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More than Meets the Eye

Colour – A Primer

Colour may initially seem uncomplicated, and we could think of colour being a property of an object. For example: “That bus is red.”

Is the bus really red? What if there was a white bus, illuminated by red light?

We can’t be sure, because we see the light reflected from objects. When we say the bus is red, we assume that it was illuminated by a white light source.

Maybe we need to think of colour as being a property of the light that is reflected by an object – a combination of the spectral quality of the light that illuminates the object, and the spectral reflectance of the object.

Ok, but what if colour is something that happens in the person observing the light?

While it’s helpful to look at it from each point of view, the answer lies somewhere in the middle – colour is the result on the incident light, the reflective properties of the object, anything that filters the light, and the response to the light by the observer.

The eyes have it

The photosensitive part of the human eye is made of rods and cones (photoreceptors). In this post, we’ll focus on cones. There are three types of cones in the retina, and each type is most sensitive to a particular wavelength (420nm – blue, 530nm – green, and 565nm – red), with some overlap.

 

"Cones SMJ2 E" by Vanessaezekowitz at en.wikipedia. Licensed under CC BY-SA 3.0 via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Cones_SMJ2_E.svg#/media/File:Cones_SMJ2_E.svg
“Cones SMJ2 E” by Vanessaezekowitz at en.wikipedia. Licensed under CC BY-SA 3.0 via Wikimedia Commons – https://commons.wikimedia.org/wiki/File:Cones_SMJ2_E.svg#/media/File:Cones_SMJ2_E.svg

 

The names we give to colours, or parts of the visible spectrum, are an abstraction – a way to describe the stimulus resulting from light of certain wavelengths entering our eyes.

Another possible complication is that each observer may experience the same light differently. For example, you might be able to perceive a more saturated red than me. Don’t worry too much about this – if the print (seen under a certain illuminant) results in colours that are consistent with the original (seen under a certain illuminant), my experience of the print will appear authentic, as will yours. For the purposes of colour management, CIE defines a standard observer.

why colour management?

Different devices also ‘experience’ colour differently. Our cameras, scanners, displays and printers all have different ranges of colours, contrast and saturation that they can capture or reproduce.

Our devices do not abstract colour the way we do – to them, colour is simply a number (or a combination of numbers).

The goal of colour management is to provide a means of translation between devices, so that the output can faithfully represent the input.

In my next post, we’ll take a look at how colour management works.

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