Lighting Research Center Lighting Research Center
    Volume 12 Issue 1
October 2013    
application - The use to which a lighting system will be put; for example, a lamp may be intended for indoor residential applications. ballast - A device required by electric-discharge light sources such as fluorescent or HID lamps to regulate voltage and current supplied to the lamp during start and throughout operation. lumen depreciation - The decrease in lumen output that occurs as a lamp is operated, until failure. Also referred to as lamp lumen depreciation (LLD). color rendering index (CRI) - A rating index commonly used to represent how well a light source renders the colors of objects that it illuminates. For a CRI value of 100, the maximum value, the colors of objects can be expected to be seen as they would appear under an incandescent or daylight spectrum of the same correlated color temperature (CCT). Sources with CRI values less than 50 are generally regarded as rendering colors poorly, that is, colors may appear unnatural. color shift - The change in a lamp’s correlated color temperature (CCT) at 40% of the lamp’s rated life, in kelvin (K). ultraviolet - Any radiant energy within the wavelength range 100 to 400 nanometers is considered ultraviolet radiation (1 nanometer = 1 billionth of a meter, or 1 X 10-9 m). wavelength - The distance between two corresponding points of a given wave. Wavelengths of light are measured in nanometers (1 nanometer = 1 billionth of a meter, or 1 X 10-9 m) arc tube - An envelope, usually quartz or ceramic that contains the arc of a discharge light source. electrodes - The structure that serves as the electric terminals at each end of electric discharge lamps. high-intensity discharge (HID) - An electric lamp that produces light directly from an arc discharge under high pressure. Metal halide, high-pressure sodium, and mercury vapor are types of HID lamps. power quality - The degree to which current and voltage wave forms conform to a sinusoidal shape and are in synchronous phase with each other. Poor power quality results when the wave forms are distorted and/or out of phase and can interfere with data communications, cause inefficient operation or failure of other electrical equipment on the same supply line, and result in excessive current in electrical distribution lines. grid - The combination of electric power plants and transmission lines operated by an electric utility. restrike time - The time required for a lamp to restrike, or start, and to return to 90% of its stabilized light output after the lamp is extinguished. Normally, HID lamps need to cool before they can be restarted. shielding - Blocking an electric or magnetic field with a metallic substance. The incident field induces currents in the metallic substance, and these currents induce a field that opposes the incident field. Shielding reduces radiated electromagnetic waves. Electronic components, wires, lamps, and devices can all be shielded. visual performance - The quantitative assessment of the performance of a visual task, taking into consideration speed and accuracy. warm-up time - The time it takes for a lamp to produce 90% of its stabilized light output when it is started, unless otherwise indicated. high-pressure sodium (HPS) - A high-intensity discharge lamp type that uses sodium under high pressure as the primary light-producing element. HPS lamps produce light with a correlated color temperature (CCT) of approximately 2000 kelvins, although CCTs for lamps having higher CRI values range from 2200 to 2700 kelvins. Standard lamps have a CRI value of 22; others have CRI values from 60 to 80. HPS lamps are among the most efficacious light sources, with efficacies as high as 150 lumens per watt, although those with higher CRI values have efficacies as low as 25 lumens per watt. initial light output - A lamp's light output, in lumens, after 100 hours of seasoning. lamp - A radiant light source. luminaire - A complete lighting unit consisting of a lamp or lamps and the parts designed to distribute the light, to position and protect the lamp(s), and to connect the lamp(s) to the power supply. (Also referred to as fixture.) frequency - The number of cycles completed by a periodic wave in a given unit of time. Frequency is commonly reported in cycles per second, or hertz (Hz). correlated color temperature (CCT) - A specification for white light sources used to describe the dominant color tone along the dimension from warm (yellows and reds) to cool (blue). Lamps with a CCT rating below 3200 K are usually considered warm sources, whereas those with a CCT above 4000 K usually considered cool in appearance. Temperatures in between are considered neutral in appearance. Technically, CCT extends the practice of using temperature, in kelvins (K), for specifying the spectrum of light sources other than blackbody radiators. Incandescent lamps and