There are two main measurement device types used in color calibration, Spectroradiometers and Colorimeters
Spectroradiometers
A spectroradiometer is a subclass of a spectrophotometer. A spectrophotometer is a device that reads the intensity of incoming light as a function of wavelength. A spectrophotometer is not specific to any wavelength range while a spectroradiometer is specific to the wavelength range visible to the human eye. This device separates light into its component spectrum and is read by a photodiode array as intensity as a function of wavelength. This data can then be transformed into the XYZ tristimulus values and used in further calculations.
What makes spectroradiometers very attractive for display calibration is that their method of reading light by measuring intensity at each wavelength makes them able to read varying display light sources accurately. They do, however, also have some serious weaknesses that affect their usefulness in regards to calibration. Each photodiode in the reading array is getting a very small percentage of the light coming into the meter. This means that spectroradiometers are much less sensitive to low light levels. Another issue with spectroradiometers is that they require frequent dark offset readings to ensure accurate results. These offset readings provide them with a way to ensure an accurate black reading and separate and sensor noise from actual reading data.
Tristimulus Colorimeters
A type of meter characterized by the use of a color filter and a photodiode array (CMOS, CCD, etc.) There are three main filters used that are designed to match the transmissive response of the CIE 1931 Standard Observer Curves. Light entering the device passes through the Red, Blue, and Green filters and is read by the photodiode as raw RGB intensity. Using a transform matrix, these RGB values are converted into the XYZ tristimulus values (corresponding to the 1931 Standard Observer) which serve as the basis for all further color computations. The derivation of this transform matrix used for this numerical conversion is what we’re talking about when referring to colorimeter “calibration.”
A tristimulus colorimeter is only as accurate as the filter set it uses and the calibration offsets it has. Meter manufacturers will use filters that match as closely as possible to the CIE 1931 standard observer. This is an illustration of the filters used by a high-quality tristimulus colorimeter as compared to the CIE 1931 Standard Observer. An accurate filter set is important, but filters still change with age, so regular and accurate calibrations are a must for a colorimeter to maintain its accuracy over time.
Spectral Power Distribution Curves
Both kinds of meters return XYZ values. Spectroradiometers calculate XYZ based on the spectral response of the intensity levels at particular wavelengths. Colorimeters derive XYZ from the RGB values calculated based on the sensor input through a filter that should match the standard observer curves. The red curve is the X function, the green is the Y function and the blue is the Z function. We determine the XYZ values for a given display by calculating the area under these curves with a spectroradiometer or measuring the light intensity through each filter with a colorimeter and processing it through an appropriate calibration table for a colorimeter.
Here are some examples of different Spectral Power Distribution curves from common display technologies. All of these SPD (Spectral Power Distribution) charts were created in a light/environment controlled environment using the same pattern (white window pattern at full luminance) and readings were taken with a reference spectroradiometer.
LCD – CCFL backlit
LCD – LED Backlit
UHP – Ultra-High performance mercury Lamp standard in most projectors
CRT – Phosphor based display
As you can see from looking at each of these, there are some significant differences in the spectral power distribution of each display. This difference is why we must calculate different calibration offsets for colorimeters based on the display technology. As they are not able to read the spectral power distribution at each point as a spectroradiometer can, they instead only have three sets of information: red filtered light, green filtered light, and blue filtered light.
Almost all colorimeters come with a standard calibration for LCD- CCFL backlit light sources as their standard calibration. Some include many more calibration matrices as well. The primary weakness in colorimeters is that they need an accurate calibration table based on how their filters read for each SPD they will read, and reading a light source with a different SPD will give results that are not accurate. What this all means for someone using a tristimulus colorimeter in the field is that to get an accurate set of readings from a given display type, they need to first be certain that there are accurate calibration offsets for that light source and also that the meter has been recently checked or recalibrated to account for any filter aging.