The determination of colour differences when calculating the colour rendering index

The paper addresses the issue of determining the reference light source when calculating the colour rendering index (CRI) and highlights a longstanding recognition of the importance of accurately conveying the shades of illuminated objects by light sources. Certain fields, such as garment and textile factories, art workshops, museums, printing houses, beauty salons, and dental offices, necessitate high-quality colour rendering to successfully perform their work. Improving the quality characteristics of lighting installations is essentially a risk-based approach to operating conditions, including the provision of the conditions that meet current international occupational safety standards. In 1965, the Colour Rendering Index (CRI) was introduced as a method to assess the colour accuracy in light sources, now mandated for testing by international standards for the LED equipment manufacturers. The practical significance of the colour rendering index is based on the fact that the perception of lamp light by the human eye depends not only on quantitative characteristics (illumination, luminous flux), but also on qualitative characteristics, the negative indicators of which may affect the entire lighting system, that is, create the impression that “the lamp is shining, but does not illuminate”. As a result, this indicator has a direct effect on the energy efficiency of a lighting installation, where, to compensate for the “lack of light”, the power consumption of the installation is increased, instead of improving the quality of lighting, by replacing light sources with other sources with a high colour rendering index. A crucial aspect of the CRI calculation involves determining the reference light source based on the colour difference determination. The approach aims to minimize the uncertainty by studying a reference emitter with a colour temperature similar to the tested light source. However, this criterion neglects the heterogeneity of colour perception, although it eliminates the need to consider changes in the eye adaptation states. To address this limitation, having analysed the standards on colorimetry, a formula for calculating the colour difference was proposed for the first time. The formula accounts for the heterogeneity in the perception of colour space, providing a more comprehensive approach in calculating the colour rendering index.