VESA Improve DisplayHDR Certification Criteria Including Positive Changes to HDR 400 Tier

Introduction

VESA are today announcing several updates to their ‘DisplayHDR’ certification programme, including a range of new tests and performance criteria for displays which will be part of their Compliance Test Specifications (CTS) v1.2. The announcement today marks their third iteration of the specifications, which is largely used for certification of desktop monitors, both LCD and OLED. It is a widely used system to help try and articulate HDR performance and capabilities for consumers and the new specifications will help bring increased testing and rigour to the scheme.

Compliance Test Specifications (CTS) Iterations

VESA’s test specifications for the DisplayHDR scheme were originally launched in Dec 2017 (CTS 1.0) and then updated to CTS 1.1 in Sept 2019. Devices have been certified exclusively under CTS 1.1 since June 2020. The new scheme CTS 1.2 is being announced today and will run in parallel for a 1 year period for certifying monitor, until the previous CTS 1.1 is then phased out. The first monitors being certified under the updated scheme are already being announced now though.

Local dimming types explained

Just before we get in to the details of the DisplayHDR scheme updates, we wanted to quickly summarise the various dimming classifications which VESA use for this programme as it will become important later as we look at the different tiers. These are applicable to LCD panels, as OLED obviously has pixel-level dimming capabilities anyway.

• Global dimming – not included in the images above but this is a simple “dynamic contrast ratio” approach where the whole backlight can be dimmed or increased as a single unit. It doesn’t do anything to improve the single frame dynamic range/contrast and so cannot be considered a technology that can assist in HDR experience. It is not very popular as a technology generally nowadays.
• 1D local dimming – simple edge-lit local dimming where the monitor’s backlight is controlled in a limited number of zones along one edge of the screen, usually the bottom. You will often get 8 or 16 zones from this kind of approach. It provides some limited improvements to the contrast/dynamic range of the image, but with very few zones available and large areas therefore needing to be dimmed or brightened at once, it’s not possible to provide a very finite control over the image.
• 1.5D local dimming – a step up from 1D where the backlight is still edge-lit, but is introduced on two sides of the panel, typically one on the top/bottom, and the other on the left/right hand side. This means it can typically have 2x 16 zones (32 total) and provides an improved level of control. There’s still not many zones, but it at least allows the screen to be split in to smaller areas.
• 2D local dimming – this is a far better approach for improving the dynamic range and contrast and involves moving the backlight from the edges of the panel to the rear of the panel where they are arranged in a two-dimensional matrix of LED’s. This ‘Full Array Local Dimming’ (FALD) is often referred to nowadays as ‘Mini LED’ and will typically have a high number of zones – the higher the better generally. 500+, 1000+ or even more are commonly available. They’re more expensive to produce, but can reach the highest contrast ratios and brightness, and provide the best level of dimming control for LCD panels.

A side note, you will also see reference to ‘active dimming’ which is an additional test VESA include to ensure that local dimming backlights are actually being controlled properly when content changes on the screen, not only when input meta data is changed. It’s basically a test to ensure things are working as they should.

Updates to the DisplayHDR 400 Tier

Any long-time readers of TFTCentral will know our feelings about the HDR400 tier (not to be confused with the ‘HDR400 True Black’ tier for OLED screens), and we are pleased to see VESA take some positive steps towards making this a more useful and meaningful HDR certification. Previously, HDR 400 basically meant that a screen needed to be able to accept an HDR10 input signal, and reach a peak brightness of 400+ nits. That’s about it!

It didn’t require any form of backlight local dimming to actually improve the contrast ratio/dynamic range of the image, to offer actual hardware capabilities to support HDR content. It accounted for “global dimming” where the backlight can be turned up and down as a single unit, increasing “dynamic contrast” but that does nothing to improve the actual single frame contrast / dynamic range so we do not consider that useful really for HDR.

