Dictionary tag: C

• The CIE XYZ colour space was the first comprehensive method able to systematise the relationship between colour stimuli and human colour perception.
• In an experimental situation, the CIE XYZ colour space is able to match any colour an observer sees with a known mixture of wavelengths of light.
• The foundation of the CIE XYZ colour space is the ability to identify the precise mixture of wavelengths of light needed to stimulate cone cells to produce any colour experience within the visible spectrum.
• Viewed diagrammatically the CIE XYZ colour space takes the form of a graph showing a volume of colour corresponding with every wavelength in the visible spectrum. The location of every colour is determined in relation to the x and y axes of the graph. The two axes are used to identify the coordinates for each colour within this two-dimensional vector space.
• The coordinates themselves are derived from tristimulus colour values.
• With the development of the CIE XYZ colour space, trichromatic colour models and their corresponding colour spaces provide methods for anticipating and managing colour reproduction in every applicable field and type of technology.
• In terms of colour management, the trichromatic colour theory underpins device-independent additive colour spaces such as the sRGB colour space and the Adobe RGB colour space and device-dependent additive colour models such as RGB, HSB and CMYK and their corresponding colour spaces.

The CIE XYZ colour space serves as a standard reference and underpins more recent colour spaces such as:

• CIELUV 1976 –  a modification of CIE 1931 XYZ used to display additive mixtures of light more conveniently.
• CIELAB 1976 –  a more perceptually linear colour space. Perceptually linear means that changes in colour values are directly related to changes in colour appearance.  CIELAB is commonly used for surface colours, but not for mixtures of light.

• Classical electromagnetism is based on the idea that electric charges and electromagnetic fields are continuous and smooth. It does not take into account the quantization of energy or the wave-particle duality of matter.
• Charged particles create electromagnetic fields, which in turn exert electromagnetic forces on other charged particles.
• The four Maxwell equations are:
  • Gauss’s law for electricity: The electric flux through a closed surface is proportional to the total electric charge enclosed by the surface.
  • Gauss’s law for magnetism: There are no magnetic monopoles, and the magnetic flux through a closed surface is always zero.
  • Faraday’s law of induction: A changing magnetic field produces an electric field.
  • Ampere’s circuital law with Maxwell’s correction: A changing electric field or an electric current produces a magnetic field.
• These equations can be used to describe a wide range of phenomena, from the propagation of electromagnetic waves to the operation of electrical and electronic devices. They are also used in many different fields, including engineering, medicine, and astronomy.

Core concepts of classical electromagnetism

• Charged Particles (Matter): These are particles that have an electric charge, either positive (protons) or negative (electrons). They are the sources of electric and magnetic fields and are affected by these fields.
• Electromagnetic Force: This force is a fundamental interaction between charged particles. It can be attractive or repulsive, depending on the sign of the charges.
• Electromagnetic Fields: These are regions where electric and magnetic forces are experienced due to the presence of charged particles. Electromagnetic fields carry energy and can exert forces on other charged particles.

Everyday examples of Maxwell’s electromagnetism

• When you turn on a light switch, the electric current in the filament of the light bulb produces a magnetic field. This in turn produces an electric field causing the filament to glow white hot.
• When you listen to the radio, the electromagnetic waves from the radio station interact with the antenna on your radio to produce an electric current. This electric current is then amplified and converted into sound, which you can hear through the speakers on your radio.
• When you use a microwave oven to heat food, the electromagnetic waves from the microwave oven interact with the water molecules in the food. This causes the water molecules to vibrate, which heats up the food.

• Classical physics describes many aspects of nature at an everyday scale but neglects to explain things at very small (sub-atomic) and very large (cosmological) scales. It is a very successful theory, and many of its predictions have been experimentally verified.
• Classical physics studies the motion of macroscopic objects, from projectiles to parts of machinery and astronomical objects such as spacecraft to the movement of planets and stars.
• For objects governed by classical physics, if the present state is known, it is possible to predict how it will move in the future (determinism), and how it has moved in the past (reversibility).
• Classical physics has its roots in:
  • Newtonian mechanics – Isaac Newton, 17^th century
  • Thermodynamics – Carnot, Joule and Kelvin, 19^th century
  • Maxwell’s electromagnetism, 19^th century.

