The Mysterious Science of Colorimetry

Not long before descending into the grey misery of the Great War, the Europeans decided to standardise the way we talk about light and colour and so in 1913 the CIE or International Commission on Illumination was born. The CIE acronym, by the way, is from its French name, la Commission Internationale de l´Eclairage (which, let’s face it, sounds much better than the English) and their first major work was the creation of the standard observer (which I’ll explain in minute), and then on to produce one of the most quoted and least understood ‘infographics’ in history. The oddly-beautiful but impenetrable 1931 chromaticity diagram…

If you take even a passing interest in colour – as a designer or photographer or artist or anyone really – you’ll come across this little gem in your reading but I’m guessing you won’t understand a bar of it unless you’re some distance down the road to colour or vision science. If that’s the case you might find this article a little pedestrian for your taste. For the rest of you, read on…
The diagram is supposed to represent the range or gamut of human colour vision. The horseshoe shape, which somehow seems wrong (surely it should be a circle or something?) is to allow for a perceptually eq
Sometimes it’s presented along with device gamuts like Adobe 1998 or sRGB, particularly with regard to colour management in the graphic arts but we’ll get to that a little later.

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Why We Can’t See Light

Now here’s a puzzle for you: why is it that we can’t see light? I mean, we can see objects that are illuminated by light, but we can’t see light itself. As a newly-minted researcher into colour perception, this question drives me nuts…
Take a torch and switch it on. Can you see the beam? Nope. Well, you might be able to see it if it’s a misty night or you’re in a dusty room or if there’s plenty of smoke about but if the air is clear of reflective particles you won’t see nothin’. Just as you can’t see light from the sun during the day, or from the distant stars at night when you dance by the light of the moon…
Of course you can see a light when you look directly at it. You can see the sun itself and you can see a lightbulb. And if that light is shone through a material which is not completely opaque, like stained glass or coloured gel, you can see its glow. But what, exactly are you seeing when you look at that light source? 
We seem to have stumbled on some kind of paradox. But on reflection (pardon), perhaps the more important question is not so much why can’t we see light? Rather, acknowledging that we can’t see radiation of any kind, we should ask: why can we see a light (source)?
The light-sensitive components of our eyes – the so-called rods and cones that respond favourably to a tiny range of radiation within the much broader electromagnetic spectrum – don’t see the ‘visible’ radiation at all, despite what it says on the box. Rather, they respond to light reflected off physical objects. And specifically, only the part of the spectrum that hasn’t been absorbed by the surface we’re looking at.
Orange and Un-oranges
It is generally accepted that we see an orange as orange because the un-orange part of the spectrum is absorbed by the surface of the fruit. The remaining radiation is reflected away from the surface and that, in turn, enters our eye. Which is where the real magic begins…
The light radiation, travelling as photons or particles-that-behave-like-waves, first arrives at the cornea – the outer part of the eye covered by contact lenses if you happen to wear them – then moves through the pupil (the aperture or hole in the iris which varies in size according to the volume of ambient light); is cleverly focused by the lens; moves on through the ridiculously-named jelly-like wasteland of the vitreous humour; then finally arrives at the retina where the heavy lifting is performed.
The retina contains a stack of cells which respond in their own special way: the thin strips we know as rods are by far the most plentiful – humans have a massive matrix of about 120 million cells per eye – which are mostly sensitive to light volume (that’s luminance or brightness in the graphic-arts world); and the 6 million-odd collection of red, green and a relative few blue-sensitive cells which kinda look conical although I, for one, would never have described them as ‘cones’ if it was my job.
If you want to get picky they’re not specifically red-green-blue sensitive at all but each tend to respond to the longer (reddish), middling (greenish) and shorter (blueish) wavelengths of the spectrum with plenty of overlap. It’s the data collected from the relative stimulation of these cells that allow our brains to discern colour.
The rods and cones do a remarkable thing: they convert the light particles into electrical signals by a process called phototransduction before passing the signal on through the optical nervous system via synapses and connections in the same way that other brain functions are transmitted. There are some double-ended (bi-polar) cells first, then an array of ganglion transmission cells. There are fewer of them than the rods and cones (about 100 cones connect to one ganglion) and we didn’t know much about them until the 90s but now we think they’re important for non-imaging reasons like psychological responses to colour, circadian rhythms etc.
Anyway, after all that is said and done, the relative electrical charges which were sympathetic to the original light source pass on to the brain by way of the venerable optic nerve. After that, it’s a process of neurology which leads to the business of perception and recognition. So when certain rods and cones are stimulated in a particular way then the brain determines it must be orange we’re seeing. Simples.
But is it orange?
We call it orange without really knowing if there’s a universal experience of orange, or whether my orange is the same as your orange, or without prejudice as to whether orange is good or bad or indifferent, or whether it makes you feel happy or sad, randy or glad etc. Strangely, we literally call it orange because of the orange fruit, not the other way around…
