Common Colour Measurement Difficulties
You have taken the utmost care with the latest ‘XYZ’ brand packaging print job.
Measured and verified the digital proof is within the required ISO 12647-7 tolerances for CMYK and for the brands spot colours.
Measured and checked that the viewing conditions on press and your viewing system in the quality department are within ISO 3664.
Measured, checked and reported that the required number of printed samples are within the customer requested tolerances (dE, TVI, Grey Balance).
Checked and completed in the in-house validation of the measurement instruments used.
Checked that the instruments used are up to date for manufacturers maintenance, service and certification.
All ok from your side, right! But your customer is complaining that the colour is no good, out of tolerance…. A bit pale or over heated like one G Ramsay on the crockery packaging boxes above!
Why is this happening?
We often get called in to help and assist sort out situations like this. The solution is fairly straight forward.
- You need a documented Colour Quality Measurement procedure that clearly defines how colour is going to be measured, analysed, reported and viewed.
- There needs to be agreement by all parties concerned.
- All parties then need to follow and adhere to the agreement.
What do you need to include into the colour assessment procedure?
You and your customer maybe be carrying out colour measurement but that does not mean you will get measurement agreement. What do need to consider about colour measurement instruments?
There are various types of instruments - Densitometers, Colorimeters & Spectrophotometers
Densitometers are mainly used to measure the ink film thickness. i.e. ink on paper, more ink = higher density. Good for process control for hitting target densities when printing and checking if the inking is even across the sheet. A densitometer can also tell you about dot% and/or dot gain-TVI (tone value increase). A densitometer is a colour blind instrument.
A Colorimeter is a low cost way to measure and do basic evaluation. A Colorimeter uses only 3 filters (RGB) to capture colour…
Spectrophotometers are at the top of the pile! They provide the most accurate capture of colour.
Geometry and Model?
There are basically three types of spectrophotometer geometry - 0/45 or 45/0, Sphere and Multi- Angle.
All three of the aforementioned have there place, but they work entirely differently. Two spectrophotometers with the same geometry will not measure colour exactly the same due to manufacturer differences and instrument specifications.
To confirm measurement consistency everyone should use the same type, and model of spectrophotometer.
What Aperture Size?
All spectrophotometers have an aperture that is a fixed size. It could range from 1.5mm to more than 20mm. The size of the aperture definitely has an affect on the measurements. You need to use the right size aperture
to suit your ‘job’ at hand. Measuring 3mm square printed patches? You will need to use a 1.5mm aperture for spot measurement.
Using a larger aperture for spot measuring 3mm patches and you run the risk of including the background or surrounding colour outside the patches.
Everyone should be using the same size aperture.
Spectrophotometers can have different measurement modes and settings. In 45/0 instruments the measurement mode is usually set and controlled by the M condition. i.e. M0, M1, M2 and M3.
It is vital that everyone uses the same mode!
For print measurement this is very important. Black or White Backing??For double sided print material, on a thin substrate (e.g. 80gsm paper) you should use Black Backing to stop the show through or to stop the influence of printed colour on the other side of the substrate 2°. Information about the standard for backing is contained in ISO 13655. You can also use some software tools to evaluate whether your backing is within the tolerances of the specification.
Everyone should the same specification of Backing. lluminant and Observer?
The Observer angle will directly affect the measurement results. For ‘print’ colour measurement the Observer is usually set to 2°. Spectral measurement data for print is usually converted to L*a*b* using D50/2° - (Illuminant and Observer).
Make sure everyone uses the same Illuminant and Observer.
In-House Validation, Calibration and Manufacturers service and re-certification?
Like all things mechanical, your instrument requires scheduled checks and service. Having a procedure and for in-house validation and keeping records is a great way to track your instrument ‘behavior’ or condition.
Your spectrophotometer is a significant investment - look after it - keep it clean, well maintained. Keep your measurement check samples clean. Follow a good calibration procedure
- Calibrate every time your start a ‘job’ and at least once a day.
No one likes to be without their favorite instrument! But sending your instrument back to the manufacturer ensures the instrument is measuring and performing within specification. Manufacturers service and re-certification provides peace of mind and also ensures you are up to date for compliance.
2. Viewing Conditions (Illuminant)
The viewing conditions used to assess your colour print results should be standardised across departments, sites and customers.
For print this means conforming to ISO 3664:2009, which means using D50 for colour assessment, evaluation and measurement (as mentioned in the previous point in this blog!).
I see all too often a casual approach taken to critical visual colour assessment being completed under non standard factory lighting or office lighting. THIS IS USUALLY THE EASIEST THING TO FIX AND CONTROL! More often than not, someone comments about ‘bad’ colour issue from looking at a printed result or digital proof under non-standard lighting conditions.
ISO 3664:2009 viewing conditions, of the same type, should be strictly adhered to wherever the print samples are to be visually checked for ‘colour’.
A JUST Normlicht Multi Light Viewer - Three different light chambers reveal colour shifts on identical samples.
A JUST Normlicht ISO 3664:2009 viewing system, with soft proof option...
3. Colour Definition and Tolerance
There are many ways to define and describe colour, which has caused a huge amount of angst among colour specifiers, quality departments, manufacturers and suppliers.
We would recommend to use L*a*b* as the means to define a colour target or reference. You will also need to describe how the L*a*b* co-ordinates were achieved.
- Illuminant and Observer?
- Aperture size?
- Measurement Mode?
- Instrument Geometry, Type and Model?
What is the tolerance amount and the tolerance methodology?
Defining a pass/fail tolerance should be in line with industry standards (i.e. ISO 12647-2), as well the capability of the production process. And, best of all discuss this at length with your customers, as they are the ones whom you are endeavouring to please and look after!
You may use a tolerance of dE 2~3 for spot or custom printed colours…
The tolerance methodology could be:
- CIELab (1976) - dE
- CIE94 - dE94
- CIE2000 - dE2000
The latter is now commonly used for brand or spot colours…
The colour definition, tolerance amount and tolerance methodology should ALL be clearly defined, documented and followed by you and your customer(s)!
We covered most of this in the previous point when we discussed about using L*a*b* and how those numbers arrived at.
Importantly the measurement method should be same across departments, sites and with your customer.
This may include using ‘averaging’ when measuring samples and creating a colour definition or standard.
5. Environmental considerations and the affects on the standards and samples
Factory and Office environments can vary widely depending the manufacturing and production process. This can obviously have an affect on the measurement instrument performance and the condition of the sample(s) to be measured.
Therefore measurement should always be done within the same environmental conditions to ensure consistency.
Printed samples can be affected by ink dry back, UV inks, aqueous coating and varnishes.
ISO 12647-2 (Offset print standard) states that the measurement is based on dry samples…
How do you eliminate customer disagreements with colour assessment and measurement?
1) You need a documented Colour Quality Measurement procedure that clearly defines how colour is going to be measured, analysed, reported and viewed. It has to be standardised!
Include specifications for:
- Viewing Conditions
- Colour Definition and Tolerance amount and methodology
- Measurement Method
- Environmental considerations
2) Identify the variables in the production process
3) There needs to be agreement by all parties concerned, and test the process
4) All parties then need to follow and adhere to the agreement.
It is possible to improve and maintain predictability with colour. It takes some effort to set up colour assessment and a Colour Quality Management
system, but the long term benefits far outweigh the on going arguments, disagreements, loss of production, job re-prints, etc…