Spectral Power Distributions

In Eqs. (7)—(10), 5(A) is the spectral power distribution of the illuminant, and R A) is the spectral reflectance factor of the object. Jc(A), y(A), and 5(A) are the color-matching functions of the observer. In the usual practice, k is defined so that the tristimulus value, Y, for a perfect reflecting diffusor (the reference for R A)) equals 100. Using the functions proposed by the CIE in 1931, y(A) was made identical to the spectral photopic luminous efficiency function, and consequently its tristimulus value, Y, is a measure of the brightness of objects. The X and Z values describe aspects of color that permit identification with various spectral regions. 

Spectral power distribution, standard illuminant, 7 313 Spectral properties... 

Figure 19.2 Spectral power distribution of UV radiation from a solar simulator (ORIEL) (SSUV) simulated solar zenithal UV (intense solar UVB domain) (DUV) simulated daily UV (attenuated solar UVB domain) (UVA) solar UVA (no UVB).

Figure 1. The brilliant orange-red emission of Eu (A), contrasted to the bluish-white spectral power distribution of average daylight (B)...

Figure 6. The spectral power distribution of a fluorescent lamp containing two rare earth phosphors, those of Figures 1 (Curve A) and 5 (Curve A), and green-emitting zinc silicate Mn. A closer approximation to Figure 3 is desirable.

The spectral power distribution of Figure 6 is as close as we can presently get to the ideal prime-color mixture. 

Figure F5.1.2 The spectral power distribution curves of standard CIE illuminants. Figure courtesy of GretagMacbeth. This black and white facsimile of the figure is intended only as a placeholder for full-color version of figure go to httptfwww.currentprotocols.com/colorfigures...

A color correction may also be achieved by using filters. Table 3.1 shows the type of filter used by professional photographers to achieve accurate color reproduction. The required filter depends on the type of illuminant and also on the type of film. The type of light source can be described using the temperature of a black-body radiator. A black-body radiator is a light source whose spectral power distribution depends only on its temperature (Jacobsen et al. 2000). The color temperature of a fight source is the temperature of a black-body radiator, which essentially has the same spectral distribution in the visible region. The concept of a black-body radiator is formally introduced in Section 3.5. 

Funt et al. (1991, 1992) use a finite dimensional linear model to recover ambient illumination and the surface reflectance by examining mutual reflection between surfaces. Ho et al. (1992) show how a color signal spectrum can be separated into reflectance and illumination components. They compute the coefficients of the basis functions by finding a least squares solution, which best fits the given color signal. However, in order to do this, they require that the entire color spectrum and not only the measurements from the sensors is available. Ho et al. suggest to obtain the measured color spectrum from chromatic aberration. Novak and Shafer (1992) suggest to introduce a color chart with known spectral characteristics to estimate the spectral power distribution of an unknown illuminant. 

Figure 2.2. Spectral power distribution of blackbodies with color temperatures of 2854 K (source A) and 6500K (Pivovonski, 1963 Billmeyer and Saltzman, 1981).

In Part III the authors, Lachman et ah, also give a more comprehensive description of their photostability cabinet, along with a plot of the spectral power distribution (SPD) of the lamp used. For "ordinary illumination" (room illumination) studies, the authors constructed their own light stability cabinet of smaller dimensions and used lamps of smaller size and wattage. 

His thesis, for example, gives the results of a survey of sources then in current use in the German, Federal Republic, the spectral power distributions of these sources, their effect on various selected test substances as a function of time of exposure, total dose exposed to and absorbed. It is apparent from this data that the sources studied are not equivalent. 

Figure 7 Three-dimensional plot of the relative spectral power distributions of the standard, ID, standard and other ICH lamps. Source-. From Ref. 288.

Option 2 is defined as, "A cool-white fluorescent lamp as defined in ISO 10977 (1993)" and "A near ultraviolet fluorescent lamp having a spectral power distribution from 320 to 400 nm with a maximum transmission energy emission between 350 and 370 nm a significant proportion of UV should be in both bands of 320 to 360 nm and 360 to 400 nm."... 

Many different irradiation sources can be used in the stability studies of drugs and drug products. The source(s) used should be comparable in spectral power distribution to those to which products are exposed in practical use. It is however, difficult to predict the actual exposure of a pharmaceutical product during practical usage. 

In some cases, a drug product can even be exposed to direct sunlight. Clearly, it is difficult to predict the amount of UV and VIS irradiation to which the product is exposed during the shelf life. The spectral power distribution and the overall illuminance used in a photostability study should therefore provide a "worst-case" exposure. 

Changes in the spectral power distribution of the lamp caused by glass aging can be an important cause of the variability in results between different devices running identical test cycles. 

The ideal source would provide a close simulation of window-glass-filtered daylight with even illumination ( 10%) over a large area. The spectral power distribution and intensity would be constant, have a low-heat output, and would be inexpensive to purchase and run. At present, the ideal source does not exist (Table 1). 

Many daylight sources are so named because they have the same color temperature although they do not simulate daylight in a spectral power distribution sense. Several papers have been published recommending the proper source to use for pharmaceutical photostability testing (6,10-16). 

As described above, most lamps age as a function of time. This can result in not only a change of spectral power distribution but also in a lower output. Lamps should, therefore, be changed at defined time intervals (e.g., xenon lamp after 1000-2000 hrs, fluorescent tubes after 5000-10,000 hrs, and metal halide after 3000 hrs) as specified by the producer. Care should be taken to ascertain that the producers and the user s rating criteria are the same, which may not be the case. 

A procedure for the calibration of radiometers against a specific light source is suggested in Table 2. It is important to remember that neither the UV filter radiometer nor the luxmeter provides information on the spectral power distribution of sources. A detailed plot of the irradiation as a function of wavelength is only obtainable by use of a spectroradiometer. At present, such equipment is not used on a regular basis. 

Spectroradiometric data should be provided by the lamp manufacturer upon request. In cases where the spectral power distribution data are obtained separately from the calibration of the narrowband radiometer, it can be difficult to place the two devices exactly at the same point in space (Table 2). It is then of great importance to at least place radiometers at the same distance used for calibrating the lamp if accurate calibration measurements are to be achieved. 

Figure 2 D65 (—) and ID65 (...) spectral power distributions. Source Reproduced with permission of Atlas Electric Devices Company.

Figure 3 shows two "cool white" lamps of the same CCT but definitely not of the same spectral power distribution. It is evident that this single criterion is not sufficient to designate a lamp for our uses. 

Figure 3 Spectral power distributions of the two cool white lamps available overlaid with detector response curve. Source. Courtesy of Dr. R. Levin, Osram Sylvania.

D65, daylight, with a CCT of 6500K is defined by the CIE. The International Standards Organization Standard ISO 10977 (1993) refers to this fact. D65 is also known as D6500 or Standard Illuminant D by the CIE, represents daylight over the spectral range 300 to 830 nm was first adopted in 1966. This standard is not a particular lamp but an internationally agreed to spectral power distribution for solar radiation, issued by the CIE as "Technical Report, Solar Irradiance," first edition... 

In discussing lamp equivalency, it should be noted that not all lamps labeled "daylight" are equivalent for chemical purposes. Their spectral power distributions, as demonstrated in Figure 8, can be different. 

Figure 8 Spectral power distributions of various daylight lamps. (A) Hytron, (B) General Electric, (C) Bahren Lichttechnik, and (D) Philips.

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