Tuesday, November 10, 2015

The Pith and the Pendulum: How do slow visual signals keep up with fast ones from a moving object?

When we watch a moving object, we experience at least two visual signals: A fast one due to luminance contrasts (which also brings with it a high spatial resolution) and a slow one due to chromatic contrasts (which has much lower spatial resolution).  It is interesting---and functionally necessary---that we see an object in motion as recognizable and locatable, without any chromatic blur due the slow visual mechanism's failure to catch up with the fast one.

Chromatic blur is just one aspect of the visual signal that is suppressed when we look at a moving object. Also suppressed is our awareness of the persistent motion of the eye itself; despite this motion, we see the motion of, say a thrown baseball, as a simple arc.

Despite the general suppression of the chromatic blur, there is a simple demonstration that shows the chromatic signal sluggishly and fuzzily lagging the luminance signal. (I did this experiment at MIT about 40 years ago.) Set up a pendulum in a generally well illuminated room, in front of a white wall.
Now illuminate the scene with a diffuse blue light that nonetheless has a localized source. An example of such light is a slide projector without its lens, illuminating through a Kodak Wratten 98 filter.  The pendulum should be between the projector and the wall. The viewer should sit in such a way as to see simultaneously the pendulum and its shadow on the wall. If the room is brightly enough lit and the shadow is well defined, the blue light will be almost invisible, but the shadow will appear a rather intense yellow. Because the shadow (illuminated only by the white light) is almost the same luminance as the background (illuminated by white and blue lights together), the shadow will appear quite blurry. To see that it is the visual system that is creating the blur, replace the blue filter by a filter of another color and see the shadow appear as a relatively sharp image. The shadow edge is indeed exciting chromatic contrast with very low luminance contrast. (Note: If we instead had contrived the contrast edge to be truly isoluminous and restricted to excitation differences of the blue-sensitive cones, the edge would be completely invisible [1]. To quote Boynton [1], “The blue-sensitive cones […] seem to be free of any serious spatial or temporal responsibilities in vision.”)

Given our stationary shadow-casting pendulum, now set the pendulum in motion. You will notice immediately that the shadow lags the motion of the pendulum. At first it will seem to be an independent object, but then a weird thing happens. With each cycle, the shadow of the pendulum somehow reasserts its phase relative to the pendulum itself. If this were an independent object, the phase difference would continue to pile on, for the two pendula would have different frequencies. Eventually the pendula would be in counterphase, and then beyond counterphase. But this doesn't happen. Instead, each time the real pendulum reaches its turning point, the shadow reaches a turning point a fixed time thereafter. If you try to stare down the shadow pendulum to capture what should be a progressive phase lag, you will notice the shadow getting blurrier as your eye's involuntary motion takes over.

At this point, present and future vision scientists should take over. It should be possible to extract some scientific pith from this pendulum.

[1] R. M. Boynton, Ten years of research with the minimally distinct border. In J. C. Armington, J. Krauskopf, and B. R. Wooten (eds.), Visual Psychophysics and Physiology. Academic, New York (1978).
[image copyrighted by the Exploratorium, www.exploratorium.edu.] Among these colored shadows, the yellow hand is the one that waves to you most slowly.

Michael H. Brill

Friday, August 7, 2015

Red Like You've Never Seen It Before

In his recent book, The Tell-Tale Brain, V. S. Ramachandran describes some cases of synesthesia.  A synesthete's  reaction to a stimulus can involve multiple senses.  If the stimulus is the sound of a trombone, for example, a synesthete hears the trombone, but might also see a colour, such as light blue.  While synesthetic cross-sensory effects are well known, p. 114 of the book reports a lesser-known phenomenon: some synesthetes see colours that are “unreal” or “Martian,” colours they have never seen previously.  This Hue Angles column hypothesizes a plausible cause and speculates about its implications.

A commonly suggested explanation for synesthesia is “cross-wiring” in the brain.  The idea is that certain stimuli activate not only the receptors they should activate, but also some that they shouldn't.  The sound of a trombone, for example, ordinarily affects only the aural apparatus and related brain regions.  In a synesthete, however, the signal pathway for a trombone's sound might overlap with the signal pathway for light blue, or the trombone circuit might cause activity in the light blue circuit, as if the two circuits were linked by a relay.

 Such a linkage might explain the perception of unreal colours.  The figure shows the Stockman-Sharpe cone fundamentals for the eye's red, green, and blue receptor cones.  Colorimetry is based on the colour-matching functions which, at least in theory, are linear transformations of these fundamentals, so ultimately these response curves determine human colour perceptions.  In particular, the overlaps in the response curves limit our perceptions.  For all light stimuli, except those very near the infra-red, the green cones will respond only if the red cones also respond.  Similarly, a physical stimulus cannot excite the blue cones appreciably without also  exciting the red and green cones slightly. 

