Guys, I found this post on collectors universe about toning , wavelengths of light. colors, this is not my work, and I only know the screen name of the person to give them credit for it but I found it really interesting. Link is here, the post is below that in case you don't want to follow links. I'm just wanting to give credit where credit is due. Anyways I found it to be thought provoking into the science of toning. https://forums.collectors.com/discu...nding-of-the-color-progression-on-toned-coins "Sunnywood Feb 27th, 2009 1:22PM Several threads have been devoted in the past to a discussion of the thin film intereference phenomenon, which is responsible for the colorful toning we see on silver coins. I would like to delve more deeply into the physics, to see if we can develop a model for understanding and predicting the actual "standard progression" of colors that we observe on silver coins. For example, why does toning start out light gold? Why does it then progress to gold, amber, russet, burgundy, cobalt blue, light blue, lemon yellow, orange, red, magenta, blue, blue-green, emerald green, and so on. Why can't a coin have gold toning that progresses to green, or some other color, rather than following the "standard progression?" It turns out that once you understand the thin film interference mechanism, it is possible to model the colors that we actually observe on the coins. As has been shown before, the light reflecting off the surface of a toned coin looks something like this: Some of the incoming light reflects off the upper surface of the toning layer, and exits as ray B. However, a portion of the light travels through the thin film layer, and then reflects off the denser metal surface of the coin underneath the toning layer, and exits as ray C. Notice that the light in ray C has to travel a longer distance than ray B. The extra distance (d) traveled is approximately twice the thickness of the toning layer (t). [More accurately, d=2t/sin(x), where x is the angle of incidence of the light hitting the coin's surface below the toning. For light coming in at a direct perpendicular to the surface, x=90°, sin(x)=1, and d=2t.] Because ray C travels the extra distance, the emerging reflected light of ray C is phase-shifted relative to ray B. If the extra distance traveled by ray C is precisely equal to the wavelength of the light (or a multiple thereof), then the two re-united beams B and C will be in phase, and there will be constructive interference, or reinforcement, of the luminosity at that wavelength. If the extra distance d is precisely equal to one-half the wavelength of the light (or an odd multiple thereof), then the two re-united beams will be precisely out of phase, and there will be destructive interference, or cancellation, of the light at that wavelength. For intermediate phase-shifts, there will be moderate cancellation or reinforcement. (I am leaving out more details, such as one-half wavelength phase shifts due to differences in the index of refraction of air vs. the toning layer.) So what we have so far, is that for a given thickness of toning layer, and a given wavelength of light, there will be either constructive interference (reinforcement), or destructive interference (cancellation) of the light as a result of the toning layer. As the toning layer thickness increases, the beams B and C go in and out of phase. The next step is to understand how we perceive light. Visible light covers a spectrum of wavelengths from about 400 nanometers (nm) for violet to about 700 nm for deepest red. A rainbow occurs when full-spectrum light (i.e. light containing all wavelengths in the visible spectrum) is refracted by a prism, and separated out into all the different wavelengths. For each color in the spectrum, there is a corresponding wavelength of light. However, the way we perceive light is peculiar to the design of our retina, with its "rods" and "cones" (the photoreceptors). The cones perceive color, and there are three types of cones in the human eye. Each one has a peak sensitivity at a particular wavelength: The three types of cones are B=blue (peak sensitivity at about 440 nm), G=green (535 nm), and R=red (575 nm). The peak sensitivity wavelengths vary in different sources and papers on the subject, but you get the idea. So for example, when our eye sees yellow light, we know it is yellow because of the way our red and green cones respond to it, and our brain translates that into a perception of yellow. The peculiar consequence of our having three specific cone receptors, rather than one that covers the entire spectrum, is that you can "trick" the human eye. If the incoming light contains only two wavelengths - red and green - the eye will perceive it as yellow, just as though the incoming light were at the wavelength for yellow. So I write: G+R=Y. Similarly, blue light plus green light will be perceived as cyan (B+G=C), and blue light plus red light will be perceived as magenta (B+R=M). The combination of all three is perceived as white: B+G+R=W. Because of this, if you take full-spectrum light, and remove