The first part of this miniseries (see Light 1/2020, pages 16-17) demonstrated increasing the general color rendering index of a white LED by adding one or two colored LEDs. This part explores improving the_{Ra of} light sources by using a filter that, in turn, attenuates the appropriate spectral region. The experiment again uses the method of shifting a Gaussian curve over the entire visible spectrum, but here its waveform is a band-stop filter and its characteristic is multiplied by the spectral power composition of the light source under investigation.

Filtration LED 840

For the first experiment, a commonly used light-emitting diode was chosen with the parameters shown in Table 1 and Figure 1. The initial filter parameters were determined empirically: an FWHM of 24 nm and a notch-bottom attenuation of 40%. For all filter offsets _λc from 400 to 760 nm in 1 nm increments, the parameters_Ra, _Rf (precision color rendering index according to CIE 224:2017),_TCP, the equivalent colorimetric deviation of the SCDM denoted by N (Δu',v'/0.001 1, see previous section), and the relative luminous flux φ were calculated at the product. In the first step, the criterion was only the maximization of Ra, which was achieved at _λc = 571 nm. Here, however, the colorimetric deviation N = 7.1 is too high. In the second step, the optimization therefore sought to maximize_Ra while keeping N ≈ 3, which was achieved for _λc = 582 nm. Detailed results are shown in Table 1 and Fig. 1. Here, the best place to filter is near the peak of the "phosphor" part of the light-emitting diode spectrum. At the cost of a 9% decrease in luminous flux, an increase in_Ra of 12 points was obtained. The real loss in the filter would probably be close to 20%, which is the usual reduction in luminous flux for an LED with_Ra > 90 compared to a comparable model with_Ra > 80. The spectrum can also be divided into intervals and a suitable combination of several band-stops can be sought.

Table 1 - Light parameters without and with correction

Fig. 1. Spectral characteristics of original (dashed) and filtered light

The advantage of the described filter is the possibility of additional improvement of colour rendering in existing or already operating luminaires with light sources in basic_Ra. The challenge may be its feasibility, stability over the lifetime of the light source and the limitations associated with the need for approximately perpendicular light passage.

An example of a successful implementation is the Verbatim Vx filter, which takes the form of a purple cover glass for a general illumination reflector. For LEDs with_Ra > 80 and a_TCP of 3,000 to 4,000 K, the filter increases_Ra by 10 points, and the manufacturer further states a 20 to 25% decrease in luminous flux, a_TCP change of approximately 10% and a lifetime of 50,000 hours [1]. In the illustration in Figure 2, the_Ra index has increased from 85 to 94.

Fig. 2. Approximate spectral characteristics of the Verbatim Vx filter, measured with permission of the exhibitor

The High End Systems TM-30/CRI dichroic filter, on the other hand, is designed to improve the colour rendering of professional LED stage lights. The manufacturer claims an improvement in_Ra from 75 to 90 and _Rf according to IES TM-30 from 71 to 80. In the illustration in Figure 3, the_Ra index has increased from 75 to 88.

Fig. 3. Approximate spectral characteristics of the High End Systems TM-30/CRI filter, reconstructed from [2]

Sulphur lamp

Another example of the use of a filter to improve colour rendering is the sulphur lamp. The discharge takes place in sulphur vapour (_S2) and is excited by microwaves (electrodeless). Its invention dates back to 1990, when a specific power of 100 lm/W (later 30% more) and the possibility of implementing small kilowatt units was an attractive alternative to high-pressure lamps. An example of the use of sulphur lamps was the lighting at the National Air and Space Museum in Washington. Sulfur lamp units have limited use as sunlight simulators (Plasma International) or stage lights (Hive Lighting). The spectrum of the discharge in sulfur vapor takes the form of a broad bell curve with a peak in the blue region, see Figure 4,_Ra is just below 80, but the light suffers from a very significant colorimetric deviation from the reference source (N = 27). Its correction and a small improvement in color rendition have opened up a wide range of applications for this unusual light source. The algorithms described in this and the previous part of the miniseries can be used to identify suitable spectral regions for_Ra enhancement. The replenishment option identified the most suitable region around 625 nm, which corresponds exactly to the enhancement of the sulfur lamp by the addition of calcium bromide (_CaBr2) described in [3]. However, the colorimetric deviation is still very high (N = 21). The filtering variant provides characteristics corresponding to the magenta filter used and additionally compensates for the colorimetric deviation (N = 2). The luminous flux loss due to the filter characteristic is 20 %. All three waveforms are shown in Figure 4.

Fig. 4. Spectral characteristic of uncorrected sulphur lamp, reconstructed according to [3], correction by magenta filter (green) and correction by added red light (red)

Conclusion

The described algorithms search the spectrum of the light source for the most suitable places for addition or filtering. Although the application of this knowledge has a number of practical limitations - in the case of filters, their feasibility, durability or directional properties, in the case of colour light supplementation, the perfection of the mixing of the components or long-term stability, and in both cases a certain loss of luminous flux, it offers a solution in cases where existing luminaires need to be corrected or a replacement light source with a high colour rendering index is not available. When optimising the spectrum, the progression of the colorimetric deviation from the reference source must be carefully monitored, as the colour rendering index is not very sensitive to it. The criterion function also includes the chromaticity temperature shift and the energy loss in the filter. The above procedures provide a quantitative framework for an intuitive view of what is 'missing' and what is 'present' in the spectrum of a light source in terms of colour rendering. The next part of the miniseries will be devoted to the announced experiments with lasers and with light-emitting diodes of different colours.

Literature:

[1] LED Lighting Product Catalogue. Verbatim Ltd, Mitsubishi Kagaku Media, July 2016.

[2] LIN, J. High End Systems Unveils TM-30/CRI Correction Filter for LED Automotive Lights [online]. 2016 [cited 2020-05-13]. Available from: https://tinyurl.com/y7hadgs5

[3] LENG, Y and D. A. MACLENNAN. Sulfur lamp with _CaBr2 additive for enhanced plant growth: KSC-11970. NASA Tech Briefs. 2000, 24(1), 20.

Author. Antonín Fuksa NASLI & Blue step
Published in Světlo magazine 4-5/2020