Why grey and not white
Uniformity faults hide on white. A bright white field saturates your vision and flattens out the small differences in tone and brightness that make a panel look uneven in real use. Mid grey sits exactly where human contrast discrimination is best, which is why every serious display review measures uniformity on grey fields rather than on white ones.
This test steps through seven levels, from near black to near white, holding each for four seconds. Different faults show at different levels: clouding is worst in the dark fields, tinting is worst in the middle, and backlight falloff is worst in the bright ones.
What you are looking for
- Tinting: regions that read warmer, cooler, greener or pinker than the rest of the same flat field. The most common large panel fault.
- Dirty screen effect: a mottled, grubby looking texture across the field, most obvious when you sweep your eyes across it. This is what makes panning shots in football look smeary on a television.
- Brightness falloff: corners and edges dimmer than the centre. A small amount is universal. A lot means the backlight or its diffuser is uneven.
- Banding: horizontal or vertical stripes that appear only at certain grey levels.
OLED uniformity is its own problem
OLED panels have no backlight, so they do not suffer from bleed or backlight falloff at all. What they do have is emitter level variation, which shows up as faint vertical banding on near black and mid grey fields, and as uneven ageing after a few thousand hours. If you are checking an OLED, the dark grey fields in this test are the ones to watch carefully. Most sets run a compensation cycle when they are idle that reduces this, so let the panel run its maintenance routine before judging it.
Judging it fairly
Every panel is non uniform to some degree, and a test like this makes you look for faults you would otherwise never notice. The useful question is not whether the field is perfect, it is whether you can see the problem while doing ordinary work. Look at a document, a spreadsheet or a dark film for a few minutes after the test. If the fault disappears from your attention, it is not a fault worth chasing.