By Abhay Sharma
By Abhay Sharma
Hallelujah! – There is finally a clear and elegant solution for the ever-present problem of doing a press-to-proof match, or doing a press check and matching a proof in the viewing booth. Most printing papers today contain optical brightening agents whose fluorescence causes the printed sample to look bluer and brighter, which is good, but then the colours do not always match the proof.
Ultraviolet (UV) radiation in a light source can hit the paper causing it to fluorescence and change colour. This UV-induced colour shift has been difficult to control causing headaches in prepress, in the pressroom and with light booth and measuring instrument suppliers. But wait – the whole industry can now breathe a sigh of relief. A new standard called ISO 13655 has recently clarified UV-included and UV-excluded measurement modes for spectrophotometers, which is likely to improve inter-instrument agreement, especially when measuring press sheets containing fluorescent brightening agents.
To avail of this new standard you have to update your equipment – you need to buy the new X-Rite eXact or i1Pro2 or the Techkon SpectroDens; and you need to put new lamps in your GTI or JUST Normlicht viewing booth. The cost of updating technology is burdensome, but any such financial pain is ultimately superseded by recent changes that allow us to now banish colour-matching woes that have plagued the industry for many, many years.
In the printing industry, one of the major considerations is the ability to deliver accurate and consistent colour to the customer. Colour matching is done using instrumentation via a process known as “printing to the numbers.” The numbers in this context are usually L*a*b* values that are measured and monitored via the use of a measuring instrument. One challenge with instrumentation has been that the UV component in the measuring illuminant of different instruments can be different, which causes different instruments to give different readings for the same sample. If the paper or ink exhibit fluorescent behaviour, then we have typically seen variations between measurement devices when measuring the same press sheet – and this causes much head scratching.
At the moment, the UV component in the light source of an instrument is not stipulated so that different devices can have different amounts of energy in the UV which lead to different measurement readings of the same sample. This means that if you used the X-Rite i1Pro instrument to make an ICC colour profile, but press side you used an X-Rite 530 handheld spectrophotometer, the press sheet and contract colour proof may not match despite full and correct application of a colour management process. Instruments up until now were not carefully regulated in the UV part of the spectrum – so different instrument families had different lamp characteristics and differing amounts of UV in their devices. As more and more printing papers started to include optical brightening agents to increase their brightness, we started to see problems which caused endless headaches in the field.
The UV in a measuring instrument represents the classic case of the observer causing an effect in the experiment that they are trying to observe because the light used to measure the sample is itself influencing the measurement. In the case of samples containing optical brightening agents (OBA), UV from the measuring light is absorbed and emitted in the blue part of the spectrum, so we are no longer independently measuring the sample because the light used to probe the characteristics of the sample is changing the sample’s characteristics. When OBAs are present, the sample’s reflectance will change with the amount of UV in the measuring instrument and different instruments from different manufacturers will compute different spectral data for the same sample.
In a simple experiment, the IDEAlliance Control Strip was printed on Epson Photo Paper and the white patch was measured using different instrument configurations. The different instruments produced varied results. The peak in the blue part of the spectrum (around 400 to 450 nm) changes depending on the amount of UV light in the measuring instrument – the more UV light in the device, the higher the peak in the blue. This difference is most evident in the white patch, but is expected to affect all colours in the control strip to a greater or less extent. We get different readings depending on the instrument used, this understandably causes problems – which reading is correct and which reading is seen by the customer in the viewing booth during the press check?
The new standard
There is an updated ISO Standard called ISO 13655:2009 graphic technology, spectral measurement and colorimetric computation for graphic arts images. This ISO standard specifies the illuminant (UV) characteristics when using an instrument to measure printed samples. A number of new devices incorporating ISO 13655 have been recently released by major suppliers, as well as a number of light booth fixtures. ISO 13655 clarifies the different UV included and UV excluded measurement options. There are now four clear and well-defined measurement modes defined as M0, M1, M2, and M3.
M0 is known as a “legacy” mode and is a standard that represents the majority of measuring instruments currently in the field today. It is directed to instruments that use an unfiltered gas-filled tungsten lamp to illuminate the sample being measured. Prior to LED-based devices, the tungsten bulb based device was the primary type of device on the market. The light contained within the instrument may approximate Illuminant A. It should be noted that in this mode the light is neither UV filtered nor polarized, and also the UV component can be very weak. In general, the M0 mode is a catch-all mode so that legacy devices can be characterized within the new ISO standard. An M0 instrument can safely be used for process control applications as it can make very reliable, repeatable measurements, but it cannot be used in situations where it is necessary to exchange information or seek correlation with other measurement scenarios because an M0 instrument may not read the same as another instrument that is measuring the same sample.
M1 is known as the “D50 mode” or “UV included” mode. A major difference (and improvement) over earlier specifications is that the amount of energy in the UV and visible wavelengths is now specified. The light source in the instrument must match CIE Standard Illuminant D50. It is useful to remember that D50 is simply a spectral curve and there may be different ways to elicit a D50 response. Generally speaking, there are two methods to achieve conformance to condition M1 – we describe these as physical (using a tailored, customized source and or imaging apparatus, as employed in the X-Rite eXact) and mathematical (using a mathematical function to approximate the required spectral power distribution, as used in the Konica-Minolta FD-7).
