"It helps to look for something that says 'FDA cleared,' and that sits in the low energy red light spectrum," she said ... are generally safe at low energy levels for everyone of all skin types ...

When photons of light pass ... with the same energy as the energy gaps in the atoms can be absorbed. This causes an absorption spectra, as shown below. Because the energy levels are the same ...

This is not a continuous spectrum as only light of specific frequencies and specific colours are produced. Different types of atoms have different energy levels. As a result each produces photons ...