4.5 Working with a Spectrophotometer to Read Standard Colour Charts for Output Profiling
Alan Martin
Armed with our fundamental concepts in colour theory, we can apply these principles to the physical process of colour management. The practical application to print production requires a procedure for measurement, colour profile generation, and the correct use of profiles in the manufacturing process. Let’s begin with measurement and discuss the working components of a standard graphic arts spectrophotometer and the colour charts we would use it with.
X-Rite i-One (i1) Pro Spectrophotometer
The i1 Pro is one of the most common hand-held spectrophotometers used in the graphic reproduction industry. It can also be mounted in the iO base for automated scanning. As described in section 4.3, the spectro works by recording spectral data from incremental slices of the range of wavelengths included in visible light. To do this properly, the spectro must calibrate its white point to establish the baseline for interpretation. It does this by reading the white tile supplied in the baseplate that comes with the spectro. Each baseplate is uniquely matched to a specific spectrophotometer and marked with a serial number that corresponds to its spectro. Make sure you confirm that you have the correct baseplate that matches the serial number on your spectro. When used commercially, the manufacturer recommends that a spectro be returned for factory recalibration every two years. The packaging will include a certificate indicating the expiry date for the current calibration.
The spectro may also be identified (on both the serial number tag and surrounding the light-emitting aperture) as a UV-cut. This indicates it has an ultraviolet filter, which acts to remove the impact of fluorescence from optical paper brighteners. If you have access to more than one spectro device, be sure that any related measurements are done consistently either with or without the filter. Best practice is to use exactly the same instrument for any series of measurements.
The USB cable provides power to the spectro and so should be plugged directly into the computer and not into a peripheral such as a keyboard. Additional typical accessories for the spectro include a weighted strap and screw-in base for hanging the spectro against the screen of a monitor and a proof mount base with sliding rail for the spectro to read printed colour targets.
Colour Charts or Targets
You will typically be dependent on the colour management software application that you have chosen to produce a pdf file of the colour chart that your spectro can read. While in the software, you select a reading device (such as the X-Rite i1 Pro or i1 iO) from the list of devices that the software supports and then provide the dimensions for your output device. The choice of reading device will determine the size of colour patches and page format, and the size of output will define how many pages are ganged to a sheet. When prompted, name the pdf with a clear identifier (output device and date or equivalent) and save it.
Once you have the pdf, use it for any direct output, such as a proofer, or to make a plate for your printing press so that the colour chart can be printed. In all cases, it is critical that no colour management be applied to the output device for the production of the chart so that the natural base state of the device is captured in the colour target produced. It is also essential that the proofer or press be in an optimal operating state so that the output is an accurate reflection of the device’s capabilities. This may require a calibration process for a proofer or standard maintenance procedure on the press.
There are several colour chart standards that you should be aware of. The chart produced by your colour management software will likely be one of these or a slight variation thereof. The original standard is the IT8.7/3, composed of 928 colour patches. This was followed by the ECI 2002, which has 1,485 colour samples. There is now an update to the IT8, the IT8.7/4, which has extended the colour sampling to 1,617 patches. The larger number of patches provides a more detailed snapshot of the colour capability of the device that you are profiling. Of course, it takes more time to read the greater number of samples, so the newer sets are more manageable with automated reading devices such as the iO table. If you are reading with a hand-held spectro, choose the IT8.7/3 or smaller patch set if it is offered by your colour management software. The other trade-off between larger and smaller numbers of colour patches lies in the smoothness of colour transitions. Fewer data points mean less interpolation and potentially smoother curves in colour modulation. Be aware that it will require some experimentation on a device-by-device basis to determine the ideal intersection of accuracy and smooth interpretation for your measurement data.
The colour charts come in two standard patterns of organization: random and visual. Random is exactly that, and comprises the vast majority of charts that are produced for measuring. The colour swatches are distributed to provide optimal contrast between adjacent patches to aid the spectrophotometer that is reading them in distinguishing where one patch ends and the next begins. Having the colours scattered over the sheet also generates relatively even distribution of cyan, magenta, yellow, and black ink which leads to better performance from the press. The visual pattern places the colour blocks in a logical and progressive sequence so it’s easy to see the chromatic ranges that are covered. Some scanning spectrophotometers can read the visual arrangement.
