## Tuesday, August 8, 2017

### The brightest crayon in the shed

People are always telling me that I am not just the brightest crayon in the shed. But which crayon is?

The yellow crayon screams out "Pick Me! Pick Me!"

Well, white is the logical answer, but yellow is pretty darn close to white in terms of brightness. And a very bright yellow can also be very saturated. In this sense, yellow is kind of an anomaly in the color kingdom. All other colors, when they get saturated (color scientist use the term high chroma), get darker.

Why is yellow such a gosh darn bright color?

Munsell agrees

I am not just making up this "yellow is a bright color" thing. Munsell agrees with me, as we can see from the Munsell color pages below, where I have circled (or ellipsed in some cases) the most saturated colors on each page of constant hue.

A selection of Munsell plates with constant hue

Some preliminary stuff

I will explain why yellow is such a gosh darn bright color, but first, I need to get some fundamentals in place.

The Cohans

Those of you who are fans of my blog (I think there are currently seven of you, worldwide) will no doubt remember a stirring blog post I wrote about the cones in the eye. The image below is a recap of the exciting opening premise of that blog, suggesting that the eye has three types of color sensors, and that they are red, green, and blue.

I looked deep into her eyes,
and suddenly and inexplicably found myself hungry for H
aagen Dazs

The excitement generated at the start of the blog post was short-lived. The whole point of the post was that the colors of the cones in the eye were not quite as black-and-white as the first guess of red, green, and blue. But, if you are taking the final exam for Color Theory 101, then red, green, and blue is the correct answer. Red/green/blue is also suitable for our purposes.

Definition of eight basic colors

The RGB Cohans in the eye (not to be confused with G. M. Cohan, who was red, white, and blue) lead to a simple explanation of the eight basic colors in Color Theory 101. This is all based on a lie, but it is a useful lie. If the red cone is the only cone that sees the light, then the color we will perceive is red. Similarly, if the incoming light stimulates only the green cones, then we see green. And guess what? If it is only the blue cones, then we see blue. I bet you had already guessed that one.

How about combinations? If red and blue cones are activated (but not the green cones) then we see magenta. If the activated cones are the blue and green ones (but not the red), then we see cyan. Finally, if blue is left out and the red and green cones get all the attention, then we see yellow.

I said finally, but really I didn't mean finally, since there are two more combinations. We see black when all the cones are inactive, and white when all three are activated.

The table below summarizes the cone responses to each of the eight basic colors in the RGB color system.

 Color Red cones? Green cones? Blue cones? Black No No No Red Yes No No Green No Yes No Blue No No Yes Yellow Yes Yes No Magenta Yes No Yes Cyan No Yes Yes White Yes Yes Yes

Oh... I forgot to mention... these eight colors are all the strongest colors. The cells in the table above that have "No" in them mean "zero light", and the ones with "Yes" in them mean full intensity. There are a zillion combinations where the amount of light captured by the three cones is somewhere between full on and full off. These are not the strongest colors.

The importance of the lightness channel

There is a famous experiment -- very famous, everyone has heard of it -- where an ace was flashed on a screen for an instant. If that instant is really small, then the subjects had no trouble identifying the object as the ace of spades. But if they slowed it down so that the ace stayed on the screen for just a little longer, people got all kinda cornfoozled. When the researcher extended the time just a bit more, then the subjects could readily understand that they were being shown a red ace of spades. (For those who did not grow up in a casino, the ace of spades is supposed to be black, not red.)

This dorky (but famous) little experiment sheds a little light on how our eye/brain works, more particularly on how the color signals are encoded in the neurons that connect the eye to the brain. There is one signal (carried on a neuron) which transmits our perception of brightness.

This is a special signal. It arrives to the brain quicker than the other signals. When the red ace is flashed quickly enough, the signals that further narrow the color down to red don't make it to the brain in time for analysis. A little longer flash, and the red signal makes it, but the signal isn't stable enough for full pattern recognition. A little longer still, and the brain has time to parse the image out and understand the weirdness.

Not only does the brightness signal show up first, but it is far more important than the other signals in terms of our understanding the scene. I am old enough to remember complaining bitterly about being the absolute last family in the whole town to get a color TV. Well, maybe not the last, but my buddy, Gary, had a color TV well before we did. His father worked for Motorola. My father gave me the lame excuse that black and white were colors, so our TV was actually a color TV. It's a wonder that I can function as an adult at all, what with the extreme deprivation and subsequent emotional trauma that I was subjected to!

People leading colorful lives, despite living in a black and white world

The funny thing about B&W television is that it actually worked. I don't recall my father ever setting me down and explaining that light gray could mean the taupe uniforms of Andy and Barney, or it could mean a Caucasian skin tone, or it could mean Ethel's blond hair. Somehow, I just subconsciously understood the color transform, and never questioned it. At least until I enviously watched Gary's TV.

So, why is saturated yellow so bright?

We now have enough background to explain the enigma of bright yellow. One sentence brings it all together: the brightness signal which is fed to the brain from the eye is a combination of the signals from the red and green cones. There are two separate signals that encode 1) the difference between green cones and red cones, and 2) the difference between blue cones and green cones.

All colors that have red and green at the same intensity have the same brightness. A quick look at the table shows that white and yellow are the only colors where red and green are at full intensity.

And that is why yellow is such a bright color.

1. Helmholtz Korlharausxh

2. Very informative, thank you!

4. Thanks for the terrific explanation John! I understood that yellow was the brightest color in the spectrum, but not exactly why. Q: Does the difference between the blue & green cones transmit special brightness information to the eye or just a specific level of brightness?

1. Yellow is what we see when the long and medium wavelength cones (what we call red and green cones) are at full intensity, and the short wavelength cones (what we call blue cones) are low in intensity.

Since yellow is just as bright as white, this tells us that our perception of brightness is strictly a function of the long and medium cones.

Our perception of hue and saturation depend on differences. Simplistically, our perception of redness to greenness is the difference between the long wavelength cones and the medium wavelength cones. That is, if red - green > 0, then we see red, and if red - green < 0, then we see green. The greater the difference, the more saturated the color.

Our perception of yellowness to blueness is the difference between short wavelength and medium wavelength cones, in much the same way.