When evaluating images on a monitor, it is important to note that light from windows, light fixtures, etc. can impair the quality of image assessment. The lighting conditions in the environment significantly reduce the maximum contrast of the image display. Reflections from windows can be avoided by positioning the monitor so that the viewing direction is parallel to the window surface. Dimmable room lighting contributes to the appropriate lighting level in accordance with the room classes (AUK). In addition, the monitors can be fitted with screens on the side walls or partition walls made of dark, non-reflective materials can be used to avoid disturbing light sources and reflections.
The viewing conditions for an image reproduction system are highly dependent on the illuminance of the environment and must therefore be included in quality assurance in monitor classes MK3 and above.

It should be noted that vision for distinguishing between dark gray tones is better after spending a long time in darkened rooms; for the highest evaluation environment class (AUK1), an adaptation period of at least 5 minutes should be observed before starting the evaluation.
Table 1 of the DIN 6868-157:2014-11 standard specifies room classes (RK) on which this guideline is based, but specifies different illuminance levels for the evaluation environment classes (AUK) due to the different environmental conditions in order to take into account the conditions of industrial radiography.
Therefore the NDT has much more varying viewing conditions:


The procedure and the measuring device for illuminance measurement shall comply with DIN 5032-7 /2017 Class C. For the AUK4, so-called gray card, which shall be used in accordance with the manufacturer's specifications, can also be used to determine the ambient light.

Also here is the discussion if the AUK 4 is needed - the majority in working group D8 and at the 2023 FAD meeting voted in favor.
In the latest revision the AUKs are renamed to Room Classes - even if the class AUK4 contains "out-door"