How to define luminance? Well, luminance (in candela per square, i.e., cd/㎡), in the field of lighting, is a physical quantity that characterizes the intensity of luminescence reflected from the illuminating surface or illuminated surface. The unit of luminance is, according to SI base units, is candela per square, i.e., cd/㎡. In layman’s terms, luminance is the intensity of light perceived by the human eyes when seeing the light source. Therefore, it is a subjective amount, the only basic lighting parameter perceived by human’s bare eyes, which speaks about one’s most straightforward feeling of a room’s lighting. When we hear someone says, “this room is too bright”, or “there isn’t enough light, it’s too dark,” we are actually referring to luminance (Fig. 1.1.1 shows the contrast of the same setting under low luminance and high luminance).