Value is the dimension of lightness/darkness. In terms of a spectral definition of color, value describes the overall intensity or strength of the light. If hue can be thought of as a dimension going around a wheel, then value is a linear axis running through the middle of the wheel, as seen below:
Drusen are tiny yellow or white deposits in a layer of the retina called Bruchs membrane. They are the most common early sign of dry age-related macular degeneration. Drusen are made up of lipids, a type of fatty protein. They may be the result of a failure of the eye to dispose of waste products that are produced when the photoreceptors of the eye drop off older parts of the cell. There are several types of drusen with different levels of risk.
Optic nerve drusen commonly cause mild side vision loss that can be detected by machines in the doctors office called visual field analyzers. The side vision loss is usually not appreciated by patients and not a functional concern. Rarely the drusen can cause severe, noticable side vision loss. It would be extremely rare for drusen to cause central vision loss, usually as a result of a choroidal neovascular membrane, which is a growth of new blood vessels near the optic nerve. These blood vessels can leak or bleed, causing loss of vision.