Anyway, we can chalk this up alongside other uncertainties of human experience like, ‘does a tree make a sound when it falls (without audience) in the forest?’ and ‘does the refrigerator light go out when we close the door’ or even ‘what happened to Schrödinger’s cat?’ But surely there’s a way we could determine whether the experience of orange is in some way universal or at least common?
Well, not exactly, but there is an area of the brain we call the colour centre which is critical in the perception and processing of colour signals received by the eye, which ultimately results in colour vision. With this in mind, we can map the stimulation of the brain in response to the presence of a colour and that, at least, is more or less consistent.
But what about the philosophical question that people always want to talk about when the conversation becomes colourful: are we all seeing or experiencing the same thing when we talk of, or see, a given colour? Is my orange the same as your orange?
Well… That’s a bit meaningless to be honest. I mean, are we all hearing the same note on the piano? Feeling the same softness of a kitten’s fur? Experiencing the same, um, orgasm? I have no idea but let’s get back to the problem of not seeing light…
If you read anything about colour within the graphic arts (Photoshop etc), you’re bound to learn about red-green-blue or RGB colour as opposed to the primary (irreducible) colours we learned about in school which were (well, still are) red, yellow and blue. If you’re a fine artist (I love that expression) – a painter or a sculptor working with physical paint or glaze – then it’s this second world of passive, subtractive, colours that appears to make sense.
The more paint you add to the page, the darker it gets, right? And you will recall that if you combine yellow and blue paint you get a kind of green; red and blue you get purple. And that goes for anything that isn’t transmitting light like our friend the orange, the clothes we’re wearing, or the paint on the wall…
But it’s the opposite effect that happens when you look at your screen or shine coloured lights onto a stage or sit in a cinema. In that case, the more coloured light you throw at the subject the brighter it gets. We call it an additive colour space because it adds up to being white light when you combine it all together.
Isaac Newton is usually credited with figuring out that you can reverse engineer white light into its component parts by encouraging the light to spread its relative wings out from slowest to fastest through a prism. He could see the same colours that you can see in a rainbow and decided there were seven colour bands with a whole lot of others in between.
It’s from Newton’s observations that we get our famous ROY-G-BIV rainbow colours although that’s a bit of a stretch really. There are more like 1o million colours that we can discern as humans – far fewer of them are in the rainbow to be fair (the spectral colours) but there are a lot more than 7. But Newton really wanted there to be 7 because there are 7 notes in the musical octave and it would be so poetic if they were a perfect match! Well they’re not but there are indeed some fascinating parallels in the perception of light/colour and sound/music which we’ll look at in another article…
Anyway, 7 or not, it is indeed possible to reduce the additive colours down to just 3 primary building blocks which when combined evenly produce white. That’s how we get colour on our computer screens and televisions and devices: tiny little charged liquid crystals (or light-emitting diodes these days) are arranged in filtered clusters of red, green and blue which from even a short distance give the illusion of the colour spectrum very effectively.
Interestingly, if you take just two of the additive primaries – red and blue – you get a blueish red or pink known to printers as magenta – the very same red as the primary colour of the subtractive space. If you combine red and green alone you get yellow; combine blue and green and you get a greenish blue known to printers as cyan and to the rest of us, as turquoise or aqua blue – the same primary colour blue as the subtractive space. A case, you might say, of moving through Alice’s looking glass and seeing everything in reverse…
Why CMY(K)?
So what about CMY? Well when offset lithography began to gain traction at the beginning of the 20th century as the most practical form or commercial printing, engineers figured out that if you put coloured dots on a page or other substrate (material) very close together then they tend to blend together to form new colours.
If you took that turquoise-cyan as the blue primary, that pink-magenta as the red primary and, well, yellow, you could pretty much get the entire spectrum of colour required to reproduce anything (well, nearly anything) we can see in the real world. You need to alter the density and frequency of the dots using a process called half-toning but that’s more or less the way it works.
Along the way they figured out that you really need to add in black (that’s the K or key colour) to get darker colours or they’d end up a bit wishy-washy. And voilà: CMYK – the basis of process colours as used in commercial lithography and likewise the components of toner-based laser printers like you may well have in your office if you have a real job (unlike me).
Converting from RGB to CMYK is something we do in the graphic arts under the banner of colour management. Now, for future reference this is absolutely not merely a matter of opening your RGB photographs in Photoshop and changing the colour mode to CMYK because in so doing you ignore the kind of printer, substrate and ink that will eventually reproduce the colour but let’s hold off on that for now…
Is RGB colour visible at all?
So can we even see RGB colours? Given that RGB colour is transmitted as light radiation how can we see it? Well we only really see the net result of its action in falling on an RYB surface which in turn appears illuminated. So that would be a no in terms of the everyday, passive world of things. But that doesn’t account for what you’re reading on your screen right now. That’s RGB light-colour you’re seeing…
OK, here’s what we know…

