Re: [Phys-l] experiments with dyes (was: happy equinox)

[John Denker <jsd@av8n.com>]

On 03/24/2008 10:32 AM, chuck britton wrote:

> I suspect that SOME of our non-understanding comes from 'symmantics'  
> while other some comes from our mixing the two (valid) viewpoints  
> that color represents.

Absolutely.

Ideas are primary and fundamental;  terminology is of secondary
importance.  Terminology is only important insofar as it helps us
formulate and communicate ideas.

To express the same idea in different words, we have _misnomers_ and 
_misconceptions_.  They're not the same, although there is often lots 
of overlap, i.e. lots of cases where misconceptions are hiding 
behind (or caused  by) misnomers.  
 -- A misconception is a bad idea.
 -- A misnomer is bad terminology


          ++++++++++++++++++++++++
          +  misconceptions      +
          +                      +
          +                      +
          +    |-----------------+----|
          +    |                 +    |
          +    |                 +    |
          +    |                 +    |
          +    |                 +    |
          +    |                 +    |
          +    |                 +    |
          +    |                 +    |
          ++++++++++++++++++++++++    |
               |                      |
               |                      |
               |           misnomers  |
               |----------------------|



Arguing whether the bluish dye in an inkjet printer should be called
"blue" or "sky blue" or "cyan" is just a problem with the terminology.
It is a serious problem in the sense that it causes all sorts of
misunderstandings.  It could be fixed by changing the terminology,
although I have little hope that this will happen anytime soon.

  I think a good start would be to concede that the vernacular term 
  "blue" refers to a range of colors, not a specific color.  No
  professional artist would think for a moment that sky blue was the
  same as cobalt blue.

  I consider terms like Red, Green, and Blue (and hence RGB) to be so
  ill-defined as to be not worth arguing about.  In contrast, if we
  talk about the sRGB color space (as used in a computer display),
  the "s" stands for standardized, and in that context we know exactly
  what R, G, and B mean.  I sometimes call them sR, sG, and sB.  We 
  can quantify them in terms of CIE chromaticity values.

  Tangential remark:  The whole idea of three primary colors is only
  a crude approximation.  The sRGB color space spans less than half
  the area of the CIE chromaticity diagram.
    http://www.av8n.com/imaging/img48/cie2.png
  The situation for dyes (as opposed to lights) is even worse.  The
  printer on my computer has *six* colors of ink, which overcomes
  some of the worst limitations inherent in using only three colors.
  (I'm working on a fuller discussion of how this works.)

Another good starting point would be to get rid of the terminology of
"subtractive" color mixing.  This is the sort of misnomer that leads
directly to misconceptions.
 -- The mixing of lights is additive.  It is linear (to a good approximation,
  under mild conditions).
 -- The mixing of dyes is not subtractive.  It is not linear, not even
  to a first approximation.  It is *multiplicative*.  (If you take
  logarithms, the logarithms are subtractive ... but that step of
  taking the logarithm is highly nonlinear, and must not be glossed 
  over!)

By way of analogy:  When we put resistors in series, the resistance
is additive.  It's a linear proposition.  Meanwhile, if we put
resistors in parallel, you wouldn't say the resistance is "subtractive".
The true relationship is not subtractive, and it's not linear.  The 
opposite of "additive" is not always "subtractive".  To repeat:
 -- The combination of lights in parallel is additive.  It is linear.
 -- The combination of dyes or gels in series is not subtractive.  It
  is not linear.  It is *multiplicative*.

This is why I am fond of the demonstration, using "yellow" food coloring,
that 
   yellow plus yellow makes orange                [1]
and
   orange plus orange makes red                   [2]

This is very easy to observe.  It instantly and totally destroys any
temptation to believe the elementary school Red/Yellow/Blue primary
color mixing theory.  There is no way you can represent equation [1]
or equation [2] on any "color wheel" of the sort that the alleged
theory uses to predict "subtractive" color mixing.

This is an example of a misconception separate from any misnomers.  In
equation [1] and equation [2], it doesn't matter what terminology you
use;  you could invent arbitrary new names for the colors and the
equations would still totally contradict the "subtractive" color-wheel
theory.

On the other side of the same coin, I like this demo because the right
answer, i.e. the observed behavior, can be nicely explained in terms
of simple physics.  The intrinsic color of a dye is not some /sui generis/ 
abstraction, subject to its own special axioms, divorced from the rest 
of the world.  It well understood in terms of its absorption spectrum.


===========

To summarize:  Suggestions:  As a passable starting point:
  --- Demo:  "yellow" dye as a function of concentration and thickness.
  --- Demo:  spoon with three shadows demonstrates secondary colors
        http://www.av8n.com/imaging/dye-spectra.htm#sec-secondaries
  --- It is better to talk about the well-specified sRGB primaries rather
   than some vague "RGB" primaries for lights in parallel.
  --- sRGB does *not* span the whole space.  Not even close.
  --- It is OK to talk about CMY primaries for dyes or gels in series.  
   Cyan, magenta, and yellow are reasonably specific (unlike "blue" which 
   is wildly non-specific).
  --- Avoid BRY primaries for dyes or gels.  Non-specific and misleading.
  --- Avoid any notion of "subtractive" color mixing.  Putting gels or
   dyes in series is not subtractive, it's multiplicative.