The brightness of a source is measured in candela per square meter. Everything that is visible can be regarded as a light source. Museum objects become light sources when they reflect light from a lamp. We will return to the unit of brightness, the candela per square meter, later. This colour gives the best visibility per watt for an object such as black text on white paper.
Good colour rendering, however, demands light sources that radiate over the whole visible spectrum.
This inevitably reduces the efficiency of the lighting, because the eye needs more energy in the red and blue-violet ends of the spectrum to give the same visual clarity. This efficiency factor goes under the curious name of the V-lambda curve. Here it is. This curve, obtained by averaging results from experiments with many people, has long been standardised as an essential component in the quantitative description of light. The curve defines the relationship between the human sensation of light and the physical concept of energy, which is the quantity which measuring instruments react to.