Method for determining a start of relaxation ( t R) when switching a pixel-by-pixel controllable optical display device from a burn-in image (EB') to a relaxation image, wherein - in the burn-in image (EB') a trigger image area comprising at least one image pixel is allocated pixel values such that a first parameter formed over the at least one pixel value in the trigger image area of the burn-in image (EB') differs from a second parameter formed in the same way over the at least one pixel value in the trigger image area of the relaxation image, - by means of a camera (3, 13) comprising a sensor field of sensor pixels, the spatial distribution of a greyscale value corresponding to the spatial distribution of a photometric characteristic variable over the display device is recorded continuously over time, wherein the camera (3, 13) is configured for reading out the totality of all sensor pixels with a camera clock rate and subareas of the sensor field can be read out with a higher clock rate, wherein o a trigger subfield comprising at least one sensor pixel is defined overlapping with respect to the trigger image area of a display image presented on the display device, and o with a trigger clock rate, a trigger characteristic value is determined continuously over time from the pixel values of the at least one sensor pixel in the trigger subfield, o wherein the trigger subfield is defined as a subarea of the sensor field and is read out with the trigger clock rate, and - a trigger threshold value ( T S) is determined, which lies between the trigger characteristic value resulting for the recording of the burn-in image (EB') and the trigger characteristic value resulting for the recording of the relaxation image, and - the start of relaxation ( t R) is ascertained as the point in time at which the continuously ascertained trigger characteristic value crosses the trigger threshold value ( T S), wherein the trigger clock rate is chosen to be higher than the camera clock rate provided for reading out the totality of the sensor pixels of the sensor field. - in the burn-in image (EB') a trigger image area comprising at least one image pixel is allocated pixel values such that a first parameter formed over the at least one pixel value in the trigger image area of the burn-in image (EB') differs from a second parameter formed in the same way over the at least one pixel value in the trigger image area of the relaxation image, - by means of a camera (3, 13) comprising a sensor field of sensor pixels, the spatial distribution of a greyscale value corresponding to the spatial distribution of a photometric characteristic variable over the display device is recorded continuously over time, wherein the camera (3, 13) is configured for reading out the totality of all sensor pixels with a camera clock rate and subareas of the sensor field can be read out with a higher clock rate, wherein o a trigger subfield comprising at least one sensor pixel is defined overlapping with respect to the trigger image area of a display image presented on the display device, and o with a trigger clock rate, a trigger characteristic value is determined continuously over time from the pixel values of the at least one sensor pixel in the trigger subfield, o wherein the trigger subfield is defined as a subarea of the sensor field and is read out with the trigger clock rate, and o a trigger subfield comprising at least one sensor pixel is defined overlapping with respect to the trigger image area of a display image presented on the display device, and o with a trigger clock rate, a trigger characteristic value is determined continuously over time from the pixel values of the at least one sensor pixel in the trigger subfield, o wherein the trigger subfield is defined as a subarea of the sensor field and is read out with the trigger clock rate, and - a trigger threshold value ( T S) is determined, which lies between the trigger characteristic value resulting for the recording of the burn-in image (EB') and the trigger characteristic value resulting for the recording of the relaxation image, and - the start of relaxation ( t R) is ascertained as the point in time at which the continuously ascertained trigger characteristic value crosses the trigger threshold value ( T S), wherein the trigger clock rate is chosen to be higher than the camera clock rate provided for reading out the totality of the sensor pixels of the sensor field.