In any discussion of lighting issues, it is important to understand the differences between the various terms and to use them with precision, as they are often misconstrued. Quantity Light is a type of radiation and forms part of the electromagnetic spectrum visible to the eye. The amount of light falling on a surface is known as the illuminance and is measured in lumens per square metre or lux . This is easy to calculate and measure and is therefore widely used. The illuminance of direct sunlight is approximately 100,000 lux, but normal daylight, which is filtered through a cloudy sky, is between 5,000 and 10,000 lux, while moonlight is as little as 0.25 lux.
Human vision is a complex process.
The eye does not see the light arriving at an object, but rather the light radiating or reflecting from its surface. This is known as luminance , or brightness. The other term commonly used by lighting engineers is luminous intensity . However, in reality, a source's luminous intensity is seen by the eye relative to the brightness of its surroundings, and this is not easy to measure. The colour appearance of light is identified by its chromaticity coordinates .