HP Embedded Spectrophotometer
How does the spectrophotometer work?
The spectrophotometer is a precision instrument which is mounted on the printer carriage and is used for colour calibration and profiling. In short, regardless of the paper type you're using you can use the spectrophotometer to generate an ICC profile in order to obtain accurate and consistent colours for your prints. Practically speaking, this is carried out by the processes described below and as detailed in the accompanying diagram:
First you add the paper type to your printers list of known papers (or follow the instructions for use of non-HP paper). Paper types that are defined in the printer also have colour profiles stored in the printer (although HP recommend that you calibrate the paper before using it). If you define a new paper type, the printer automatically leads you through the colour calibration and profiling stages - see points 2 and 3 below
Next, you colour calibrate the paper type to ensure consistent colours. You perform this calibration either when a printer alert recommends this calibration (typically every few weeks for each paper type you use) or before a particularly important print job where colour consistency is vital
You then colour-profile the paper type to ensure accurate colours. Profiling does not normally need to be repeated because after you have a profile for a particular paper type, you can continue to use it. However, re-profiling does no harm and some users repeat the profiling process every month to ensure that the profile is up-to-date
When printing, select the correct colour profile for the paper type that you are using
How does it work exactly?
The spectrometer is able to determine the exact composition of the light that is reflected from a colour patch. It then splits the reflected light into different wavelength components and measures the strength of each component. It has a built-in white calibration tile (which is protected by an automatic shutter) which ensures reliable measurements meet international standards. Your Designjet, together with the colour imaging pipeline and spectrophotometer are integrated with the HP Colour Centre Software.
It then allows Designjet users to create custom RGB ICC profiles for any illuminant on any given media - with the exception of fluorescent media. (ICC was created by the International Color Consortium and allows for colour matching when moving between applications and colour systems). The spectrophotometer returns information about densitometry and colorimetry. The profiling ability is important because the standard illuminant used in graphic arts is different from photographic reproduction (which is slightly bluer and more closely resembles natural daylight).
The measurements taken allows for printer calibration which in turn delivers colour consistency with less than half the colour error of earlier HP Designjets giving an average print-to-print tolerance of dE00<1.5 and printer-to-printer tolerance of dE00<2. dE is a measure of colour difference calculated to give perceptually uniform values over the CIELab colour space.
A densitometer (see left) can be used to calibrate a printer by correlating measurements of the printed density for each ink to input ink values. This is done by printing and scanning a set of patches printed with varying amounts of ink area coverage. This allows determination of the maximum print density obtainable on a particular type of paper. The densitometer also performs other useful functions such as finding paper edges to locate the top of a sheet and determining paper width. With two photodiodes, it can also measure the specular and diffuse reflections from an unprinted paper surface to provide automatic paper type detection. Comparing the amplitude and ratio of these signals to preset values can determine whether the sheet is plain paper, coated inkjet paper, transparency film or photo paper.
By comparison, a colorimeter uses red, green and blue light sources (or filters) to approximate the response of human colour vision.
Why has HP embedded the Spectrophotometer into their Designjet?
In order to ensure colour accuracy for commercial print production, measurements using a spectrophotometer are used. HP has revolutionised professional colour workflows by automating media profiling with an embedded spectrophotometer (and associated firmware and software). The key thing here is the word 'embedded' as spectrophotometers have been around for a quite a while now. Prior to having an embedded spectrophotometer, customers requiring accurate colour calibration primarily did one of two things
Employed an "off-line" profiling solution - this is where colour calibration charts were printed, removed from the printer, dried and then scanned using hand-held or automated spectrophotometers. This solution however proved expensive, difficult to use and not easily integrated into the workflow. For example, the software that printed the colour patches may not have had access to the printers internal imaging pipeline to precisely control the ink limits and colour separations and repeated measurements had to be made to regenerate accurate profiles when changes occured in environmental conditions, inks, media or when printheads were replaced
Downloaded standard ICC profiles - this is where ICC profiles could be downloaded from a manufacturers website or outsourcing profile creation. With this method, users often got good results when a profile was available for a specific combination of ink and media, but accurate colour reproduction was still a matter of trial and error. This was due to profiles often being generated on a different printer, using different production runs of inks and media, under different environmental conditions. The other drawbacks were factors such as turn-around time, inconvenience and expense.
By contrast HP have upped the ante by embedding a spectrophotometer into all of their Designjet Z series printers. This now allows you to create your own ICC profile and makes it convenient and reliable while providing you with the tools to achieve greater control over quality, workflow, costs and schedule.