Integrative lighting: Keys to understanding its metrics

The importance of colour rendering: CRI vs TM-30

The colour rendering index (CRI) is probably the most widespread way of evaluating the colour rendering of a light source or luminaire. In other words, it is the capacity to qualitatively reproduce a colour, in relation to eight initial sample colours (Ra), reaching a total of 15 sample colours in the last update by CIE (the International Commission on Illumination) in 2004, and taking a standard source as a reference. In this extension, one of the shades considered is the red tone, expressed through the R9 index and introduced by the WELLTM standard. This will help us to understand how it will affect the perception of people's skin, among other elements.

However, this means of measurement was still incomplete. Because of this, in 2015, and then in a  subsequent revision in 2018 and 2020, the Illuminating Engineering Society (IES) published the IES TM-30-15 standard, a new colour measurement method that improved the CRI and R9. By introducing 99 colour samples of different types instead of eight, it was now possible to assess the colour fidelity and saturation of a light source more accurately.

This metric allows us to evaluate two dimensions of light:

• Fidelity Index (Rf). This is used to measure the proximity of a light source to a reference source. Similar to CRI, but with a higher precision, as it uses 99 colour evaluation samples.
• Gamut Index (Rg). This provides information on the average saturation of a light source.

Proper colour perception affects human beings mainly in terms of their nervous system. It is even more essential when it comes to visually demanding activities, such as artistic tasks and medical activities. 

However, we shouldn’t only think in terms of carrying out work activities successfully here. If we consider the world of education, we can also recognize that it is a very important factor in children’s cognitive development. This is because an adequate chromatic perception of their school environment will give them a better all-round educational experience and a better relationship with their surroundings.

The importance of spectral power distribution (SPD) over and above the choice of correlated colour temperature (CCT)

Colour temperature is a very relevant factor when it comes to creating a certain atmosphere and is usually related to the task to be performed, the lighting levels, the surroundings, and even the location.

However, when we look at the spectral composition of two light sources or luminaires with the same colour temperature, they can actually differ greatly.

The spectral power distribution (SPD) shows us either visually through a graph, or in a set of values, the power of each of the emissions in the different wavelengths that make up the visible spectrum of a light source or luminaire (between 380 and 760 nm). 

One of the most relevant factors to take into account when choosing the light spectrum is the efficacy, from a circadian stimulation point of view. This affects the non-visual aspects that influence the design of the circadian lighting.

New metrics in integrative lighting: Equivalent Melanopic Lux (EML) vs Circadian Stimulus (CS)

Equivalent melanopic lux

Equivalent Melanopic Lux (EML) is a metric used by the WELLTM building standard to measure the light indexes that are considered effective in helping us synchronise our internal clocks and circadian activation, especially during the early hours of the day. In order to make these calculations, we need to have the correct information regarding the spectral power distribution; this will give us a figure known as the Melanopic Ratio. This data, which is specific to the luminaire and not just to a light source, will help us to calculate whether the melanopic lux levels we are obtaining, measured on a vertical plane at a height of 1.2 metres, are enough to stimulate the ipGRC (intrinsically photosensitive glands). These are the photoreceptors responsible for processing the light stimuli that regulate our biological clocks.

These photoreceptors are most sensitive to a specific wave emission (480 nm), which is why it is so important to know the spectral information of the lighting solutions you are planning to use. This is the only way to ensure that the technology used is efficient enough from the point of view of circadian stimulation.

Circadian stimulus

Another institution committed to lighting research is The Lighting Research Center (LRC) at Rensselaer Polytechnic Institute. This institution has developed a different metric to help designers understand and use circadian lighting in purpose-built areas. This is called "circadian stimulus" (CS).

The scale for measuring circadian stimulus (CS) ranges from a minimum value of 0.1 to a maximum of 0.7, with 0.3 generally being the minimum value considered necessary during the first few hours of the day.

To measure the effectiveness of this type of lighting, experts measured the amount of melatonin in the saliva of people subjected to a given level of lighting. For a value of 0.1, no suppression of melatonin in saliva was observed, and the assumption is that lighting is not contributing to circadian activation, while 0.7 is considered a saturation value, and no variation was observed from this value upwards.

The main differences between one type of measurement and the other are that the circadian stimulus (CS) makes it possible to calculate the influence of different light sources in the same area, while the melanopic lux (MLE) calculates the influence of just one light source.

However, the use of the Wellbeing or Multispectral technologies available within Lamp's solutions would be the best option to cover this non-visual lighting dimension.

Flickering

This is the name of the effect produced by a visible and repetitive change in a light source’s luminous intensity. The frequency, shape and magnitude depend on factors such as the quality of the power supplied or the type of driver being used. 

Depending on each individual person’s sensitivity, and the type of activity they are carrying out, flickering can cause certain health effects in some people. It has been known to cause fatigue, decreased concentration levels, and even dizziness and headaches. 

To create lighting conditions that favour visual comfort, Well Certification takes this concept into account, based on the IEEE 1789-2015 standard "Recommended Practices for Modulating Current in High-Brightness LEDs for Mitigating Health Risks to Viewers".

All luminaires (except decorative, emergency, and other special-purpose luminaires) should meet at least one of the following flicker requirements:

• Non-LED luminaires. A minimum frequency of 90 Hz at all light output intervals from 10% to 100%.
• LED luminaires. “Low risk" flicker level of less than 5%, especially below 90 Hz, as defined by IEEE 1789-2015 LED.

Experts recommend that manufacturers provide supporting data using the Modulation (%) vs. Frequency (Hz) table included in the IEEE standard.