Tunable White – biologically effective lighting – HCL

Tunable White

Tunable White refers to the luminaire's ability to produce warm-white and cool-white light colours. These two colour temperature circuits can, for example, be independently dimmed. The user selects their preferred light colour which stays set until it is readjusted manually. Tunable White therefore does not have a colour temperature that depends on the time of day. If a dynamic light is desired, a control system is needed.

Biologically effective lighting

... is described in DIN/TS 67600:2022 and in WELL BUILDING Standard, Circadian Light. It aims to completely suppress the sleep hormone MELATONIN during the morning. This boosts the CORTISOL hormone level, ensuring people are awake and efficient. For a standard working day from 8:00 – 16:30, there must be a cylindrical, melanopically weighted, daylight-equivalent light intensity of > 250 MEDI Lux (Melanopic Equivalent Daylight Intensity) for at least four hours in the morning at the observer's head position. A high indirect component and additional wallwashers can deliver this. Neither Tunable White luminaires nor circadian control are needed for biologically effective lighting. Let's look at the requirements in detail. The observer's head position is assumed to be H=1.2m for seated activity and 1.6m for standing activity.

"Cylindrical" means that four vertical measuring surfaces are inserted: in head position looking forward to the screen, left, right and back. Conveniently, this can be represented in a light calculation program with a cube of 30cm edge length. The lux values from the front, back, left and right are added and divided by four to obtain the mean value, the "cylindrical" illuminance.

Each light source has a specific melanopic effect factor (Melanopic Ratio = MR). MR expresses the different intensity within the visual human sensitivity curve v λ and the melatonin suppression sensitivity curve Smel λ in the spectrum of the light source.

"Daylight equivalent" means that the melanopic effect factor (MR) has been multiplied by the daylight effect factor of 0.906. This gives the Melanopic Daylight Equivalent Ratio (MDER). MDER is calculated from MR x 0.906. Just like MR, MDER is a luminaire-specific value. Dividing the required 250 MEDI by MDER gives the cylindrical illuminance needed to synchronise the inner clock. Example: 250 / 0.833 = 300 lx

In the XAL luminaire data sheets, the melanopic effect factor, abbreviated with MR, for melanopic ratio, as well as the daylight equivalent melanopic effect factor, abbreviated with MDER, for melanopic daylight equivalent ration, can be found.

A melanopic effect factor of 1.0 means that the visually assessed illuminance has the same biological effect as the melanopically assessed illuminance. A melanopic effect factor of < 0.4 has a relaxing effect; 0.4 - 0.8 for office, industry, gastronomy; > 0.75 recommended over the day for optimal concentration, cognitive work, light therapy.
A melanopic effect factor of 1.0 means that the visually assessed illuminance has the same biological effect as the melanopically assessed illuminance. A melanopic effect factor of < 0.4 has a relaxing effect; 0.4 - 0.8 for office, industry, gastronomy; > 0.75 recommended over the day for optimal concentration, cognitive work, light therapy.


Neither a Tunable White luminaire nor a time-of-day-dependent control system are necessary for biologically effective lighting. Tunable White luminaires with circadian control are only needed if the workplace operates beyond 8:00 – 16:30. DIN/TS 67600 contains exemplary light sequences that recommend a day length of > 8 hours in winter and < 14 hours in summer. Outside these times, the lighting level and color temperature should be significantly reduced.. This does not suppress the production of the hormone Melatonin, which is necessary for regeneration.

HCL - Human Centric Lighting

Human Centric Lighting means lighting that focuses on the visual, emotional, and biological needs of people.

The minimum visual requirements for people in workplaces are defined in EN 12646-1. Furthermore, the biological needs are described in DIN/TS 67600: in the morning for at least four hours > 250 MEDI Lux.

The emotional needs are psychological in nature. Below are some recommendations. It is important that nobody feels dazzled and that the work area is considered pleasant. Pointed-angle luminance contrasts like those produced by square LED panels have a disconcerting effect. Round luminaires with an indirect component, for example, are therefore recommended. Colours should be vibrant. The highest possible colour rendering is recommended. Bright wall and ceiling surfaces look pleasant during the day. Consequently, additional indirect wall and ceiling luminaires are required. By changing the light, a person is oriented regarding the time of day. Circadian lighting control makes this possible. Customisable intensity and colour temperature settings can have a positive effect on an individual's mood. Workplace dimmer switches make this possible. HCL lighting is not limited to visual and biological requirements. The light planner needs to have a sure instinct for the emotional aspects.

Examples of emotional lighting components are: Dynamic Daylight, Highest Colour Rendering, Full Spectrum LEDs, Double Dynamic Lighting (intensity and beam angle change), Natural Shape of Light Emitting Surface, Large Area High Illuminance on Wall and Ceiling Surfaces and Lighting Design according to the Limbic Group.

Colour and room design is an essential component in meeting the emotional needs of the user. Walls and ceilings with dark paints absorb incident light and dampen the sense of well-being, as do rooms without colours, which are kept in black, white, and grey. Colours appeal emotionally and are motivational. Long visual axes from the workplace relax the eye during visual tasks. This prevents myopia. Relaxing views of the outside sky and trees are to be ensured. However, lighting design is not capable of correcting significant errors in colour and room design. In such cases, the lighting designer should point out that the lighting system cannot meet emotional needs under these conditions.


Tunable White luminaires are not inherently biologically effective nor HCL. For biologically effective lighting, high melanopic, daylight-equivalent illuminance levels are needed. A lighting design that is coordinated, and can even be realised with one light colour, achieves this. Tunable White luminaires with circadian control and high melanopic illuminance levels meet the biological requirements for long office working hours. HCL lighting design requires the fulfilment of visual, emotional, and biological needs. Tunable White luminaires with circadian control and further measures to meet emotional needs are necessary.