Pre-Calibration Display & Meter Setup for Accurate Meter Profiling Procedure
There 5 initial pre-calibration steps to provide the most accurate meter profile creation and calibration measurements:
Warmup times of an hour may be required before a display reaches a stability that will guarantee that the colors and luminances don’t change relative to one another during the time while the set of measurement results are obtained.
The luminance and chromaticity characteristics of displays change over the period of time that a display warms up after its powered on.
The change is most significant in the first 30 minutes, becoming less significant in the next 15 minutes. After 45 minutes of warmup, most displays, while not fully stable, continue to change only a small amount due to thermal changes.
If some of the profile measurements are made before the display becomes thermally stable, those measurements won’t accurately represent the luminance and chromaticity performance of the stabilized display.
That will cause the final calibration data to be less accurate than it could have been. Any display technology acts differently the first minutes after its powered on. CRT's/LED's are brighter and become darker, projectors do the opposite, OLED's have very low gamma initially; but after about one hour the most of displays/projectors have stable output.
Meters will require to be connected/powered from their USB plug for about 20-30 minutes before starting any measurement. The exact time depends from the temperature conditions. It can be longer if the device came from outside in winter time and will be shorter if stored at room temperature.
Usually users confuse warmup procedure and leave the meter on-contact with the display panel to become warmer from display's panel heat, which is a bad idea.
The meter must be at the room temperature, when the meter has stable temperature it will provide stable readings over the time, heat is not good for the sensors, when the meter is becoming warm, more noise is added to the sensor and this reduce the instrument S/N ratio.
Light emitted from displays is a electromagnetic radiation which will alter the temperature of the detection system of a meter, so the sensors themselves will heat up, one reason why meters like i1PRO require dark readings; to lower the drift caused by heat.
i1PRO2 has better thermal isolation due to its aluminum at the side. i1Display PRO using robust filters against prolonged exposure to heat.
basICColor DISCUS has aluminum unibody for added robustness and thermal stability and also features internal thermal compensating circuitry which will ensure that changes in the physical temperature around the device will not influence measurement results.
Konica-Minolta CS-2000 features a peltier thermo-electric cooling system to reduce dark current and improve S/N ratio.
Because of the inherent accuracy of the Four-Color-Matrix-Method, alignment of the colorimeter and spectroradiometer with the normal of the display surface can be critical. This is particularly important for displays that have significant viewing-angle changes of their properties.
Depending upon the technology, it can be beneficial to assure alignment of the measurement system to a small fraction of a degree. Related to this viewing-angle sensitivity is the subtense of the detector.
If the angular aperture is too large or if the measurement field is too large, then it may be collecting light from too wide an area where by viewing-angle deviations can influence the measurement. Similarly, for screens that are influenced by mechanical pressure such as many LCDs, contact colorimeters that place too much force on the surface can influence the measurement.
If such contact devices are used, it can be important to assure that they are touching the screen lightly enough to avoid any changes in the measurement results from mechanical contact. Additionally, any stray light entering the colorimeter from the surround must be eliminated.
When you are taking on-contact measurements of CRT displays with X-Rite i1Display PRO, a small magnet inside the ambient light cover which X-Rite is using to signaling using a Hall effect sensor what's is the cover position, it can interfere the response of the CRT display, particularly if the ambient cover is in its natural position at 180-degree away from the measuring lens.
One way of minimizing this is to swing the cover down so that it touches the display adjacent to the lens, thereby moving the magnet away from the display surface. A more thorough but inconvenient way of avoiding this problem is to unclip the ambient light cover and slide it down the cable. Another solution is to take non-contact measurements with CRT displays.
EBU TECH.3325 Publication (Methods for the Measurement of the performance of Studio Monitors) recommends the distance measuring geometry rather than the contact method.
For projectors, we have two main concerns when deciding where to place the meter. We need the meter to be far enough back that it doesn’t cast a shadow on the area of the screen we’re reading from. But we also want the meter to be as close to the screen as possible so that we’re not adding inaccuracies such as off angle light or gain differential.
SMPTE recommends to take readings from the viewing position, but that's is not possible using consumer probes, it will require high-end spectroradiomenters with very narrow (~2.0°) field of view equipped with viewing optics (PhotoResearch/Konica-Minolta) or with laser aiming (JETI).
Meter Field Of View (FOV)
When you are measuring projectors, or displays with uniformity issues it's better to measure the same spot during the meter profiling procedure. Each meter has its own Field Of View (FOV) so their optics see different diameter area of pixels during a measurement.
Meter Profile Verification
When you create a four-color meter matrix correction table, it's recommended to always check if the calculated correction table is accurate and then start to use that correction table for all your other display calibration measurements.
A lot of users very familiar with the meter profiling procedure will be surprised when they will perform that verification test for their first time; they may be found that their generated meter correction table is not so accurate.
NIST's accepted tolerances are below xy +-0.001 and +-1.5% Luminance when you will compare the 4 color (W,R,G,B) xyY chromaticity and luminance readings between the reference meter used (spectrophotometer/spectroradiometer) and profiled colorimeter (colorimeter with four color matrix correction loaded).
Sometimes it will be required to re-perform the meter profiling procedure a second time to get better verification results with tighter tolerances.
A lot of CalMAN 5 users of Ted's LightSpace CMS Calibration Disk, they are using the Meter Profiling Check
layout page to perform a meter profiling accuracy verification: