It is defined as a ratio of the luminance of a surface in a given direction to the luminance in the same direction of an identically illuminated and oriented Lambertian surface. Though reflectance must range from zero to one, luminance factor can take on any positive value. See figure to understand how luminance factor is measured. The physical characteristics of the surface. The transmission of light through any real medium will result in a reduction in flux relative to the incident flux.
Transmittance is the ratio of transmitted flux to incident flux.
The ratio of absorbed flux to incident flux is the object's absorptance . Luminous efficacy is the ratio of the total luminous flux emitted by a source to the total electric power input to the source. Efficiency is a unitless ratio of two quantities measured in the same units, such as rating the efficiency of a motor by the ratio of input power to output power. Since the output power of a light source is measured in lumens, not watts, then the lumen per watt ratio is not an efficiency. The larger the efficacy the higher the light output for a given power consumption.