

For jewellery, a distinction is made between 2700 K for gold with reddish amber-coloured stones and 6500 K for silver colours with diamonds and bluish stones. For a mixed inventory, 3500 K is a good solution. 3500-4000 K is a popular choice for business fashion, allowing high-quality presentation of white textiles and muted fabric tones. If black and white contrast predominates, the product-specific light colour "brilliant white" can be used. White fabrics appear white as a sheet, black fabrics appear deep black. For predominantly colourful casual fashion, the "vogue" light colour is recommended, with a colour temperature of 3250 K. Colours are thus rendered particularly vividly and white fabrics bright white.

The light colour "brilliant white" has additional violet wavelengths. In addition to the very good colour reflection, "brilliant white" stands out above all with its excellent white reproduction. Nuances of white textiles or paper types can be distinguished under "brilliant white" lighting.

The strength of the "gallery" light colour, on the other hand, lies in the yellow-red range. Warm colour tones achieve an incomparable colour saturation with "gallery". With the small disadvantage that white surfaces appear slightly yellowish.

Light colour for living

We spend just as much time at home as we do in the office, albeit mainly at off-peak times.

For the first and last look of the day, for example in the bathroom mirror, the light colour should be very warm white. A neutral white colour temperature is ideal for applying make-up in the morning. In the kitchen, the recommended colour temperature is 3000 K; food appears colourful and fresh. The colour temperature can be 2700 K at dinner. When chilling afterwards, the light colour can drop to very warm white. Very few people have a smart home lighting control system. But even a mix of several luminaires that switch according to the occasion can craft a pleasant atmosphere that corresponds to the time of day. Combined with some Tunable White and Colour Warm Dimming luminaires, you can create a feel-good atmosphere from the bathroom to the lounge area.

A precise analysis of the space to be illuminated and the choice of the appropriate light colour is therefore crucial for successful lighting.