Lecture Objectives

This component of the Lecture will have these learning outcomes: • An introduction to colour theory ~15 mins. • Colour Definition ~10 mins. • Colour Spaces ~5 mins. Lecture Objectives Damian Schofield

Imagine yourself in an art museum. Several different sensory processes occur in your brain during this trip to the art museum; the majority of them involve visual inputs. How does your brain put together all the information that your eyes receive? The ideas of colour perception and colour theory are interesting ones. How do humans account for colour and does it truly exist? By examining not only the neurological goings-on in the brain, but by learning about colour through philosophy, and even art, a greater understanding of it can be reached. Introduction to Colour - Art Damian Schofield

In the art museum, you enter a room full of Impressionist paintings. Colours can take on a different appearance depending on the context in which they are viewed. It is not that these artists were the using different kinds of paint, but that their placement and choices of colour (specifically colour saturation and value) create an incredible lightness that other works lack. Impressionists were one of the first groups of painters who consciously took the idiosyncrasies of colour perception into account. They used not just the colours of pigments, but the "colours" of light in their work. Introduction to Colour - Art Damian Schofield

Think of Seurat's Sunday Afternoon on the Island of la Grande Jatte or Monet's Waterlilies and the difference between viewing the painting from a distance and up-close. Introduction to Colour - Art Damian Schofield

Seurat uses a technique called pointillism, where paint of different colours are placed next to each other, but generate a completely different "colour" in the brain. A red dot beside a green dot is read as "yellow" by the photoreceptors in the eye. What is the mental "leap" between the physicality of these two distinct colour pigments and the perceived "colour" of the image seen? Introduction to Colour - Art Obviously Seurat did not want viewers of his work to see a canvas full of opposite colour dots, but a cohesive image. There is a distinction between pigment colour and light colour. Damian Schofield

The Red Room Experiment Damian Schofield

The science of color is a very complex subject. There are untold numbers of text books that deal in great detail with all the facets this science and they are stuffed with advanced mathematical concepts. What we will attempt here is a short overview of some human computer interaction aspects relevant to the science of color. Introduction to Colour Damian Schofield

Colour is that characteristic of a visible object or light source by which an observer may distinguish differences between two structure-free fields of the same size and shape, such as may be caused by differences in the spectral composition of the light concerned in the observation. In other words, colour is that perception by which we can tell two objects apart, when they have otherwise similar attributes of shape, size, texture, etc. G. Wyszecki and W.S. Stiles, Colour Science: Concepts and Methods, Quantitative Data and Formulae 2nd Ed., Wiley, New York, 1982. Definition of Colour Damian Schofield

This text book answer is unsatisfying, because colour is an inherently subjective experience. Colour only exists in our minds, and putting a scientific definition together is no easy task. The usual definition, given above, is really a circular argument. It amounts to: "Colour is that attribute of an object leftover when you eliminate all attributes except colour." So, if two objects look different, but have the same size, shape, texture, etc., then the way you are telling them apart is their colour. Definition of Colour Damian Schofield

When contemplating how many colours we can see, people usually believe they can distinguish "millions and millions!" However, it is better to ask the question in a more specific way to get a more comprehensive answer. Many of us can select a setting for our computer monitor that displays millions of colours and we see an improvement in image quality with this setting. However, if you select the colours correctly you can reduce the number of colours to a couple of hundred or even fewer (depending on the image) without noticing any degradation in quality. This would indicate that we can't see millions of colour variations simultaneously. Definition of Colour Damian Schofield

One way to answer the question is to measure the ability of people to discriminate colours. Many researchers have investigated chromatic discrimination by varying the wavelength of two monochromatic lights until they are just noticeably different. Other studies have used the variability of colour matching to gauge discriminability and yet others have directly measured threshold differences throughout colour space. Using these measures we find that our visual systems can discriminate millions of colours. Definition of Colour Damian Schofield

In fact, the human eye can distinguish something on the order of 7 to 10 million colours. That's a number greater than the number of words in the English language (the largest language on earth). This is why car owners may often reject a certain touch-up paint because it's not a close enough match to the colour of their car Definition of Colour Damian Schofield

But we can't always see millions of colour variations simultaneously. Definition of Colour Damian Schofield

16,000,000 colours Definition of Colour Damian Schofield

64,000 colours Definition of Colour Damian Schofield

32,000 colours Definition of Colour Damian Schofield

256 colours Definition of Colour Damian Schofield

16 colours Definition of Colour Damian Schofield

In 1669, Isaac Newton, aged only 27, was appointed Lucasian Professor at Cambridge. His first work in this post was in the area of optics. During the two plague years, he concluded that white light is not a simple entity. This idea contravened every scientist since Aristotle, but chromatic aberrations seen by Newton in a telescope lens convinced him otherwise. When he passed a thin beam of sunlight through a glass prism Newton noted the spectrum of colours formed. History Damian Schofield

