Quantity vs. quality of LED lighting. Evaluation measures with consideration of physiology of vision, efficiency, and photobiological safety.

LED technology and its application in luminaires pose a new challenge for quantitative and qualitative evaluation of lighting, taking consideration of its influence on the comfort of performing visual tasks, on photobiological effects, and on photobiological safety. The field of LED lighting is in a particular way interpenetrated by issues related to the design, construction, light efficiency, psychology, and physiology. This leads to the need of introducing new useful measures and standards, enabling us to take a specific measure or make a specific evaluation, and determine equipment used to address such needs.

It seems obvious, that illumination which has direct influence on people should be evaluated for its impact on visual and cognitive processes, on memory, state of readiness and performance, circardian rhythm, sleep quality, and general health status. Regretfully, our knowledge on how to determine and measure positive or negative impact of illumination on these processes is still very limited and is the subject matter of
many studies and R&D projects. In addition, the evaluation must also take into account the illuminance levels, exposure time, and spectral power distribution.

It also has to be recognized that the design and application of lighting systems should take into account other matters, seemingly unrelated to heath and biology, like spectral power distribution, proper color rendering, and practical aspects related to the manufacturing processes, cost price, electrical efficiency, etc.

What should be considered then, when selecting lighting components and products in order to meet market demands and user needs? To better understand available ways of verification and the factors influence humans, as well as quality and quantity measures, we can distinguish three, somewhat overlapping, categories:

  1. Vision related quality measures,
  2. Photobiological safety related measures,
  3. Photobiological efficiency related measures.

The highest number of available studies and measures which help better evaluate  LED illumination products are related to vision.
The most obvious ones are the well known photometric and colorimetric values, specifically:
– illuminance and luminance [ lx i cd/m^2 ],
– colorimetric values, including color temperature closest to CCT,
– Ra color rendering index (CRI) and the new color fidelity index Rf.

New color rendition index
In the last few years, due to LED light specific spectral distribution, some traditional measures were replaced with others, and some were expanded. At the end of April 2017, the CIE (International Commission on Illumination) published a new standard, i.e.: CIE 224:2017 Colour Fidelity Index for accurate scientific use about a new color fidelity index called Rf. This index takes into account the specificity of LEDs as a source of white light. The commonly recognized and earlier used color rendering index CRI has limitations when evaluating LEDs, which in recent years led to controversies on this matter. The lighting market demanded an update or expansion of the method. The rendering of colors depends on the light spectrum distribution, which is the amount of radiation within a specific wavelength which reaches out vision after being reflected from illuminated objects. This allows us to see colors and shapes. If the light consists of only specific wavelengths, corresponding to specific colors, the perception of colors 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 color index at that time, did not take into account the specific radiation from LED sources. CIE’s new publication presents the Rf index as a
supplementary index for scientific purposes. This addition is largely in keeping with publication TM-30 by IES (Illumination Engineering Society of North America) published in 2015. For the purposes of developing the new methodology thousands of different LED sources and lamps were analyzed, and the number of color samples was increased from 8, used to calculate the Ra, 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 color temperature. The new index has been introduced as an addition to Ra ( Rendering index averaged ) for scientific purposes,
rather than its replacement, due to the fact that all standards about lamp functional requirements, e.g. the ECO regulation, refer to Ra (CRI). Thus, it would be difficult to introduce an immediate change. In addition, TM-30 does not take into account all color rendering aspects and it would be too early to announce the end of the Ra (CRI), and switch only to the new index. Nevertheless, it is a very good step towards adapting standards to available technology and market expectations. Presently, all professionals from the lighting industry and demanding customers can compare and select LED products for color rendition quality in an additional and more objective way.

Optical radiation safety measures
With respect to photobiological safety we have the provisions of the existing IEC 62471 standard to evaluate photobiological safety of light sources and luminaires (including LED ones).

Photobiological effects have been studied for many years, to better understand their spectral efficiency function. Of special importance for human safety and health is an in-depth understanding of the photobiological impact of optical radiation on human eyes and skin.