 If, however, cone combinations were not limited by physical stimuli, colours could be produced that were outside our usual perceptual limits.  Synesthesia, it is suggested, can produce cone combinations that could not be produced by physical light sources, so the resulting colours are physically unreal.  Suppose, for example, your brain was wired so that only the red cone response was received.  Then the red signal would exist without the green signal, which, as we have seen, is an impossibility for physical stimuli.  You would see only a red, but since the red was unadulterated with other signals, it would be a red like you've never seen red before.  

 Any attempts to find such a red in the real world would be doomed to failure.  Whether the stimulus came directly from a light source, or indirectly, after reflection off an object colour, none of its wavelengths would be able to stimulate just the red cone any significant amount, without stimulating the other cones, too.  The new red would be “unreal,” because it could never have the same colour as any physical stimulus.

 A similar unreality occurs with so-called ''supersaturated'' colours, discussed by Glenn Fry, and also by Hurvich and Jameson.  A receptor can be temporarily adapted so that it is not responsive; this adaptation is often offered as an explanation for complementary afterimages.  If the green receptor were briefly inactive, then a stimulus between 550 and 700 nm would produce a red that was more chromatic than any real red.  
 An interesting question is the hue of a synesthetic or supersaturated colour.  The chromaticity of a pure red cone perception would be a point R outside the standard chromaticity diagram.  Suppose a neutral chromaticity N is chosen in the chromaticity diagram.  Join R and N by a straight line, that intersects the spectrum locus (which, along with the purple line, makes up the diagram's boundary)
at some point D.  By definition, D would give the dominant wavelength of the perception R.  Dominant wavelength is roughly equivalent to hue, but the two are not identical.  In fact, lines of constant hue curve noticeably as they radiate outward from N.  If R is far enough outside the diagram, then the curving might be significant, so D might not be a very good hue indicator at all.  It is possible, perhaps, that the perception resulting from a pure red cone might actually be an orange or a purple---which would be a whole new hue angle.

Paul Centore

Dr. Centore is a freelance colour scientist who is available for colour-related projects. He combines technical ability with an art/design/graphics background. For more information, see www.99main.com/~centore or send an email to centore@99main.com .

Friday, May 1, 2015

Equal-Energy White: Does it Illuminate or Obscure?

All illuminants are equal-energy, but some are more equal than others.

Once in a while I have to rant about a theoretical point that requires some mathematical discussion. I apologize in advance to those who expect a minimum of math in this column. But I believe some editorial space is warranted by something that consensus has given an undeserved special status.

We are often cautioned by the CIE not to confuse lights with illuminants: Lights are physically real but illuminants are just lists of numbers juxtaposed with wavelengths. The distinction can at times appear pedantic: Illuminants are derived from physical measurement, aren’t they? Yes, but there is more to say. Some illuminants are more closely tied to physical reality than others. For example, D65 emerged from a principal-component analysis of measured spectra, but nobody has ever actually seen D65 because it is a statistical distillation---like the average family having a fractional number of children. Even farther from physical reality is the equal-energy white illuminant, which is not even a statistical distillation of any real measurements. Yet the CIE has dubbed it “Standard Illuminant E”, it is sometimes called EE, and it has had an interesting history.

For many decades, the equal-energy white illuminant has been a bulwark of color science. Defined simply as a constant-in-wavelength spectral-power distribution, E was used in both the 1931 and 1964 CIE systems to area-normalize the color-matching functions (so E gives the “white” X = Y = Z by definition). It is the adaptation state that some believe we awaken in after a long sleep---and the adaptation state to which all colors are transformed in modern CIE color-appearance models.1 Colorimetrists’ affinity for a unit weighting function has also led to such functions being used to define colorimetric quantities that are not illuminant spectra. A unit weighting function such as heralds EE underlies all discussions of orthogonal color-matching functions from D. L. MacAdam to Matrix-R analysis, and is a hidden part of the structure of principal-component analysis of reflectances. (The principal-component eigenvectors are, after all, orthogonal with respect to the unit weighting function.) I will say no more about these non-illuminant choices here.

All seems well in using the equal-energy white as a preferred spectrum, until one thinks: What if we had adopted the EE illuminant as equal-energy per terahertz of frequency ν  instead of per nanometer of wavelength λ? The colorimetric world as we know it would be quite different numerically. For example, the figure below shows the current equal-energy white illuminant (the horizontal black line) and also the wavelength-density plot of the spectrum that is constant in frequency-density over the visible range (the red curve, normalized to 1 at 550 nm). Relative to the former, the latter emphasizes the short-wavelength end of the spectrum. [The red curve is f(λ) = c/λ2 , derived from ν = c/λ and the equal-energy-in-ν relation f(λ) |dλ| = 1 |dν||.]

