Effect and correction factors for biologically effective lighting

To plan biologically effective lighting, the effect and correction factors must be known. The table on the right shows the weighting of the v λ- and Smel λ-function in numbers.

Visual sensitivity curve v λ (green) and melatonin suppression curve Smel λ (blue).
Visual sensitivity curve v λ (green) and melatonin suppression curve Smel λ (blue).

Quick info:

MR is the abbreviation for Melanopic Ratio.
The melanopic effect factor describes how much of the intensity in the spectrum evaluated with v λ is also in the spectrum evaluated with Smel λ is also in the spectrum evaluated with Smel λ.

MDER is the abbreviation for Melanopic Daylight Equivalent Ratio.
The melanopic daylight equivalent effect factor takes into account, in addition to the melanopic effect, the different intensity of the melanopic and visual effect curve is present in standard daylight D65.

To convert from MR to MDER, multiply MR by the constant daylight correction factor 0.906: MR x 0.906 = MDER

555nm wavelength light appears brightest. 490nm wavelength light is biologically most effective.

The light spectrum is measured to evaluate a light source's light. The light meter determines an intensity value for each individual wavelength between 380 and 780nm. The 400 values are collated as the measured radiation value in mW/m². This radiometric quantity takes into account the total radiation. The human eye, however, does not perceive all wavelengths with equal brightness. The human brightness function v λ is used to convert the radiometric quantity into a photometric quantity. This determines a subjective, human value from a technical value. The intensity remains constant at 555nm with this conversion. All other wavelengths' values are devaluated according to the v λ curve.

Example 1: Typical melanopic effect value for a 2700 K LED Ra ≥ 80. MR = 0.45
Example 1: Typical melanopic effect value for a 2700 K LED Ra ≥ 80. MR = 0.45

Room surfaces' illuminance can be calculated with lighting calculation software using the luminous flux in lumens thus evaluated. This evaluation of the measured spectrum is also used to determine the biological effect of light. The maximum shifts from 555nm to 490nm from the visual effect curve to the biological effect curve.

The standard light calculation software only calculates the visual illuminance. The melanopic illuminance is determined from the visual illuminance using the light source's melanopic effect factor. The melanopic effect factor can be found in the luminaire data sheet and is abbreviated as MR (melanopic ratio).

Example 2: Typical melanopic effect value for a 4000 K LED Ra ≥ 80. MR = 0.75
Example 2: Typical melanopic effect value for a 4000 K LED Ra ≥ 80. MR = 0.75

Only the illuminance striking the eye is relevant for the biological effect. This is specified as cylindrical illuminance (Ez). DIN/TS 67600 requires a cylindrical illuminance of Ez > 250 MEDI Lux as the biologically effective lower limit. MEDI= Melanopic Equivalent Daylight Illuminance.

How do you calculate the visual illuminance needed to ensure the required 250 MEDI?

Conversion from daylight equivalent illuminance (Ev,mel,D65) to visual illuminance (Ev) The 250 MEDI are divided by the light source's melanopic effect factor and then divided by the daylight effective factor 0.906.


For the biologically effective 250 MEDI at the colour temperature of 2700 K, much more luminous flux is needed than at 4000 K. The biological effect is strongly dependent on the light's colour. Warm white light is not very suitable for achieving a biological effect, see example 1. The devaluation due to the melanopic effect factor is too high. A natural white light colour < 5300 K is suitable, which naturally predominates in the morning hours that are important for waking.

If the lighting system is equipped with a control system and Tunable White luminaires, a biological effect can be achieved. However, this natural light colour should only be used in the morning hours; from midday onwards, the light colour should become increasingly warmer towards the evening.

The biologically effective illuminance of 250MEDI refers to an average 32-year-old standard observer. The biological effect of light is also relevant for older people. There are two correction factors to achieve the same effect in older people (DIN/TS 5031-100).

1. Age-dependent correction factor for the human eye's spectral light transmission

With increasing age, the human eye's light transmission decreases as the eye’s lens yellows. Therefore, less biologically effective light reaches the retina in older people.

2. Age-dependent correction factor for the pupil diameter of the human eye

Older people's pupil diameter is smaller than younger people's. Therefore, less light falls on the retina.

Since these two factors can be multiplied, this effect can be represented by a single factor.

3. Age-dependent correction factor for spectral light transmission and pupil diameter

These requirements vary considerably according to age. The above factors should be considered when planning lighting for offices or lounges in nursing and retirement homes.

Example 2) mel. Effective factor 0.75:

A 32-year-old needs 368 lx

For a 25-year-old 368 lx/1.145 = 321 lx

For a 50-year-old 368 lx/0.664 = 554 lx

For a 90-year-old 368 lx/0,193 = 1907 lx

Luminaires with a high direct component are primarily suitable for illuminating horizontal surfaces. This makes them less suitable for Human Centric Lighting-lighting designs. The high cylindrical illuminance levels required in HCL lighting concepts require a different choice of luminaire.

Which light distribution curves can be used to achieve this high cylindrical illuminance?

Wallwashers, ceiling floods, circular, and linear direct/indirect luminaires with wide indirect light distribution can be used for this purpose, see sketch.