Measuring Your Colour Chart
Once you have used the pdf to generate the printed sample, you have to measure the sample. For charts from proofers, it is critical to wait a minimum of 30 to 90 minutes after the sample has been produced before measuring in order for the colour to stabilize. To create a measurement file, you need three things: colour management software; a target from that software; and a measuring instrument supported by the software. After connecting your measuring device to the computer (remember to connect directly to a USB port on the computer, not to a peripheral such as a keyboard), enter the measuring step of your colour management software. You will need to point to the correct measurement chart that was used, which can be easily identified if you have named it distinctively, and confirm the right measuring device is indicated. If you are getting errors once you begin measuring and can’t proceed, the typical culprit is the selection of an incorrect colour chart or incorrect measuring device.
When you begin the measurement process, there are a few option prompts you may have to respond to. Some software allows for the averaging of multiple press sheets to produce an improved characterization of the device. This software can scan the colour bars placed at the bottom of the sheet and indicate which sheets are the best candidates for measuring. If you have chosen to do multiple sheets, then you will have to record the sheet number on each of the pages that you cut from the full press sheet in order to enter the sheet number correctly as you carry on measuring.
Proofing devices are stable enough that it is not necessary to average multiple sheets. You may still be cutting out single pages from a larger sheet (depending on the size of your proofing device) and should label the individual pages to stay organized. You can skip any of the prompts that deal with choosing multi-sheet options.
Once past the initial options, you will be prompted to calibrate the measuring instrument. Make sure the i1 Pro is correctly seated on its base plate and push the button. After successful calibration, you will be instructed to mount page 1 of your colour chart and begin measuring. For hand-held measuring with the i1 Pro, use the white plastic proof mounting base it came with. Secure your page under the spring clip at the top of the mounting base, positioning it so that the sliding clear plastic guide the spectro rides on can reach the first row of colour patches. The clear plastic guide has a horizontal opening that the head of the spectro nestles into to traverse the horizontal row of colour swatches. There is a cradle for the base of the spectro to rest on that slides horizontally across the guide. The spectro must begin on white paper to the left of the colour patch row, and finish on white paper to the right of the row. If your target has a semi-circle printed to the left of its colour rows, use it to position the chart under the scanning opening of the plastic guide. The left semi-circle on the opening in the guide should align with the printed semi-circle on the chart. If there is no printed semi-circle on the chart, allow ¼ – ½ inch of white space showing in the opening to the left of the first colour patch.
With your chart properly placed in the mounting base, position the spectro all the way to the left on the plastic guide with its base in the sliding cradle and reading aperture in the cut-away channel. Press the button on the left side of the spectro and keep it pressed. Wait one second while the white of the paper is read (there may be a beep to indicate this is done), slide the spectro smoothly and evenly to the right until you reach white paper past the colour patches, and then release the button. You should be rewarded with a prompt to scan row B. Slide the entire plastic guide down one row, move the spectro all the way back to the left, and do it all over again! If you receive an error, clear the prompt and try scanning the same row again. Success at hand-held scanning comes with a little bit of practice and a Zen approach. Your degree of success will be inversely proportional to the amount of anxiety you feel. The techniques that contribute to getting it right are smooth and even passage of the spectro; consistent pressure (make sure you are not pushing down on the head); white paper at either end; and keeping the button pressed from start to finish.
After you receive a couple of errors on the same row, the software may switch to individual patch reading. In this case, the prompt will now say “Scan patch B1” instead of the previous “Scan row B.” You must position the read head over each individual patch and press the spectro’s button when prompted on screen. After completing one row of individual patches, you will be returned to full row scanning mode. This procedure allows the measurement process to go forward when the software is having trouble detecting colour boundaries in a particular row.
Having completed all rows on all pages (which may take some time), you will be prompted to save the measurement file. Some colour management software will take you directly to the additional settings required to define a profile and then proceed with the profile creation.