The light phenomenon of color object generally has transmission, reflection, selective absorption. As shown in figure 3-20 (color), O is the color object. When the light energy of incident light is Q, it is selectively absorbed by object O. The light energy transmitted or reflected is Q. Therefore, the density of object O is D=lg (q-out/q-in). Therefore, the density represents the nature of the amount of light absorbed by the object. If an object absorbs a lot of light, its density will be high: if an object absorbs a little light, its density will be low. Density is a commonly used physical quantity for measuring film transmittance and color depth in printing industry.