Anyone mastering images for later viewing on any display must use an accurately calibrated grading display. There is no alternative to this, regardless of the expected viewing environment or relative accuracy of the final displays the material will be viewed on. This includes material expected to be viewed in a theatre; on home TVs; on the web; on mobile phones & tablets, with digital signage - in other words, anywhere.
Calibration is achieved by the use of a probe, which measures the amount of light in the environment as a first operation. Steve Shaw of Light Illusion states that the ambient light for Home Entertainment should be 10 nits (just under 3 foot lamberts). The room is set at 10 nits by regulating the lights and using black cloth. Care should be taken to avoid as far as possible reflections on the grading screens, and the room itself should be preferably without windows to exteriors with their associated changing light. One fairly inexpensive solution to expedite set up time and the amount of ambient light is to use a pop up gazebo such as the which creates a "room within a room", also affectionally known as the "Punjabi Tent" within the Indian Film Industry.
Once the ambient light level is ascertained and then set up in the Light Illusion software, then the probe is used to measure the screen's brightness, gamma and colorimetry with a sequence of specific colour patches displayed. Finally, the software elaborates a 3d LUT from the readings, and this is applied to the images sent to the Apple XDR screen so that it will display the colours, gamma and intensity we need for specific projects.
For theatrical deliverables, image calibration can be achieved if there is a room that can be darkened so that it can become as dark as a Cinema. The afore-mentioned pop up gazebo may work well for this too. Calibration for digital signage needs to take into account whether this is going to be displayed in interiors or exteriors and the Apple XDR screen rotates easily to become a vertical display for this and other vertical formats. Events will also have a particular calibration depending on the lighting at the event itself.
See for further information.
Calibration with Steve Shaw's Light Illusion