Multimedia – Light and colours

Dr. Lina A. NimriLebanese UniversityFaculty of Economic Sciences and Business Administration 1st branch

Plan

•Light•Coulors •Colour representation

What is light?•Light is a form of energy that has two aspects:▫An electromagnetic wave aspect▫A corpuscular aspect (photons)

•The light emitted by the sun travels at a speed of approximately 300,000 km/s with a frequency of approximately 600,000 GHz.

The concept of colour

•The colour of light is depends on its frequency, which itself depends on the wavelength and the speed of the wave front.

•The wavelength of an oscillatory phenomenon is usually characterized by the relation: λ = CT where: ▫λ represents the wavelength▫C represents the speed of the wave fronts▫T indicates the period of the wave (in seconds)

Light and colour• Monochromatic radiation: radiation

comprising only one wavelength.• Polychromatic radiation: radiation which

contains several wavelengths. • Spectrum: is the collection of all the

wavelengths composing polychromatic radiation (and their respective luminous intensities).

• The human eye is not capable of distinguishing the various components of this radiation and perceives only the result, which is a function of the different wavelengths which it comprises and their respective luminous intensity.

• The human eye is able to see radiation with wavelengths between 380 and 780 nanometres.

• The radiation with wavelengths below 380 nm is called ultraviolet radiation

• The radiation with wavelengths above 780 nm is known as infra-red radiation.

• The range of wavelengths that are visible to the human eye is called the "visible spectrum"  : It is possible to separate the spectral colours using a crystal prism.

Light and colour

The operation of the human eye• Thanks to the cornea (the translucent envelope

of the eye) and the iris (which by closing allows the amount of light entering the eye to be regulated), an image is formed on the retina. The latter is made up of rods and cones.

The operation of the human eye• The rods, which contain a pigment called

rhodopsine and are located in periphery of the retina, make it possible to perceive luminosity and movement (scotopic vision)

• The cones, located in a region called the fovea, make it possible to differentiate the colours (photopic vision).

• There are actually three kinds of cones: Those mainly sensitive to red radiation (570 nm), called

erythrolabes Those mainly sensitive to green radiation (535 nm),

called chlorolabes Those mainly sensitive to blue radiation (445 nm), called

cyanolabes

Daltonism

•When a type of cone is lacking, the perception of the colours is imperfect. This condition is known as daltonism (or dichromasy).

•Depending on the type of defective cone, people with this vision anomaly are known as:

Protanopes, who are highly insensitive to red Deuteranopes who are highly insensitive to

green Trinatopes who are highly insensitive to blue

sensitivity of the human eye

•The sensitivity of the human eye to luminous intensities related to the three primary colours is not the same:

Additive and subtractive synthesis• Additive synthesis results from the addition of

light components. The light components are directly added to the emission; ▫ this is the case of monitors or colour televisions.

• When the three components, red, green, blue (RGB) are added, white is obtained.

• The absence of components yields black. • The secondary colours are cyan, magenta and

yellow because: Green combined with blue yields cyan Blue combined with red yields magenta Green combined with red yields yellow

Additive and subtractive synthesis

• Subtractive synthesis makes it possible to restore a colour by subtraction, starting from a white light source, with filters for the complementary colours: yellow, magenta, and cyan.

• The addition of filters for all three colours yields black and their absence produces white.

• When light falls on an object, some wavelengths are subtracted since they are absorbed by the object.

• What we see is the combination of the wavelengths that are reflected or transmitted (i.e. those that are not absorbed).

• This process is used in photography and for the impression of colours.

• The secondary colours are blue, the red and the green: Magenta (primary colour) combined with cyan (primary colour) yields blue Magenta (primary colour) combined with yellow (primary colour) yields red Cyan (primary colour) combined with yellow (primary colour) yields green

• Two colours are said to be "complementary" if: white is obtained by their additive synthesis, or black is obtained by their subtractive synthesis.

Computer representation of colour

• In order to be able to handle colours correctly and to exchange colorimetric information it is necessary to have the means to categorize them and to choose them.

• A palette is presented, in which the appropriate colour is selected.

• Likewise in IT, it is essential to have a means of choosing a colour among all those that are usable.

• However, the possible colour range is very vast and the image processing chain passes through various peripheral devices: ▫ for example, a digitizer (scanner) followed by image

improvement software, and finally a printer.• It is thus necessary to be able to represent the colour

in a reliable way in order to ensure coherence between these various peripheral devices.

colour space

•The mathematical representation of a set of colours is called the "colour space". There are several, among which the best known are: ▫RGB Coding (Red, Green, Blue). ▫HSL Coding (Hue, Saturation, Luminance). ▫CMYK Coding.

•The colour spectrum that a peripheral display device allows to be shown is called a gamut or colorimetric space.

•The colours not belonging to the gamut are called out-of-range colours.

Selection of colours in software•Most graphic

software offers means of selecting a colour in an interactive way.