Additionally it also did not require the screen to offer a wide colour gamut, or support for a 10-bit colour depth to give you that colour boost associated with HDR experience. Basically the HDR400 tier was, in our opinion, largely meaningless. We’ve been vocal about that over the years, as have other reviewers and outlets.

Improved colour criteria

Thankfully VESA have taken note and are making some welcome positive changes to this tier in CTS 1.2:

1. Displays must now offer a 10-bit colour depth support (8-bit +FRC is acceptable like it is on the higher tiers) whereas previously only 8-bit was required
2. Displays must have a colour gamut covering ≥ 90% of the DCI-P3 colour space, whereas before they didn’t need to stretch beyond sRGB / SDR gamut. They’ve also slightly improved the requirements for sRGB gamut coverage, specifying ≥ 99% instead of ≥ 95% as it was previously.

This means that those screens certified under the new CTS 1.2 scheme that achieve HDR400 classification will now at least have the colour capabilities to support HDR content much better.

No requirement for backlight local dimming but some stricter contrast ratio specs

Despite the positive improvements to colour specifications, this tier still does not require any local dimming capabilities from the screen. This continues to leaves a gap in terms of hardware support for improving the actual dynamic range/contrast of the image in HDR. The scheme is better than before, and we are pleased VESA have made some changes, but this is still a gap in terms of delivering a true ‘High Dynamic Range’ experience – if the screen doesn’t have the hardware capabilities to improve the dynamic range (i.e. via local dimming), how can it be classified as “high”? We’d really like to see every tier require some kind of local dimming really to provide hardware capabilities to improve the dynamic range.

VESA have however taken some steps to improve the specs for the contrast ratio (CR) requirements of a screen. There were previously no specs at all for CR defined in the scheme, so you end up with a load of low-contrast <1000:1 IPS panels being certified as “suitable for HDR” under the old HDR400 tier.

The HDR400 tier does at least now list a 1300:1 minimum static contrast ratio requirement. That will actually now start to exclude the vast majority of IPS (and TN Film) panels, and encourages advancements in contrast ratio in those technologies like we’ve seen in recent times from LG.Display with their ‘IPS Black’ panels, and AU Optronics’ equivalent. That’s a good thing, as it should at least help drive better contrast ratios from panels if the manufacturers want to continue to reach any of the DisplayHDR certifications. VA panels can already surpass this and so meet the contrast ratio requirements easily.

We will talk about the contrast ratio specs for the other tiers a bit later.

Other improvements across all DisplayHDR tiers and testing

There’s also been a wide range of updates that apply across all the DisplayHDR tiers, including HDR 400. These include:

Increased white point accuracy criteria

Luminance and white point accuracy	Previous	New CTS 1.2
Max dE ITP for 1 nit	Not included	20
Max dE ITP for 5 nits	20	20
Max dE IPT for 15 nits	20	15
Max dE for 50 nits and above	15	10

Previously the scheme specified tests for white point including dE ITP accuracy thresholds at various low luminance levels and for brighter luminance levels of 50 nits and above. This ensures white point is close to a specified target of D65 (6500K).

VESA have now tightened those up as indicated in the table above to ensure a more reliable performance. Note that dE ITP is a different accuracy measurement to dE 2000 which is commonly used for familiar colour accuracy measurements, including in our reviews, and the numbers cannot be directly compared. Like that approach though, the lower the dE-ITP number, the better. The takeaway here though is that accuracy has been enhanced and these updated measured apply across all the DisplayHDR tiers.

X-rite Colour Error Tests

As well as the white point accuracy tests discussed above, VESA have now introduced a range of colour tests based on patches from well-known calibration experts, X-rite. These tests are taken at luminance levels of 50 and 100 nits, as well as at 50% of the tiers luminance. So for example an HDR 1000 display would include a test then at 500 nits too.

The test includes nearly 300 colour patches for all tiers and the criteria is that the display must achieve a Delta-TP figure of under 8 for all tiers, except for the HDR 1000 and HDR 1400 tiers where it is more strict and must reach under 6. It’s good to see some additional tests added to measure colours now as well, helping give higher levels of assurance around HDR accuracy.

