Brightness - Light, Colour, Vision

In this resource, the term brightness is associated with the intensity of light an object such as the Sun or a lightbulb emits.
The brightness of a light is always determined by comparing it with the brightness of other light sources.
As light propagates through a vacuum it is invisible but its brightness becomes apparent when a light source shines directly into our eyes or it is reflected towards us.
The perceived brightness of a light source depends on how the photoreceptive rod and cone cells in the human retina respond to wavelengths of light (rather than the way that translates into the experience of colour).