•The main one is often the swatch chart, that is:▫the colours are

presented in a table in which they are classified by nuances.

Selection of colours in software•More and more software

is integrating more powerful tools however, allowing the choice of a colour from a vast range: ▫the hue is represented by

a chromatic disc,▫the luminance is

represented by a vertical selector offering the colour nuances ranging from black to white.

Selection of colours in software•In the selector below on

the other hand:▫the hue is presented as an

X-coordinate of the selector on the left,

▫the saturation is presented as a Y-coordinate,

▫the selector on the right-hand side makes it possible to regulate the luminosity.

Gamma factor • The criterion that defines the nonlinear

character of the luminous intensity of an element is called the Gamma factor .

• Thus, the luminance of a computer screen is nonlinear as : ▫ the luminous intensity which it emits is not

linearly proportional to the voltage applied, but corresponds to a function curve of the screen gamma (usually between 2.3 and 2.6):

I ~ Vgamma ▫ the luminous intensity perceived by the eye is

not proportional to the quantity of light actually emitted

Gamma factor• In order to overcome this and to obtain a

satisfactory reproduction of the luminous intensity, it is possible to compensate for luminance by applying a transformation called a "gamma correction".

• Thus, there is a gamma transformation corresponding to each peripheral display device, which is itself capable of being adapted to the perception of the user.

Colour management• It is easy to understand the interest regarding image colours

when passing through several peripheral devices (a digital chain made up for example of a scanner, an image processing software and then a printer) in order to make sure that the image at the end of the processing chain has colours close to those of the original image.

• Colour management: The group of operations that are necessary to guarantee the good conservation of the colours of an image.

• In order to be able to guarantee colour coherence, it is essential to calibrate all the devices or tools of the digital chain. The calibration of a device or tool consists in describing all the colours which it is capable of acquiring or producing (i.e. its gamut) in an independent colour space in a file called an ICC profile (International Color Consortium).

RGB coding• The RGB coding (Red, Geen, Blue), developed in

1931 by the International Lighting Commission (Commission Internationale de l'Eclairage, CIE) consists in representing the colour space with three monochromatic rays, with the following colours:

Red (with a wavelength of 700.0 nm), Green (with a wavelength of 546.1 nm), Blue (with a wavelength of 435.8 nm).

• This colour space corresponds to the way in which the colours are usually coded on a computer, or more precisely to the way in which the computer screen cathode tubes represent the colours.

RGB coding• Thus, the RGB model proposes each colour

component to be coded on one byte, which corresponds to:▫256 intensities of red (28), ▫256 intensities of green and▫256 intensities of blue,

• thus, there are 16777216 theoretical possibilities of different colours, that is, many more than the human eye can distinguish (approximately 2 million).

However, this value is only theoretical because it strongly depends on the display device being used.

RGB coding

•Since RGB coding is based on three components with the same proposed value range, it is usually graphically represented by a cube of which each axis corresponds to a primary colour.

RGB coding•Problems:

▫Indeed, the RGB model does not allow colours to be selected easily.

▫Indeed, RGB colour adjustment with computer tools is generally done by using three slide blocks or of three cells with the relative values for each primary component, however the lightening of a colour requires the respective values of each component to be increased proportionally.

•Thus, the HSL model was developed in order to overcome this issue with the RGB model.

The HSL Model• The HSL Model (Hue, Saturation, Luminance),

based on the work of painter Albert H.Munsell (who created the Munsell Atlas), is a representation model known as “natural”, that is, close to the physiological perception of colour by the human eye.

• The HSL Model consists in breaking up the colour according to physiological criteria: ▫Hue, corresponding to the perception of the

colour (mauve or orange T-shirt), ▫Saturation, describing the purity of the colour,

that is, its sharp or dull character (new or washed-out T-shirt),

▫Luminance, indicating the amount of light of the colour, that is, its light or dark appearance (T-shirt in the sun or in the shade).

The HSL Model• The following is a graphical representation of the HSL model,

in which the colour is represented by a chromatic circle and the luminance and saturation are represented by two axes:

Other models• The HSL Model was developed with the aim

of allowing fast interactive colour selection; however, it is not adapted to the quantitative description of a colour.

• There are other natural representation models closely related to the HSL model: ▫HSB: Hue, Saturation, Brightness. The

brightness describes the perception of the light emitted by a surface.

▫HSV: Hue, Saturation, Value. ▫HSI: Hue, Saturation, Intensity. ▫HCI: Hue, Chrominance, Intensity.

CMY Coding• CMY Coding (Cyan, Magenta, Yellow) is to

subtractive synthesis, while RGB coding is to additive synthesis. This model consists in breaking up a colour into values of Cyan, Magenta and Yellow.

• The absence of these three components yields white while their sum yields black.

• However, since the black obtained by the sum of the three colours Cyan, Magenta and Yellow is only partially black in practice (and is expensive).

• Printers add a black ink component called pure black. This is known as the quadrichromy, or CMYK model (Cyan, Magenta, Yellow, BlacK).