First, every active pixel creates extra drain on the battery. So Martin and his colleagues worked at optimising how the image would be displayed. For example every second vertical pixel is removed on LCD screens, to cut down on the power used. On AMOLED screens, horizontal pixels were also removed to further aid with power consumption.
Second, there was a risk of screen burn – the background image creating a permanent ‘after-image’ on the display. To resolve this, the displayed pixels are alternated every hour to the ones that are otherwise turned off, so that pixel burn won’t happen. This has quite a happy consequence. The image used for the background almost looks as though it’s on another, lower level than the clock figures. Less pixels = less light = it looks further away.