Ask A Scientist Chemistry Archive

name         Pat J.
status       educator
age          40s

Question -   1. What is the actual chemical added to paints and dyes
that causes the effect of neon color which is beyond the normal spectrum
of brightness for conventional paints and dyes.
2. Where can I find a diagram of the spectrum of these colors in the form
of sine waves?
Neon colors in the form of paints and dyes are always of interest to
newly minted painting and art students. In an attempt to direct specific color
choices and interests into their proper channels creatively, I always
attempt to explain the physics of color and what effects colors will have
on the viewer based purely on their physical impact on the eye. Neon
color is difficult to explain. I attempt to discuss interference that gives the
effect of multiple layers of luminosity but this is confusing.
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There are many "neon" colors. They have a common feature. They contain a
chemical substance that absorbs ultraviolet radiation just at shorter
wavelengths that visible light (about 400 nm) and re-emit light in the
visible range of wavelengths (400-700 nm) [nm = nanometers]. Mixed with
other dyes and pigments gives the "dazzling" colors. There are many
web sites dealing with "fluorescence" "fluorescent pigments" where you can
find a lot -- probably more than you or your students care about. Do a
Google search on the topic "Interrogation de Seconde Licence 2002-2003" and
you will find an explanation of intermediate difficulty.

Vince Calder
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I think you are referring to "fluorescent" colors, like unnaturally bright 
reds/orange/yellow/green, including hunter orange, and some not-as-impressive 
fluorescent blues.


What they do is absorb some shorter-wavelength (bluer) light,
and give some large fraction of it back as longer-wavelength (redder) light, emitted in 
random directions.
So fluorescent colors can have diffuse reflectance slightly greater than 100%, for one or 
more of the eye's three color bands (RGB, Red, Green, Blue).
Our eyes learn from life experience that we never see reflectance greater than 1, so when 
we see one it is a sensory novelty.


The actual chemicals are usually organic dye molecules not much different from all the 
other dyes used in clothes.
Most dye molecules waste as heat, all the power they get from absorbing a little light.
A relative minority happen to be structured right, so that some of the power of one 
absorbed blue or UV photon
is kept alive a short time as an "excited" molecule, and then often escapes as light of 
about 30%-longer wavelength.
Sodium Fluorescein is a very commonly seen bright green fluorescent dye, possibly 
yellow-green in smaller concentrations,
and variations of Rhodamine make a lot of common and good reds and oranges.


Fluorescent re-emission has a limited efficiency and strength, so it is usually 
important to the brightness of neon colors that
they are placed on a very white substrate.  Mixed with any duller colors, they easily 
become nothing special
(with the possible exception of an unusual fading of some colors in yellowish 
illumination).


Fluorescent blues are usually unimpressive because they must be fueled by farther-UV 
light,
and there is not always an abundance of that in the ambient illumination.
Red is typically fueled by green, yellow by blue, green by violet/near-UV.
In a well-lit room you are guaranteed plenty of blue light, so fluorescent oranges have 
it made.


I think you and your art students should dabble with the easiest 3-D rendering software 
you can find,
because these have a concise and very logical way of expressing visual appearances, with 
much "generic scientific" validity.
"POV-Ray" is downloadable freeware, and using words in a text file you can set up
blocks and spheres in a 3-D  arena, with light and "camera" (your eye) in designated 
places around it.
They can be flat-pigmented, glossy/mirror-like, or transparent, in any combination you 
wish to see the effect of.
Rephrased, every surface has Diffuse Reflectance, Specular Reflectance, and 
Transparency,
each on a scale from 0-1 or 0%-100% (or 0-255 for geeks),
for each of the eye's 3 color-bands, Red, Green, and Blue.
Associated with the Transparency is a parameter "Index of Refraction",
which works just like the science term of the same name,
determining how much light rays get bent as they pass through this surface.
(You can also do more detailed things, like plastering a surface with a 
picture/pattern.)



With these numbers you can make painted blocks, colored mirror-balls, and clear glass 
objects.
In {R, G, B} terms, a painted glossy red block might be   [diffuse={80%,10%,0%}, specular
{10%,10%,10%}, trans{0%,0%,0%}]
depending on your taste in red.  It is up to your common sense that the sum of all R's is 
less than 100%.
But you can violate common sense at will, by specifying values of 100-200%, to represent 
fluorescence or glowing.
A neon yellow block with no gloss would be [diffuse={130%,130%,40%}, specular{0%,0%,0%}, 
trans{0%,0%,0%}].
That set of numbers probably looks about like the brightest neon yellow that is 
physically possible,
because you will notice that the light absorbed is 60% of the blue,
and the "extra" light emitted is 30% in Red plus 30% in Green.  Their sum is 60%, 
matching the total absorbed.
Thus we can retain the limited brightness range that conservation of energy imposes on 
real neon paints.


"interference" and "multiple layers of luminosity" sounds more like pearliness or 
iridescence to me.
That is distinctly different than neon colors.
I would say those refer to rough, complicated (ripply or crystalline) surfaces
with high percentages in the specular reflection and most of the remainder in 
transmission to another layer behind it,
plus some random change in these numbers across the surface.
Diffuse reflection makes opals white and pearls dull.


Fogginess, scattered transmittance within a volume, is a more advanced feature not 
available in all rendering software.
It takes more computation time, and the language used to specify it is not yet 
standardized everywhere,
and there is another set of resemblances to physical behavior to be learned and 
understood by the user.


Jim Swenson
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