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From: jbm@eos.UUCP (Jeffrey Mulligan)
Newsgroups: sci.electronics,comp.graphics
Subject: Re: Perceptual Color (was: Re: Looking for Blue LEDs)
Message-ID: <1658@eos.UUCP>
Date: 5 Oct 88 19:00:58 GMT
References: <619@ardent.UUCP>
Organization: NASA Ames Research Center, California
Lines: 57

From article <619@ardent.UUCP>, by sleat@ardent.UUCP (Michael Sleator):
 
> I've lost the trail leading up to this, but the discussion was about
> apparent color due to flashing white light.  If that's what the above
> is suggesting, then I think there's a problem.  I believe that the
> phenomenon of perceived color from flashing white light does not give
> anything like the range of color that you can get with mixed primaries.

Damn straight.  Let's drop it.

> 	Suppose you have three LED's, red, green, and blue; the light
> 	from all three being combined by some diffuser.  If you control
> 	the current through each one (in an analog fashion), you can control
> 	the brightness of each, and hence the resulting perceived color.
 
> 	Now, for a single LED, you can also control the apparent brightness
> 	by varying the duty cycle (the pulse width modulation referred to
> 	above).  However, it's not obvious to me that varying the apparent
> 	brightness of each LED in this manner would result in the same
> 	control of color as would the analog method.

Above the critical frequency for flicker fusion (CFF), all that
matters is total light flux.  CFF for humans is around 60 Hz;
this is why movies, which are exposed at 24 frames/sec (an adequate
sampling rate for the perception of smooth motion) are projected
through a chopping shutter which causes each frame to be flashed
three times, upping the flicker rate to 72 Hz.

> Here's what I *think* will happen:  If you do this at a high enough
> frequency (a couple of MHz oughtta be safe :-) ), it will probably work
> pretty well.  But as you decrease the frequency, things will start to get
> weird somewhere, possibly well above the nominal flicker fusion frequency
> for a constant-spectrum source.
 
> Either control technique will give you reasonably precise control over the
> energy output of the LED (integrated over some reasonable time period).
> However, almost nothing in nature is linear.  The human eye is decidedly not.

The eye is an excellent linear integrator for durations less than 16 ms.

PWM techniques were used by John Krauskopf in the laser colorimeter
he built at Bell Labs (he moved to at NYU shortly after the divestiture).
Same idea:  three lasers, red (HeNe), green and blue (both argon),
controlled digitally by acousto-optic modulators.  As I remember,
pulse widths were controlled by dedicated 12 bit counters; I forget whether
the controller had it's own memory or fetched data over a DMA interface.
The clock for the counters probably ran at a few MHz, making the pulse rate
about 1 KHz.  This pulse rate is more than an order of magnitude larger
than needed to eliminate flicker, but it does allow you to make nice
smooth sinusoidal waveforms up to 60 Hz, as long as you can afford the
memory.

-- 

	Jeff Mulligan (jbm@aurora.arc.nasa.gov)
	NASA/Ames Research Ctr., Mail Stop 239-3, Moffet Field CA, 94035
	(415) 694-6290