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Name: Pat J.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 6/4/2004


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.


Replies:
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


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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