M2 is defined as a “UV-cut” mode. ISO 13655 states, “To exclude variations in measurement results between instruments due to fluorescence of optical brightening agents… the spectral power distribution of the measurement source… shall only contain substantial radiation power in the wavelength range above 400 nm.” How is this mode used in practice?
There will be times when a customer will request a print to be measured using M2 because the lighting used to display the job is expected to be free of UV content. A museum is an example of one of the major places that uses UV-free lighting. In colour management circles OBA induced colour shifts were often dealt with by removing UV light from both the measuring system and the viewing conditions. Now with the new standard we have a specific definition for “UV-cut” and the wavelength at which it happens. Note that the rest of the illuminant spectral power distribution for M2 is not specified – it does not have to be, as in this spectral range we are in a situation where the instrument illuminant does not interact with the specimen or change the emission characteristics, so it is not necessary to define the spectral power distribution of the source from 400 to 700 nm and a measuring instrument can simply compute the spectrum of the sample in this range.
M3 is a polarizing mode and consists of UV-cut up until 400 nm and then a polarizing filter is applied to the remaining wavelengths. As above, the illuminant spectral power distribution from 400 to 700 nm for M3 is not specified – it does not have to be, as in this spectral range we are in a situation again where the instrument illuminant does not interact with the specimen. The main use of M3 is to limit or completely remove surface reflections.
In the offset printing industry, the customer pays for the final dry product. One of the main concerns is that the press sheets come off the press wet and as they dry the density of the ink drops. The M3 mode can aid printers in cutting the surface gloss from wet inks, and if drying is primarily represented by a change in surface gloss then by removing the gloss we may have a better prediction of the final expected dry density. Because of the polarizing filter the measured density values using M3, may be different to the density achieved from the other modes. In fact, in any measurement we see that each mode (M0-M3) can produce a very different spectral response and thus any computed metrics (CIELAB, CIEYxy, density) can be different for each measurement mode.
Measuring and viewing
The instrument manufacturers have responded to ISO 13655 with the introduction of a suite of devices that all meet the M0 – M3 measurement modes. Konica-Minolta Sensing has entered the prepress market with the FD-7 spectrophotometer and a paired automatic chart reading table called the ColorScout A+. The device also measures ambient lighting – a feature not found in other similar devices. Techkon released the SpectroDens, which can be used in spot mode but also uniquely has four wheels allowing the user to roll it over control patches. Techkon also has two clever iPhone apps: iRegister Pro can be used to measure the register on a press sheet and the ColorCatcher app can measure the L*a*b* of a sample. You can use your iPhone as a measuring instrument! Note that both the iPhone apps require a small kit costing around $100.
We note that market leader X-Rite of Grand Rapids, Michigan, offers us the eXact for the pressroom and a new version of the i1Pro called the i1Pro2 for colour management users. It is important to note that neither the chart reader iSis device or the press-side IntelliTrax scanning system is compliant with the new standard and neither can be retrofitted to comply with the new standard. Users can, however, adapt their i1iO table to accommodate the new i1Pro2. When buying an instrument or upgrading your system, make sure you are using an instrument from the above list that is ISO 13655 compliant.
The clarification for illuminant in measuring instruments (ISO 13655) is accompanied by a similar clarification in the standard for viewing booths called ISO 3664. Via the updated viewing booth standard, emphasis has turned to requiring a closer simulation of Illuminant D50 thus clarifying the amount of UV illumination in the viewing booth too. The new viewing booth standard refers to issues such as excluding stray light and that the walls of the booth should be a type of neutral gray, but in the current context, ISO 3664 has called for tighter tolerances on the quality of the light source to ensure that it closely matches the D50 (M1) curve especially in the UV spectrum.
In terms of light booths, two major manufacturers in GTI and JUST Normlicht have had new light fixtures available for a couple of years now. All users should check with their representative or on the supplier Website to ensure they have ISO 13655/3664 compliant lighting.
We are at a truly exciting juncture in colour management systems – finally we have a clear specification for the UV component in both the viewing booth and the measuring instrument. Together, these systems are able to deal with the challenges of OBA-induced colour changes that have plagued our industry for a long time. If a viewing booth is fitted with the new light sources and we use a new measuring instrument in M1 mode, then visual appraisal of press sheets and contract colour proofs will always be in agreement.
Measuring instruments are supposed to provide a reliable and robust method for colour measurement. Unfortunately, in the case of UV and OBAs there has been considerable confusion and lack of inter-instrument agreement. The new ISO 13655 standard for instruments and ISO 3664 standard for viewing booths will greatly reduce the colour matching problems currently faced in the field.
Dr. Abhay Sharma is a professor at Ryerson University’s School of Graphic Communications Management. Dr. Sharma is active in print media research and recently coordinated the IDEAlliance Wide/Grand Format Inkjet RoundUP study. He can be reached at firstname.lastname@example.org.