Commentary & Analysis
Color Measurement in Operation
It could be said that drupa 2008 will be a spectrophotometer drupa based upon the wide spread use of this type of instrument,
By Andrew Tribute
Published: May 20, 2008
It could be said that drupa 2008 will be a spectrophotometer drupa based upon the wide spread use of this type of instrument, and how the spectrophotometer is being see 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 in ensuring high-quality printing operations.
Plate readers, densitometers, colorimeters and spectrophotometers are all measuring instruments in use within the printing industry. Each of these instruments has a role to play and each has very different characteristics. Two of these devices are not devices that measure color. These are plate readers and densitometers, both of which have no understanding of color.
This is also known as a dot meter 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 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 spot sizes from highlight to solid areas of an image. Plate readers have to be able to work with all different plate emulsion colors, as well as with the latest process free plates where there is a very low level of visible image. Plate readers have a major role in quality control to ensure printing can be maintained to a defined standard such as GRACoL, SWOP and FOGRA.
A densitometer is a very commonly used instrument within the printing industry, and has been the predominant instrument up to now use 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 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 in terms of density and shows the more light blocked or absorbed the higher the density. Most presses are set up to run to specific densities for each color as well as total ink values for the four colors. In printing standards such as GRACoL, SWOP and FOGRA working to support the ISO 12647 standard for process control of color printing there are specified ink densities that have to be maintained to ensure a consistent level of print quality.
As stated earlier a densitometer has no understanding of color, just light transmission or reflection. A colorimeter is very different and is an optical instrument that responds to color in a manner similar to a human eye by filtering light into red green and blue colors, or tristimulus colors. In this way a colorimeter works in much the same way as a color scanner or a monitor. The colorimeter will measure the amount of red, green and blue light reflected from the object. A color’s numeric value is then determined using the CIE XYZ color 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 it will depend upon the lighting, the environment and the substrate that the reading is taken from. As such this is not device independent color but is device dependant.
These are the most accurate of all color-measuring systems. In this 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 color. 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 color 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 color 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 colors 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 main product for the future in graphic arts applications. It ability to generate both density and chromatic measurements makes it a very versatile instrument. We are now seeing it coming down in price to move into creative and photographic environments as seen with the recent introduction of the new Pantone ColorMunki instruments. One of the key developments recently has been the move 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 color 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 taken to a scan table.
Spectrophotometric measurements have now also moved into digital printing devices to ensure consistent color quality and to ensure color control across multiple devices. This is particularly relevant for color proofing and approval applications. HP is showing spectrophotometric measurement in its latest Z2100 and Z3100 wide format ink jet printers. In the sheet-fed digital printing area the latest HP Indigo 7000 press has an inbuilt spectrophotometric system constantly calibrating the press and ensuring consistent color quality. Xerox will show a similar approach in its enhancement package to be shown at drupa for the iGen3 presses.
I have stated before I see spectral measurements will become increasingly important for truly device independent color and I see X-Rite’s CxF Color Exchange Format as a possibility as a future standard for moving spectral data between systems as a part of this. Spectral data is the key for the holy grail of device independent color.