The number of shades available on the VFD is determined by a combination of the rate at which it is refreshed and the persistence of the cells. The display is refreshed at around 200Hz, and this should be more than adequate to dither cells over time to increase the number of perceived shades. Each cell is either on or off, and the persistence of each VFD cell is such that dithering over time is only effective for darker shades. We found in early prototypes that there was little difference between cell shades except at the lower end of the brightness scale, and that for moving images a large number of shades made it appear flickery.

We are still playing with different pixel timings and find that on a 16 shade kernel, only the lower shades are noticably different, and there is therefore little point in using more shades than the current 3.