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The Importance of Calibration

Posted September 14, 2015 by Mike Huda

Why calibrate?

For the most part, today’s color measurement instruments are 100% digital. In fact, there are very few analog components inside, except for the light bulbs. Although they’re more stable than their analog predecessors, their tolerances are much narrower, and they need regular calibration to stay within these tight specs.

digital color measurement instrument

Bulb stability

As you use your instrument and the bulb turns on and off, it starts to change its character. Calibration compensates for these changes in the bulb’s ability to create light. Careful calibration will keep the instrument stable for many months – even years – until the bulb reaches the end of its life.

calibration compensates for changes in bulbs ability to create light

Tile and lens cleanliness

Inside the instrument are the settings for two known points in color space – a defined white, and a defined black. Back at the factory, a full reflectance curve was built to define the white tile, and a full reflectance curve was built to define the black point inside the black trap. Those exact locations for white and black are programmed right into the device.

If the white tile gets a little dirty, the white point will drop down the horizontal axis toward black. Not only does the white point drop, every point along that line drops along with it in differing degrees.

If the black point goes up because there’s dust in the trap, the black point moves upward toward white. As the slope of the line is changed, every point along that line is degraded. False passes can cause you to send products to your customers that won’t pass inspection. False rejections will cause rework that isn’t necessary. This is why you need to keep the trap free of dust, and the tile meticulously clean.

Treat the tile as if it’s a precious optical device, because it is. Generally manufacturers suggest cleaning it with warm water and a clean, lint-free, non-scuffing towel, but follow the user manual instructions for your specific device.

If you’re in an environment such as a printing area where there’s ink, or where paint films can be a problem, or where compressors are blowing unknown gasses and oils into the air, water alone may not do the trick. You can use wipes that are specifically designed for eyeglasses or camera lenses that are moistened with isopropyl alcohol. Gently buff the white tile until it’s absolutely clean, and it will dry quickly without residue.

How often to calibrate

Most instruments have a built-in timer that you can set to monitor the interval between calibrations. You can set it for a number of hours, such as 8, 12, or 24, or you can set it for no interval at all and calibrate manually when you think it’s necessary. Set a schedule that’s based on your color workflow and environment. If you come on shift to an instrument that someone else has been using, you should calibrate.

Some instruments allow you to verify calibration from within the device, without any software. For example, on the Ci6x  you’ll find a green tile with associated numbers built right into the device. Make sure the tile is clean, set the instrument to the appropriate values, and measure it. If your instrument reads within 10 – 15 hundredths of the L*a*b* values for both specular included and specular excluded, then you don’t need to calibrate. But if you find that your target does not measure within at least 2/10ths of the printed values, then it’s time to calibrate.

When calibration isn’t enough

A regular calibration schedule will keep your device functioning as it should for a period of time, but eventually you must send it for service to maintain its performance and ensure accuracy. Check out our Caring for your Spectrophotometer  post to learn what’s involved, and how X-Rite can help.

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