Top Five Attributes that Can Affect Spectro Measurements

Posted July 07, 2015 by Jackie Weeks-Atkin

To establish a solid quality control program, you need good instrumentation, robust software, and trained users. But even with everything in place, there are some common pitfalls you must watch out for when using a spectrophotometer to analyze color quality.

attributes spectrophotometer measurement

Measuring plastic parts with the X-Rite Ci7800.

1. Bad Samples

When samples are first manufactured, it’s a good idea to immediately make backup samples from the same batch. Verify the master and working standards to ensure that they’re the same shade. If a working standard becomes damaged, you can just throw it away and replace it with a backup. Then, standardize, document, and communicate your sample preparation procedures to everyone involved in the workflow to ensure that each user does each task the same way, every time.

Standards and samples don’t last forever. To preserve life, you must protect them from exposure to light, heat, humidity, airborne contamination, and fingerprints. Careless handling and dirty devices can quickly destroy a standard. Store your standards in a black plastic bag without plasticizers or in non-acidic paper envelopes in a cool, dark place. Refrigerators and freezers will also extend sample life.

2. Optical Brighteners

White isn’t always what it seems to be. This effect is caused by the presence of optical brighteners, which are fluorescent materials that manufacturers add to make their products look whiter.

spectrophotometer optical brightening agents

Here we have two white shirts. The fabrics look identical under daylight, but the whites are quite different when viewed under UV lighting. This can be a real problem for manufacturers who assemble products with parts from different factories… it’s pretty obvious these shirts were sewn together with fabric from different suppliers. If there isn’t someone inspecting the raw materials under many different lighting conditions, you won’t know you have a problem until the product is assembled and upset customers start calling customer service.

This isn’t just a problem for textiles. Optical brighteners can also be found in paper, packaging, plastics, paints and coatings, and liquids. Check out our recent post on OBAs to learn more.

3. Metamerism

metamerism baseball fieldThis is Yankee Stadium. How do the groundskeepers force the grass to grow in two different colors? Do they carefully spread two different types of grass seed, light and dark green?

In this case, it’s true the grass is greener on the other side. Literally. If you force it to lay in one direction, you see more of the side of the blade. Force it to lay down in another direction and you see more of the ends. That’s exactly what we’re seeing here on the grounds at Yankee stadium. This phenomenon is called gonio-appearance, or geometric metamerism.

Gonio-appearance is seen quite often in fabrics, especially piles, corduroy, and satins. If you brush the nap in one direction, you see one color. Brush it in another direction and you see a different shade. As you can imagine, textured surfaces are difficult to measure. Spherical spectrophotometers like X-Rite’s Ci7800 can measure light reflected at all angles and calculate color measurements that closely match what the human eye would see.

4. Unknown Characteristics

There is no way to list all of the variables that can affect your measurement data.

Is your sample flat or curved? Translucent or opaque? Wet or dry? (Wet items usually appear darker.) Does it have texture or weave? (If pressing the sample against the instrument causes it to puff up or push inside, the measurement will be affected.)

Dust, oil and fingerprints can cause inaccurate measurement data, and even temperature can play a role. Many pigments are thermochromatic, which means the samples actually change color as they become warmer. Think about how the color changes when you iron a shirt. Sometimes the hot dry section looks less chromatic until it cools down and regains some moisture.

You need to know the characteristics of your samples and consider them when selecting a measurement device.

5. Lack of Operating Procedures

It’s critical to manage all of the variables when it comes to spectro measurements. Make a detailed list of every possible scenario and create procedures that everyone must follow when measuring samples. Otherwise your data will be ambiguous, and your color control will suffer.

Right the first time. Right every time.

By taking these pitfalls into consideration as you develop your color measurement and management procedures, you will be more likely to achieve accurate color, right the first time, right every time!

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