In General
Light and Color in the Electromagnetic Spectrum
All About Chroma Stuff
Industry Specific Chromaticity Charts


In General

This page augments my page on railroad lenses:

On this page you will find chroma charts for various services such as the railroads, how to read them, and a little bit about light in general.

So what does any of this "stuff" have to do with railroads?

Chromaticity is how the color of a lens is specified.

From Wikipedia: Chromaticity is an objective specification of the quality of a color regardless of its luminance. Chromaticity consists of two independent parameters, often specified as hue (h) and colorfulness (s), where the latter is alternatively called saturation, chroma, intensity (or excitation purity). End Wiki

Specifying lenses in the railroad industry, or any other industry for that matter, has gotten better over time as the charts below from my lens page shows us.  This comes from a PRR publication from the early 1900's.

Today, they are specified in terms of color/hue and saturation, and any combination of the values within a defined area are acceptable.  This is especially noticed with green traffic light lenses, where some look more blue than others.

Websites and other additional information sources of interest:

Light and Color in the Electromagnetic Spectrum

The human eye can perceive wavelengths of the light spectrum between 380nm and 750nm, with violet at the shorter wavelength end, and red at the higher end.

The human eye has two types of light receptors: rods and cones.  Rods transmit mostly black and white information to the brain, and as it gets darker around us, are responsible for what we can see in the dark, this is why it gets harder to see color in the dark.  Rods also make up the majority of light sensors around the eye and give us our peripheral vision.  Cones on the other hand, are mostly responsible for being able to see in color, and there are three different kinds, seeing light in short (blues), medium (greens), and longer (reds) wavelengths.  Blue is the hardest color to see as more light energy is required for a full response from blue-violet cones, compared to green or red. (from the Pantone page)

Objects that appear white, reflect all wavelengths of light.  Black objects absorb all wavelengths of light.  If you have a red apple, the only color getting reflected is red, all others are absorbed.

Additionally, If you compare mixing colors with light, as opposed to paint, you will get different results.  This has more to do with the property of the paints than it does with light.  If you mix red and green light sources, such as with an RGB LED, you will wind up seeing yellow.  If you mix those two with paint, you get a yucky brownish color.  But that's whole 'nuther subject :-).

Lenses for the railroad industry have to take a (basically) white light source, and filter out the unneeded colors.  If you're into radios and RF stuff, this is referred to as a bandpass filter.  A bandpass filter only lets through signals of a certain frequency, and in the case of light, it is specified as wavelength instead of frequency.  In early specifications, this is referred to as the "transmission value" - Today, they would be ordered as AAR Green and so on, using the chromaticity chart shown below for the railroads.  The chart below illustrates the bandpass concept:

Below are a few charts that show where the visible light spectrum is in relation to other "things" of/in the electromagnetic spectrum. The spectrum starts off at "the bottom" with sound waves (frequency wise).  We can not only hear sound waves, but that wavelength can also be transmitted as radio waves.  Frequencies this low are known to penetrate water, so they are used to communicate with submarines and require big antennas due to their wavelength.

As we progress in frequency, going ever higher, we go thru a variety of radio frequencies - low, to medium, to high, then VHF (Very High Frequency) and UHF (Ultra High Frequency), and then finally microwaves before getting into the light spectrum with infra-red.  Above purple, we have UV (Ultra Violet), x-rays, and then cosmic rays.  If there is anything above this, we so far, don't have the technology to know.

Keep in mind that when we talk about anything on the electromagnetic spectrum, there are several ways of expressing the signal - frequency and wavelength, and to a lesser degree, energy in Ev.  When we get to the light spectrum, we can also add the usage of another term into the mix: angstroms, but its usage is declining, being replaced by the nanometre (nm).  One nanometre equals ten angstroms.  From Wikipedia: The nanometre is also commonly used to specify the wavelength of electromagnetic radiation near the visible part of the spectrum: visible light ranges from around 400 to 700 nm. The ångström, which is equal to 0.1 nm, was formerly used for these purposes. End Wiki

So here are a few charts to help visualize where the visible light spectrum falls in the electromagnetic spectrum.  Keep in mind, that many charts will start with low frequencies on the left, increasing as we go to the right, which puts infra-red on the left side of the color bar.  Those charts that use increasing wavelength as the main measurement (in nanometres), will have ultra-violet on the left side of the color bar because the frequency is decreasing as we go to the right.

Going beyond, and slightly off topic....  You will notice, in the third chart above, that they mention "temperature".  You will see this when you go to buy light bulbs, as white light is often expressed in these terms, with "cool white" having a bluer tint to it, and "warm" light bulbs will appear more red.

I like this representation because it's well done artsy-fartsy :-) :-)

Now Onto This Chroma Stuff

There is a lot to chew on here.  For additional information on the chromaticity chart, and why it looks the way it does, please refer to the links at the top of the page, for they go into much more detail than I have room for here.  I will attempt to touch on some of the main points so you have some idea how all this stuff works and relates to lens color.

So this is a Chromaticity Chart, this one nicely displays the colors pretty much as they actually are, and the center of curve is at "C", which is pure white.

