Colour Management - Part Ten

Colour measurement has come a long way from subjective visual measurement and now any process of colour measurement, management, and reproduction follow instrument measurement systems for objective colour reproduction. Instrument systems help the process to work scientifically, eliminate guess work, reduce wastage, and increase productivity. There are three basic systems of instrument measurements

1. Densitometers

2. Colorimeters

3. Spectrophotometers

Densitometers

Densitometers replace the human eye and measures the printed ink based on reflected or transmitted light from the substrates. Densitometers, basically measure one third of the visible spectrum at one time and, therefore, are useful for the cyan, magenta, yellow, and black

process colour measurements. It is not very effective for other special colour measurements. See figure 1. On printing machine ink is controlled through density measurements.

When no ink is deposited on the good white substrate, it reflects all colours and is the reason why it appears white. Many substrates are not good white and some have strong colour bias. However, since the ink is deposited on the substrate, doesn’t matter what its colour, it is taken as base reference. Ink deposition is measured with reference to this base.

The densitometer is made to read ‘zero’ on base substrate. This means it is calibrated to read as 100 % reflection or transmission of basic colours red, green, and blue. When process inks cyan, magenta, yellow, and black are deposited on the substrate it selectively absorbs the part of the visible spectrum. As seen from figure 1 cyan, magenta, and yellow absorb red, green, and blue part of the spectrum, respectively.

This absorption of part of the spectrum begins with ink deposition and the amount of light absorbed depends on ink film thickness. As the ink film thickness increases so does the amount of light absorbed. It increases ink opacity that makes it look stronger (similar mechanism operates when inks are highly pigmented).

To be able to measure the ink deposition effectively it is measured with the colour of the light that is absorbed by a particular ink. This makes ink appear black due to absorption of light and, therefore, better indication of ink deposition. Red light measurement for cyan ink and green and blue for magenta and yellow, respectively. See figure 2.

Densitometers are used to measure and quantify several quality parameters in the printing, graphic arts, and many other industries, such as engineering, x-rays, dyes, pigments, etc. All quality parameters are measured as reflected / transmitted, absorbed quantities of light, and

nothing else. These are then expressed as densities. Density values are used to interpret the quality parameters using mathematical formulae. Modern densitometers have these formulae programmed and does not need any manual calculation.

Expressing reflection / transmission and absorption values in opacity is unwieldy and very inconvenient. These values expressed in log of opacity, i. e. density is more convenient. See table below.

                          Reflected light

Reflectance = -------------------------

                           Incident light

Transmittance is ratio of transmitted light to that of the light incident on it.

The density values obtained depend on the filter system used while measuring the print and values obtained will differ. Therefore, it is important to specify this when quoting density or any of the quality values. There are different colour filtration systems in use as given below.

 

New generations of densitometers are spectrally based. This means modern instruments measure spectral curve given by diffraction gratings and exact the information using mathematical filters to calculate the density. This system continues to quote this mathematical filtration based on the earlier physical filters. Contradictory to this the responses of these

Densitometers have slight deviations compared to the responses defined by ANSI / DIN / ISO, because the accuracy in manufacturing gelatine and glass filters is limited.

Status E / DIN wide band densitometer response, mainly used in Europe.

Status T / ANSI T wide band densitometer response, mainly used in the United States.

Status I / SPI, DIN NB narrow band densitometer response.

Status A / ANSI A wide band densitometer response, used mainly used in the photographic industry for measurements of prints and slides.

Status M wide band densitometer response, used in the photographic industry for measurements of negatives.

Status Ax, Ex, Tx earlier generation instruments that used filters made from gelatine or glass.

HIFI Status E densitometer responses for CMYK plus additional filters for red, green, blue and orange colours for high fidelity colour printing.

Wide band and narrow band are the terms used to denote the light band transmission through the optical filters. Medium bands give higher density values compared to wide band.

 

Note: What is now called wide band be is really the medium band and real wide band have still wider transmission spectrum, but not used for colour measurement. These are used for special applications. See figure 3.

 

Another main component in density measurement is the polarization filter. Waveform of the light creates glare and give false reading. Polarisation filters that cut out the specular reflection of light reduces the glare effect on density measurements. The principle is same as the Polaroid eye glasses. Two polarisation filters are placed at 900 angle to each other, see figure 4. When printed is wet it gives specular reflection due to most of the ink vehicle still on the substrate, resulting in higher density values. Once the vehicle is either absorbed or dried the specular reflection is reduced resulting in lower density values. Most American users carry out measurements without polarization filters whereas Europeans use polarisation filters.

 

 

Quality parameters measured using densitometer based on density values

Dot area measured using densitometer collect reflected or transmitted light from substrate and printed inked halftone dots. These two are combined to give effective total light falling on densitometer sensor and ultimate density. This is called integrated density. There are two types of dot area, physical and optical. See figure 5.

Physical dot area is size of inked dot area on the substrate. Optical dot area is the optical size of dot area which always bigger than the physical dot area. It appears bigger than actual size on the substrate due light scattering mechanism within the substrate. This light scattering is indicated by ‘n’ factor. For example, for coated papers is something like 1.65 and for coated papers 2.70.

Mathematical formulae programmed in densitometer display dot percentage either physical dot area or optical dot area. In the following formulae Rt is reflectance from dot area, Rs is reflectance of solid ink area, Dt is density of dot area, and Ds is density of solid ink area.

 

Dot gain is the enlargement or reduction of the dots after printing as compared to the dots provided by the pre-press department. Programmed dot values in the densitometer display dot gain on display, either physical or optical dot gain. See figure 6.

Print contrast is generally the contrast between the dot area in the shadow and solid ink area. Higher the contrast better details in the shadow and vice versa. See figure 6.

Ink trap is the ability of first printed ink to accept the next ink. This overprinted ink is compared to itself when directly printed on substrate. This 2 nd ink over 1 st, or 3 rd ink over first 2, or 4 th ink over first 3. See figure 7. Due to more complicated nature of ink trap due to various factors different formulae are recommended and used.

Measurement using the major filter of the second down colour. Dop is density of overprint minus paper density, D1 is density of first-down ink minus paper density, D2 is the density of second-down ink minus paper density.

Measurement using the major filter of the second down color. Dop is solid density of overprint minus paper density, D1 is the density of first-down ink minus paper density, D2 is density of second-down ink minus paper density.

Measurement using the major filter of the second down color. Dop is the density of overprint minus paper density, D1 is density of first-down ink minus paper density, D2 is density of second-down ink minus paper density, Dm is maximum printable density for a given substrate minus substrate density.

Hue error is colour error. Good process colour ink should have equal reflection of the two of the spectrum colours. When these two are not equal, colour bias is introduced. This is called hue error.

Grey error occurs is when none of the spectrum colours reach 100 % energy. This is difference between 100 % and maximum reflection of the highest reflected colour.

Efficiency is the combined effect of hue and grey errors. High hue and grey errors result in lower efficiency and vice versa.

 

High, medium, and low densities are three densities of each ink through three colour filters where maximum reflected light gives low density, minimum reflected light gives high density, and third one is medium.

Future article will deal with colorimetry and spectrophotometry.