Also called "refresh rate," it is the number of times an entire CRT screen was refreshed, or redrawn, per second. Measured in Hertz, display systems typically ranged from 56 Hz to over 100, but at least 70 was recommended to help prevent eyestrain. The earliest CRTs were unable to support both the highest resolution along with a high refresh rate; however, later CRTs provided adequate rates at all resolutions. Vertical scan frequency is not an issue with LCD displays because they do not use phosphors that have to be continuously energized. However, the data from the frame buffer is continuously sent to the LCD panel in order to display animation and video. Thus, the flat panel does have a maximum refresh rate to support motion, but not to refresh static images. Contrast with horizontal scan frequency. See interlace.