1. Answer the following questions – I am looking for proof that you have understood what you are writing about. • Pick an illusion that you like and explain why humans are fooled by this particular illusion ? Assignment 1 Damian Schofield

2. In any study of colour it is important to understand the system defined by the Commision Internationale de L'Eclairage (CIE). In order to understand this system it is necessary to know the fact that no finite set of real primary colours can be combined to produce all possible colours. This fact can be illustrated using a Maxwell triangle. The Maxwell Triangle is a demonstration of additive colour mixing, it is created as follows.  Three coloured lights are placed at the vertices of an equilateral triangle so that they shine inwards over the triangle and that each light becomes uniformly weaker until it reaches the opposite edge at which point it has 0 intensity. The Maxwell Triangle Damian Schofield

3. The triangle may seem like a good representation for a colour system, as we can easily represent the coordinates of any point in an equilateral triangle.  However, the triangle has a major drawback.  If we compare the points around the sides of the triangle (the sides are by definition the most saturated) with the equivalent pure spectral hues, we find that although the hue is a perfect match, the saturation is not.  The Maxwell Triangle Damian Schofield

4. In fact the only way we can get a match is by diluting the spectral colour with the third primary.  For example the center point on the blue/green edge is not as saturated as the spectral cyan so the only way to make the two colours the same is to desaturate the spectral hue by adding the third primary, red.  In mathematical terms this is equivalent to adding negative red to the colour in the triangle, thus moving the point outside the triangle. The Maxwell Triangle Damian Schofield

5. It is impossible to choose three real primaries such that all possible colors can be matched with additive mixtures of those primaries. Thus, in a real additive color reproductive system such as color television only a limited gamut of colors can be displayed. In 1931, when the CIE system was specified, it was decided to use three imaginary primaries such that the tristimulus values X, Y, and Z, are always positive for all real color stimuli. There is only one way around this problem, and that is to use primaries that can't be found in the spectrum. The CIE chose three primaries called X,Y and Z which are theoretically defined super-saturated colours, which lie outside the bounds of the spectral locus, and because of this fact the XYZ system never has to use negative values. CIE Imaginary Primaries Damian Schofield

6. These imaginary primaries (called stimuli, or tristimulus values) are shown in the figure to the right. As we are trying to find a good way of pin-pointing a colour we need to transform this triangle into a right angled triangle so that the coordinate system is simpler. We can now define a colour in terms of the amount of X,Y and Z it contains, more formally we can say that a color C of wavelength lambda is expressed as CIE Imaginary Primaries Damian Schofield

7. Visible colours are found in a cone-like shape in the XYZ colour space CIE Imaginary Primaries Damian Schofield

8. We can now produce the CIE chromaticity chart (shown on the right). This chromaticity chart is useful for a number of things such as identifying dominant wavelengths, identifying complementary colours, and comparing gamuts. CIE Chromaticity Chart Damian Schofield

9. A gamut is the range of colours that can be produced by a set of primary colours. Gamuts are represented on the chromaticity chart as straight lines, or polygons. CIE Chromaticity Chart Damian Schofield

10. A property of the chromaticity chart is that the gamut of a set of primaries will lie within the shape or along the line. This introduces an important point - as there is no triangle that can lie within the chart and encompass all the colors, then there are no 3 primary colours whose additive mixtures can represent all colours. CIE Chromaticity Chart Damian Schofield

11. CIE Chromaticity Chart Damian Schofield

12. Last Words on Colour and Illusions Damian Schofield