daylight closely approximate the spectra of black body radiators at different temperatures and can be designated by the corresponding temperature of a blackbody radiator. The spectra of fluorescent and LED sources, however, differ substantially from black body radiators yet they can have a color appearance similar to a blackbody radiator of a particular temperature as given by CCT. efficacy - The ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt. electromagnetic interference (EMI) - The interference of unwanted electromagnetic signals with desirable signals. Electromagnetic interference may be transmitted in two ways: radiated through space or conducted by wiring. The Federal Communications Commission (FCC) sets electromagnetic interference limits on radio frequency (RF) lighting devices in FCC Part 18. horizontal illuminance - The average density of luminous flux incident on a horizontal surface, measured in footcandles (fc) or lux (lx). One fc equals 10.76 lx. mercury vapor (MV) lamp - A high-intensity discharge lamp type that uses mercury as the primary light-producing element. Mercury vapor lamps produce light with a CCT from 3000 to 7000 K. Mercury vapor lamps with clear outer bulbs have CRI values from 15 to 25, whereas phosphor-coated lamps have CRI values from 40 to 55. Mercury vapor lamps are less efficacious than other HID lamp types, typically producing only 30 to 65 LPW, but they have longer lamp lives and lower initial costs than other HID lamp types. metal halide (MH) lamp - A high-intensity discharge lamp type that uses mercury and several halide additives as light-producing elements. Metal halide lamps have better color properties than other HID lamp types because the different additives produce more visible wavelengths, resulting in a more complete spectrum. Metal halide lamps are available with CCTs from 2300 to 5400 K and with CRI values from 60 to 93. Efficacies of metal halide lamps typically range from 75 to 125 LPW. operating position - The manufacturer-recommended operating position for a lamp. power - The power used by a device to produce useful work (also called input power or active power). In lighting, it is the system input power for a lamp and ballast or driver combination. Power is typically reported in the SI units of watts. power line carrier (PLC) - A system that transmits high-frequency (50 to 500 kHz) analog or digital signals via the power lines of a building. These signals control devices such as luminaires or contain voice transmissions such as intercom messages. Some commercial and residential energy management systems also use power line carrier systems. glare - The sensation produced by luminances within the visual field that are sufficiently greater than the luminance to which the eyes are adapted, which causes annoyance, discomfort, or loss in visual performance and visibility. color rendering - A general expression for the effect of a light source on the color appearance of objects in conscious or subconscious comparison with their color appearance under a reference light source. sky glow - Brightening of the sky caused by outdoor lighting and natural atmospheric and celestial factors. ambient temperature - The temperature of the surrounding air that comes into contact with the lamp and ballast. Ambient temperature affects the light output and active power of fluorescent lamp/ballast systems. Each fluorescent lamp-ballast system has an optimum ambient temperature at which it produces maximum light output. Higher or lower temperatures reduce light output. For purposes of lamp/ballast tests, ambient temperature is measured at a point no more than 1 meter (3.3 feet) from the lamp and at the same height as the lamp. system efficacy - Also referred to as relative system efficacy, system efficacy is a measurement of a system'’s ability to convert electricity into light. Measured in lumens per watt (LPW), system efficacy is the ratio of the light output (in lumens) to the active power (in watts). rated lumen - Also referred to as rated light output from lamp in lumens. Lumen refers to a unit measurement of the rate at which a lamp produces light. A lamp’s light output rating expresses the total amount of light emitted in all directions per unit time. Manufacturers rate their lamps’ initial light output after 100 hours of operation. lumen maintenance - The ability of a lamp to retain its light output over time. Greater lumen maintenance means a lamp will remain brighter longer. The opposite of lumen maintenance is lumen depreciation, which represents the reduction of lumen output over time. Lamp lumen depreciation factor (LLD) is commonly used as a multiplier to the initial lumen rating in illuminance calculations to compensate for the lumen depreciation. The LLD factor is a dimensionless value between 0 and 1. uniformity - The degree of variation of illuminance over a given plane. Greater uniformity