History Damian Schofield

Newton showed that not only can a triangular prism separate a beam of sunlight into rainbow colours (that had already been known), but also that, when a second prism brings the different colours together again, white light is once more obtained. Therefore white light is a combination of all the rainbow colours, and the prism separates its colours because the angle by which a beam of light is bent, when it enters glass, differs from one colour to the next. History Damian Schofield

ROYGBIV – RICHARD OF YORK GAVE BATTLE IN VAIN R – Red O – Orange Y – Yellow G –Green B –Blue I –Indigo V –Violet History Damian Schofield

ROYGBIV – RICHARD OF YORK GAVE BATTLE IN VAIN R – Red O – Orange Y – Yellow G –Green B –Blue V –Violet Isaac Newton named seven colours for his spectrum - red, orange, yellow, green, blue, indigo, violet. One does not really see indigo as a separate colour, and orange is a bit doubtful. Newton came from a culture where specific numbers were regarded as having mystical significance, so he added the names orange and indigo to make the magic number seven. History Damian Schofield

Colour is a function of the human visual system, and is not an intrinsic property. Objects don't "have" colour, they give off light that appears to be a colour. Spectral power distributions exist in the physical world, but colour exists only in the mind of the beholder. Visible light is the range of wavelengths within the electromagnetic spectrum that the eye responds to (from roughly 400 to 700 nm). Wavelengths Damian Schofield

In a rainbow or the separation of colours by a prism we see the continuous range of spectral colours (the visible spectrum). A spectral colour is composed of a single wavelength and can be correlated with wavelength. Wavelengths Damian Schofield

This progression from left to right is from long wavelength to short wavelength, and from low frequency to high frequency light. The wavelengths are commonly expressed in nanometers (1 nm = 10-9 m). The visible spectrum is roughly from 700 nm (red end) to 400 nm (violet end). Indigo is no longer commonly used as a colour name. Three typical waves of visible light are shown in the figure. The wavelength is the distance from one wave crest to the next, and is represented by the Greek letter lambda, l. Wavelengths Damian Schofield

The retina, as explained in the previous human vision section, is a neural sheet, containing the photoreceptors called rods and cones that is located at the back of the eye. Colour vision is complex, colour is perceived in the retina by three sets of cones which are photoreceptors with sensitivity to photons whose energy broadly overlaps the blue, green and red portions of the spectrum. Cones of Colour Damian Schofield

The three types of cones are each sensitive to a different region of the visible spectrum. They are known as the Short-wavelength (b) sensitive cones, the Middle-wavelength sensitive (g) cones and the Long-wavelength sensitive cones (r). Sometimes they are referred to as R, G, and B cones but these are misnomers based on the colours in the spectrum. For example, very short wavelength light can uniquely stimulate the S-cones but the sensation associated with this light stimulation has a reddish and bluish component. Cones of Colour Damian Schofield

Short-wavelength (b) sensitive cones, the Middle-wavelength sensitive (g) cones and the Long-wavelength sensitive cones (r). Although the Short-wavelength (b) and Middle-wavelength (g) sensors correspond closely to blue and green, the Long-wavelength (r) sensor (the red curve) isn't even close to red. An ink with the same reflectivity spectrum would appear yellow-orange. Cones of Colour Damian Schofield

The relative abundance of the three types of cones is r:g:b = 40:20:1 If a monochromatic light source is viewed there are wavelengths which will evoke a response from two or even all three types of cones. Fundamentally our colour vision derives from comparisons between the amount of light being absorbed by each cone type. Our visual system compares the outputs of the cone types to process colour. In addition, colour appearance is influenced by the ratios of cone excitations in surrounding regions and by the overall levels of cone excitation caused by the prevailing illumination. Cones of Colour Damian Schofield

Colour Illusions Cones of Colour Damian Schofield

Cones of Colour Damian Schofield

Colour Illusions Cones of Colour Damian Schofield

If a person can see, but not very well, we would not call him or her blind. A blind person is someone who simply can not see. It is true that legally blindness is defined differently. Legally a person is considered blind if they have 20/200 (or worse) vision in their best corrected eye. We often refer to people who misperceive colours as being colour blind. That may or may not be correct. For example, if a person has no cone receptors they are truly colour blind. Such people are called achromats and are very rare. Colour Vision Problems Damian Schofield