We cannot see radiation of any kind.
Radiation within a certain, tiny spectrum of around 400 to 800 nanometers in wavelength, causes objects to be seen by we humans.
We can’t see the reflected light from those objects but we can see the objects.
If the light shines through something translucent it appears to glow.
We can see the glow of a light source but not see the light leaving it: once it leaves it disappears!
RGB light falling on a coloured RYB surface allows that surface to be seen and the resulting colour we see is a consequence of the bias of the incident light (the colour of the light falling on the surface) and the colour of the surface.
If that surface is highly reflective, light can leave that surface and excite another surface with its RGB light but we still can’t see the light – only the surface of the objects which the reflected light falls on and that surface is RYB in nature.
We can’t see RGB light.

And the answer?
I suspect there may be some kind of explanation in terms of energy which disperses on leaving its source. Visible light is technically non-ionising, or relatively weak electromagnetic radiation. That is, it only carries enough energy to excite electrons and sub-atomic particles rather than enough for them to break their covalent bonds, leave their homes and families, and wreak havoc with DNA and anything else in their path… That’s the stuff of ionising, nuclear radiation such as X-rays and gamma rays which, by the way, we can’t see either but do allow us to see further than light can travel under its own steam. That’s why we x-ray our limbs to see beneath the skin to the bone within and why the operators of such machines protect their private parts with dense metals that can withstand their penetrating gaze…
But while therein might lie some kind of descriptive, technical explanation, it does little to explain such an odd paradox: no matter how hard we try, we can’t see light. And if we accept that in good faith and humility, why can we see a light at all? 
Feel free to comment, if you can shed some, um, light…

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Generative Design

Rune Madsen is an artist-cum-programmer who uses, as he puts it, “programming languages to create things with the computer” and believes in “simplicity, organized complexity, and that the pragmatic and poetic are inseparable”. Nice. Me too. I think.
Anyway, I’ve been very interested in the idea of generative design and art lately. The notion that the representation and repetition of form and shape can be generated by some kind of algorithm gets me going…
There are lots of examples in the fine arts: MC Escher, Sol Lewitt and Piet Mondrian all had this repetition of form in their work but more recently, people have been using code as a kind of art form in and of itself..
Here are some examples I’ve been working on myself using the p5 JavaScript libraries.
Rotating Cubes

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Filling in the Blanks: A Prehistory of the Adult Coloring Craze

Its dizzy heights may have passed, but the fad for adult coloring books is far from over. Many trace the origins of such publications to a wave of satirical colouring books published in the 1960s, but as Melissa N. Morris and Zach Carmichael explore, the existence of such books, and the urge to colour the printed image, goes back centuries.

Uncolored portraits of the artists involved in the production of Leonhart Fuchs’ De historia stirpium commentarii insignes — Source.
For many publishers around the world 2015 was, fiscally speaking, an excellent year — a welcome boost in an otherwise uncertain decade. But this upturn had a perhaps surprising source: coloring books for grown-ups. What strange winds conspired to suddenly urge adults in their droves to take up colored pencils again? Whatever the reasons, sales rocketed: Nielsen logged sales of 12 million for the category in 2015, up from a measly 1 million the year before. In February 2016, with the craze still going strong, New York Academy of Medicine Library gave birth to a new initiative called Color Our Collections Week, a scholarly take on the coloring trend. Now in its third year, the campaign sees, on the first week of February, archives, special collections, and libraries take to social media with individual images and even entire books compiled from their holdings for the public to color. While these chosen works are all in the public domain, and so can technically include (in the US at least) works published up until 1924, the images in these coloring books more typically hail from the fifteenth through eighteenth centuries. And it is in these images — published in the centuries prior to the advent of color printing — that we can see a precedent for this seemingly modern fad. While it may seem like simply jumping on the adult coloring bandwagon, Color Our Collections Week, with its naturally historical focus, is actually tapping into (and shedding light on) a tradition much older.
Last year, the New York Academy of Medicine Library chose an image from Leonhart Fuchs’ monumental 1542 botanical work, De historia stirpium commentarii insignes (“Notable Commentaries on the History of Plants”), to promote the event. An archivist from the History of Science Collections at the University of Oklahoma chimed in on Twitter to say their own copy of this book had already been colored in.

