Colour Management - Part Three

After  learning  the  importance of colour  and  the  human  vision  mechanism the next  of importance are the self luminous objects, i. e. light sources - the origin of colours.

Without light there is no colour. When sun, the major natural light source, comes out the colours of this beautiful earth become visible and at night when the sun sets no colours are visible without the artificial light sources. Some artificial light sources appear white viewed on its own but appear coloured when seen in comparison in presence of other light sources. Everyday example is a tungsten lamp (electric bulb) or a fluorescent tube. Both appear white on its own but in presence of both bulb appears yellow and a tube blue. Strongly coloured light sources appear coloured as in case of red or other colour leds (light emitting diodes), a theatre lighting, or fireworks.

The nature of light sources has profound effects on their appearances or of the objects illuminated by them. This   is experienced when we get into the photographic darkroom illuminated with red light or platemaking room with yellow light. Some colours just disappear while others appear to have changed their appearance. This phenomenon is one of the major causes of disagreement in colour reproduction, assuming all parties involved have no colour defective vision. So what are the reasons?

For any colour to appear exactly the same, three factors are important.  A normal colour vision (discussed in article 2), light source with good output of wavelengths throughout the visual colour spectrum (see article 1), and reflection or transmission of the print or an object. The colours  are  denoted  by what  is  called ‘colour temperature’, normally applied  to  self luminous objects.

Colour temperature is always marked by the manufacturers on the bulbs, lamps, and tubes we buy in the market. This indicates the colours output these self luminous devices and help us in comparing the light sources as far as its colour characteristics are concerned. A perfect black object theoretically does not reflect any light, therefore, appears black.

For example, a candle’s wick is black or a blacksmith’s tongs are black, ie no light is reflected. When a candle wick is lit or tongs become hot it appears to emit light. So the colour light it gives out is purely a function heat converting heat waves in to light waves.
 

Figure 1: Black body radiator

Therefore, the heat which is measured in degree Celsius, the measurement can be extended to colour characteristics of light. Candle light has yellow flame that we use to give us light and not so much of heat whereas the gas stove has blue flame that is needed to cook the food and not to give us light. When the heat applied is low we get yellow flame and as the heat applied increases we gat blue flame. The emission of light from the heated object starts only when sufficient heat is applied and as we increase the heat application further colour given out that begins at red goes towards blue through the colours of the spectrum. See figure 1.  To differentiate from heat degree Celsius the colour is measured in degree Kelvin. See figure 1.

The Kelvin scale starts at minus 2730 Celsius or 00 Celcius is equal to 2730 Celsius.

00 Celcius = 2730 Kelvin or minus 2730 Celcius =  00 Kelvin

From figure 1 it can be seen that graph indicating 10000 Kelvin gives red colour whereas

10,0000 Kelvin gives predominantly blue colour. Height of the graph indicates light output in that particular region of the colour spectrum. This is the fundamentals in describing the light sources.

In 1931, CIE (Commission Internationale de l’Eclairage) was established to firmly define colour science as colour perception perceived by the human eye, see article 2. In fact, printing industry is one of the last one to apply this after all other industries. CIE further worked on defining the possible light sources that should be developed for the industrial as well as other lighting applications. These are called CIE Illuminants.

CIE defined illuminants. See figure 2.

A (2,8560 Kelvin) -  most common tungsten lamp

C (6,7740 Kelvin) -  represent average daylight

D50  (5,0000   Kelvin),  D55  (5,5000   Kelvin),  D65  (6,5040   Kelvin),  D75  (7,5000   Kelvin)

represent different phases of daylight, from cloudy sky to clear sky at different times of the day and season. D50 (5,0000 Kelvin) is more balance with roughly equal amounts of all visible spectrum colours. Lower the temperature yellowish the colour and higher the temperature bluer the colour.

However, the manufactured light sources do not exactly match the CIE recommendations due to the limitations of raw materials and components available to manufacture lighting. See figures 3 to 5. There are various ways the light can be produced.

 

Figure 2 : CIE Illuminants

Light producing methods

Incandescence - Substance emits light when heated to above 1,0000 Kelvin. These lamps output colour spectrum is continuous and closely resembles the black body radiators. This includes sun, tungsten and tungsten halogen lamps. See figure 3.

