Basic concepts of colour measurement
Colours in our Daily Life
People involved with colour and colour reproduction needed a unique and unambiguous specification of colour, leading to colours being expressed in numeric codes, which is explained in detail in this article like the perception of colours, the representation of colours, and other interesting semantics of it.
In colourant production and application industries, colours are to be communicated, compared, recorded and formulated on a regular basis. This necessitates systematic classification of colours. The objects can be classified in various ways in terms of colour. The classification may be based on visually or instrumentally-assessed colour parameters (Roy Choudhury, 2000).
While communicating or talking about colour, a language which is understandable by both the parties must be followed. A logical scheme for ordering and specifying colours on the basis of some clearly defined attributes is known as a colour notation system. The attributes are generally three in number as our vision is trichromatic and they constitute the coordinates of the resultant ‘colour space’. Colour notation systems also encompass ‘colour order systems’ which are typically comprised of material standards in the form of a colour atlas. Due to constraints of colourant gamut, the atlases may depict only a physically realisable subset of a colour order system.
For instrumental colour measurement of an object, additive primaries are used to predict the number of subtractive colours required to reproduce that colour. This entails the use of a standard source of light, viewing geometry and a standard observer. The expression of colour as numerals has been standardized by Commission Internationale de l’Eclairage (CIE) set up in 1931 for standardisation of measurement of colour. Thus the instrumental measurement of colour is helpful in, communication of colour across the entire supply chain as well as comparing samples and setting a pass-fail criterion for sourcing and supply of products. In this form, it is a useful tool for quality control.
It is very convenient, easily understandable and memorable when the colours are called by names similar to common objects orange, ultramarine, olive, malachite green, bottle-green, peanut-green, sea-green etc. But such colour names are very approximate, unreliable and temporary. Their meaning also changes with the observer, time, place, style, technology, language, culture etc.
When we deal with a reasonable number of a specimen, say a few thousands, to cover the whole range of possible colours (1millions or more), the specimen must be selected according to a system or plan. It is well known that the colours are three-dimensional. However, the dimensions of colour are expressed in various ways in different fields. For systematic arrangements, the dimensions should be independent of each other.
Visual Description of Colour
A colour order system is a systematic and rational method of arranging all possible colours or subsets by means of material samples. Once the colours are arranged systematically they are named in some descriptive terms and/or are numbered (Graham, 1985).
The colour order systems are of three types (Wyszecki, 1986):
- The colourant-mixture system based on a subtractive mixture of colourants e.g. Pantone (Figure 2)
- The colour-mixture system based on an additive mixture of colour stimuli e.g. Ostwald system.
- Colour appearance system based on the principles of colour perception or colour appearance e.g. Munsell (Figure 3).
Examples of colourant-mixture systems are the colour atlases developed by different dye manufacturers. ICI colour atlas (1969) was a collection of 1379 original colours and 27,580 variations printed on papers.
Pantone Colour Matching System (Figure 2) is basically a colourant mixture system. The Pantone system (www.pantone.co.uk) began life in 1963 in the USA, for defining colours for printers, but expanded into other fields later, e.g. textiles in 1984, plastics in 1993, and architecture and interiors (1925 colours) in 2002, each of which has a 6-digit numerical notation (e.g. # 19-1764) and an inspirational colour name. This is widely used in graphic art and also in the textile industry mainly because of its low cost, though the colours are not equally spaced. The shades are prepared on paper using printing inks. It is not a colour order system since it does not include a continuous scale. It is more appropriately considered a colour naming system.
Colour appearance systems are based on the perception of colours by an observer with normal colour vision. The scales of these systems are chosen to represent attributes of perceived colours. However, attributes represented in various systems are different.
The main emphasis of appearance-based systems is the uniform visual spacing. The systems thus allow easy interpolation between the samples represented and extrapolation of colours not illustrated in a given collection. The collections of samples are generally represented in pages of constant hue.
Six popular colour order systems, country of origin and their respective colour attributes are as follows (Roy Choudhury, 2010) :
- Munsell (USA) – Hue, Value and Chroma
- Natural Colour System (Sweden) – Hue, Blackness and Chromaticness
- Ostwald system (Germany) – Hue, Lightness and Saturation
- DIN system (Germany) – Hue, Saturation degree and Darkness degree
- OSA-UCS (USA) – no separate scaling of three attributes
- Coloroid System (Hungary) – Hue, Saturation and Lightness.
Most popular appearance-based colour order system is the Munsell system. The system (Figure 3) consists of the following three independent dimensions which can be represented cylindrically in three dimensions as an irregular colour solid.
- Hue (H), measured along the circumference of the horizontal circles
- Chroma (C) or purity of colour, measured radially outward from the neutral (grey) vertical axis
- Value (V), measured vertically from 0 (black) to 10 (white).
The complete Munsell specification of a sample is expressed as H V/C (e.g. 5R 4/8).
Munsell system divides each horizontal hue circle into five unique or principal hues: Red (5R), Yellow (5Y), Green (5G), Blue (5B), and Purple (5P), along with 5 intermediate hues (5YR, 5GY, 5BG, 5PB, 5RP) halfway between adjacent principal hues.
The Munsell atlas is usually available on painted paper in glossy (1488 chips) and matt forms (1277 chips). A method for specifying opaque object colours such as textiles, painted panel etc. by Munsell colour system has been described by ASTM (1980).
SCOTDIC, a textile version of Munsell created by a fusion of two quite different systems – Standard Colour of Textile (Japan) and Dictionnaire Internationale de la Couleur (France), is adopted by over 8,000 companies worldwide.
Most of the material based atlases are now available in digitised form e.g. NCS Digital Atlas (www.ncscolour.com), Digital Colour Atlas 3.0 (www. dtpstudio.de) etc.
Instrumental Measurement of Colour
Newton said that (light) rays are not coloured, but merely has the power to simulate certain sensations in the mind of the observer. The human eye is a highly versatile detector of light and colour. An observer can perceive chromatic attributes and various geometric factors (direction, texture, shape and many others) simultaneously. An instrument till date is far behind in versatility. It can measure only one attribute at a time. In other words, we need several instruments to measure various aspect of visual perception.
Basically, there are three types of colourimetric instruments in use – colourimeter, spectrophotometer and spectroradiometer. They are available in the market with varying degrees of sophistication and specialisation. While the spectroradiometer measures in illuminant-mode, the other two generally measure in object mode. The recent trend on instrumental process control has resulted in the use of on-line instruments. However, a majority of the colourimetric instruments till date are off-line and mostly used in laboratories. Laboratory instruments should be highly accurate and standardised, while on-line instruments should be rugged under various environments and should have good precision and firmness.
The colourimeters measure colour in terms of the quantities of the three primaries required to match the colour. On the other hand, spectrophotometer measures per cent reflectance or transmittance of the object plotted against wavelength at regular intervals of 1 nm, 5 nm, 10 nm, 20 nm throughout the visible range of light i.e. 380-750 nm or for practical purposes 400-700 nm.