• Newton’s laws of motion, the law of conservation of energy, and the law of conservation of momentum are all fundamental laws of Newtonian mechanics. Newtonian mechanics is a branch of physics that describes the motion of objects under the influence of forces.
• Thermodynamics is a branch of physics that deals with the relationship between heat and work. Thermodynamics is based on the two laws of thermodynamics, which were developed in the 19th century by Carnot, Joule, and Kelvin.
• Maxwell’s electromagnetism is a branch of physics that describes the interaction of electric and magnetic fields. It is based on the four Maxwell equations, which were developed by James Clerk Maxwell in the 19th century.
• These three branches of physics are all related to each other. Newtonian mechanics can be used to describe the motion of objects in thermodynamic systems, and thermodynamics can be used to describe the energy changes that occur in electromagnetic systems.

Here are some examples of how these three branches of physics are related:

• The heat engine is a thermodynamic device that converts heat into work. The efficiency of a heat engine is limited by the laws of thermodynamics and the laws of Newtonian mechanics.
• The electric generator is an electromagnetic device that converts mechanical energy into electrical energy. The operation of an electric generator is based on the laws of electromagnetism and the laws of Newtonian mechanics.
• The light bulb is an electromagnetic device that converts electrical energy into light energy. The operation of a light bulb is based on the laws of electromagnetism and the laws of thermodynamics.
• Classical physics has some limitations. For example, classical physics cannot explain the behaviour of light at the atomic and subatomic levels. This is because light behaves both like a wave and a particle, which is something that classical physics cannot describe. These limitations are explored in the field of quantum mechanics (quantum physics).

• The primary colours in the CMY colour model are cyan, magenta and yellow.
• The CMY colour model is a subtractive colour model used with transparent or translucent inks or filters.
• Meanwhile, the CMYK colour model (sometimes called four-colour or process printing) uses the same three primary colours as CMY but uses a fourth component, black ink (K), to increase the density of darker colours and blacks.
• The CMY colour model along with its system of notation enables an exact and reproducible approach to colour printing and other similar applications.
• The CMY colour model is deeply embedded in all contemporary digital printer technologies and underpins industrial standards for the printing industry.

Subtractive colour mixing

• The CMY colour model can be explained by imagining that an observer is presented with a well-illuminated surface such as a highly reflective sheet of white paper.
• In the diagram below a torch is used to illuminate the paper, producing a round pool of light.
• The surface appears white because it is illuminated by white light, so by light containing all the wavelengths of the visible spectrum.
• Cyan, magenta and yellow filters or inks are then placed between the light source and the paper or between the paper and the observer.
• The diagram below shows the effect of placing the filters or patches of ink onto the paper so they partially overlap.

• Where pairs of the primary coloured C, M and Y filters/inks overlap they produce secondary colours and where all three overlap, all wavelengths of light are blocked producing a dark area in the centre.
  • A red filter will transmit red light but absorbs all other colours including green and blue.
  • A green filter will transmit green light but absorbs all other colours including blue and red.
  • A blue filter will transmit blue light but absorbs all other colours including red and green.

• Where two overlapping filters are placed between the light source and the paper or between the paper and the observer the results are as follows:
  • A red filter absorbs both green and blue and a green filter absorbs blue and red, as a result, red, green and blue are all absorbed where they overlap and that area appears black.
  • A green filter absorbs both blue and red and a green filter absorbs blue and red, as a result, red, green and blue are all absorbed where they overlap and that area appears black.
  • A blue filter absorbs both red and green and a red filter absorbs green and blue, as a result, red,  green and blue are all absorbed where they overlap and that area appears black.

• Where all three filters are placed between the light source and the paper or between the paper and the observer the result is that red, green and blue are all absorbed where they overlap and that area appears black.

• Cyan, magenta and yellow filters that correspond with the secondary colours in the RGB colour model but are the primary colours in the CMY colour model behave as follows.
  • A cyan filter absorbs red but transmits green and blue light. Green and blue together appear cyan to the human eye.
  • A magenta filter absorbs green but transmits red and blue light. Red and blue together appear magenta to the human eye.
  • A yellow filter absorbs blue but transmits red and green light. Red and green together appear yellow to the human eye.

• Lastly, where two overlapping CYK filters are placed between the light source and the paper or between the paper and the observer so that they overlap, the results are as follows:
  • A cyan filter transmits green and blue light whilst a magenta filter transmits red and blue. Green and red cancel out producing blue.
  • A magenta filter transmits red and blue light whilst a yellow filter transmits red and green. Blue and green cancel out producing red.
  • A yellow filter transmits red and green light whilst a cyan filter transmits green and blue light.  Red and blue cancel out producing green.

