by Dinesh Thakur

When choosing a monitor, one of the factors that the customer usually considers is the refresh rate. A high refresh rate is important in providing a clear picture and avoiding eye fatigue.


 What is a refresh rate and why is a monitor's refresh rate important?


An image appears on screen when electron beams strike the surface of the screen in a zig-zag pattern. A refresh rate is the number of times a screen is redrawn in one second and is measured in Hertz (Hz). Therefore, a monitor with a refresh rate of 85 Hz is redrawn 85 times per second. A monitor should be "flicker-free meaning that the image is redrawn quickly enough so that the user cannot detect flicker, a source of eye strain. Today, a refresh rate of 75 Hz or above is considered to be flicker-free


 How are refresh rates calculated?


Factors in determining refresh rates


 A refresh rate is dependent upon a monitor's horizontal scanning frequency and the  number of horizontal lines displayed.The horizontal scanning frequency is the number of  times the electron beam sweeps one line and returns to the beginning of the next in one  second. Horizontal scanning frequency is measured in kilohertz (kHz).A monitor with a  horizontal scanning frequency of 110 kHz means 110,000 lines are scanned per second.


The number of horizontal lines on the screen depends upon the monitor's resolution. If a monitor is set to a resolution of 1024 x 768 then there are 768 horizontal lines (1024 is the number of pixels on one line). For a monitor set to a 1280 x 1024 resolution, there are 1024 horizontal lines.

Additionally, the time it takes for the electron beam to return to the top of the screen and begin scanning again must be taken into account. This is roughly 5% of the time it takes to scan the entire screen. Therefore, the total is multiplied by  0.95 to calculate the maximum refresh rate.


 How to calculate maximum refresh rates?


The following formula is used to calculate maximum refresh rates fV = fH / # of horizontal lines x 0.95

fV = vertical scanning frequency (refresh rate) fH = horizontal scanning frequency

 Example: A monitor with a horizontal scanning frequency of 96 kHz at a resolution of 1280 x 1024 would have the following refresh rate based on the calculation above.

 fV = 96,000 / 1024 x 0.95

fV = 89.06

This figure is rounded down to produce a maximum refresh rate of 89Hz.

 If the same monitor is set to a resolution of 1600 x 1200, then the equation will be as follows:

fV = 96,000 / 1200 x 0.95
fV = 76

The maximum refresh rate at this resolution is 76 Hz.