What Aperture Size is Right for Print Colour Measurement?

When using a spectrophotometer for colour and density measurement of printed matter, what is the right aperture size? Sometimes, depending on the print process, the colour patches to be measured are much smaller than normal.

Colour management is never going to be as easy as 1, 2, 3 so performing your colour measurement correctly is very important to remain on top of digital colour management.

Here are 6 key things to consider when deciding what size aperture to use to suit your colour management requirements.

1. What is the Size of the Colour Patches to be Measured?

   The patch size can vary depending on the print process and whether or not the patches make up a colour bar for ink zone scanning. As a general rule we would recommend that patches be kept to a minimum of 3 x 3 mm, otherwise they will be too small in order to get the colour information required.

   For 3 x 3 mm patches the smallest aperture would be 1.5 mm for spot measurement. It is generally accepted that 4 x 4 mm patches would be the minimum for scan measurement with a 1.5 mm aperture.

   With a 3 mm aperture you should be using 6 x 6 mm patches for scanning measurement. For spot measurement with a 3 mm aperture you can use 5 x 5 mm patches.

2. What is the Screen Ruling You are Going to Measure?

   The screen ruling also has an impact on the selection of aperture size. If you have to use a small 1.5 mm aperture the minimum screen ruling will be 175 LPI, and anything above that will be fine.

   With a 3 mm aperture you can safely measure 85 LPI or above. Low LPI or coarse screen rulings makes it difficult for a small aperture to properly sample the colour correctly.

3. Is a Large Aperture More Accurate Than a Small Aperture?

   Interesting point! Comes down to basic physics... A larger aperture will allow more light in, which translates to the measurements being slightly more repeatable. Why?

   Well a 3 mm aperture can allow up to 4X as much light in when compared to a 1.5 mm aperture. More light translates to lower sensor noise. And what you will find is that if you average 4 measurements using a 1.5mm aperture you will achieve around similar precision as a 3mm aperture.

   Best to choose a spectrophotometer that uses a sensor with extremely low noise characteristics. making it acceptable to use 1.5 mm aperture in most situations. Lastly a smaller aperture can be more convenient to use in most situations.

4. Will You Need to Compare Measurements From Wwo or More Instruments?

   There will always be some measurement differences between different instrument manufacturers which can add to the equation, but in this blog we are discussing about aperture size.

   Best practice dictates that you should always use the same aperture size when comparing measurements between two or more instruments, if possible. The TECHKON SpectroDens is calibrated and designed for tight agreement between 1.5 mm and 3 mm apertures.

5. Is Measurement Condition M3 or is a Polarisation Filter Required?

   Measurement condition M3 uses a polarisation filter or a polarisation function. A polarisation filter reduces surface reflections caused by ink dry back and can reduce the gloss differences in colour measurement, to a degree. But a polarisation filter can reduce the light by up to 90% which means a larger aperture will work better.

   Sometimes an instrument manufacturer will not offer the polarisation option with a small aperture as there is a chance of higher variation in the measurements.

6. Will You Need Different Aperture Sizes for Different Jobs?

   Sometimes you may just need the flexibility of being able to change apertures to suit your job task at hand.

   However some manufacturers require you to choose your aperture size, up front, when purchasing the instrument. Being able to swap apertures, based on your needs, can provide you with great flexibility! The TECHKON SpectroDens provides the ability of changing apertures by the end user.

To stay on top of colour measurement management we recommend you clearly define and describe your colour measurement samples and reports to include ALL the relevant measurement information.

For more information about Colour Management training and the full TECHKON instrument range please contact us at info@colourgraphicservices.com