all the blue wavelengths, that means only the red and green cones can respond, and the eye sees yellow. This is called the "subtractive" property of light, which refers not to the light itself, but the way we perceive it. Yellow paint, for example, actually looks yellow because it contains pigments that absorb blue light. We can represent this algebraically as follows: W-B = (B+G+R)-B = G+R = Y So, if the toning layer has a thickness that results in the cancellation of blue light, then the coin will appear yellow. Depending on which wavelengths get destructively cancelled or constructively reinforced at a particular toning layer thickness, the exit light will have some combination of components of light that appears white (R+G+B), yellow (R+G), cyan (B+G), magenta (B+R), blue (B), green (G), and red (R). Example: if the incoming light has 4 units each of B, G, R; but the exit light after interference has 3 units B, 2 units G, and 1 unit R, what will we see? We will see a combination of one unit white (B+G+R), one unit cyan (B+G), and one unit blue (B). The result will be a light cyan-blue. If the exit light contains four units B, five units G, four units R, we will see four units white (B+G+R), plus one leftover units of green. This will give us a bright light green. If the exit light contains one unit B, five units G, one unit R, we will see one unit white (B+G+R), plus four leftover units of green. This will give us a stronger deeper (less bright) green. In this way, I can model and predict the color of the exit light. [For the technical among you, the model depends on choosing the right peak wavelengths to represent blue, green and red as seen by our cone photoreceptors. There is some latitude in the numbers chosen, particularly for red; I'm not sure whether the peak sensitivity of 575nm is the right choice, or perhaps a higher number up to 660nm. For our purposes I am assuming the angle of incidence "x" equals 90°, and I am ignoring the half-wavelength shift off the upper surface due to the refractive index change, which actually would imply that light gold toning becomes visible at even lower layer thicknesses.] Here then is a sample spreadsheet: It still requires some tweaking, but this approach is on the right track, and basically predicts what we see on the coins. So for example, why does toning start out in the gold-amber category, before we see any blue? Because blue light has the shortest wavelength of the three cones, at 440 nm. That means the phase shift to cancel blue light is about 220 nm, which requires a toning layer of about 110 nm (ignoring the angle x mentioned above). So the very thinnest toning layers cancel blue before they cancel anything else, and when you cancel blue, you are left with R+G=Y, so toning always begins with faint yellow (gold), deepening to amber, russet, and burgundy, before we suddenly get blue, as predicted in the spreadsheet. This is not quite complete or perfect, but I hope it helps to understand the origin of the magnificent colors we see on toned silver coins, such as this one: Sunnywood" Again, I just want to restate, this isn't my work, in my opinion it's well done and thought provoking, and I just found it really interesting. there are charts and stuff on the link I couldn't get to translate over to here. so if it interests you I suggest you click the link and take a look over there to his charts and graphs.
Hi halfcent1793, Yeah, that's what I thought about it also, it seemed pretty on the money with what I knew avbout rainbows, prisms, light and lumens and Kelvin of light. I'd just never approached it from this angle, and toning was sort of a mystery to me on how it worked and what stage it was in at any given time. I don't know about the nm of thickness, and cones and wavelengths of light but seems plausible at the least. I wonder at how many coins hit the gold to russet color at the beginning of toning, and were deemed "ugly toning" and dipped to remove it when all it really needed was another few decades to keep progressing......
Okay, who's going to be the first one to do an extended-spectrum study on toned coins? Those thin films ought to start absorbing shortwave UV long before they get thick enough to absorb blue, and then there's the whole IR spectrum as they continue to thicken...
It can. And in point of fact, toning does not always follow the "standard progression". It's actually quite common for toning to not follow standard progression. The only thing that is necessary for toning to skip colors or even repeat colors out of progression is for the environmental variables to change. Change the environmental variables and the color progression will change.
Sunnywood’s toned Morgan set was probably the most beautiful set in existence. https://www.pcgs.com/setregistry/do...irculation-strikes-1878-1921/alltimeset/71635
doug was well known and acknowledged for his expertise in toning, and was one of the persons that were instrumental in questioning chemical gassing techniques that coin doctors were scamming the hobby/market with. enjoyable to discuss morgans with him.
