Implications for Red-Enhanced Surgical Lighting

The development of the Quasar Qi surgical lighting product, and specifically its red boost function, provides a practical driver for reviewing how added red spectral content may affect surgical visualisation. The addition of red content to a surgical illuminant should be evaluated as a spectral-design intervention rather than as a simple improvement in colour quality. Surgical fields contain blood, fat, vessels, instruments and tissue surfaces whose appearance depends on both the spectral power distribution of the light and the reflectance of the target. The literature therefore separates three questions: whether colours are rendered faithfully, whether observers prefer the appearance, and whether clinically relevant colour differences become easier to discriminate. These outcomes can align, but they can also diverge.

Colour Fidelity

Colour fidelity describes how closely object colours under a test source match their colours under a reference illuminant. Traditional surgical lighting specifications have relied heavily on the CIE general colour rendering index, Ra, because it provides a convenient indication that colours are not grossly distorted. Mundinger and Houser note that IEC 60601-2-41 requires surgical luminaires to meet colorimetric limits including Ra of at least 85 and CCT between 3000 K and 6700 K (IEC, 2000). However, they also stress that CIE Ra is a fidelity measure, not a complete account of surgical visibility (Mundinger & Houser, 2019).

The limitations of single-number fidelity metrics are central to CIE TC 1-91 (CIE, 2023). The report reviews methods beyond colour fidelity, including TM-30, the Colour Quality Scale, the Feeling of Contrast Index, memory-colour approaches and a skin-colour preference index. It explicitly states that these methods do not replace Ra or CIE 2017 Rf, but can provide a more complete description of colour rendition characteristics. Royer et al. similarly argue that TM-30’s Rf, Rg and hue-specific measures are useful because colour quality cannot be reduced to one number (Royer et al., 2020).

For red-enhanced surgical lighting, this means that R9 or red-local chroma information should be reviewed alongside Ra or Rf. A spectrum may increase red chroma, blood salience or perceived tissue richness while also shifting hue or reducing fidelity for other tissue colours. In a clinical setting, high fidelity is still desirable because clinicians use colour as information, but fidelity alone does not answer whether a particular tissue boundary, vessel or foreign object is easier to detect.

Colour Preference

Colour preference is a subjective judgement of how acceptable, natural, vivid or appealing a lit scene appears. It is relevant because a surgical mode that users dislike or find fatiguing is unlikely to be adopted, but preference should not be equated with diagnostic accuracy. Deng et al. found that lighting had a strong effect on colour preference across objects including fresh pork and skin tones, and that experimental protocol and object selection changed the strength of some conclusions (Deng et al., 2022). This is important for surgical lighting because a surgical field is a complex, textured scene rather than a set of isolated colour patches.

The meat-display literature is a useful analogue for flesh-like materials. Liu et al. showed that fresh-pork display lamps significantly increased perceived freshness and related sensory ratings compared with ordinary LEDs (Liu et al., 2024). This supports the general claim that spectrally biased lighting can alter the appearance and acceptance of red or flesh-like targets. However, the purpose of retail display is to improve appeal, while the purpose of surgical lighting is to support reliable visual judgement. A preferred appearance can therefore be beneficial for comfort and usability but is not sufficient evidence of clinical benefit.

Royer et al. further show why preference should be specified carefully (Royer et al., 2020). Their TM-30 preference criteria combine Rf, Rg and red local chroma shift rather than relying on fidelity alone. This is relevant to red-enhanced lighting because red saturation or chroma enhancement may be preferred up to a point, but excessive or uneven spectral emphasis may produce unnatural colour appearance. The design target should therefore be a controlled change in red-related appearance within a broader white-light spectrum, not a one-dimensional increase in redness.

Colour Discrimination

Colour discrimination concerns the observer’s ability to detect and separate colour differences. It is the most directly safety-relevant area for surgical lighting because surgical tasks often require differentiation between tissue types, perfusion states, blood, fat, vessels, sutures and metallic instruments. Bustamante et al. compared colour rendering metrics with Farnsworth-Munsell testing under seven light sources and found divergence between colour rendition and hue discrimination; their results also indicated that the lighting condition preferred psychologically was not necessarily the best for hue discrimination (Bustamante et al., 2021).

The same tension appears in Liu et al. (Liu et al., 2024). The pork-display lamps improved perceived freshness, yet colour discrimination performance measured with the Farnsworth-Munsell 100 Hue Test was poor under those lamps. The authors concluded that inappropriate spectral proportions can influence judgements of meat quality and weaken the ability to distinguish freshness level. However, note that this evidence used uniform hue caps for discrimination task rather than real flesh, textured tissue, blood or a complex surgical field where texture, gloss, spatial boundaries, luminance contrast and memory colour may all contribute to discrimination.

Surgical-lighting research supports task-specific spectral optimisation, but with caution. NIST’s early work proposed using tissue spectral reflectance and computational modelling to design illumination that enhances contrast between tissue of interest and surrounding tissue (Litorja et al., 2007). Wang et al. likewise framed surgical lighting as a problem of simultaneously optimising colour contrast and colour rendering (Wang et al., 2016). For pig-heart tissue, their unconstrained optimised spectrum emphasised wavelengths around approximately 470, 510 and 650 nm, and produced higher calculated tissue contrast than D65 or a 5500 K white LED (Wang et al., 2015). Mundinger and Houser found that variable CCT alone did not provide clear utility for surgeons, but their discussion supports the broader concept that empirically guided spectra may enhance discrimination for particular tissues or conditions (Mundinger & Houser, 2019). The key is that task performance must be tested directly.

Implications for Red-Enhanced Surgical Lighting

Adding red spectral content is plausible as a way to alter the appearance and salience of blood-rich or flesh-like targets, and preference studies suggest that such changes can be noticeable and sometimes desirable. However, colour-quality research also shows that fidelity, preference and discrimination are distinct dimensions. A red-enhanced mode should therefore not be described simply as improving colour rendering unless fidelity metrics and visual tests demonstrate that claim.

A defensible evaluation should combine physical metrics and psychophysical task tests. The physical side should include SPD, CCT, Duv, illuminance, Ra, R9, Rf, Rg and hue-specific local chroma or hue shifts, especially in the red region. The visual side should include observer preference, glare/comfort ratings, adaptation time, and surgical task performance using realistic tissue models or ex vivo tissue. Useful tasks could include distinguishing tissue boundaries, detecting vessels, identifying needles or sutures on blood-rich backgrounds, and ranking staged tissue or blood colour differences. Presentation order should be randomised and the red mode should be compared with an otherwise similar high-fidelity white mode at matched illuminance.

In summary, red-enhanced surgical lighting may be promising as an optional, task-specific visualisation mode. The strongest literature-based claim is not that red light automatically improves colour fidelity, but that carefully optimised spectra may improve selected contrast or salience tasks while maintaining acceptable fidelity. The preference-discrimination trade-off should therefore be considered: a field that looks more vivid or fresh may not provide better colour information. Future product evidence should therefore demonstrate improved task performance, not only improved appearance.