With spectrophotometry playing a major role at drupa, Andrew Tribute looks at current developments in colour management, believing that spectral data is the key to the holy grail of device independent colour

It could be said that drupa 2008 was the drupa of spectrophotometry, based upon the wide spread use of this type of instrument that we saw, and how it is being built into a wide range of different printing equipment.
This includes wide format inkjet printers, xerographic and electrophotographic printers, and offset and flexographic presses.

However, before discussing this situation I want to first look at all forms of measuring instruments used today to ensure high-quality printing operations.
Plate readers, densitometers, colorimeters, and spectrophotometers are all widely used within the printing industry.
Each of these instruments has a role to play and each has very different characteristics.
Two of these devices do not measure colour. These are plate readers and densitometers, both of which have no understanding of colour but provide other valuable measurements in the printing process.

Plate Readers
This group of instruments may also be referred to as dot meters, and as the name implies the role of these devices is for use in quality control and set up operations for the imaging of printing plates. Today this is done predominantly by computer to plate (CTP) imaging. The devices use either a CCD or video camera to read information from the surface of an imaged printing plate to check halftone dot sizes and shapes. These are shown on a display or can be uploaded to a PC. The use of a plate reader is to provide information that can be used by the rip of a CTP imager to control the imaging of the plate in order to give the required dot sizes from highlight to solid areas of an image. Plate readers have to be able to work with all different plate emulsions and chemistries, as well as with the latest process free plates where there is a very low level of image visible after the plates have been exposed. Plate readers have a major role in quality control to ensure printing can be maintained to a defined standard such as GRACoL, SWOP, or FOGRA.

Densitometers
A densitometer is a very commonly used instrument within the printing industry, and has been the predominant instrument used up to now to check the print quality on offset presses. It is a photoelectric device that simply measures and computes how much of a known amount of light is reflected from, or is transmitted through an object. In printing terms, an object normally means a film of ink printed on a substrate such as a solid ink patch. The reading from this is given in terms of density - indicating light blocked or absorbed to give the density of the ink film on the substrate. Most presses are set up to run to specific densities for each colour, as well as total ink values for the four main colours - CMYK. When printing to standards such as GRACoL, SWOP, or FOGRA, and when working to support the ISO 12647 standard for process control of colour printing, there are specified ink densities that have to be maintained to ensure a consistent level of print quality.

Colorimeters
A colorimeter is very different from a densitometer in that it is an optical instrument that responds to colour in a manner similar to the human eye - by filtering light into what are known as the tristimulus colours of red, green, and blue. In this way a colorimeter works in much the same way as a colour scanner or monitor. The colorimeter will measure the amount of red, green, and blue light reflected from the object. A colour's numeric value is then determined using the CIE XYZ colour space or one of its derivatives such as CIE L*a*b or CIE L*u*v. The reading that is taken will be similar to that taken by a human eye in that the results are dependent upon the lighting, the environment, and the substrate that the reading is taken from. As such this is not device independent colour but is device dependant.

Spectrophotometers
These are the most accurate of all colour-measuring devices. Light is reflected from the sample and is divided into the sections of the visible spectrum. The visible light spectrum is measured as wavelengths and a wavelength is a colour. The visible light spectrum goes from 380 through to 780 nanometers. This is between the invisible ultraviolet and the infrared wavelengths. A spectrophotometer measures the value of colour by reading its wavelength. In a spectrophotometer the total visible spectrum is divided into sections each with a bandwidth of 10-20 nm. Each of these sections gives what is termed a remission value. The measured colour is shown as a remission value with a remission curve on the spectrophotometer. This measured value can be held as a spectral measurement for future use or is converted using special calculations into chromatic colours shown as a CIE XYZ, l*a*b or L*u*v values. The remission curves can also be used to calculate densitometric values. The spectral readings from a spectrophotometer are not light dependant as with a colorimeter and for best use should only be converted to chromatic values required specifically for the particular output, taking into account the characteristics of the output device and the substrate to be used.

The spectrophotometer is becoming the primary tool for the future of measurement in graphic arts applications. Its ability to generate both density and chromatic measurements makes it a very versatile instrument.
We are now seeing it coming down in price thus allowing it to move into creative and photographic environments as seen with the recent introduction of the new X-Rite/Pantone ColorMunki instruments. One of the key developments recently has been the embedding of spectrophotometric instruments into production equipment. We have seen spectrophotometric measuring in offset presses.
This started with Heidelberg with their Image Control and Axis Control instruments with closed loop colour control for controlling ink key setting, when other suppliers were only using density measurements.
Today most press suppliers offer spectrophotometic measurement of the printed sheets to update the ink key settings. The latest developments in this area again comes from Heidelberg with its Prinect Inpress system where the spectrophotometer reads sheets running on the press rather than ones removed from the print run and taken to a scanning table.

Spectrophotometric measurements have now also moved into digital printing devices to ensure consistent colour quality, and to ensure colour control across multiple devices.
This is particularly relevant for colour proofing and approval applications.

HP is showing spectrophotometric measurement embedded in its latest wide format ink jet printer.
In the sheet-fed digital printing area the latest HP Indigo 7000 press has an inline spectrophotometric system constantly calibrating the press and ensuring consistent colour quality.
Xerox showed a similar approach in its enhancement package at drupa for the iGen3 presses.

I have stated before I see spectral measurements will become increasingly important for truly device independent colour and I see X-Rite's CxF Colour Exchange Format becoming a key approach for communicating spectral data between systems as a part of this. Spectral data is the key to the holy grail of device independent colour.