Final answer:
Automatic Brightness Control (ABC) is the feature in modern fluoroscopic units that adjusts radiation levels to maintain consistent illumination across different densities in a virtual image.
Step-by-step explanation:
The functionality that allows modern fluoroscopic units to automatically adjust radiation levels for different body parts of varying density is known as Automatic Brightness Control (ABC). This feature ensures consistent illumination of the virtual image produced during fluoroscopic procedures. Automatic Brightness Control works by varying the X-ray intensity in response to the image brightness detected by the image receptor. It compensates for more dense and more radiolucent areas of the body, maintaining a steady level of illumination and optimal image quality.