20160126 - Local/Global Dimming, Blooming, Contrast Ratios - CRT/LCD/Plasma/OLED


Global Dimming
OLED HDTVs drop to about 33% of peak brightness on a full white screen. My Samsung Plasma HDTV has similar issues. The random made-in-1996 CRT I just profiled at home drops to about 60% of peak brightness on a full white screen. Global dimming has been around for a long time.

LCD Local Dimming vs CRT Blooming
LCD TVs often have LED back-lights split into zones (on the order of dozens for poor quality and low hundreds for better quality), with individual control of the brightness per zone. This zone brightness control simultaneously moves up or down the {black level, peak brightness} as a joined pair. A single-pixel thickness peak brightness line on a black background forces black level high in a blooming pattern of where the line intersects zones. For this reason I always turn off local dimming if it is an option on a LCD TV display. ANSI contrasts can still reach 4000:1 with the best LCD panels without any local dimming.

CRTs in contrast have local blooming (talking about the large diffuse effect, not the shadow-mask-scale effect). Lets look at some measured numbers from my Sony Wega CRT TV, with ANSI contrast being the standard 4x4 rectangle checkerboard pattern of black and white boxes.


ANSI contrast ratio for this Wega is a poor 158:1 (my random 1996 CRT gets 265:1 in comparison). And if one stops here and compares to the great LCD, the LCD has over 4 extra stops of dynamic range? But this isn't the full story. For the Wega as Average Picture Level (APL) drops from 50% with ANSI contrast test, to something more natural to look at, contrast ratio improves. Seeing 2500:1 for black with the large white bar case, and then almost up to 32000:1 measuring a black screen with only a white mouse away from the sensor. And all these tests done in a room at night lit by a few bright lamps.

The CRT does not suffer from the LCD local dimming problem, a single pixel line won't cause blooming as it does not add enough energy (APL stays low). The CRTs blooming looks totally natural in comparison.