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Practical tips for Print Inspector

Tips for Print Inspector

The practical solution on how to use Print Inspector for quantifying precision and accuracy of your output devices.

 

Test form/ control strip

The first step is to choose an optimal test form to capture output devices performance.
We developed ChromaChecker control strips/test charts which are designed to optimize performance results. There is a compromise between a number of patches versus accuracy and cost (time, consumables and dedicated print run) for best analysis.  Although ChromaChecker a work with any target, we built the ChromaChecker PI-64 control strip and VI-816 test charts to work in both digital and conventional environments and provide the detail required to determine precision and accuracy of output devices.

 

First, to determine your devices variation, we recommend running multiple print runs with your control target to collect this data. We recommend that the run length should be typical of your device.
Large Format= 5, digital press= 500, offset press= 2000. You will want to measure at least the first sheet,
the 10th sheet (or last), and 100th, and then every 250 if less than 800, every 500/1000 if more making sure, you measure the last sheet. From this information, you will be able to calculate Intra-Page variation and Inter Page variation from the same print runs.

Generally, you will want to capture this data from multiple runs, and ideally on different substrates.
Make sure that you hand label what sheet number, and what device condition each sheet was produced.
You will have a lot of sheets and it is easy to get confused. Poorly labeled and entered data is worse
Then doing nothing...

 

Choosing the right target for the job...  

 

Offset Press: PI-64,

 

The PI-64 target consists of 64 patches which are optimal for assessing a device's performance and iterating a TVI or G7 curve. Based on your measurement instrument, this target is capable of being printed as small as .5”x10” which small enough to be printed in line with regular production so no dedicated print run is required.

There are no repetitive patches in the PI-64 target for:

  • intra-page analysis so it is necessary to place multiple targets on the same page when desiring an
  • intra-page analysis. Our VI-816 test chart has repeating patches, so it works well by itself to analyze
  • intra-page variation. The unique layout allows for in-depth calculations and can also be used for color conformance and iterate a TVI or G7 Curve. This test form is specially dedicated for digital presses and
    large format technology, but can be used for any printing technology, especially when the
    requirement involves understanding the spatial uniformity (intra-page) variation.

But this target is not conducive for a traditional printing press due to the cost of running a dedicated press run. For this purpose, we can use multiple copies of the PI-64 target spread around the page in line with production work. Remember that in order to calculate variation,
it is necessary to analyze measurement targets with repetitive patches. Once the multiple targets
which were on the same page are measured and uploaded as separate files,

ChromaChecker allows you to group the targets on the same page and call the group
“Page3 Intra-Page conformance” to assess the Intra-page variation on the given page.
This group will consist of enough repetitive patches to provide variation analysis.

We don’t recommend the use of an IT-8 or P2P25 for this purpose - keep those test charts for
profiling or G7 calibration purposes. Both Test Charts (IT-8 or P2P25) have few repetitive patches
- too few to get representative results.

 

 

Second Step - collecting data for analysis

In most cases, you should start from intra-page evaluation. If pass goes to the inter-page case, next inter-job.
For example in a case of an offset press, you should have at least four PI-64 control strips placed in
different locations on the sheet (top/bottom and left/right) in line with regular production jobs.
For better accuracy, we recommend to increase the number of unique localizations – but there is the
tradeoff for locations on a production sheet in terms of cost and time.

 

Digital or Large Format: VI-816

INTRA–PAGE Analysis and INTER PAGE Analysis

 

a.
Print multiple pages (Ideally use sheets that represent a start, mean time and end of print-run to get
a picture of how the press is stable during a longer production period) of VI-816 on Digital or Large Format,
for sheetfed/web, place the PI-64 target in multiple locations on press sheet with live work. 
Then label it, measure it, and upload data to your Qualification track for targeted device. 

b.
Go to Variation Inspector make a Qualification track for this device and name it Intra/Inter
Page variation and analyze the results. If you used the PI-64 target you will need to group the measurements that were from the same pages.

c.
If your sheet is much bigger than VI-816 test form you may want to duplicate the VI=816 multiple times
up on the same page (corners and center) to get data from different locations. 


If satisfied with the results, you can now profile your device, otherwise, fix variation problems until satisfied. 

 

INTRA—JOB ANALYSIS 

d.
Repeat the above test by printing the same targets over multiple days to check inter-job repeatability.
Ideally, keep track of how long since the last calibration or other maintenance has been done.

 You may observe long term system stability issues by comparing various production jobs along a long time.
Some devices (e.g. digital presses) are very sensitive to environmental factors. You may want to use
Event tool to mark on timeline critical intervals. Environment Inspector may be very useful to find a
potential correlation between ambient temperature and humidity and their influence on process stability.
Substrate Inspector will monitor problems coming from inconsistencies with paper or other media.

 

Checking for Accuracy with Print Inspector

Once uploaded data can be analyzed by Print Inspector – just switch the tool, but
as Print Inspector make an average of data (excluding curves) we strongly recommend
to use Variation Inspector first. Otherwise averaged data may look good but
due to instability it can hide real problem – 
An average as simple statistical gives the results that man with a dog has three legs!

 

Basic principles on how to judge variation

 
Analyze how primaries solids are reproduced is important to compare each primary unit  (C, M, Y and K) - to themselves. If one or more Units is inconsistent with itself - it is probably due to a mechanical issue with that unit. If one unit prints consistently than the other units should too.


If all units have similar inconsistent results consult if another device of the same kind can perform better.
Some output devices are not very stable, manufacturers are not forthcoming about weaknesses of their printers. You may have to change the device or lower your expectations.

 

Next analyze the secondaries solids (overprints Red, Green, Blue traps)

You may expect that overprints will show problems already recognized by primaries analysis. If the secondary solid has an unexpected difference - you may have a problem with inks or substrates.

Next, analyze Grey patches, they are often the worst part of the report – they combine errors from all C, M and Y units plus problems with proper tints reproduction. This type of issues is easy to minimize by regeneration of
the black component in your color conversion process.

Next, analyze the Tint variation section. This is focused on dot gain issues. Any problems like slurring or
doubling will be reflected by high values in the graphs. If one or more units is visibly different from
another you should perform an additional test and start maintenance procedures on the press.
Slurring/Doubling Tool or NIP Control Tool may solve this issue very efficiently (see more information on these two tools).

Always evaluate Variation before any other step – stability problems are very common – and it directly affects your ability to get accurate results from your device. This can save a lot of time and effort. 

 

The expectations are simple - we expect that color patches/builds are reproduced the same way
- on various location the same sheet (intra-page variation), on each sheet during a single print run
(intra-page variations), on all current jobs (inter-job variation). But this is NEVER the case.
No Printing device in 2017 is capable of 0 E-Factor precision. Which means you are only as
accurate as the weakest device. Your desired E-Factor can be me if the print condition is stable and
repeatable for all jobs. That means that all the variables related to color reproduction substrate,
inks or toners, instrumentation, lighting, plate setter all have to be consistent in order to get the desired results.

You need to be sure that device is in repeatable, stable condition before you start to calibrate and color manage it. That why variation analysis is the FIRST pre-qualifying step on any device that needs to be color controlled.