Visual color evaluation in a light booth

1. Standardization of visual color evaluation

Color perception is dependent on personal experience as well as illumination and surrounding observing conditions. As the ambient conditions are highly variable and not consistent at all, it is required to standardize common lighting situations. The illumination should be easily switchable in order to observe and avoid so-called "Metamerism", i.e. differences in color matching when lighting conditions change. In order to guarantee reliable testing and evaluation conditions, international standards specify testing procedures defining the following components. [1][2][6][7][10][12][13]

1.1 Observer

The observer must have normal color vision and should be properly trained in evaluating colors. To avoid eye -fatigue the color decision should be made within seconds. Additionally, small breaks must be taken between evaluations. As people describe color differently, the following order should be used for communication and documentation of color: Hue ? Chroma ? Lightness.

1.2 Object

The specimens should be flat and uniform in color, gloss and surface texture. Standard and sample shall be placed next to each other without any distance in between and be reversed from time to time. The preferred sample size is approximately 10 cm to 15 cm. The viewing distance between eyes and the specimen should be 50 cm. This distance corresponds to a 10° field of vision.

1.3 Surrounding

The visual field immediately surrounding the specimen as well as the ambient visual field, used to let the observer’s eyes rest, are of high importance for color appraisals. The interior of the light booth should have a matte grey surface, and the appraiser should wear neutral colored clothing to avoid disturbing chromatic reflections.

1.4 Illumination

The level of illumination at the color-matching position shall be between 1000 lx and 5000 lx, depending on the respective international standard. A diffuser panel shall normally be used to avoid direct reflection and ensure a uniform distribution of lighting over the entire inspection field.

2. Terms for characterization of light

Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word light usually refers to radiation that is visible to the human eye (wavelength: 400nm to 700nm). In order to characterize light emitted by a light source several terms are used: color temperature (CT) and correlated color temperature (CCT), color rendering index (CRI Ra) as well as spectral power distribution (SPD) and metamerism index MI_VIS.

2.1 Color Temperature (CT) and Correlated Color Temperature (CCT)

The concept of color temperature is based on the fact that the color of an object changes when being heated because the emitted radiation changes. According to ISO/CIE 10526:1991 (E) a color temperature Tc is defined as “The temperature of a Planckian radiator whose radiation has the same chromaticity as that of a given stimulus.” Simply explained, the color temperature is a temperature describing the color characteristics of visible light based on the temperature of a Planckian black-body. Figure 1 shows the CIE x,y chromaticity space. The black line in the graph pictures the chromaticity values of black-body light sources of various temperatures.[8] [11][12][13]
Color temperature is usually expressed in Kelvins (K). The Kelvin scale is a measure for absolute temperature (0°C=273K). Color temperatures over 5000K are defined as cool colors, with hues of bluish white, where lower color temperatures (2700K – 3000K) are defined as warm colors, with hues of yellow and red. Most natural light sources (e.g. sun, stars) very closely follow the Planckian locus. When sources are to be described, which do not emit light exactly matching the black-body radiator, the term Correlated Color Temperature is used. According to ISO/CIE 10526: 1991 (E) a correlated color temperature, Tcp is defined as “The temperature of the Planckian radiator whose perceived color most closely resembles that of a given stimulus at the same brightness and under specified viewing conditions.” [1][2][8][12][13]

2.2 Spectral Power Distribution

Color and color temperature are not unique parameters to describe a light source. Two light sources with the same chromaticity coordinates xy and color temperature might have a different spectral characteristic and therefore, create different color impressions. Thus, the most accurate way of characterizing a light source or an illuminant is by their spectral power distribution curve (short: SPD curve). [1][2][6][8][10][12][13]
The curve shows the radiant flux (amount of energy) emitted by the light source at various wavelengths across the visible spectrum. The SPD curve is normalized at the wavelength of 560nm so that the assessment is independent of the absolute level of illumination.

2.3 Color Rendering Index - CRI

The color rendering index is a quantitative measure of a light source and its ability to reveal colors of objects in comparison with an ideal or natural light source. The term CRI is often used on commercially available lighting products. By proper definition, it should be called Ra – the General Color Rendering Index, or Ri – the Special Color Rendering Index, corresponding to the number of test-color samples that are evaluated.
The CRI is calculated by comparing the color rendering of the test source to that of a defined source. For test sources with a CCT under 5000K a black body radiator is used as the defined source. For test sources above 5000K daylight (D illuminants) are used. The detailed calculation of Ri and Ra is explained in the technical report of CIE 13.3-1995. [4] The test method is using a set of eight (Ra) or fourteen (Ri) CIE-1974 test-color samples from an early edition of the Munsell Atlas. The first eight samples are moderate in saturation, cover the hue circle and are approximately the same in brightness. The other six samples provide supplementary information about the color rendering properties of the light source.

2.4 Metamerism Index – MI

CIE publication 51.2 describes a method to rate the quality of daylight sources. For the visible range the method uses five theoretical sample pairs, where each pair consists of the standard and metameric sample with a MIVIS = 0 for standard illuminant D65. The higher the mismatch between the MIVIS of standard illuminant and MIVIS of the daylight-simulating source, the worse is the quality of the light source. [5][13][14]
Pursuant to MIVIS for the visible range, the CIE also defined MI_UV for the fluorescent range, with three theoretical metameric sample pairs. The formulas for MI_VIS and MI_UV are shown below in Figure 2. Where ΔE_i (L*a*b*) and ΔE_j (L*a*b*) are the color differences between the i^th and j^th pair of metamers.

