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A data-font is a form of optical computer interface used by Cylons to access information in their datastream, as well as control various other Cylon technologies. Humanoid Cylons need only to place their palm on the illuminated port to instantly access the data stream (Exodus, Part I).
Caprica-Valerii performs a rough version of this technique with Colonial technology by inserting a fiber optic cable (which uses light as a data carrier) into an incision in her left arm to connect to Galactica's computer mainframe (Flight of the Phoenix). Another Eight interfaces with a Raptor's computer in the same manner (The Face of the Enemy). These abilities, including other observed interactions or reactions such as glowing spines and photo-sensitive eyes shows that many cells in their bodies are photo-sensitive and/or bio-luminescent.
Data-fonts are found on basestars in the ship's command and control center for data access and other ship functions. These data ports have a layer of water over the interface, which may have an aesthetic or functional purpose. These data-fonts (and the database circuitry used throughout the ship) are at least partially organic in design, as they are damaged after an infection kills or shuts down almost every Cylon entity and device on the basestar (A Measure of Salvation).