  Found at, but the page doesn't exist anymore.

Notice that in the chromaticity chart, that the spectrally pure colors are on the outer boundary, or edge of the horseshoe shaped curve....
This is the curved line A-B-C on the left drawing below.

Points A and C are considered the endpoints of the curve.

Straight line A-C, between purple on the bottom left, and red on the bottom right, is known as the non-spectral "line of purples".  Colors along this line do not have any wavelengths associated with them, because these are "not real colors" --  these colors do not correspond to a "natural" color, so you won't find those colors in a rainbow or the bands of light made from a prism splitting white into its component colors.

These charts offer you an idea of the colors, where they are on the chart, what their wavelength is in relation to the color, and the fact that as we go in towards the center, things get progressively whiter (less saturated/more diluted, but of the same color).  Most of the charts way below are self labeled as to the industry they cover.


You'll notice in these charts, that the X and Y axis' do not have units associated with them other than the numbers - zero to 0.8 and 0.9.  That's because, to specify a color, we only need to give the coordinates of a point on the chart.

Let's say we want to have some green lenses made, we'll refer to the green portion of the FRA chart (A).

If we specify a point such as 0.1x, 0.6y (at the yellow dot), we will get a green lens made of a certain color and saturation (B).  But it won't be a "pure" green, it will be slightly diluted because it is not on the outside edge of the green envelope.

If we want the lens to be a "pure" green, we could select the point 0.01x, 0.65y (C), which puts the green saturation at 100%.  In fact, if we picked a point anywhere along the purple arc, the green color would be "pure".  Maybe this diagram can help to explain:

If we move the point to the orange dot as in (D), the lens would be slightly more "yellowish", but not a lot because the envelope for FRA green doesn't go all the way over to the yellow-green area until we get to the less saturated region towards the white.

One thing most of the chromaticity charts don't do well, is to show an accurate representation of the saturation level as we move from the outside to white in the center.  Using Corel Draw to help illustrate what I mean, look at the following drawing.  I use the points 1, 2, and 3 in (A) above as my starting points.  We'll call point 1 as the greener of the three starting points.  If we move to 2, it becomes slightly more yellow, and if we start at 3, it becomes more blue.  The color dots correspond to the points in A, B, and C, and the yellow and orange dots are not completely saturated - they are ever so slightly "diluted", or less green, but of the same color.  Are you confused yet?  The bottom bar gives us the colors (wavelengths in nm) as we go around the outside of the chroma chart, and the arrows point to where the other three color bars are on that spectrum between bluish green and yellowish green.

I touch on this on my lens page, but another thing to keep in mind as we think about railroad signal lenses, is the light source in relation to the perceived color of the signal.

If we look at lenses made for semaphore signals when they were lit with a flame, you will notice the lenses appear typically more blue than green, when comparing them to a typical green lens of today.

This is because the design of the lenses back then had to take into account the spectral purity of the "white" light produced by a flame in comparison to that of an incandescent light bulb.  A flame puts out very little in the blue and a little in the green portions of the spectrum, but a lot in the red and yellow portion.  So, to get an indication that looked green, they made the lens "more blue".

Industry Specific Chromaticity Charts

Above, AAR chroma chart for the railroad industry.


I love trains, and I love signals.  I am not an expert.  My webpages reflect what I find on the topic of the page.  This is something I have fun with while trying to help others.

Please Note:  Since the main focus of my two websites is railroad signals, the railfan guides are oriented towards the signal fan being able to locate them.  For those of you into the modeling aspect of our hobby, my indexa page has a list of almost everything railroad oriented I can think of to provide you with at least a few pictures to help you detail your pike.

If this is a railfan page, every effort has been made to make sure that the information contained on this map and in this railfan guide is correct.  Once in a while, an error may creep in :-)

My philosophy: Pictures and maps are worth a thousand words, especially for railfanning.  Text descriptions only get you so far, especially if you get lost or disoriented.  Take along good maps.... a GPS is OK to get somewhere, but maps are still better if you get lost!  I belong to AAA, which allows you to get local maps for free when you visit the local branches.  ADC puts out a nice series of county maps for the Washington DC area, but their state maps do not have the railroads on them.  If you can find em, I like the National Geographic map book of the U.S..... good, clear, and concise graphics, and they do a really good job of showing you where tourist type attractions are, although they too lack the railroads.  Other notes about specific areas will show up on that page if known.

Aerial shots were taken from either Google or Bing Maps as noted.  Screen captures are made with Snagit, a Techsmith product... a great tool if you have never used it! 

By the way, floobydust is a term I picked up 30-40 years ago from a National Semiconductor data book, and means miscellaneous and/or other stuff.

Pictures and additional information is always needed if anyone feels inclined to take 'em, send 'em, and share 'em, or if you have something to add or correct.... credit is always given!  Please be NICE!!!  Contact info is here

Beware: If used as a source, ANYTHING from Wikipedia must be treated as being possibly inaccurate, wrong, or not true.


NEW 3/15/2009, SEP01/2013, OCT02/03/04/2022
Last Modified  04-Oct-2022