The EN 62471 standard lists values of hazards defined with the use of three functions: spectral efficiency and hazards to eyes from near ultraviolet (UV-A), hazards to eyes from infrared radiation in the 780-3000 nm spectral range, and thermal hazards to skin caused by radiation in the 380-3000 nm spectral range. The values of hazards should be given either as values of irradiance or effective irradiance, or as values of effective radiance.

Given the very broad spectral range specified in the standard and the fact that such an evaluation is a very complex measurement process requiring specialized equipment and high qualifications, the practical application of the standard was very limited. Many manufacturers marketed their products only declaring compliance with the standard, or ordering only partial measurements in specialized laboratories.

LED lamps and luminaires used for general lighting and in industrial applications emit mainly optical radiation in the visible wavelength range. Consequently, differently from other types, LED lamps and luminaires create photobiological hazards only from blue light.

The methods of evaluating photobiological safety of light sources and luminaires, emitting blue light, were presented in technical report IEC TR 62778. The document gives a lot of practical information which may help understand measurement principles, simplification of measurement, and consequently – common application of the standard’s provisions. This may help improve the safety of products introduced to the market. Details of the methods and instruments were described in this post 

The principles of lamp classification into risk groups with respect to hazards were specified in the standard, and lamps and luminaires were divided into four risk groups:

  • risk-free group (RG0),
  • risk group 1 (low risk) (RG1),
  • risk group 2 (moderate risk) (RG2),
  • risk group 3 (high risk) (RG3).

To learn more about the available instruments please visit the GL Optic application page here 

Table 1 Blue light emission limits for individual risk groups

The evaluation of photobiological risk from blue light usually requires quite labor intensive measurements, however, existing correlations between photometric and colorimetric values vs. blue light hazard effective values in some cases permit a significant simplification of the measurement. The measurement result determines the risk group, RG0 or RG1, or – if the source of light or luminaire is classified as RG2 risk group – it determines the threshold illuminance Ethr.

The method described herein uses dependencies existing between values describing blue light hazards, and photometric and colorimetric values of a light source. Naturally, this method applies only to sources of white light. Having determined the color temperature of a light source closest to T cp , the result should be cross-referenced in Table 2 to find the corresponding threshold illuminance E thr .

Table 2 Conservative estimation of E thr as a function of color temperature closest to T cp

Human Centric Lighting
The impact of illumination on human biological functions, including the circardian rhythm, is a new topic, presently under in-depth analysis. The very fast growing in popularity concept of Human Centric Lighting or Circadian Lighting, in one of its assumptions, uses the influence of changing color temperature and the illuminance value on the human alertness and performance. With the discovery of a new receptor, i.e. the ipRGC (Intrinsically Photosensitive Retinal Ganglion Cells) the era of evaluating the influence of non-image forming mechanisms on the human organism began. The process of melatonin secretion and suppression in the human organism is responsible for the change of human activity, and is stimulated by exposure to light of a specific wavelength (not a color temperature). On the basis of studies, several different effectiveness curves were published in the recent CIE S026 standard.

In the US, the WELL organization studying issues of designing buildings, including lighting that takes into account human needs, published a standard showing a graph of spectral effectiveness of suppressing the secretion of melatonin EML (Equivalent Melanopic Lux). Also technical report CIE TN 003-2015 on neurophysiological photometry forms a basis to standardize the methodology of measuring biologically effective illumination.

By using the available methods we can install conversion tables in measurement equipment and calculate the EML intensity, verifying in this way what kind of light will have the most effective influence on the human organism.

However, it must be remembered, that color temperature is not enough. According to the rule of additive color mixing, there are many ways of obtaining the same color resulting from mixing different constituents. Therefore, even if we are able to obtain a specific color temperature, it may not necessarily be the most efficient one to influence the melatonin level. For example, if by mixing RGB colors we obtain white light with a cold temperature, it may turn out that specific wavelengths are missing in the distribution, and this light will only seemingly “look” like biologically efficient. Read more about the available spectral light meter offered by GL Optic here

Therefore, what is decisive in suppressing the secretion of melatonin, consequently influencing work productivity and concentration, is the value of optical radiation of a specific wavelength. One should also bear in mind the possible negative impact of artificial illumination on the human organism which during the day also requires the right amount of rest and sleep. An imbalance in that respect may also lead to very serious health problems.

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