COLOR ASSESSMENT
The three main methods of analyzing the color in fibers are visual examination, chemical analysis, and instrumental analysis. Each of these methods has strengths and weaknesses that must be considered by the fiber examiner. The most basic method is simple visual examination of single fibers with the aid of a comparison microscope. Visual examination is quick, and comparison is an excellent screening technique. However, this method is subjective and because of day-to-day and observer-to-observer variations, it is not always a repeatable method. Additionally, the dilemma of metameric colors exists. Metameric colors are those that appear to match in one set of lighting conditions but do not in another. By their nature, metamers are difficult to sort out visually. Visual examination must be used in conjunction with an objective method. Chemical analysis involves extracting the dye and characterizing or identifying its chemistry. Typically, thin layer chromatography is the method of choice although oth ers may be employed. Chemical analysis addresses the type of dye or dyes used to color the fiber and may help to sort out metameric colors. It can be difficult to extract the dye from the fiber, however, because forensic samples typically are small and textile dyers take great pains to ensure that the dye stays in the fiber. Dye analysis is also a destruc tive method, rendering the fiber useless for further color analysis. Because very light or very small fibers have little dye in them, weak or equivocal responses may result. Instrumental analysis offers the best combinations of strengths and the fewest weaknesses of the three methods outlined. Instrumental readings are objective and repeatable, the results are quantitative, and the methods can be standardized. Impor tantly, it is not destructive to the fiber, and the analysis may be repeated. Again, very light fibers may present a problem with weak results, and natural fibers may exhibit high variations due to uneven dye uptake. The microspectrophotometer (MSP) is an instrument that allows for the color measurement of individual ibers. The MSP is essentially a standard spectropho tometer with a microscope attached to focus on the sample. A spectrophotometer compares the amount of light passing through air with the amount of light transmit ted through or reflected off a sample. The ratio of these measurements indicates the percentage of light reflected or transmitted. At each wavelength of the visible spec trum, this ratio is calculated, stored, and recorded. The light is broken into smaller regions of the visible spectrum by a monochrometer, which acts like a prism dividing the light into its spectral components (see Figure 1). Color is a major factor in comparing textile fibers. If tape lifts are being searched, it is the predominant factor in selecting fibers for further comparison. Very fine grada tions of color difference can be seen once fibers have been mounted; it is necessary, however, to train the observer’s eye to make these distinctions in a uniform manner. The MSP is crucial to the comparison process because it can segregate colored ibers that appear visually the same but are subtly different. Objectively distinguishing between otherwise identical fibers is necessary to ensure a reliable comparison method.
FIGURE 1 Schematic of a microspectrophotometer, what the instrument looks like, and an absorbance spectrum from a red fiber (fiber photo inset).
