The eyes have it: Extron screen checker scores big
- By John McCormick
- Jul 28, 1997
This device, the size of a pack of cigarettes, can measure the scan rates of any CRT
monitor or television screen.
To activate it, just pull out the plastic strip protecting the factory-installed
internal batteries. That's all--no calibration needed. Once you pull out the isolator
strip, the device will run for about a year before it needs new batteries.
Use the Extron SRI 200 to check the vertical frequency, or refresh rate, of monitors
installed in your office and those you might buy.
Refresh rate is the critical factor that determines whether workers suffer eyestrain
and fatigue from flickering images.
Flicker occurs whenever an electron beam refreshes the screen pixels so slowly that you
can actually perceive them turning on and off. Pixel phosphors are formulated to light up
quickly and dim just as quickly.
If they glowed any longer, the screen would show streaks.
To test vertical refresh frequency, hold the SRI 200 in contact with the center of the
screen and read the scan rate from the small LCD display. To measure horizontal scan rate,
hold the SRI 200 with the company name pointing upward.
Don't worry if you mix this up, because directions are printed on the back of the unit.
If you don't want to wade through all four sentences, there are red horizontal and green
vertical LEDs that light up to show which frequency is being tested.
The On button is only for backup in case the unit doesn't turn on automatically when
exposed to a fluctuating magnetic field. Store the SRI 200 away from magnetic fields to
keep the batteries from running down when not in use.
Note: The SRI 200 can't tell you whether a display is interlaced or noninterlaced. It
only counts fluctuations in the magnetic field.
Extron Electronics representatives told me the SRI 200 is used mostly by video
If a projector works only at VGA horizontal frequencies up to 31.5 KHz, then trying to
drive it from a computer with a 35-KHz SuperVGA frequency will cause failure or image
distortion. The SRI 200 helps troubleshoot this distortion.
Government offices will find several desktop uses for the SRI 200.
For example, monitors seldom indicate scan rates on their labels. If an employee
complains of eyestrain or other problems that might by caused by a monitor, the SRI 200
will show whether slow scan rate might be at fault.
For offices that make purchasing decisions about monitors and video cards based in part
on claims of fast scan rates, the SRI 200 is the perfect verification device.
Of course, monitor performance also depends on other factors such as whether a given
graphics card can support all the speeds available in a multisync monitor. So before you
file a complaint with the supplier, find out whether the monitor's factory settings were
unnecessarily high, forcing a lower refresh rate.
It's also possible that employees inadvertently created their own flicker problems by
tweaking the display settings in Microsoft Windows.
A monitor in MS-DOS mode usually gives the fastest refresh rate, because it forces VGA
Switching to any other graphics mode can cause a slowdown, as can selecting greater
color depth or resolution.
The SRI 200 gives an instant reading on whether a particular computer's resolution has
been set too high and whether reducing it would improve the refresh rate.
Here's another handy use: Perhaps your office has two or three different brands of
video boards installed in dozens of systems, and you want to find out which machine
contains which board, in preparation for upgrading.
There's no need to unscrew all the cases to read the names on the boards.
It takes only a few seconds per machine to check for differing scan rates with the SRI
No, it can't verify vendor claims of screen size or dot pitch--two current legal
controversies in the monitor industry--but the SRI 200 definitely can show whether your
monitors and video boards are capable of the promised scan performance.
The SRI 200 passed the accuracy test I gave it. On an old Compaq Computer Corp. QVision
172 monitor in MS-DOS mode, it measured an average horizontal scan rate of 31.3 hertz
(31.5 hertz is the VGA standard), and a vertical rate between 69 hertz and 70 hertz.
Switching to Microsoft Windows dropped the vertical rate to 60 hertz. An 8-year-old,
19-inch Sampo TriSync monitor also ran up to spec.
The SRI 200's simplicity and good performance make it an almost perfect information
If I absolutely had to criticize, I might suggest a larger LCD display.
John McCormick, a free-lance writer and computer consultant, has been working with
computers since the early 1960s.