The page from the University of Oklahoma’s colored version of Leonhart Fuchs’ De historia stirpium commentarii insignes — Source

Another colored edition of Leonhart Fuchs’ De historia stirpium commentarii insignes — Source (Wellcome Library)
Should we be surprised by this? Color Our Collections Week might give the impression that these images, from the era before colored printing, are at last being colored — rescued from their hitherto drab monochrome existence. Yet printed images from the early modern period were regularly colored by hand.
The practice goes back to the earliest days of print in the fifteenth century. Artists, printers, booksellers, consumers, and readers all applied color to originally black-and-white images. Before Gutenberg’s innovation of the moveable-type press, both woodblock and engraved prints, single sheets with printed images, were popular in Germany and parts of Central Europe. They were used in various ways, and many people did what we might do with them — hung them on the walls of their home.
With the emergence of the printed book the coloring trend continued. Colored illustrations were common in medieval manuscript books, most notably in the intricately illuminated manuscripts produced by monastic institutions. The early printed books from the fifteenth century and after often imitated the textual design and illustrations of these medieval manuscript books. Indeed, illuminated manuscripts and printed books were not mutually exclusive: some printed books contain illumination, while some manuscripts have painted prints pasted into them. It would seem that at least some early printers and readers attempted to create color illustrations for these works the only way they knew how: by coloring the pictures themselves.

This 1493 herbal shows how early printed works imitated manuscripts — Source (Wellcome Library)
The images below further demonstrate this transition from medieval to early modern book production, and the role colored illustrations played. Both are from De Claris Mulieribus, a fourteenth-century book by Giovanni Boccaccio (author of the Decameron). This work was a compilation of biographies of women, real and mythical, famous and infamous. It was first circulated as a manuscript, and surviving examples are richly illustrated with images of the women they discuss. The book was among the first to make the leap from manuscript to print, and the illustrations came with it. In order to recreate the feel of previous versions of the work, it needed colored illustrations. The images below are, fittingly enough, of the painter and sculptor (and apparently prolific creator of self-portraits) Iaia of Cyzicus (also known as Marcia). The first two are from manuscript versions of the work, showing Marcia sculpting and painting.

Marcia sculpting, image from a 15-16th century version of Giovanni Boccaccio’s De claris mulieribus — Source.

Maria painting, detail of page from a 1403 version of Giovanni Boccaccio’s De claris mulieribus— Source.
These next two are from printed editions of the work. The Latin edition has some illumination of the letters, while the German book’s image is fully colored.

Image of Marcia, uncolored, from a 1473 Latin version of Giovanni Boccaccio’s De claris mulieribus — Source.
Image of Marcia, colored, from a ca. 1474 German version of Giovanni Boccaccio’s De claris mulieribus — Source.
Most illustrations found in books from the early days of print are in the form of woodcuts and etchings. Woodcuts were most compatible with moveable type because both used relief printing, and early printers could easily print a page with both text and illustrations.
Because of the carving and printing process, woodcuts have simpler designs with less shading. They therefore make for excellent coloring pages, and Color Our Collections participants frequently choose woodcuts for their images. Moreover, art historian Susan Dackerman argues that they were meant to be colored. Many of these color prints were created in a workshop setting, with an engraver, printer, and colorist working together. The “vast majority” of surviving fifteenth-century woodcuts are hand colored, and they were produced in the tens of thousands in the fifteenth century.1
Some images, like this fifteenth-century German woodcut of Christ on the cross, are only complete once colored. In this case, angels hold cups to catch blood that needs to be added with paint. The National Gallery of Art owns a number of examples of this woodcut, each differently colored. Some have been left uncolored, and a couple have only the requisite blood added to complete the image. Among those more fully colored, we can see that quite a bit of artistic license was taken.

