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64 The PCB Magazine • February 2016 direction. Since this is a surface property, it is affected by things like the surface topogra- phy, the refractive index of the material being measured, and the angle of incidence. The angle of incidence is important for differ- ent types of gloss. For instance, it is referred to as "sheen" when the gloss is measured at low angles to the substrate. The "luster" is the perceived brightness of the specular light and the diffuse light relative to the sur- face. "Specular gloss" is the light reflected at the surface opposite the incoming angle of light. The strict definition of reflectance is that it's a measurement of how well a surface reflects ra- diant energy. The surface is exposed to a light source, and the reflectance is how much and at what wavelength that light is reflected back. You typically see reflectance noted as a graph over the color spectrum with the reflectance number typically noted as the % reflectance at 450 nm. The graph in Figure 2 for a generic white prod- uct would be noted as having between 90–95% reflectance for the red and green lines with the spectrum reflectance decreasing towards red colors at around 650 nm. Figure 1: Examples of gloss measurements. Figure 2: Reflectivity of white solder mask at various thicknesses. solder Mask for led appliCations: forMulation 101