Figure 3 : Incandescence lamp

Figure 4 : Gas discharge lamps

Gas discharge - Gases emit light when electric current passed through. The colour given is due to characteristic of element present in gas. This includes sodium, mercury, and xenon lamps. These lamps as opposed to tungsten lamps have conspicuous peak emission at some wavelengths, the only exception being a xenon gas lamp. See figure 4.

Electroluminescence - Semi-conductors or phosphors emit light when electric current passed through. These are various types of fluorescent tubes available on the market. Here again some peaks are prominent with differing height for different type tubes. See figure 5.

Figure 5 : Electroluminescence lamps
 

Photoluminescence - Some substances absorb radiation and emits light with a change of wavelength. If emission is immediate it is called fluorescence but if after considerable gap then called phosphorescence.

Cathodoluminescence - Phosphors emit light when bombarded with electrons. These are used in television, video display tubes, and oscilloscopes.

Chemiluminescence - Some chemical reactions emit light without generating heat.

The lamps recommended in printing industry are D50 and D65. See figures 6 and 7.

Figure 6 : D50 lamps

 

Figure 7 : D65 lamps

These lamps have basically well balanced output throughout the visible spectrum; of course, D65 is slightly bluer than D50. D65 originated from the idea that to judge yellow ink on the printing machine this gave better judgement than D50 and therefore, recommended for the press room. While calibrating colour monitors D65 is always preferred as D50 appears too yellow. Televisions have a colour temperature of 6,5000 Kelvin.

However, the most  important is colour should appear acceptable even in  other lighting temperature as available in the shopping malls and departmental stores. It is advisable to have a light booth with different lights to judge how the print looks under different light temperatures.

The colour temperature of lamps is also called co-related colour temperature as the lamp is not actually heated to that high temperature. It only relating the colour output of the lamps to the colour emitted by black body, see figure 1, at that temperature.

Light intensity

Apart from colour temperature the amount of light available to see colour is very important. This is called luminous intensity and is the result of lamp wattage output.

If light is very dim the colours do not appear bright enough, on the other hand excessively high luminous intensity produces glare and desaturation of colours to some extent.

Luminous intensity of 500 lumens per square foot should be available for proper colour matching.

Colour rendering index

Different light sources have different colour rendering properties. In case of fluorescent tubes spectral distribution can be varied over a very wide range. See figure 5. Therefore, it is required to have some means of expression whether light source will give satisfactory colour rendering.

A colour rendering index of 100 indicates that colours considered in its evaluation, the source is closest to black body radiator. Light sources having colour indices of 90 or more are considered to be very good for practical applications. Some fluorescent tubes have indices of as low as 50 or less and are quite deficient in matching work.

Light sources                     Colour temperature             Colour rendering

Computer monitor                                          9300 K

North-sky light                                                 7500  K

Average daylight                                             6500  K

Fluorescent lamps                                        6500  K              93
(Northlight, colour-matching)

Television monitor                                         6500  K

Xenon                                                               6000  K              93

Sunlight plus skylight                                    5500  K

Blue flash-bulbs                                             5500  K

Balance light                                                   5000  K

Carbon arc for projectors                              5000  K

Sunlight at altitude 200                                    700  K

Fluorescent (cool white)                                4200  K               58

Sunlight at altitude 100                                   4000  K

Clear flash-bulbs                                            3800 K

Fluorescent lamps (white, natural)              3500  K              92

Photoflood tungsten lamps                           3400 K

Tungsten-halogen lamps (short life)           3300 K             100

Projection tungsten lamps                             3200  K

Studio tungsten lamps                                    3200  K

Tungsten-halogen lamps (normal)              3000  K

Fluorescent lamps (warm white)                   3000  K            51

Tungsten lamps floodlights                            3000  K

Tungsten lamps (domestic 100 watts)         2800  K          100

Tungsten lamps (domestic 40 watts)           2700  K          100

Sunlight at sunset                                             2000  K

Candle flame                                                     1900  K

 

Light sources                                                 Lighting level

Bright sun                                                  50,000     -  100,000   lux
Cloudy dull                                                 2,000      -    10,000    lux
Shop windows                                          1,000      -      5,000    lux
Offices, living rooms                                      50      -         300    lux
Sunset                                                                1       -        100    lux
Street lighting                                                   0.1     -          20    lux
Full moon                                                       0.01     -        0.1     lux

Click here to read Colour Management - Part One on Importance of colour management

Click here to read Colour Management - Part Two which explains how the human eye is a source for colourful world