The first generation of special-effect paints for automobiles used aluminum flakes to provide what the industry calls “dark and light flop.” When such a sample is tilted, its appearance changes. An instrument that has one viewing angle cannot capture all of the effects.
For non-colorful effects (aluminum flakes mainly) you need one angle for each regime to assess color (i.e. lightness) change between observation angles (Figure 1). Face is typically the main color information you see perpendicular to the sample surface, which is the angle where you see your face reflection, therefore it is called “face”. So, this angle is typically 45°as45° or r45°as45°. Flash is any of the three angles close to specular: 45as25°, 45as15 or 45°as-15° (not required for Al-Flakes). Flop is any of the two angles “from the lamp direction”: 45as75° or 45as110°.