Static contrast ratio tests

	DisplayHDR tier (for LCDs)
Static contrast ratio spec	400	500	600	1000	1400
1D local dimming	1300:1	7000:1	8000:1	n/a	n/a
2D local dimming	n/a	n/a	n/a	30,000:1	50,000:1

The specifications now list static contrast ratio requirements for all tiers. The lowest HDR 400 tier is the only one which can get away without backlight local dimming to reach its spec, with 1300:1 being achievable for some modern IPS Black panels and of course VA technology, but not older IPS or TN Film panels. Note that these contrast ratio measurements are taken using a single frame test pattern, so there’s no cheating with dynamic contrast ratio / global dimming allowed.

For HDR 500 and 600 the specs of 7000:1 and 8000:1 will only be possible with some kind of backlight local dimming although VESA list these as being enabled by 1D / 1.5D local dimming approaches.

For the HDR 1000 and 1400 tiers the static contrast ratio specs are very high at 30,000:1 and 50,000:1 respectively. VESA list these as being only enabled by 2D local dimming backlights (i.e. FALD / Mini LED). They tell us that the test patterns that will be used are expected to exclude edge-lit local dimming screens from reaching these tiers, whereas previously in the past you could find some HDR 1000 certified monitors which only had this basic level of dimming. That’s a positive step forward as well, as it should mean that those achieving HDR 1000 / 1400 under CTS 1.2 must have FALD / Mini LED backlights.

Colour Gamut tweaks

	sRGB / BT.709 coverage		DCI-P3 coverage
DisplayHDR tier	Previous	New CTS 1.2	Previous	New CTS 1.2
400	95%	99%	not included	90%
500	99%	99%	90%	95%
600	99%	99%	90%	95%
1000	99%	99%	90%	95%
1400	99%	99%	95%	95%
True Black tiers	99%	99%	90%	95%

We already discussed the positive change being made to the HDR400 tier above, but we should note here that minor tweaks are also being made to the colour gamut requirements for the other higher tiers as listed in the table above. This basically means that all tiers above HDR400 now need to offer ≥95% DCI-P3 colour gamut coverage.

Black Crush Test

In a newly added test the screen will be tested at 5 different dark levels – 0, 0.05, 0.1, 0.3 and 0.5 nits. The simple requirement is that it needs to be possible to measure the difference between each of these dark grey shades and that they should be distinguishable from one another, and the difference between each level should be appropritate. It can be quite common for these dark shades to be crushed to 0 (black) and so this is a test to ensure that doesn’t happen.

This is not however an accuracy test, so does not check the accuracy of the brightness levels, it only checks whether any black-crush is happening or not. Because this is carried out as a full-screen test, HDR400 tier monitors can use ‘global dimming’ to help acheive these dark shades, with the display being dimmed via dynamic contrast to help. Higher tier monitors can.

Subtitle flicker test

This is another new test to ensure that when bright subtitle text is shown on the screen during a dark scene, it doesn’t impact the luminance of the content as the subtitle appears and disappears. This could cause visible flicker, and this tests the local dimming algorithm, which needs to adjust both the backlight and LCD transparency based on the content on the screen so as to drive enough luminance for bright areas (i.e. the subtitles in this case) while avoiding negatively impacting the rest of the content.

HDR vs SDR Black level test

This is a new test which is designed to ensure that when you enable HDR on your screen, the contrast ratio within SDR applications like the Windows desktop, is at least the same as it was when you were running in SDR mode. They define “the same” as being within 10% leeway of the contrast ratio performance from SDR mode to account for the complexities in measurement and differences in the measured patterns.

VESA explained to us that one problem they’ve encountered on some displays in the past is that when you enable Windows HDR mode, it’s common for the backlight intensity to be turned up higher (or to full), and then the LCD transparency to be turned down so that the appearance of white isn’t too bright. This causes a drop in the contrast ratio, and is the same issue we explained in our article discussing why you should only enable HDR when viewing actual HDR content.

