The number of times per second that a video display system redraws the image on screen; rates below about 75 images per second can cause flicker, depending on the image size, lighting and image content.

The vertical scan rate of a video display; number of times a video display can paint an entire screen with a video signal. The standard refresh rate for video (television, DVD, VHS, laserdisc) is 60 Hz. This means that the display goes through 60 cycles refreshing or redrawing the screen 60 times in a second. he maximum vertical resolution of a video display (the number of horizontal lines drawn) can be determined by dividing the horizontal scan rate by the refresh rate (the vertical scan rate). By this formula, a graphics projector with a horizontal scan rate of 63,000 Hz can produce up to 1,050 horizontal lines or a vertical resolution of 1,050. A data grade projector (31,500 Hz horizontal scan rate) can furnish 525 horizontal lines and a video projector or traditional television display, including LCD and DLP projectors, (15,750 Hz horizontal scan rate) can furnish 262.5 horizontal lines. he interlaced video format used by the analog NTSC television standard specifies that two fields create a full frame so that there are 30 frames per second. Each field thus contains 262.5 lines (each line spaced apart by one blank line; all the odd lines are drawn and then all the even lines are drawn in the next field to create a complete frame). When the two fields are combined, the NTSC standard allows for 525 horizontal lines of resolution (although only DVD approaches using those 525 lines with laserdisc using only 425, cable television around 300 and VHS tape just over 200). he highest format of digital HDTV uses an interlaced format featuring 1,080 horizontal lines. Thus every one sixtieth of a second, a field of 540 lines is created with two fields combining for a total of 1,080 lines in a frame. Graphics grade projectors can easily produce HDTV video as they can output 1,050 horizontal lines of resolution (when only 540 lines are created at a time). Data grade projectors will generally be able to show HDTV signals as well. Conventional televisions and video projectors are limited in their horizontal scan rate and cannot produce HDTV signals (they also are not digital in nature and still could not reproduce HDTV signals – data and graphics grade video projectors will need digital tuners to receive the HDTV signals).