Summary
This is caused by the digital back and capture software sensing a change in the capture conditions from the previous shot. Most often these are changes in the integration time and/or changes in temperature.
In-Depth
Integration time:
This the total amount of time that the CCD in your Phase One digital back is active. When integration time varies slightly, or not at all, between one shot and the next the capture software can use a previously measured black calibration file. Integration time is the sum of two components of the exposure process;
1. Time from "wakeup" to the beginning of Exposure.
-This is caused when the back is "awakened" by the Camera (i.e. Phase One Camera) or manually when using Large Format solutions (i.e. in two-shot mode with a Phase One FlexAdaptor, a Hasselblad Flexbody or a Kapture Group True Wide).
2. Exposure time.
-The time the CCD is being exposed.
Heat:
CCDs are sensitive to heat as well as light. Heat can cause image noise in the file. The longer the integration time the more heat can build up on the CCD and create noise. The black calibration is a capture file that creates a profile of the thermal noise on the CCD. The noise profile is then substacted from the image data of your capture file in order to create a very high quality, noise free image.
Prevention
The best way to avoid unnecessary black calibrations is to avoid unnecessary shutter speed changes. Make sure the shutter mechanism in your lens or camera body is in good condition and is consistent from shot to shot.
Below, an example of a Black Calibration on a P+ back.
Note: the moon shape indicates a black calibration is in process and the timer is counting down to completion