This is a combination of continuous and discontinuous spectra. This particular type is emitted by light sources with modified electrical discharge, such as fluorescent tubes (Figure 2.5).
Figure 2.6 The three main emission wavelengths of the argon-ion laser
These tubes are the most common source of artificial lighting in public places today. They are sometimes wrongly referred to as neon tubes because of an earlier version, which was filled with neon gas. The form of these spectra is important for PV. For example, they show that combined sources like fluorescent tubes include an important element of blue light, which is well absorbed by amorphous silicon, and this enables solar modules using this material to produce current when exposed to this type of lighting.
Some light sources like lasers or laser diodes emit only in a few wavelengths (Figure 2.6). When associated with the narrow bandwidth filters, these sources become practically monochromatic.