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Purple: It doesn't exist. How do we see it?

I first came across this topic when I was looking into how rose lenses work and why they are so contrast-enhancing in some environments, but downright useless in others.

What I discovered is that magenta, rose, purple, etc. do not exist at all.

Magenta is an ‘extra-spectral color’, meaning that it is not found in the visible spectrum of light, which is why it is not in a rainbow.

You can “see” an object when light reflected from an object enters your eyes and strikes the photoreceptors inside them. I explain this in much more detail here LINK. But briefly, we have three types of cones in our eyes that are sensitive to blue, green, and red light.

When your brain sees red light your red cone fires. Same with blue and green. If your brain gets yellow light, it makes both the red and green cones fire partially, and your brain will interpret that combination as yellow, since it is in between red and green. This is really astounding, as your brain is perceiving something real it cannot actually measure.

In physics, you cannot mix photons. But in biology, the brain can mix what the eyes pick up. That's how we can see hundreds of shades of color. And that's why you can see yellow even if there is no yellow light present since a mix of red and green would result in the same yellow perception as yellow light itself. And that is why computer or phone screens only need red, blue, and green lights to make all the colors you are seeing right now.

When it comes to the color of materials there is a lot of misunderstanding. Many people think that a yellow object, for example, reflects only yellow wavelengths (those at about 580nm) and absorbs the rest. But it isn't like that.

Most of the colors that we see do not correspond with one wavelength of light but are composed of a mixture of wavelengths.

This can be shown using a reflectance spectrum, which is the best way to see what light makes up the color that we see from an image.

We can measure a reflectance spectrum using a reflectance spectrophotometer. This instrument illuminates the object with light at all the wavelengths in the visible spectrum and measures how much light is reflected by the sample at each wavelength. This gives us spectral reflectance factors which generally range from 0 (total absorbance and no reflectance) to 1 (total reflectance) at each wavelength.

Let’s take something purple and see what the spectral reflectance looks like. First, this is the purple I am talking about.

Now let’s look at the reflectance spectrum.

Notice that reflectance is lowest at around 540nm. This is in the green region of the spectrum. Purple objects generally absorb in the middle of the spectrum and reflect both long and short wavelengths. But notice that the reflectance factors are less than 1 at every wavelength. This means that to some extent the object is absorbing light right across the visible spectrum but the absorption is strongest in the middle of the spectrum. Most crucially, notice that the purple object does not only reflect the blue and red wavelengths. It reflects strongly in the orange region (around 620nm). There is appreciable reflectance (around 20%) in the yellow region (around 580nm). There is even about 5% reflectance in the green region.

Light is not colored; it just looks colored. The distinction is critical to understanding color. Objects don’t look a particular color because they reflect the wavelength that corresponds to this color. The vast majority of things (look around your room now; almost all of those) have quite broad reflectance spectra; they reflect lots of different wavelengths.

Purple is different than yellow though. Yellow wavelengths do exist, though we cannot see them. Our brains see a mix of red and green light and ascertain that what we are seeing is yellow - and it gets it

right. Purple is a mix of blue and red, short and long wavelengths, and our brain makes up the color purple, but there is no actually corresponding wavelength. It wants to see a color that makes sense to be in between blue and red, which would be green. But our green cones are not firing, so it cannot be green. In other words, purple can be described not only as a mix of red and blue but in terms of optics, it is also simply the relative lack of green light.

And that is why rose lenses are some of the most interesting and useful lenses out there (you knew we would get here eventually).

The sensitivity of the human eye varies with wavelength or color. Because of the physiology of the human photoreceptors, the sensitivity of the eye falls off rapidly for colors in both the blue and red ends of the visible spectrum and is highest for wavelengths near the middle of the spectrum. This means that considerably higher quantities of blue or red light are required to elicit the same sense of brightness as, for instance, yellow-green light. A plot of the relative sensitivity of the eye as a function of wavelength is known as the relative luminous efficiency function. During the day, the eye is maximally sensitive to yellow-green wavelengths near 555 nm; this is the photopic response of the eye.

Now look at a rose lens spectral profile.

And when we overlap the two:

Rose lenses almost perfectly inversely correspond to the sensitivity of the human eye. While we get a much higher brightness from green light relative to red or blue light normally, with the green light reduced relative to the red and blue light the result will be a similar brightness through all colors.

Rose lenses and their effects and best uses are discussed much more here LINK.


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