Standard Observer

The GIE Standard Observer. The CIE standard observer is a set of curves giving the tristimulus responses of an imaginary observer representing an average population for three primary colors arbitrarily chosen for convenience. The 1931 CIE standard observer was deterrnined for 2° foveal vision, while the later 1964 CIE supplementary standard observer appHes to a 10° vision a subscript 10 is usually used for the latter. The curves for both are given in Eigure 7 and the differences between the two observers can be seen in Table 2. The standard observers were defined in such a way that of the three primary responses x(X),jy(X), and X), the value ofjy(X) corresponds to the spectral photopic luminous efficiency, ie, to the perceived overall lightness of an object. 

CIE used the 1931 CIE standard observer to estabUsh a color representation system in which the hue and saturation could be represented on a two-dimensional diagram. Three tristimulus values X, Y, and Z are first obtained, based on the standard observer, so that the hue and saturation of two... 

Hunter L,a,b and Other Color Spaces. The CIELAB and CIELUV color spaces were the outgrowth of a large and complex group of interrelated early systems and have replaced essentially all of them except for the 1942 Hunter Y,a,b group of color spaces (3). This was the earliest practical opponent-based system which is still widely used. In this system, for illuminant C and the 2° standard observer ... 

Colorimeters. Also known as tristimulus colorimeters, these are instniments that do not measure spectral data but typically use four broad-band filters to approximate the jy, and the two peaks of the x color-matching functions of the standard observer curves of Figure 7. They may have lower accuracy and be less expensive, but they can serve adequately for most industrial color control functions. Examples of colorimeters are the BYK-Gardner Co. XL-835 the Hunter Lab D25 series the Minolta CA, CL, CS, CT, and CR series (the last of these is portable with an interface) and the portable X-Rite 918. 

Objective Evaluation of Color. In recent years a method has been devised and internationally adopted (International Commission on Illumination, I.C.I.) that makes possible objective specification of color in terms of equivalent stimuli. It provides a common language for description of the color of an object illuminated by a standard illuminant and viewed by a standard observer (H). Reflectance spectro-photometric curves, such as those described above, provide the necessary data. The results are expressed in one of two systems the tristimulus system in which the equivalent stimulus is a mixture of three standard primaries, or the heterogeneous-homogeneous system in which the equivalent stimulus is a mixture of light from a standard heterogeneous illuminant and a pure spectrum color (dominant wave-length-purity system). These systems provide a means of expressing the objective time-constant spectrophotometric results in numerical form, more suitable for tabulation and correlation studies. In the application to food work, the necessary experimental data have been obtained with spectrophotometers or certain photoelectric colorimeters. 

Once this fact was rccdized, it was understood that an average of what each person saw would have to be made if a standard system weis to be formed and promulgated. This led to the concept of the "Standard Observer". Thus, the research required to define and measure color took a completely different path from the original methods such as the Munsell Color Tree. 

In this case, the relative response of the observers are summed into a response called "THE STANDARD OBSERVER" and is normalized for easier usage. Now, let us examine the effects of colors (chroma) as perceived by the human eye. Keep in mind that each person perceives "color" somewhat differently from other persons. 

Since color matching is meant for humans, it is natural to define color in terms of an average, or "Standard Observer". Our first step is to build an instrument which contains three colored lamp sources, a place for the individual observer, intensity detectors, and a monochromator, as shown in 7.8.19. on the next page. 

The difficulty in setting up the initial system for color comparisons cannot be underestimated. The problem was enormous. Questions as to the suitability of various lamp sources, the nature of the filters to be used, and the exact nature of the primary colors to be defined occupied many years before the first attempts to specify color in terms of the standard observer were started. As we said previously, the Sun is a black-body radiator having a spectral temperature of about 10,000 °K (as viewed directly from space). Scattering and reflection... 

The result is finalized response curves for the Standard Observer, also called "Tristimulus Response curves", and is shown as follows ... 

However, these analogues are actually hypothetical. The reason for this is that it is nearly impossible to obtain optical measurement components, such as the source and the detector, whose response to light across the visible spectrum is flat (or nearly so). However, this is not an impossible task and we find that an excellent match can be obtained to the transmission functions of 7.8.21., i.e.-those of the Standard Observer. This is typical for commercially available instruments. Now, we have an instrument, called a Colorimeter, capable of measuring reflective color. 

This diagram shows the energy spectrum of a given source, coupled with a filter of defined transmittance, which is established by a detector of known spectral response, as modified by a standard source and modified to that of a Standard Observer. Once an instrument has been set up properly with the proper optical... 

