ARCHIVED: What is refresh rate?

On this page:

• Introduction
• CRT monitors and refresh rate
• LCD monitors and refresh rate

Introduction

A monitor's refresh rate, or vertical scan rate, is the number of times in a second a monitor redraws a picture.

Refresh rate is not the same as frame rate, an attribute that concerns many video game players and some movie viewers. Frame rate measures how many times per second the source image changes. Refresh rate is how many times per second the monitor redraws the image. A monitor will redraw an image at the same rate no matter what the frame rate of the game, movie, video, or other source. If the frame rate of the source is slower, for example, 35 frames per second, and the monitor's refresh rate is 70Hz (70 redraws per second), then the monitor draws each individual frame twice before the frame changes.

CRT monitors and refresh rate

Refresh rate is an important attribute of cathode ray tube (CRT) monitors and any sort of cathode ray display, including TVs, because of the way the CRT renders the image. The glass in the screen is coated with phosphors. The CRT draws the image by shooting electron beams at the glass, electronically exciting the phosphors until they glow. The beam continues to scan left to right, descending one pixel each cycle, until it reaches the bottom. Then it returns to the top and starts again.

The phosphors, once glowing, will start to fade after the electron beam passes. This is the reason the CRT redraws the whole image instead of just the parts that change. The higher the refresh rate, the less this fade will be apparent, as the phosphors won't have as much time to dim between refreshes.

The lower the rate, the more apparent the fade will be. The "flicker" you see is the repeated brightening and dimming of the phosphors. When the refresh rate is high enough, you will no longer perceive the flicker.

Most people stop perceiving flicker at refresh rates of about 75Hz (75 screen redraws per second). Some people need refresh rates in the 80s. Refresh rates in the low 60s hurt most people's eyes; in fact, health care professionals have documented eye strain and headaches that occur in workplaces using refresh rates in this range. Most CRT monitors can do 75Hz for common resolutions, and a fair number of them can easily handle 85Hz. A few high-end examples exceed 100Hz at midrange resolutions, and an even smaller number exceed that at high resolutions.

To change your refresh rate settings, follow the instructions in the Knowledge Base document ARCHIVED: What should I do if my monitor display flickers or wavers?

LCD monitors and refresh rate

Refresh rate does not apply to LCD (liquid crystal display, now commonly referred to as flat panel) monitors, because they do not render images the same way CRT monitors do. Pixels in LCD monitors remain open or closed as needed until the image changes, and the light that passes through from behind stays constant. Therefore, the pixels don't fade. The rate that affects LCD displays is "response time", the time it takes for a pixel to go from fully open (the brightest intensity) to fully closed (black). Response time is a fixed property, not a configurable setting.

In Windows, Display Properties includes settings for refresh rate, but they are only there for compatibility with the video card, which must be able to handle a CRT that can do multiple rates. An LCD monitor has only one optimal refresh rate setting, so it expects a specific value. Changing this setting for an LCD monitor will affect the image (e.g., it may shift it off center), but will not affect the monitor's actual refresh rate.