A reader asks:

When you write about the light “spectrum,” what do you mean?

Answer:

Think about it as the range of light wavelengths, the elements of physical energy that can excite visual activity in the retina of our eyes. These wavelengths fall into the range of about 400 to 700 nanometers. When you view white light through a prism (or look at a rainbow) you observe the colored spectral components, ordered from short- to long-wavelength light. The shortest wavelengths appear violet and blue, and the longest wavelengths appear orange and red, with green and yellow in between. A broad-spectrum light source, as used for light therapy, will contain all of these components, but can be balanced in different ways to produce “harsh” or “cold” white (with a higher balance of short wavelengths) or “soft” or “warm” white (with a higher balance of red). On either side of the visible spectrum we have a range of invisible wavelengths that comprise ultraviolet and infrared radiation; because of their distinct hazard risk for exposure to the eyes, we need to take care that ultraviolet and infrared are minimized in light therapy apparatus.

The Light Spectrum