Some people have rods and one kind of cone. These people are also unable to make any discriminations based on colour and are called monochromats Some people are born with only two classes of cone receptors. They are called dichromats. Most people have three classes of cone receptors in their retina (short wavelength, middle wavelength and long wavelength receptors). Such observers are called trichromats. Colour Vision Problems Damian Schofield

Normal colour vision is not precisely defined. The problem is that there is considerable variation among those who test normal for colour vision. Similarly those who test as ‘colour blind’ are not carbon copies of one another. They also exhibit considerable variation as a class. Hence, it is probably more accurate to speak of colour variant vision when describing colour vision characteristics rather than colour blindness or colour defective vision. Colour Vision Problems Damian Schofield

One of the most important conclusions of the biology of vision is that colour is not technically generated by physical reality. Colour appears to be a mental construct, and therefore, everyone views colour differently. The rationale one is often given for the colour of particular objects is the following: - Light consists of colours. - When light strikes an object in absorbs most of the wavelengths of light, - But those that it reflects correspond to the colour one sees. Summary Damian Schofield

This component of the Lecture will have these learning outcomes: • An introduction to colour theory ~15 mins. • Colour Definition ~10 mins. • Colour Spaces ~5 mins. Lecture Objectives Damian Schofield

James Clerk Maxwell, the Scottish physicist who also gave us the basic equations of electricity, the ones that predicted electromagnetic waves. • Maxwell showed, while still a student, that two kinds of colour existed, depending on whether it was perceived by an instrument or by the human eye: • "Spectral colour" i.e. the colours of the rainbow and their combinations. The amount which each part of the rainbow spectrum contributes to a beam of light can be determined by splitting the beam with a prism. • "Perceived colour" reported by the human eye to the brain. • An instrument using prisms ("spectrograph") will reveal that the human eye can be fooled: different combinations of rainbow colours may look the same to the eye. The Painter’s Colour Wheel Damian Schofield

The painter's colour wheel is an historical artifact that refuses to die. The primary colours are not always red, yellow, and blue. Painters and art teachers promote this scheme. It is a convenient way to understand how to mimic one colour by mixing red, yellow, and blue. But these colours do not satisfy the definition of primary colours in that they can't reproduce the widest variety of colours when combined. Cyan, magenta, and yellow have a greater chromatic range as evidenced by their ability to produce a reasonable black. No combination of red, yellow, and blue pigments will approach black as closely as do cyan, magenta, and yellow. The Painter’s Colour Wheel Damian Schofield

The theory of colours, in particular, has suffered much, and its progress has been incalculably retarded by having been mixed up with optics generally, a science which cannot dispense with mathematics; whereas the theory of colours, in strictness, may be investigated quite independently of optics. That all the painter’s colours mixed together produce white, is an absurdity which people have credulously been accustomed to repeat for a century, in opposition to the evidence of their senses. The Painter’s Colour Wheel Damian Schofield

The Painter’s Colour Wheel Damian Schofield

Depending on the application and environment Primary Colours fall into three families... • RGB (Red / Blue / Green) • CMY (Cyan / Magenta / Yellow) • YRB (Yellow / Red / Blue) • RGB is used by most electronic and transmissive-light technologies such as TV and film, • CMY (actually CMYK including Black) is used with reflected light technologies such as printing inks. • The primaries traditionally taught in art school for painters are YRB (Yellow / Red / Blue). • There's no point in arguing over which primary system is best — they each have their place in a specific discipline. The Painter’s Colour Wheel Damian Schofield

This representation of colour is called the additive colour system. It explains how we see objects that emit their own light. This system states that all perceivable colour hues can be created by mixing different amounts of red, green and blue light. Equal amounts of red, green and blue give the sensation of white. The absence of red, green and blue gives the sensation of black. The Adaptive Colour System Damian Schofield

Since additive colour mixing refers to the mixing of different coloured lights, it can be easily demonstrated by the superposition of lights (primaries) on a projection screen. When this is done using red, green, and blue primaries, the colours yellow, cyan, and magenta are produced where two of the primaries overlap. Where all three primaries overlap the sensation of white is produced if the spectral distributions and intensities of the three primaries are carefully chosen The Adaptive Colour System Damian Schofield

Hence the RGB colour space is an additive colour system, it is based on trichromatic theory. This system is commonly used by CRT displays where proportions of excitation of red, green and blue emitting phosphors produce colours when visually fused. The Adaptive Colour System Damian Schofield

Lecture Objectives