Electronic Design

Bob's Mailbox

Dear Bob:
You were talking about the difference in dynamic range of the senses of sight and hearing, and came to the conclusion that there was not a lot of difference. (I just said that the ranges were COMPARABLY wide. /rap) There are two other ways in which those senses differ enormously:

One is the resolution of two simultaneous frequencies. Many people can listen to a two-note chord, and sing back each note separately. (Musicians can manage many more, and tell you which instrument is playing which note.) But I don't know anyone who could color-match each component of a colour produced by combining two spectral lines. Yet the eye has more resolution in space! And—remember—the ear can hear fairly well over a 10-octave span of frequency—with perhaps 12 notes per octave—whereas the eye can hardly see over one octave!!

The other way is the spatial resolution of a light or sound source. The eye can resolve position to a fraction of a degree (or, more correctly, a fraction of 1/60th of a degree—a fraction of a minute /rap). The ear, meanwhile, is accurate only to tens of degrees. (Well, the ear can detect an angle a LITTLE better than 10 degrees. /rap) The reason is obvious when you think of the actual transducers. The eye has only three types of color discriminator, but they are repeated spatially thousands of times. The ears, on the other hand, have many different frequency discriminators, but they are only repeated twice, once in each ear.
PETER HESKETH
via e-mail

You are right: Each transducer is optimized for us to survive and thrive! Thanks for the comments!—RAP

Dear Bob:
The Nov. 22, 1999 "Bob's Mailbox" was quite timely. On Thanksgiving at the in-laws, I wandered into my nephew's eerily "backlit" room. Lots of little glowing dust motes between the garish posters. I turned to face the blacklight tube and got an eyeful of white glow. It was pretty obvious that the light was coming from inside my eye as it filled my entire field of view.

I've also experienced discomfort looking at neon advertising signs with gases that emit at opposite ends of the visible spectrum. Our eyes have sufficient chromatic aberration that we cannot simultaneously focus on red and violet. A sign that contains only those colors (providing the violet is really violet and not a mix of blue and red) causes our visual system to continually hunt for focus. The eye strain is immediately felt.

That brings up another curiosity about our vision. Recalling ROY G BIV, we see that violet is as far from red as it can get and blue is between them. Yet as children, we all learned that you get violet by mixing red and blue. I haven't had the time to check the literature, but it seems obvious that our red and blue cones are both sensitive to violet. That's why we have a color circle with violet right between red and blue.

I recently had to get a gallon of "off-white" paint to match an old one. The manufacturer had changed their base and colorants since the first gallon. To my surprise, they were unable to match the old paint across different lighting. If it matched in sunlight, it didn't match under incandescent lighting and vice versa. I hypothesized that the new base paint's "peaky" absorption spectrum was quite different from the old one, and that it was impossible to pull them into alignment with the colorants. The perceived color of a surface is the product of its absorption spectrum, the source spectrum, and the eye's spectral sensitivity. Since all of these spectra are complex, with peaks and valleys, it is certainly possible to create a new base paint with peaks or valleys in places that simply cannot be compensated for. By examining the equations for a few other off-white colors, I determined that the new base was actually a little purple. To get my requested off-white, they added lots of yellow. The original paint was tinted with raw umber (orange-brown). So they were trying to make orange from yellow and blue. I don't know what the absorption spectrum for raw umber looks like, but I bet it doesn't look like yellow plus purple. Under sunlight, you might get the two paints' absorption spectra to light up your eye the same way. But change the source spectrum and all bets are off.

As an extreme example of this, imagine having a perfect red paint (absorbs all but one red wavelength) and a perfect blue paint (absorbs all but one blue wavelength). Mix them together and view them under sunlight. You see violet. View them under violet light and you see nothing. So much neat stuff to know. Life's too short!
SCOTT KROEGER
via e-mail

What can I say? That is wild!!! Thanks for the insight.—RAP

All for now. / Comments invited!
RAP / Robert A. Pease / Engineer
[email protected]—or:

Mail Stop D2597A
National Semiconductor
P.O. Box 58090
Santa Clara, CA 95052-8090

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