Colour Rendering of Smart Glass for Healthcare Facilities

The colour rendering capabilities of smart glass facades in healthcare facilities can directly influence medical diagnoses that rely on a visual observation of the colour of human tissue.

What is Colour Rendering?

The colour rendering capability of a light source specifies the accuracy with which it can faithfully reproduce the colours of an object, when compared to a reference light source.

If you place a red apple in broad daylight, it looks red.

If you place that red apple under green light, the green will be entirely absorbed by the apple, and there will be no ‘red component’ to reflect back to your eyes. The apple will now look black.

Colour Rendering Index (CRI)

Light sources are measured by the Colour Rendering Index (CRI), expressed as a number from 0 to 100. 

The CRI is standardised by the International Commission on Illumination (CIE) in Austria.

Daylight has a CRI of 100 by definition, while most LEDs have figures between 80 to 95. One typical reference light source is daylight at 6500K colour temperature, called D65.

Does CRI apply only to lighting luminaires?

Colour rendering is applicable to natural and artificial light sources, and also applies to ‘modified daylight’, i.e. window facades, that transmit daylight into a building.

Window facades may incorporate certain ‘enhancements’, such as:-

  • reduced iron content (since iron gives a green colouration)
  • UV- or infrared-rejection interlayers
  • anti-reflectance coatings
  • switchable interlayers (which can change the transmittance of the whole glass unit)

In all of these cases, it is clear (pun intended) that the glazing will need to be characterised for its overall colour rendering, given its complex optical properties.

The Problem with Colour Rendering for Healthcare

The industry norm at the moment is for colour rendering to be specified by the CRI ‘General Index’ (called ‘Ra’) but Ra does not include a test for the colour red.

The problem for the medical sector occurs when diagnosing patient health from the direct observation of human tissue, such as skin, gums, eyes and nails, since underlying blood vessels contribute to their overall colouration. Any light sources which do not render red accurately could lead to incorrect diagnoses. In other words, the patient might seemingly look ‘off colour’.

Note also that the colour of an object can be changed by surface roughness or texture, and that human skin can produce subsurface scattering of light, further changing its appearance.

There is good news though, because there is an ‘Extended CRI Index’ (called ‘Re’) which includes red in its test swatch, and using this would lead to a more accurate assessment of colour rendering, assuming all other spectral properties are unchanged (such as colour temperature). 

Not everyone knows this though, and many manufacturers, even standards bodies, do not always use it.

At this time, the most common CRI method is Ra, which raises issues for how smart glass manufacturers can ensure colour quality for healthcare applications.

The Use of Colour in Healthcare Facilities

Older buildings often have more brick and less fenestration, leading to dingy, dark interiors lacking natural daylight. 

Conversely, newer builds feature floor-to-ceiling glazing which then encumbers the building with excessive cooling and heating loads due to the copious quantities of UV, IR and visible light entering the building.

Smart glass glazings can alleviate these issues of course by changing the optical properties of the glazing, in real-time, and based on building-integrated sensor networks. 

However smart glass manufacturers often do not publish third-party test data stating key parameters such as CRI, correlated colour temperature (CCT) or spectral power distribution (SPD) data. 

This issue is especially important for the healthcare sector. 

As stated by the Society of Light and Lighting (SLL), working in conjunction with the UK National Health Service (NHS):-

Colour perception is very important in a medical setting. Because the surgeon is looking mainly at red and yellow tissue, the sources of light must have a good mixture of red and yellow light if the surgeon is to see human tissue clearly. Most modern operating luminaires generally have a strong yellow and red content, and colour rendering values greater than Ra=90.

As you can see, even the SLL guide uses ‘Ra’ instead of ‘Re’. 

Whereas most operating rooms are devoid of natural light, this is not the case in other parts of the healthcare facility, such as in reception areas, circulation routes, wards and of course clinical areas.

The above SLL guide defines clinical areas in terms of where ‘surgical or medical procedures are carried out, usually by a surgeon or doctor’.

Examples include consulting rooms, treatment rooms and operating departments. According to the SLL, clinical areas must have a ‘minimum colour rendering of Ra=90 and a colour temperature of 4000K’. 

Colour Rendering of Smart Glass Samples

Research conducted by the Dublin Institute of Technology centres on measuring the CRI and the correlated colour temperature (CCT) of daylight when transmitted through a panel of smart glass. The technology used was Suspended Particle Devices (SPD) smart glazing, patented by Research Frontiers Inc (Nasdaq: REFR).

When the SPD smart glass panel was powered to under 14% transmittance, the CRI was less than 80 but when it was above 24% the transmittance reached 91. When maximally powered to 55% transmittance, the SPD smart glass panel showed a CRI of 95, a very satisfactory rating.

Conclusion

Architects, building services engineers and lighting designers need to insist on accurate (and preferably third-party) colour rendering test data, which uses the extended CRI ‘Re’ index (rather than ‘Ra’), as Re takes into account the colour red in its test swatch.

Using the CRI ‘Re’ value will ensure more accurate colour rendering of daylight transmitted through a smart glass building facade, reducing errors in medical diagnoses which rely on visual observation of human tissue.

References

1. “CIE International Lighting Vocabulary”, CIE S 017/E (2011), URL.

2. “Architectural Environment and its Effect on Patient Health Outcomes”. Lawson B, et al. , University of Sheffield, 2002, URL.

3. “Lighting Guide 2: Lighting for healthcare premises”, Society of Light and Lighting (SLL), CIBSE, URL.

4. “Let there be light – the importance of lighting and colour in hospitals”, Hospital Healthcare Europe, July 2010, URL.

5. “Lighting and Colour for Hospital Design” (2004), Hilary Dalke, NHS Wales, ISBN 0113224915, URL.

6. “Method of Measuring and Specifying Colour Rendering Properties of Light Sources”, International Commission on Illumination (CIE), Austria, URL.

7. “How Natural Sunlight Can Positively Affect the Operating Room”, Becker’s Hospital Review, URL.

8. “CIE Color-Rendering Index”, János Schanda, University of Pannonia (Hungary), URL.

9. “Interior colour rendering of daylight transmitted through a suspended particle device switchable glazing”, A. Ghosh, B. Norton, Dublin Institute of Technology, Ireland, URL.