Christ in Color (ca. 1490), more fully colored, with clouds and other details — Source
According to Dackerman, twentieth-century art historians and collectors denigrated color, seeing it as nothing more than a way to hide the flaws of poorly-executed engravings and woodcuts. Well-executed prints, they argued, needed no color at all. This disdain for colored prints helped to obscure their place in art history. This line of argument harkened back to the debates that emerged during the Italian Renaissance over whether design or color were most important (disegno/colore).
In many of these images, the paint seems hastily applied. This haphazard coloring was often a result of the artist having many prints to paint rather than a lack of skill. Artists applied paint freehand, using a brush, but they sometimes employed stencils made from extra impressions of the images in order to paint more quickly.
Many works were colored not by professionals, but by readers. A lot of the examples we have found of hand-colored illustrations come from botanical works and herbals. For example, a copy of John Gerard’s Herball (1636), with selective images colored in, suggests it was the reader who painted it, perhaps as a way to record plants he or she had seen in person. Botany and painting were favored pursuits of genteel men and women in this period, so it’s not surprising that the same people would share both hobbies.
While publishers may have informally expected these monochrome images to be colored by some readers, it wasn’t until the eighteenth century that the practice was formalised in the first purpose-made coloring books. And in these the link between botany and painting persisted. Robert Sayer’s The Florist, published in London in 1760, was one of the first books where the author explicitly intended readers to color in the images. Comprised of pictures of various flowers, the author gives his (presumably) adult readers detailed instructions for paint mixing and color choice (including the delightful sounding “gall-stone brown”).

Page from Robert Sayer’s The Florist (1760) — Source
Botanical works were particularly suitable for readers who wanted to engage directly with a physical book, because they offered images of things that could be observed in the natural world. Although the images in this particular copy of The Floristwere left uncolored, the owner used the book to press actual plants. Many botanical works were heavily annotated, sometimes by several different owners, and pressed plants are often found in their pages.
The Florist was produced “for the use & amusement of Gentlemen and Ladies”, but most subsequent coloring books were created with children in mind. By the nineteenth century, these books became increasingly popular. Although they helped children develop artistic skills, creativity was not particularly prized. In The Young Artist’s Coloring Guide, a series published in the 1850s, a fully-colored version accompanied the uncolored image, ostensibly to imitate.

Two pages from The Young Artist’s Coloring Guide. No. 12 (ca. 1850) — Source.
In Walter Crane’s Painting Book, originally published in 1880, there’s also color companions to copy, though one could argue in this case, they being from the hand of one of the nineteenth century’s greatest illustrators, such an approach made for a significantly more beautiful object and one likely enjoyed by adults as well as children.

Two pages from Walter Crane’s Painting Book (1889 edition) — Source.
Crane wasn’t the only noted illustrator of the time to lend his name to such a book. A year earlier came The “Little Folks” Painting Book, published by the McLoughlin Brothers, with illustrations by noted artist Kate Greenaway. With no accompanying colored example to copy it was a bit less didactic than Crane’s but it still cautioned children to use a “fitting choice of colours”, and there was a pre-colored frontispiece which would have acted as a guide of sorts to the color scheme.

Two different colourings of the same image in The “Little Folks” Painting Book (1879) — Source
Of course, in the case of these Victorian examples, and earlier offerings such as The Florist, the coloring-in is the very raison d’etre of the book. The early modern examples less so. Though that’s not to say a similar enjoyment was not taken by early modern readers wanting to colorize their wooducts or etchings, that same thrill of bringing color to what was once blank. It seems the therapeutic effects were not unnoticed at the time either. In his 1622 work The Compleat Gentleman Henry Peacham, in a chapter encouraging the practice of coloring-in printed maps, talks of how “the practise of the hand, doth speedily instruct the mind, and strongly confirme the memorie beyond any thing else.”2
As for the modern trend in adult coloring books, critics havecharged marker-wielding grown-ups with being childish, and have alleged that the success of these books is a product of a dumbed-down culture. It may indeed be a fad, but it also has a longer history. So, the next time you buy an adult coloring book or get excited about Color Our Collections Week, know that you are not being childish. Rather, you are taking part in a long tradition of printed images that were meant to be colored.
This article was originally published in The Public Domain Review under a Creative Commons Attribution-ShareAlike 3.0. If you wish to reuse it please see:
About the authors
Melissa N. Morris is an Assistant Professor of History at the University of Wyoming. She has a PhD in History from Columbia University, where she wrote a dissertation on how plants mediated relationships between Europeans and Indigenous peoples in the seventeenth century Americas. On Twitter here.
Zach Carmichael is Local History and Genealogy Specialist II at the Carnegie Library, Muncie, IN. He has an MA in history from Miami University (OH), where he studied colonial New England taverns, and an MLIS from the University of Pittsburgh, where he specialized in archives. On Twitter here. 
1. Susan Dackerman, Painted Prints: The Revelation of Color in Northern Renaissance and Baroque Engraving, Etchings, and Woodcuts (University Park, PA: The Pennsylvania State University Press, 2002), 10.↩2. Henry Peacham, Peacham’s Compleat gentleman, 1634., with an introduction by G.S. Gordon (Oxford: Clarendon Press, 1906), 65.↩

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