Gordon Mohr posted a note [0] to FoRK which mentioned "Polyvision"
PDLC panels [1], which switch between transparency and translucency
upon electrical stimulus.

This stimulated me to think about building large displays.  Liquid
crystal units can switch between opaque blackness and relative
transparency at tens of Hz.  Old passive-matrix LCDs used rows and
columns of electrodes to selectively darken some pixels on a display.
But you could just as well use rows and columns of liquid crystal that
darkened all together; if all but one row was dark at a time, then the
darkness or lightness of the columns would appear as darkness or
lightness of that row.  Each row and column would have to be able to
turn all the way from dark to light or back within the time between
adjacent rows.

This is a tall order for LCDs, and you wouldn't have great contrast
anyway --- each pixel would only be transparent for one scan-line
duration at most, cutting your nominal contrast ratio by a factor
equivalent to the number of scan lines.  So I got to thinking about
other related items.

If the columns were made of, say, electroluminescent cable rather than
LCD, you could have a much better contrast ratio.  The LCD rows --- in
front of the EL cable --- would become transparent in sequence, while
the EL cable was switched on and off to produce the pattern of pixels
on each row in turn.  Your contrast ratio would therefore be limited
only by the opacity of your LCD.

But I'm not sure how fast EL cable can turn on or off --- I'm fairly
sure it's not much less than a millisecond, and it might be much
longer; and I'm not sure how expensive it is.

Doing, say, a 2048x1536 screen refreshing at 70Hz requires refreshing
the whole screen every 14.29 ms, which means refreshing every row
every 9.3 microseconds.

Garden-variety inorganic crystalline LEDs can switch on or off in, nI
think, 100ns or so.  So they would be ideal --- indeed, they would be
able to deal with another factor of five increase in vertical
resolution if they had to switch on and off once per row.  If they had
to switch on and off once per pixel, they would only be able to deal
with about 14 kilopixels at 70Hz --- about 103x138.

Suppose you have a linear array of LEDs.  If you scan the pixels of an
image over them, you can see the image if you rapidly move your point
of focus past the array at right angles to it --- or rapidly move the
array past your point of view.  Novelty "write in the air" devices
working on this principle have been available for sale for several
years now.

The same is true if you simply project an image of the LEDs on a
screen --- you can still move your eye past the images of the LEDs.
But moving the images of the LEDs rapidly is a much easier thing than
moving the actual LEDs.  A spinning mirror will do it, and a hexagonal
mirror spinning at 12 revolutions per second, or 700 RPM, is not at
all difficult to engineer, yet provides a 70Hz refresh rate.

I seem to recall [2] that some "laser" printers actually use linear
arrays of thousands of LEDs running all the way across the
photoconductive drum; rather than modulating an actual laser, they
modulated the array of LEDs much more slowly, leading to smaller,
cheaper printers with fewer moving parts.  Perhaps an exhausted drum
unit from such a printer could inexpensively yield an array of LEDs
for such a display.  (It appears that all Okidata "laser" printers
work this way, in fact.)

If it is desirable to reduce the number of LEDs (for example, if
assembling the LED array by hand), a linear array of LEDs can be
diagonally scanned across the image area by a pair of spinning
mirrors.

Spinning 14-sided first-surface motorized mirrors from www.sciplus.com
(what used to be JerryCo), with each side 3/16" by 3/4", cost $9.50
each.

And, of course, there's the traditional two-mirror scanning laser
setup.

[0] http://www.oreillynet.com/cs/weblog/view/wlg/1540 "DRM Helmets: An
Idea Whose Time Has Come"

[1] http://www.polytronix.com/pdlc.htm "PDLC --- Polymer Dispersed
Liquid Crystals/Polyvision/Privacy Glass"

[2] http://www.pctechguide.com/12lasers.htm says:

    LED (light-emitting diode) page printing - invented by
    Casio, championed by Oki and also used by Lexmark - was
    touted as the next big thing in laser printing in the
    mid-1990s. However, five years on - notwithstanding its
    environmental friendliness - the technology had yet to
    make a significant impact in the market.

Lexmark apparently sold a 12ppm LED printer in 1998, which implies 2.2
inches per second, or 660 pixels per second per LED (at 300dpi) or
1320 pixels per second per LED at 600dpi.

-- 
<[EMAIL PROTECTED]>       Kragen Sitaker     <http://www.pobox.com/~kragen/>
Irony and sarcasm deflate seriousness, and when your seriousness becomes detum-
escent, you're not held responsible for your thoughts. Irony beats thinking like
rock beats scissors. -- http://www.hyperorg.com/backissues/joho-june2-98.html


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