The design and application of lighting systems should take into account not only illuminance levels but also spectral power distribution and proper colour rendering, which are connected to human wellbeing. The highest number of available studies and measures which help evaluate LED illumination products better are related to vision. The most obvious ones are the well-known photometric and colourimetric values, like illuminance and luminance, Ra colour rendering index (CRI) and the new colour fidelity index Rf
Standard meets standard
The CIE (International Commission on Illumination) recommended the procedure of measuring and specifying colour rendering properties of light sources, based on a test colour sample method in 1965. This method was updated in 1974 (CIE 13.2-1974) taking the chromatic adaptation shift into account. In 1995 CIE published a new standard CIE 13.3-1995, as a corrected version of CIE 13.2-1974. The standard has been brought up to date to present spectroradiometric and calculation practice and introduced CRI (Ra) parameter.
At the end of April 2017, the CIE published a new standard, CIE 224:2017 Colour Fidelity Index for accurate scientific use, introducing a new colour fidelity index called Rf. The index takes into account the specificity of LEDs as the source of white light. CIE’s publication presents the Rf index as a supplementary index for scientific purposes. This addition is largely keeping with the IES (Illumination Engineering Society of North America) publication of TM-30-15 from 2015.
The IES has recently issued TM-30-18, the IES Method for Evaluating Light Source Colour Rendition. It presents a comprehensive method for evaluating light source colour rendition. It is a revision of IES TM-30-15 containing three modifications that achieve consistency with CIE 224:2017. CIE 224:2017 defines the General Colour Fidelity Index (Rf), which is based on the Fidelity Index (Rf) calculation within IES TM-30-15, but with a few minor changes. These changes involve the extrapolation method used for the colour evaluation samples (CES), the transition zone for the reference illuminant, and the scaling factor for fidelity calculations.. With the issuance of TM-30-18, the measures have been harmonised so that the CIE 224:2017 Rf and the newly revised IES TM-30-18 Rf are identical. It is a key step towards broader acceptance of the index.
CRI (Ra) vs Rf
The commonly recognized and earlier used colour rendering index (CRI) has limitations when evaluating LEDs. The rendering of colours depends on the light spectrum distribution, which is the amount of radiation within a specific wavelength that reaches out vision after being reflected from the illuminated objects. This allows people to see colours and shapes. If the light consists of only specific wavelengths, corresponding to specific colours, the perception of colours from other ranges is limited or impossible. It should be noted that the CRI was developed over 40 years ago, when LEDs were not used for general lighting purposes. The studies and methodology of calculating the colour index at that time, did not take into account the specific radiation from LED sources.
For the purposes of developing the new methodology, thousands of different LED sources and lamps were analysed. The number of colour samples was increased from 8, used to calculate the Ra (CRI), to 99, used to calculate the Rf. This improved the universal nature of the index when applied to the evaluation of LED illumination with a different spectral distribution and different colour temperature.
The new index has been introduced as an addition to Ra ( Rendering index averaged ) for scientific purposes, rather than its replacement. The introduction can be contributed to the fact that all the standards on lamp functional requirements, e.g. the ECO regulation, refer to Ra (CRI). It is a very good step towards adapting standards to available technology and market expectations.
Our systems are future-proof
Our light measurement software GL SPECTROSOFT gives the user an opportunity not only to measure light, but also to experiment using the data collected. Testing (e.g. PAS/FAIL), analysing according to international standards (CIE, ISO, IES etc.), comparing with other data (reference window) and making calculations using different parameters (e.g. counting luminous intensity values) are available on one platform. Spectrosoft works as a plug-and-play tool for all our spectrometers, integrating sphere systems or goniometer systems.
Accurate quantification of the colour rendition characteristics of a light source is a complex problem. By the end of 2020 we upgraded the GL SPECTROSOFT (pc software) as well as the firmware onboard the GL SPECTIS Touch series to include the latest changes with respect to the IES TM-30-18. The software generates a colour vector graphic that indicates average hue and chroma shifts, and also helps with interpreting the values of Rf and Rg.
SOURCES
IES position statement about TM-30-18
lightguality.blog post about evaluation measures with consideration of physiology of vision, efficiency, and photobiological safety.
CIE 1 colour fidelity index for accurate scientific use
CIE 2 method of measuring and specifying colour rendering properties of light sources
