Custom PC - UK (2020-03)

Conventional LCDs leak light even when a fully dark

screen is shown (top). Higher contrast LCD types

likes VA can reduce the leakage (middle) but variable

backlight control can stop it completely (bottom)

The DisplayHDR standards also account
for the difference between OLEDs and
LCDs by defining the dimming technology.
That is, the ability for a display to alter the
brightness of its pixels or backlight. For
OLEDs, the dimming technology is the
inherent ability for individual pixels to turn
on and off, so the two True Black standards
require pixel-level dimming control.
However, for LCDs, the only way to
provide the greater range of brightness
levels is to change the brightness
of the backlight, which can either
be done across the whole backlight
or in zones. DisplayHDR 400 only
requires full backlight control, which is
basically the same as those dynamic
contrast settings we’ve had on TVs and
monitors for years. It just makes the
whole backlight a bit dimmer when the
whole image is darker then brightens
it again for wholly brighter images.
For the rest of the LCD DisplayHDR
levels, some degree of zonal backlighting
control is required. However, this is perhaps
the single most crucial area where the
standards don’t go into enough detail,

In the

Zone

The Samsung CHG90 has just eight backlight zones so a single cursor lights up a whole row of backlighting

With FALD LCD displays, a cursor only illuminates one small patch of backlight, although you still get

some glow around it

as they don’t specify how many zones

are required, and having enough zones

is crucial for getting a true HDR effect.

The problem with not having enough

backlight zones is that the most visible

scenarios in which you’ll really notice HDR

are when you have areas of darkness next to

areas of brightness. That is, bright stars on a

night sky, a flashlight in a dark corridor or those

black bars above and below widescreen

video. These are the moments where you

want a display to show close to a true black

right alongside something much brighter.

With just a handful of backlight zones,

you’re still unlikely to have enough granular

control to be able to match the brightness of

the backlights closely to what’s on screen.

A perfect example of this is the Samsung

CHG90 display pictured above right. This

enormous 49in display has just eight

backlight zones meaning that just the handful

of pixels needed to illuminate a cursor in

the centre of the screen results in a huge

portion of the backlight having to be turned

on, creating a distracting bar of grey light.

The problem comes down to the way

LCD displays tend to work. Most use a single

row of LEDs along the edge of the display,

along with a series of filters and reflective

surfaces to create an even backlight for

the whole LCD. The single row is cheap to

make and allows for thin screens. However,

there’s almost no meaningful way you

can control the brightness of the backlight

in a specific zone – you can at most dim a

whole column or row. It’s these sorts of

panels that only achieve DisplayHDR 400.

You can also get displays that use two rows

of LEDs along opposite edges. Here, you

can at least design the backlight so that each

opposing row only illuminates half the length

of the backlight. In this way you can split the