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This guide is intended for use in selecting terminology, measurement scales, and instrumentation for describing or evaluating such appearance characteristics as glossiness, opacity, lightness, transparency, and haziness in terms of reflected or transmitted light. This guide does not consider the spectral variations responsible for color, but the geometric variables described herein can importantly affect instrumentally measured values of color. This guide deals with the reflected and transmitted light and the selection of geometric conditions for its measurement. There are five kinds of measurement scale used in this guide. First is the regular scale which indicates that only light that has been reflected or transmitted without scattering or diffusion is included for measurement. Second is the specular scale which indicates that only the light that is mirror reflected is included for measurement. The third scale is the diffuse scale which indicates that only the light reflected or transmitted in directions other than the specular or regular direction is included in the measurement. The total scale, the fourth scale, indicates that the light reflected or transmitted in all directions is included for measurement. Last is the directional scale which indicates that the light reflected or transmitted in specified directions only is included for measurement. Directional values depend on the illumination and viewing angles and refer to light reflected or transmitted in directions that differ moderately from the centroid direction or axis of the beam. There should be only one aperture stop in any instrument. This stop determines the cross-sectional area of the incident beam on the specimen. All incident rays within the limits of the illuminator aperture angle, and all rays within the receiver aperture angle, should reach the receiver and be given equal weight by the measurement system. When vignetting occurs, the illumination, viewing, and aperture angles do not adequately describe the geometric properties of the instrument. Regularly and diffusely reflected and transmitted light are often not adequately differentiated and identified to enable their separation for measurement. Most objects and material distribute some light both regularly and diffusely; consequently the regular and diffuse components of reflection and transmission cannot be separated precisely for measurement.