Re: VDT Displays Distorted by H Fields

["John Moulder" <jmoulder@post.its.mcw.edu>]

> ... A number of 
> the faculty and graduate students then began complaining about 
> color shifts and distortion in their VDT displays.  We have been able 
> to subsequently verify that many of the girders used in the building 
> have somehow become magnetized.  The DC H fields we are seeing are in 
> the range of 0.4 - 2.0 mTesla (4 - 20 gauss)...

That's a big field for a girder.  Are you _sure_ thats the source?

That's quite enough to cause color distortion or a large monitor.  In fact, a 
DC field as low as several 100 mG (20 microT) will cause distortion -- easily 
demonstrated by turning a monitor on it's side, and hence changing the 
orientation relative to the Earth field.

> Do any of you folks happen to know of any studies that have focused 
> on variable H field (DC) intensities and their effects on specific 
> models of VDTs

Not that I could find when dealing with problems caused by the fields from our 
MRI/MRS units.  Several possibilities:
1)  Reorienting the monitors may help
2)  Monitors with metal cases are often less susceptible.
3)  Low-emission monitors _may_ be less susceptible.
4)  Repeated degaussing of the monitor may help.
5)  Effective monitor shields are expensive, but they work.
6)  Some monitors may be adjustable to cope with distortions.

I don't suppose you could degauss something the size of a building.

Beware that effects on monitors can lead to paranoia about "health hazards"


John Moulder (jmoulder@its.mcw.edu)