What is light?

Ever since we are born, we are familiar with light. The existence of nearly all life on Earth is fueled by light from the sun. During the day, the sun makes it possible for us to see our surroundings, but as the sun sets, life and work need to continue. In order to meet this need, man invented artificial light.

The sunlight is the most ideal light in terms of both light intensity and light quality (colour). This is not true for artificial light which mimics actual daylight. In order to enhance visual comfort we need to take into account the different aspects of artificial light.               

Light has a beneficial biological effect on the body, it stimulates mood and creativity, and it enhances emotional wellbeing. We become aware of this effect when the sun reappears against a clear blue sky after a couple of cloudy days. Appropriate lighting is proven to increase productivity and to be less tiring. It also reduces the number of errors and accidents dramatically.

Light is part of the electromagnetic spectrum, which ranges from radio waves, over heat to gamma rays. Electromagnetic radiation waves, as their names suggest, are fluctuations of electric and magnetic fields, which can transport energy from one location to another. Visible light is not inherently different from the other parts of the electromagnetic spectrum with the exception that the human eye can detect visible waves, with wavelengths that range from 380 nm to 760 nm.

The human eye consists of rods and cones. The cones are sensitive to colour (spectral distribution) and provide for clear vision and  quick observation in a brightly lit space. They occur in different proportions: 66% for red, 33% for green and 1% for blue. They make it possible to see by daylight. With sunlight, the eye’s highest sensitivity is to 556 nm, which corresponds to a yellow-green colour. The rods allow us to see at in dimly lit spaces. They are not colour sensitive but react even when light is low. Thank to these rods, we are able to see in the dark.

The eye’s sensitivity to the different wavelengths is not identical. During the day it is highest for a wavelength of 556 nm, which corresponds to a yellow-green colour. In the dark it shifts to the left, and the colour blue will be seen best.