The spectral power distribution (SPD) of a light source is a measurement of the energy output at points (usually 31) along the visible spectrum. Some spectrophotometers can measure increments as fine as 1 or 2 nanometers. Values are used to calculate other specifications and then plotted to demonstrate the spectral attributes of the source. This can be a helpful tool in analyzing the color characteristics of a particular source.
A source may appear "white" to the eye, simulating a full-spectrum light , when in fact it may have as few as 3 high energy bands, perhaps in the red, green, and blue, regions, which together simulate white to our eye. Metamerism is an issue with theses high energy spike lamps. Incandescent lamps have a smooth spectrum, although low in the blue region.