The classification of MI_VIS and MI_UV is done according to the following table.

As there are no actual D65 light sources available, the challenge is to develop a D65 simulator as close as possible to the CIE D65 illuminant. The quality of a simulator is objectively assessed with the CIE Metamerism Index MI_VIS, corresponding to quality classes A to E with class A being the most accurate simulation.

Figure 1 Planckian locus in the CIE x,y chromaticity space

Figure 2 Calculation of the metamerism indices MIvis and MIuv

3. Simulating CIE illuminants by artifical light sources

3.1 Definition light source and illuminant

When speaking about light sources and illuminants it is important to know what the difference between these two terms is. A simple and proper explanation comes from “Billmeyer and Saltzmann”, where a light source is explained as “Physically realizable light, whose spectral power distribution can be experimentally determined”. An illuminant is defined as a “Light defined by a relative spectral power distribution that may or may not be physically realizable as a source.”
The CIE (Commission Internationale de l’Éclairage) defined a number of SPD curves to provide reference spectra for colorimetric applications, the so-called CIE standard illuminants.

3.2 CIE standard illuminant A

Standard illuminant A was introduced by the CIE in 1931 and represents an incandescent tungsten filament lamp. Its relative SPD is that of a Planckian radiator at an approximate temperature of 2856K. To simulate standard illuminant A in a light booth, conventional light bulbs or nowadays quartz tungsten halogen lamps (CT˜3000K) are used (Figure 3).

3.3 CIE standard illuminant series F 

CIE defined 12 fluorescent lamp types, named F1 to F12, classified into three groups. They differ in the bandwidth and wavelength peak of their emission spikes. Consequently, different color rendering indices and color temperatures are achieved.

Among these twelve, F2 and F11 are most commonly used in the industry. F2, also named “CWF” (cool white fluorescent) has a CT of about 4000K and a CRI Ra ˜ 60. F11, also known under TL84, has a CT of about 4000K and a CRI Ra > 85. To simulate standard illuminant F series in a light booth, commercially available fluorescent tubes are used (Figure 4). [11] [16][17]

3.4 CIE standard illuminant D65

In 1964 CIE defined a series of daylight illuminants by numerous measurements of real daylight shown in the below table. Out of 622 measurements, the theoretical SPD curves of daylight from 330nm until 700nm were constructed.  CIE standard illuminant D65 is the most important representation of an average daylight at 6504K corresponding to a midday light in Western/Northern Europe, comprising both direct sunlight and the light diffused by a clear sky including UV component.

Figure 3 Simulation of CIE standard illuminant A with tungsten halogen lamp in byko-spectra pro

Figure 4 Simulation of CIE illuminants F2 and F11 with fluorescent tubes in byko-spectra pro

4. Simulation of D65 in a light booth

In order to simulate daylight, different types of light sources are used in commercially available light booths and luminaires for visual appraisals. Depending on how good the match between artificial light source and standard illuminant CIE D65 is, a sample pair can look very different.
In the following example, a sample pair is evaluated in light booth 1 with fluorescent tubes with a CT of about 6500K and a CRI Ra of about 96. The daylight simulation is an approximation of the CIE defined D65 with CIE MIVIS  Class B quality level (see Chapter 2.4). The difference between this sample pair is very significant under these evaluation conditions (Figure 5).
Next, the sample pair is evaluated in light booth 2, byko-spectra pro, using halogen lamps with a specially developed blue filter glass combined with an LED array. This combination creates an excellent simulation of CIE D65 without any spikes (see blue curve in Figure 6) resulting in a CIE MIVIS quality class A. Under these conditions our sample pair will show only a small difference like under natural daylight.

Figure 5 Daylight simulation with fluorescent tube

Figure 6 Daylight simulation with byko-spectra pro patented lighting setup

5. Summary: Best in class daylight with light booth byko-spectra pro

BYK-Gardner’s new light booth byko-spectra pro masters the challenge of bringing true daylight into the lab by a smart combination of filtered halogen lamps and an LED array. This combination guarantees best in class daylight simulation – tested according to CIE.
Additionally, the built-in sensors in the light booth permanently control the lamp performance and automatically adjust the voltage to guarantee lamp stability. In order to control the lamp condition and trigger lamp replacement, the actual color temperature, light intensity and lamp-operation-time are shown on the display of the light booth. To reduce maintenance interval, the class A daylight lifetime is extended to 600 hours.
Depending on a product’s usage, color needs to match under a variety of illuminations. To be prepared for metamerism the light booth byko-spectra pro offers daylight simulation with class A performance (D65), incandescent lighting (A), three fluorescent illuminants (CWF, TL84, U30) and a UV light for evaluation of fluorescent samples.
As diffused light is essential in the evaluation of solid colors, diffuser panels mix the light to ensure uniform lighting over the entire inspection field. To comply with international standards, the interior walls are painted with a matte light grey color to eliminate influence of the surrounding on the observer.
Technical performance is not the only key criteria for a light booth. Efficient and comfortable operation also play an important role for the user. The large color display not only allows the switching of illuminants, but also enables an easy menu guided operation. The included remote control allows switching illuminants from a distance of up to 10 m. An Auto Sequence Mode progresses automatically through a user-defined sequence of illuminants for hands free operation.
byko-spectra pro is available as a regular light booth and as luminaires. The luminaires can be hung from the ceiling as a set or as multiple units to outfit an entire color harmony room to evaluate system components or complete products, e.g. car bodies. Luminaires also have the same class A illumination specifications as the light booth, ensuring the needed precision in visual color appraisals.