Color Rendering Index of Smart Glass for Healthcare

Color rendering index (CRI) of smart glass influences medical diagnoses that rely on a visual observation of the color of human tissue.

What is Color Rendering?

The color rendering capability of a light source specifies the accuracy with which it can faithfully reproduce the colors 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.

Color Rendering Index (CRI)

Light sources are measured by the Color 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 color temperature, called D65.

Does CRI apply only to lighting luminaires?

No, in fact color 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 coloration)
  • 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 that the glazing will need to be characterised for its overall color rendering, given its complex optical properties.

The Problem with Color Rendering for Healthcare

The industry norm at the moment is for color rendering to be specified by the CRI ‘General Index’ (called ‘Ra’) but Ra does not include a test for the color 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 coloration. Any light sources which do not render red accurately could lead to incorrect diagnoses. In other words, the patient might seemingly look ‘off color’.

Note also that the color 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.

Extended CRI

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 color rendering, assuming all other spectral properties are unchanged (such as color 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 color quality for healthcare applications.

The Use of Color 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 color 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):-

Color 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 color 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 color rendering of Ra=90 and a color temperature of 4000K’. 

Color Rendering of Smart Glass Samples

Research conducted by the Dublin Institute of Technology centres on measuring the CRI and the correlated color 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.


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

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


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.

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