§ 163.4. Definitions.
The following words and terms, when used in this chapter, have the following meanings, unless the context clearly indicates otherwise:
Entrance angle—The angle having as its sides the line through the center, and normal to the face, of the object to be tested, and the line from the center of the object to the center of the source of illumination, Figure 2 of Appendix A.
Fluorescent—The property of emitting visible light due to the absorption of radiation of a shorter wavelength which may be outside the visible spectrum.
Observation angle—The angle having as its sides the line from the observation point to the center of the object to be tested and the line from the center of that object to the center of the source of illumination, Figure 2 of Appendix A of this chapter.
Reflex reflective—Reflective of light in directions close to the direction of incident light, over a wide range of variations in the direction of incident light.
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