So, this new test requirement ensures that the black level for SDR apps within HDR mode is similar or better than the black level for those same apps when running in SDR mode. They’ve selected 200 nits as the reference for white in this test, so the contrast ratio must remain “the same” when viewing that luminance level in both SDR and HDR mode.

Identifying displays in the future that meet CTS 1.2

VESA tell us that like when they updated from CTS 1.0 to the more stringent 1.1 specifications, there will be no changes to the tier naming conventions or badges with CTS 1.2. This means that you will continue to see certifications listed like ‘VESA DisplayHDR 400’ but it will not be immediately clear whether that screen conforms to the older, very limited specifications, or the newer updated specifications. This makes it tricky for a consumer, although introducing loads of different names and badges does get complicated we admit.

To get around this, some manufacturers may choose to list the CTS version on their product pages, and we think this would be a useful addition to make that distinction that a new CTS 1.2 product should be better/more accurate than an older CTS 1.0 / 1.1 product. We’d like to see manufacturers include the CTS version in their specs ideally. VESA also provide a database of certified products on their website as well where confirmation of the CTS version is included, so that’s another place to check.

New CTS 1.2 certified products should be showcased from as early as SID Display week (14 – 16 May 2024)

Updated test patterns

Within the DisplayHDR test programme, which you can download and use yourself from their website, there are some test patterns that simulate a typical, industry-standard 10% APL image. That is currently a centrally positioned white square which covers 10% of the screen area, against a black background. That’s common in lots of different calibration software programmes too. The question they raised when considering their new CTS 1.2 was whether this fairly represented a 10% APL load in HDR viewing.

As a result a new pattern has been introduced which has a white central square that covers an 8% area, and there are two “starfield” areas now added, each occupying 30% of the screen and bringing the total image APL load back up to 10%. So it’s still a 10% APL test pattern, just with 2% of it spread out over the screen. This approach could also be useful to avoid any (rare) situations where screens can “trick” their figures in test patterns, something which we mitigate against in our testing through various test processes.

Summary

There’s some positive changes here to the DisplayHDR scheme and it’s good to see VESA and their partners continue to develop the test specifications and provide more stringent metrics that monitors should reach to achieve these certifications. It’s all about providing consumers with a badge that they can have some faith in which represents reliable HDR performance.

We like the range of new tests that have been added including stricter white point accuracy and new tests for colour accuracy, subtitle flicker and black crush. It’s also good to see a new contrast ratio requirement that should force 2 positive things. Firstly, it will force manufacturers to invest in higher static contrast ratio panels, or add local dimming, to meet the lowest HDR400 tier. No longer will low contrast IPS panels be eligible! Secondly, it will also require the use of high-end FALD and Mini LED backlights for displays to reach the upper HDR 1000 and HDR 1400 tiers. This starts to make those tiers a bit more “exclusive”.

The updates to the bottom HDR 400 tier are welcome too, and finally it at least now includes the colour enhancements associated with HDR like a wide DCI-P3 colour gamut and 10-bit colour depth. That is a good step in the right direction. We’d still like every HDR tier, including this one, to mandate some form of backlight local dimming to actually provide some hardware level capabilities to enhance the dynamic range though. Although on the flip side, we don’t particularly want to see a load of low-zone monitors released, as they aren’t much better. Perhaps a future CTS should create some guidelines around the number and density of dimming zones?

At least now all the tiers mean you will get the colour capabilities needed for HDR content. “Proper” HDR hardware dimming capabilities really start at the HDR 1000 tier as well now.

We’d like to see display manfuacturers list the CTS version their product has been certified against, so buyers can understand if it’s an “old” HDR 400 or a “new” HDR 400 certified display for instance. In the absence of that, buyers will need to rely on the DisplayHDR website to check.

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