An F-center or color center or Farbe center (from the original German Farbzentrum, where Farbe means color and zentrum means center) is a type of crystallographic defect in which an anionic vacancy in a crystal lattice is occupied by one or more unpaired electrons. Electrons in such a vacancy in a crystal lattice tend to absorb light in the visible spectrum such that a material that is usually transparent becomes colored. The greater the number of F centers, the more intense the color of the compound. F centers are a type of color center.

This is used to identify many compounds, especially zinc oxide (yellow).

Before the discovery of point defects it was already known that some crystals can be discolored using various methods. In 1830 T.J. Pearsall discovered that fluorspar could be discolored using violet light. Thirty years later similar results were achieved by melting crystals together with a specific metal. In 1921 Wilhelm Röntgen extensively measured rock salts. One set of these tests measured a photoelectric conductivity 40,000 times larger, after the salt was radiated with x-rays. A similar result to x-rays was accomplished by coloring the crystals with metal vapors. The photoelectric effect mainly happened around specific wavelengths, which was later found to be non-colloidal in nature.

The discolorations were later named F centers, as in Farbe, the German word for color. These defects were extensively studied by Robert Wichard Pohl and his institute at the University of Göttingen since 1920. One of his assistants, Erich Mollwo [de] concluded in 1933 that these F centers are atomic crystal defects. Around this time people started to assert these defects were unpaired electrons. The vacancy model was first described by Pohl in 1937 but still was considered tentative. It was formalized theoretically by Nevill Mott and Ronald Wilfred Gurney in 1940. It took until 1957 to prove find conclusive experimental evidence using electron spin resonance.^{[citation needed]}

F centers can occur naturally in compounds (particularly metallic oxides) because when heated to high temperature the ions become excited and are displaced from their normal crystallographic positions, leaving behind some electrons in the vacated spaces. This effect is also exhibited by ionic compounds containing metal-excess defects.

Often F centers are paramagnetic and can be studied by electron paramagnetic resonance techniques. The F centers most commonly studied are those that occur in alkali metal halides. Alkali metal halides are normally transparent; they do not show absorption from the far ultraviolet into the far infrared. Thus any changes in optical absorption can easily be detected and studied. The absorption band of F centers in sodium chloride is located in the blue part of the visible spectrum, giving a sodium chloride crystal with sufficient F center defects a yellow tinge. In other alkali chlorides the wavelength of the F center absorption band ranges from violet to yellow light. The formation of F centers is the reason that some crystals like lithium chloride, potassium chloride, and zinc oxide become pink, lilac and yellow, respectively, when heated.

Though F centers have been observed in other materials, they are generally not the cause for coloration in those materials. There are few examples of naturally occurring F centers causing colorations. One possible candidate is the mineral Blue John. This is a form of fluorite, CaF₂. Although it has not been confirmed, it is believed that the color is caused by electron F centers. It is thought that this F center is formed due to nearby uranium deposits in the rock: the radiation from radioactive decay is energetic enough to form the F center.

Another example of an F center found in nature is a relatively long-lived F center found in sapphire through luminescence, which had a duration of about 36 ms in one study.