When Munsell devised his color space, he did so on the basis of minimum observable color perception steps. But the problem with the Munsell System was one of reproducibility, which the CIE Standard Observer cured. In formulating a color match, one wants to be able to predict the correct concentration of colorants required, whose scattering and absorption properties are known, i.e.- the lightness, so as to match the sample submitted, starting with their spectrophotometric curves. In practice, this is not so simple, since two colors must have identical spectrophotometric curves to be exactly equal. It turns out that the human eye will identify the two colors to be equal if their spectrophotometric reflectances are reasonably close. Two colors may appear to be equal under Daylight illumination, but quite different under incandescent lamp illumination. These colors are known zus "metamers" and the phenomenon "metamerism". 

The CIELAB system (1976) strictly standardizes the light source and the observer. CIE recommends three standard sources, A is an incandescent lamp, and B and C are lamps provided with different two-cell Davis-Gibson liquid hlters that simulate noon daylight and average daylight, respectively. Since the main object of the system is to obtain colorimetric results for normal tri-chromats (people with normal color vision), the standard observer must represent the human population with normal... 

The A(A), y(A), and z(A) terms were derived by the CIE from data obtained in visual experiments where observers matched colors obtained by the mixing of the blue, green, and red primary colors. The average result for human observers were defined as the CIE 1931 2° standard observer, and the wavelength dependencies of these color-matching functions are illustrated in Fig. 6. 

Fig. 6 Color-matching functions Jc(A), and z(A) of the CIE 1931 2° standard observer. (Data adapted from Ref. 40.)...

CIE color system, 7 304. See also Commission Internationale de l clairage (CIE) chromacity diagrams, 7 313-315 standard illuminants, 7 315-316 CIEDE200 color difference equation, 7 322 CIELAB, 7 304 CIELAB Color Space, 7 320 CIELAB color values, of flax fiber, 11 614t Cielab dye nomenclature system, 9 244 CIELAB Metric Color Spaces, 7 320 CIELAB system, 19 428, 451 CIELUV Color Space, 7 319-320 CIE Standard Observer, 7 311-312 Cigarette filters... 

Calibration of Gel Permeation Chromatograph Polystyrene Calibration. A plot of molecular size in (S) versus elution volume for polysty-rene standards in dichloromethane showed deviation from linearity at about 2,200 which may be attributed to Imperfect column resolution, peak broadening, axial dispersion and skewing. The extensive tailing of the chromatograms of high molecular weight polystyrene standards observed in dichloromethane has also been reported in the literature (23-26). 

Lord, C., Rutter, M., Goode, S., Heemsbergen, J., Jordan, H., Maw-hood, L., and Schopler, E. (1989). Autism diagnostic observation schedule a standardized observation of communicative and social behavior. / Autism Dev Disord 19 185-212. 

Figure 3. Spectral tristimulus values according to the 1931 CIE standard observer. These values are dimensionless.

The development of the observer response functions is the foundation for color measured by an instrument. The Standard Observer established a recognized method for converting... 

Tristimulus colorimeters are used to measure color in certain food applications. They combine light source, filters, and photodetectors to reproduce the CIE Standard Observer response functions (Fig. F5.1.8). Colorimeters, having broad band-pass filters, do not measure the spectral data. Without spectral data they cannot offer the choice of either observer or... 

The International Commission on Illumination (CIE) has defined a standard observer to be used for accurate color reproduction (International Commission on Illumination 1983, 1990, 1996). In Chapter 2 we have seen that the rods mediate vision when very little light is available. This type of vision is called scotopic vision. The cones mediate high acuity vision in bright light conditions. This type of vision is called photopic vision. The sensitivities for a standard observer as defined by the CIE for scotopic and photopic vision are shown in Figure 4.3. The scotopic function is denoted by V (k). The photopic... 

Figure 4.3 Sensitivities as defined by CIE, which are used to model scotopic V (X) and photopic vision VM(k) of a standard observer (International Commission on Illumination 1983, 1990, 1996) (data from International Commission on Illumination 1988).

The normalization projects all colors onto the plane at X + Y + Z = 1. Therefore, the third coordinate is redundant and we have to only specify the coordinates x and y. The third coordinate is then given by = 1 — x — y. Since the weights as defined by the standard observer are all positive, the coordinates x, y. and will also be positive. Owing to the constraint x + y + = 1, they will all be smaller than one. Instead of specifying a color by the triplet X, Y, and Z, we can also specify the color by the chromaticity coordinates x and y as well as the absolute luminance Y. The absolute coordinates X and Z are then given as (Poynton 2003)... 

---