Of course, looking at wavelength- and frequency-based equal-energy spectra doesn’t exhaust the possibilities. Any positive-definite spectrum g(λ) is a viable equal-energy white in a function of wavelength w(λ) = the integral of g(x) dx from 400nm to λ.  [This is implied by the equal-energy-in-w condition g(λ) |dλ| = 1 |dw|.]  But of all the possible white spectra, wavelength-based EE is the one considered special by color research. To paraphrase from George Orwell’s Animal Farm: All illuminants are equal-energy, but some are more equal than others.

Would some benefit be conferred by re-selecting the domain in which E is equal-energy—perhaps an improved visual performance of principal-component approximations? Regardless of the answer, it may be beneficial to re-frame colorimetry so as not to depend on the primacy of the wavelength domain over, say, frequency. Or perhaps it is enough to be careful that our scientific conclusions don’t depend on E as a special illuminant.

What do ISCC readers think?

Michael H. Brill
1 Of course, a spectrum of light is not uniquely connected to an adaptation state, as any metamer of that spectrum would produce the same adaptation state.

Wednesday, February 11, 2015

Cobalt Blue - from runway to road

Once in a while it is educational and fun to hear from the fashion industry, so now CAUS Executive Director Leslie Harrington makes a case for the staying power of a new color trend.

Figure: Cobalt blue in fashion (from http://penelopesoasis.com/wp-content/uploads/2012/08/cobalt-blue.png).

While the business of forecasting new color trends has been more an art than a science since the Color Association first started 100 years ago, it is often hard to validate how “on trend” we have been.  Once in a while we see what we might call a “mega" color trend where a color becomes so prolific that it seems to be everywhere.  Such past color successes were Wasabi in the late 90’s and most recently chartreuse in the early 2000’s.  These colors penetrated all industry sectors but most noticeably in fashion and consumer goods. 

Everyone wants to know what the next “mega” color will be: for many, to have the right color in market at the right time means top-line growth.  Now, it is crystal clear that the “mega” color will be blue.  We have been forecasting blue for the last 5 years, but it is finally hitting its stride, and it is not all blues: specifically it is a deep rich, Cobalt blue.
Today you can find it on the runway. Designers like Ralph & Russo showcase it in their Couture Fall 2014 and Anna Sui in her Ready-to-Wear Spring 2015 collection.  In beauty it is most prevalent in nail polish. Leaders such as OPI and Essie both introduced this color in 2013/14, and shadow and eye liners were not far behind as the color gained popularity.  In consumer goods, it has long been popular in glassware, but we now see it in everything from kitchen appliances to electronics and it was even named Color of the Year by Kelly Moore Paints for 2015. 

But it doesn’t stop there.  Cobalt blue has had its biggest impact on the roads, first seen on pimped-out roadsters, then moving to rims and auto accessories. In 2014/15 it has become the hottest new color.  Lamborghini introduced its Asterion LPI 910-4 concept model at the 2014 Paris Auto show in Cobalt blue. Porsche, BMW and Audi are displaying this color at shows. Even Bentley---long known for its classic and conservative silver position---introduced its new Continental GT Supersports in what they call Moroccan Blue, a shocker for many.  From Bentley to Honda, all car manufacturers seem to be putting money on blue for 2014-15. Just watch while you are out next time: there is a sea of Cobalt Blue. In all types, makes and models, it is a universal color that seems to have cracked all boundaries.  And while it might not always be called Cobalt, the name doesn’t really matter, as you will know it when you see it.  And if I have not made my case for Cobalt as the next color darling then let’s look at the new campaign for ABSOLUT Vodka - known for its annual unique promotion: the brand has rolled out across all international markets a new artistic edition of four million individually designed bottles [1]—ABSOLUT Cobalt. 

Leslie Harrington
The Color Association of the United States

1.  Yes, four million individually designed bottles. As explained on http://www.absolut.com/us/Originality/:  “Taking inspiration from traditional Swedish glass crafts, every bottle of Absolut Originality has a drop of cobalt blue infused into its glass. This colouring technique has been used for centuries in hand-made art glass, but never before has it been applied to create four million original bottles. Added just as the molten glass goes into the mould at 1100°C, the drop of cobalt blue streams down inside the glass creating a unique streak of blue. At that temperature the cobalt is invisible, but as the glass cools off, a beautiful and unique blue infusion appears.” M.H.B.