means less variation of illuminance. The uniformity ratio of illuminance is a measure of that variation expressed as either the ratio of the minimum to the maximum illuminance or the ratio of the minimum to the average illuminance. footcandle (fc) - A measure of illuminance in lumens per square foot. One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. nadir - In the lighting discipline, nadir is the angle pointing directly downward from the luminaire, or 0°. Nadir is opposite the zenith. rated lamp life - The number of hours at which half of a group of product samples fail. The rated life is a median value of life expectancy; any lamp or group of lamps may vary from the published rated life. Rated life is based on standard test conditions. driver - For light emitting diodes, a device that regulates the voltage and current powering the source. heat sinking - Adding a material, usually metal, adjacent to an object in order to cool it through conduction. spectral power distribution (SPD) - A representation of the radiant power emitted by a light source as a function of wavelength. rated light output - The sum of the initial rated lamp lumens of the lamp(s) that were supplied with the luminaire. chromaticity - The dominant or complementary wavelength and purity aspects of the color taken together, or of the aspects specified by the chromaticity coordinates of the color taken together. It describes the properties of light related to hue and saturation, but not luminance (brightness). gamut area - A measure of color rendering based upon volume in color space. It is the range of colors achievable on a given color reproduction medium (or present in an image on that medium) under a given set of viewing conditions. photopic - Vision mediated essentially or exclusively by the cones. It is generally associated with adaptation to a luminance of at least 3.4 cd/m2. CIE - Abbreviated as CIE from its French title Commission Internationale de l'Eclairage, the International Commission on Illumination is a technical, scientific, and cultural organization devoted to international cooperation and exchange of information among its member countries on matters relating to the science and art of lighting. standard deviation - A measure of the average distance of a set of data points from their mean. A set of data points that are all close to their mean will have a smaller standard deviation than a set of points that are further from their mean. MacAdam ellipse - Researcher David L. MacAdam showed that a just noticeable difference (JND) in the colors of two lights placed side-by-side was about three times the standard deviation associated with making color matches between a reference light and a test light (MacAdam 1942, Wyszecki and Stiles 1982). These JNDs form an elliptical pattern of "constant discriminability" in a chromaticity space, centered on the chromaticity of a reference light, known as MacAdam ellipse. fluorescent lamp - A low-pressure mercury electric-discharge lamp in which a phosphor coating on the inside of the glass tubing transforms most of the ultraviolet energy created inside the lamp into visible light. light-emitting diode (LED) - A solid-state electronic device formed by a junction of P- and N-type semiconductor material that emits light when electric current passes through it. LED commonly refers to either the semiconductor by itself, i.e. the chip, or the entire lamp package including the chip, electrical leads, optics and encasement. illuminance - The density of luminous flux incident upon a surface. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux. lamp life - The number of hours at which half of a large group of lamps have failed when operated under standard testing conditions. lumen (lm) - A unit measurement of the rate at which a lamp produces light. A lamp's lumen output rating expresses the total amount of light the lamp emits in all directions per unit time. This estimate of expanded uncertainty uses a coverage factor of k = 2, covering two standard deviations.

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.

 

LUXIM STA-41-01 Topanga APL400-4500-DK
CRI CCT CRI CCT
Orientation Rated Measured (+/- SD) Rated K Measured K (+/- SD) Rated Measured (+/- SD) Rated K Measured K (+/- SD)
Base Up 75 71 (0.2) 5871/
5200*
5950 (80) 70 69 (0.3) 4500 4930 (70)
90° Not specified 73 (0.1) 5815 5860 (30) Orientation not recommended
by manufacturer
Base Down Orientation not recommended
by manufacturer
Not specified 71 (0.3) Not specified 5580 (90)
* Rated CCT for base-up orientation per Product Bulletin LEP STA Series (LUXIM 2010b).

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.

GAI

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).

Tint

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.

Figure 11.

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.

 

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