Colour Management - Part Two

 

 

Having looked at the importance of colour in the previous in the first article let us understand the structure of the human eye – the organ that enables us to see the colourful world, without which no colour reproduction can take place. Outside our physical body there is no such thing as colour. Colour creation mechanism is a nature’s wonder in the human eye. It is only when the light rays enter our eyes they are converted into colour. Thus eyes are the basis for all the colour reproduction systems and are based on the eye structure, whether in printing, photography, computer monitors, or television systems.

In rainy season or near the waterfall many times we experience a colourful spectrum of colours what is commonly refered to a rainbow.  What actually happens is that light rays from the sun while passing through the water drops, which act like tiny prisms, are refracted (bent) according to the law of refraction and during this process the white light is distinctly separated according to the wavelength of various component rays (Figure 1).

 

These different wavelength rays are converted into different colours in the human eye that we call VIBGYOR – violet, indigo, blue, green, yellow, orange, and red. Thus human eye is the natural colour reproduction system (Figure 2).

 

 

 

Light

Light is a part of the long belt called electromagnetic spectrum. This spectrum consists of various waves in the atmosphere, something like water waves in the sea. The length of these waves varies very widely from 10 kilometres for radio waves on the longer side down to .01 nanometer for cosmic rays. One nanometer is one millimicron (i.e. one millionth part of a millimeter). Thus .01 nanometer is hundred millionth part of a millimeter.

This is a ratio of 1 to 1,000 trillion at two ends of the spectrum as we know today (Figure 3). Out of this a normal human eye can perceive only .00000000004 percent of the waves, i.e. 380 to 780 nanometers, and convert into the VIBGYOR colours in the human visual processing system (Figure 4). Rest 99.99999999996 percent of the waves outside 380 to 780 nanometers are not visible to the human eyes, nevertheless the human being has been very clever to make use of these waves to perform various other tasks (Figure 5).

 

 

 

 

 

Wavelength range of 380 to 780 nanometers which is visible to human eye creates the impression of colours from violet to red depending on the wavelength perceived. Short wavelength end creates the sensation of violet and long wavelength end red. All the other colours of the rainbowlie between these wavelengths (Figures 2 and 4).

Human Eye

Human eye is a wonderful instrument that enables us to see the world in colour. Two eyes are two light tight boxes (construction of a camera is similar to human eye) allowing the light rays to enter in a controlled manner and then processing these rays in a very complicated manner to create impression of colours in the brain. The structure of the eye is shown in Figure 6.

The major parts that make  in processing colour signals possible are :

Cornea – The most significant image forming element of the eye. Its curved surface at the interface with air represents the largest change in refractive index in the eye’s optical system. Refractive errors in its curvature along with the lens may not focus sharp images on the retina.

 

 

Lens – Lens is supported with ciliary muscles to provide flexibility to the lens to focus sharp images on the retina. In most cases this flexibility is lost at the age of 50 making it difficult to focus the nearer objects. Lens absorbs and scatters short wavelength, blue-violet, energy. Colour vision is best at the age of 18. The lens becomes more and more yellow with the age. The effect is noticeable in critical colour matching, especially when judging blue and purple colours. This exaggerates the difference between red and blue colours making reds appearing extra reds with advancing age.

The cornea and lens together act like a camera lens to focus the outside images on the retina in the eye.

Retina – This is a thin layer of cells at the back of the eye and it incorporates the visual system’s photosensitive cells and initial signal processing and transmission circuitry. Retina acts like a film in a camera or image sensing sensors in a digital camera. Retina contains two types of light sensitive cells, known as rods and cones (Figure 7).

 

 

Rods contain a photosensitive substance called visual purple that gets affected at very low levels of illumination, below 1 candela per square meter (cd/m²), such as weak moonlight and starlight, scotopic vision (Figure 8). One candela is a standard unit of light intensity one lumen per steradian. The bleaching of visual purple in rods give faint vision but do not generate any colour sensation. There no rods in the fovea region and its concentration increase as we go further away from fovea in both directions. There are approaximately 130 million rods in each eye.

As the lighting level increases cones take over and at luminance levels of more than 100 cd/m2 only the cones function, photopic vision (Figure 8). At luminance levels between 1 and 100 both rods and cones are responsible for the vision, mesopic vision.

 

 

Cones are of three types, normally termed as L (long), M (middle), and S (short) denoting the sensitivity to the wavelengths within the visible spectrum. For convenience, we shall call these red (r), green (g), and blue (b) to indicate the colour impression it generates in the human eye (Figure 9). The maximum concentration of the cones is in the fovea and reduces as we go further away from fovea in both directions. Thus, while looking from the corner of the eye no colour sensation is present. Good colour reception is experienced looking directly at the object.

 

 

 

Three types of cones together present in each eye can go upto 7 millions. This number is shared by r, g, b sensitive cones in the ratio of as high as 40 : 20 : 1, i.e. for every 40 red sensitive cones there are 20 green sensitive cones and 1 blue sensitive cone. This is what makes us notice red colour first from a distance and blue last (Figure 10).

 

 

Visual Signal Processing through the neural system is a very complex process from retina until the signals reach the brain. Light falling on retina is absorbed by the photopigments in rods and cones. On absorbing light photon the pigments in photoreceptors changes its structure setting off a chemical chain reaction which produces electrical signal based on concentrations of various ions. These electrical signals are processed through retinal neurons, namely horizontal, bipolar, amacrine, and ganglion cells (Figure 11). It is interesting to note that rods and cones put together are 130 million in each eye. However, when ultimate signals passed on to the brain are one million. A very fine example indeed of data compression.

 

 

As seen in figure 7 there are tree types of cones each of which senses different region of the visible electromagnetic spectrum giving either red, green, or blue sensation and combining this in various proportions to enable us to sense all possible colours. Due to this phenomena the entire colour reproduction is based on three colour principle, trichromatic vision, whether in printing, photography, or television.

In actual fact the human colour vision is a combination of this trichromatic principle and a complementary colour vision that actually takes place when ultimate signals travel to the brain. However, to avoid complications and to keep information simple this complementary colour vision will be dealt with in future articles.