Volume 12 Issue 1
What are the color characteristics of plasma lighting systems?
CRI and CCT
Specifiers regard color rendering index (CRI) and correlated color temperature (CCT) as important color metrics for light sources (NLPIP 2004). Published information, as of June 2013, from LUXIM and Topanga show that commercially available plasma lighting systems have CRI values between 70 and 95 and CCT values between 3200 and 7650 K.
NLPIP measured the chromaticity coordinates of the two plasma lighting systems it purchased, at full power, and used these chromaticities to calculate CRI and CCT. The results are shown in Table 2. The LUXIM plasma lighting system was measured in base-up and 90° orientations, while the Topanga plasma lighting system was measured in base-up and base-down orientations. NLPIP conducted all measurements in the LRC's lighting laboratories, which are accredited by the National Voluntary Laboratory Accreditation Program (NVLAP Code: 200480-0).
The measured LUXIM system has a rated CRI of 75. NLPIP measured a CRI of 71 for the LUXIM plasma lighting system when operated in a base-up orientation, and a CRI of 73 when operated at a 90° orientation. The measured Topanga system has a rated CRI of 70 in the base-up orientation. NLPIP measured a CRI of 69 for the Topanga system when operated in a base-up orientation, and a CRI of 71 when operated in a base-down orientation.
According to LUXIM's LiFi STA 41 01 Data and Reliability Package, the measured LUXIM system had a rated CCT of 5871 K in the base-up orientation and a rated CCT of 5815 K in the 90° orientation (LUXIM 2010c). These rated CCT values can be compared with the CCT values measured by NLPIP of 5950 ±30 K for the base-up orientation and 5860 ±30 K for the 90° orientation, considering that the uncertainty of CCT measurements is approximately ±30 K for this type of spectrum. Both the rated and measured CCT values are lower for the 90° orientation by approximately similar amounts. The rated CCT of 5200 K in the base-up orientation reported in LUXIM's Product Bulletin LEP STA Series (LUXIM 2010b), however, is 750 K lower than that measured by NLPIP and 650 K lower than that reported in LUXIM's LiFi STA 41 01 Data and Reliability Package (LUXIM 2010c).
According to literature from Topanga, the measured Topanga system has a rated CCT of 4500 K in the base-up orientation (Topanga n.d.). NLPIP measured a CCT of 4930 ±30 K which was 430 K higher than the rated CCT in the base-up orientation and operating at full power. Operating the Topanga plasma lighting system in a base-down orientation increased the CCT to 5580 ±30 K, 650 K above the measured CCT in the base-up orientation, and 1080 K above the rated CCT in the base-up orientation.
Although differences as large as 1100 K in CCT for "white" light sources are noticeable, peripheral visual performance may not be measurably affected because the corresponding differences in scotopic/photopic (S/P) efficacy ratios can be small (ASSIST 2009).
Table 2: Rated and measured CCT and CRI values for two plasma lighting systems. Standard deviations (SD) for the measured values are also shown.
Table 2 shows that operating the plasma lighting systems with different orientations can change the CCT of these sources. Plasma lighting systems are not the only light sources to exhibit color shifts with different operating orientations. MH lamps can also exhibit color shifts when operated at different positions (NLPIP 2003). LED sources may also experience color shifts at different operating orientations if the orientation changes the operating temperature.
Gamut Area Index (GAI) is an adjunct measurement to CRI for assessing color rendering. GAI is defined as the enclosed area in the Commission Internationale de l'Eclairage (CIE) 1976 chromaticity space determined by the 8 CIE standard color chips used for CRI calculations relative to the enclosed area when the chips are illuminated by an equal energy spectrum (EES) (Rea and Freyssinier 2011). GAI was not reported by the plasma lighting manufacturers.
For the measured LUXIM system, NLPIP measured a GAI of 63 for the LUXIM plasma lighting system when operated in a base-up orientation, and a GAI of 64 when operated at a 90° orientation.
For the measured Topanga system, NLPIP measured a GAI of 51 when operated in a base-up orientation, and a GAI of 63 when operated in a base-down orientation.
Typically, CRI values of 80 and higher and GAI values between 80 and 100 indicate excellent color rendering (Rea and Freyssinier 2011).
Recently, the LRC determined that light sources that provide untinted "white" illumination do not fall on the blackbody locus (Rea and Freyssinier 2013), which may be contrary to popular belief. Research shows that for CCTs greater than 4000 K, light sources perceived as minimally tinted have chromaticity coordinates that lie slightly above the blackbody locus, while for CCTs less than 4000 K, light sources perceived as minimally tinted have chromaticity coordinates that lie well below the blackbody locus. The minimum tint locus, shown in Figure 11, is described by Rea and Freyssinier (2011).
As shown in Figure 11, the chromaticity coordinates of the measured plasma lighting systems are well above the minimum tint locus, indicating that these light sources will appear greenish-white (Rea and Freyssinier 2013). NLPIP confirmed, by visual inspection, that these light sources appeared to be greenish-white. Although, light sources that appear tinted (greenish-white) are noticeably different than light sources that are untinted, their effect on peripheral visual performance may not be measurably important unless the differences in S/P efficacy ratios are large (ASSIST 2009).
Figure 11. Chromaticity coordinates of the LUXIM and Topanga lighting systems at full power. Also shown are the American National Standards Institute (ANSI) chromaticity quadrangles used to specify tolerances for solid state lighting (SSL) products used in general indoor lighting applications (ANSI 2011). The size of each quadrangle corresponds to a range from 8-step to 13-step MacAdam ellipses.
At full power, changing the orientation of the LUXIM plasma lighting system changed the chromaticity coordinates by less than a 2-step MacAdam ellipse. This change is less than the 4-step MacAdam ellipse tolerance allowed for fluorescent lamps (ANSI 2001) and the chromaticity tolerance allowed for SSL products, shown in Figure 11. Changing the orientation of the Topanga plasma lighting system at full power changed the chromaticity coordinates by more than a 6-step MacAdam ellipse.