CMYK colour model

• The CMY colour model is helpful in developing an understanding of how combinations of cyan, magenta and yellow primary colours can be used to produce a wide range (gamut) of colours when light is reflected off a surface and wavelengths of light are filtered out by the inks before and reaching the eyes of an observer.
• The CMYK colour model (sometimes called four-colour or process printing) uses the same three primary colours as CMY but uses a fourth component, black ink (K), to increase the density of darker colours and blacks.
• CMYK printing typically relies on:
  • Using white paper with good reflective properties to produce the brightest possible highlights by reflecting the maximum amount of light back towards the observer.
  • Creating highlights by using the minimum amount of coloured ink and printing without black.
  • Producing fully-saturated mid-tones by relying on the brilliance and transparency of printing inks and dyes.
  • Adding black ink when the maximum amounts of cyan, magenta and yellow are insufficient to produce rich black tones in areas of shadow and where black text is required.

Half-tone printing with CMYK

• CMYK is based on the CMY colour model and is the standard model used for colour printing.
  CMYK refers to the four ink plates used in colour printing: cyan, magenta, yellow, and a ‘key’ colour (black).
• Half-tone printing (half-toning) using CMYK involves printing tiny dots of CMY and K in a pattern small enough that humans see solid areas of colour.
• Half-toning allows for a continuous variation in the colour perceived by a viewer through the use of dots, varying either in size or in spacing, thus generating a gradient-like effect.
• Half-tone printing can be used to reproduce black-and-white images by using only black ink and varying sizes or spacing of the dots. This simulates shades of grey, allowing for the representation of continuous tones and gradients in black-and-white images.
• Half-tone black-and-white printing is widely used in newspapers, magazines, and other printed materials.
• The term halftone is sometimes also used to refer specifically to the image that is produced by half-tone printing.

• CMYK refers to the four inks or inked plates used in colour printing: cyan, magenta, yellow, and black. Black is often referred to as the ‘key’ colour because it is used to enhance the depth and detail of the printed image.
• The CMYK model works by overlaying colours that partially or entirely mask the background colour which is usually white paper. The inks reduce the amount of light that would otherwise be reflected, thereby creating the desired colours through the absorption of specific wavelengths.
• CMY and CMYK are called subtractive colour models because the inks “subtract” the colours red, green and blue from white light. In essence, the inks absorb certain wavelengths of light and reflect others, which combine to produce the perceived colours.
• When an observer looks at an image printed using CMYK inks on paper, they see the light that has first passed through the inks to the paper, been reflected off the paper surface, passed through the layers of ink again, and then reached the observer’s eyes. This interaction of light with the ink and paper creates the final visual image.
• This means that:
  • When incident light contains all the wavelengths of the visible spectrum (white light) is reflected from areas of paper that have no ink on them an observer sees white highlights in an image.
  • When white light is reflected from areas of an image that have cyan ink on them the observer sees red.
  • When white light is reflected from areas of an image that have magenta ink on them the observer sees green.
  • When white light is reflected from areas of an image that have yellow ink on them the observer sees blue.
  • Where different amounts of cyan, magenta, and yellow overlap, they create other colours by subtracting some wavelengths of light but allowing others through.

• We see colour as a result of:
  • The range and intensities of wavelengths of visible light emitted by a light source.
  • The path that light takes and the different media and materials it encounters on the journey from its source to the retina of a human eye.
  • Optical phenomena such as absorption, dispersion, diffraction, polarization, reflection, refraction, scattering and transmission.
  • An observer’s predispositions, including their personal and social experience and cultural context.

• Colour is what humans see in the presence of radiated or reflected light.
• The brightness of the colour of an object or surface (its colour brightness) depends on the intensity of the incident light, as well as the wavelengths of light the object absorbs and reflects.
• The colour brightness of a transparent or translucent medium may depend on the intensity of the incident light, the wavelengths of light it absorbs and transmits and the amount it reflects.
• Colour brightness can differ depending on the difference between the way a colour appears to an observer in well-lit conditions compared with its subdued appearance when in shadow or when poorly illuminated.
• The perception of colour brightness can be influenced by hue, as some hues, such as fully saturated yellow, can appear brighter to human observers than others, like fully saturated red or blue.

• Optical factors affecting colour brightness include:
  • The angle at which incidence light approaches a medium, object or surface.
  • The composition of incident light in terms of wavelength, frequency and intensity.
  • The polarization of incident light.

• Material properties affecting the colour brightness of a medium, object or surface include:
  • Chemical composition
  • Three-dimensional form
  • Physical structure, including three-dimensional form and texture.
  • Reflectivity

• Perceptual factors affecting colour brightness include:
  • The influence of adjacent colours, which can alter the perceived hue and brightness of a target colour.
  • Individual attributes of visual perception.