Major Variola (ret) wrote:
At 02:17 AM 7/21/03 +0200, Thomas Shaddack wrote:
On Sat, 19 Jul 2003, Tyler Durden wrote: There is some minuscule proportion of X-rays produced by CRT displays.
Produced by the ebeam decelerating on the shadow mask, but adsorbed by the glass.
a_b_sorbed. Absorb is a widely used word meaning 3to drink in, to soak up,2 both literally and figuratively. Adsorb is a specialized technical term, meaning only 3to collect a condensed gas or liquid on a surface.2 The glass of CRT's absorbs so much of the X-rays that it might be hard to detect a signal at all at any distance, but then the signal is not swamped by noise from the not-immediately-illuminated areas, unlike the optical emissions. "0.5 milliroentgens per hour at a distance of five (5) centimeters from any point on the external surface of the receiver" is the US legal limit[*], and low voltage (and thus very low x-ray emission) crt monitors are common now, if not a de-facto standard. However, I expect shot noise to be a limiting factor here. Unfortunately, the Roentgen is such a wierd unit it's not that easy to convert it to photons and do the math! A light background on a CRT screen image will give out enough delayed light to give problems in the s/n ratio of an optical TEMPEST attack. It's much easier to "see" white text on a black background than black text on a white background. I use 180:210:210[**] (r:g:b) text on a 255:255:255 window background at present, with very light wallpaper, though I speckle both slightly. It's a little hard to read, but much better than some other suggested combinations. [*]< Probably far too high for safety! Originally for TV's, where the viewing distance is much higher. But most modern monitors will emit much less than that. I hope! > [**]< I replaced the black in Marcus's anti-em-tempest fonts with 180:210:210, and varied the other colours in proportion. > -- Peter Fairbrother