Test Patterns 101: What We Measure
To create a calibration test suite, one needs a method of pattern generation. Most patterns are simple squares of color or gray that the meter reads to determine what adjustments are needed. The accuracy of these patterns is terribly important since measuring the wrong thing will result in a monitor that looks worse than before!
We use a standalone signal generator that sends reference patterns to the display. It’s made by Accupel and costs around $1500. While extremely useful in the lab, it isn’t exactly practical for most computer users. The alternative of course, is to use your computer to generate the patterns.
In the case of phones and tablets, we didn’t have a decent solution until recently. Datacolor recently came out with a product called SpyderGallery, which not only generates patterns on iOS- and Android-based devices, but also creates calibration look-up tables for them. While it’s impossible to actually and fully calibrate a smartphone or tablet because the controls simply aren’t there and the OSes are locked down, this method compensates via software, allowing the user to view more accurate images through the SpyderGallery app.
All modern fixed-pixel displays create images in RGB format. That is, each pixel is made up of a red, green, and blue sub-pixel. These sub-pixels are lit to one of 256 luminance levels to render the required color. Thanks to this, it’s easy to create a pattern from simple RGB values. For example, this much red plus that much green plus so much blue equals forest green. In the case of reference patterns, a red primary would be R255, G0, B0, in other words, the maximum red signal without the other two primaries in the mix. Gray patterns are no different, simply input the appropriate RGB values for whatever shade of gray you desire.
Calibration software like CalMAN or Spyder4Elite can generate patterns on the fly for a meter to measure. Automating the process in software is a tremendous time-saver, especially when measuring 30+ patterns for a color saturation test!
The patterns themselves are either fields or windows of a particular color or gray level. For instance, when measuring grayscale, 11 patterns are displayed one after the other, from full black to full white, in 10 percent increments. When measuring color, the patterns are color primaries (red, green, blue) and secondaries (cyan, magenta, yellow) and can vary in saturation and brightness level. We’ll cover those terms in more detail in our next article. All you need to know now is that these patterns are what the meter is measuring.
Now, let’s look more at the software itself and how it turns the measurement data into something we can actually use to improve our display.
