PQ and HLG: Two Different Ways to Build HDR Television

HDR television uses two main transfer-function systems.

They are called PQ and HLG.

Both are standardized in Recommendation ITU-R BT.2100-3. Both can carry HDR television pictures. They do not describe luminance in the same way. ^[2]

The simplest useful distinction is:

• PQ is absolute. A normalized PQ signal value maps to a defined display luminance on the reference EOTF.
• HLG is relative. Its signal represents scene-light relationships, and the final display luminance depends on the display peak, black level, system gamma, and viewing environment.

PQ is useful when production and delivery want a stable absolute display-luminance scale.

HLG was designed for an evolutionary television workflow that can use conventional production infrastructure and, under defined conditions, serve compatible SDR and HDR receivers from one signal.

Neither is automatically better.

They solve different system problems.

First, separate the transfer functions

A complete television system includes more than one transfer relationship.

OETF

The opto-electronic transfer function, or OETF, maps scene-linear light into a nonlinear video signal.

EOTF

The electro-optical transfer function, or EOTF, maps the nonlinear signal into display light.

OOTF

The opto-optical transfer function, or OOTF, maps scene light to displayed light across the complete system.

PQ and HLG place these relationships differently.

PQ is usually discussed through its absolute display EOTF. The creative and perceptual rendering from scene light into that display-referred signal occurs in production.

HLG begins with a scene-referred OETF. Its system gamma contributes to the OOTF and adapts the displayed result to the target display and viewing conditions.

This is why comparing only the visible shapes of the PQ and HLG curves misses much of the difference.

Why SDR gamma was not simply stretched

SDR television uses nonlinear encoding and display functions optimized around a much smaller reference luminance range.

A wider HDR range needs enough digital precision in:

• very dark detail
• midtones
• diffuse white
• bright highlights
• transitions across the full color volume

Stretching a conventional power function across 0 to 10,000 cd/m² is not an efficient use of a practical digital code space.

SMPTE ST 2084 defines a different EOTF with a foundation in human visual perception. Its informative introduction states that the function can map as few as 10 bits efficiently across an absolute range from 0 to 10,000 cd/m². ^[1]

That wording should not be turned into a promise that all ten-bit images are free of visible contouring.

Quantization visibility depends on the image, noise, color encoding, compression, processing, display, and viewing conditions.

What the Barten-based explanation means

PQ's developer history is often explained with Peter Barten's contrast-sensitivity work.

Dolby's published technical account says the proposed perceptual EOTF was based on the human contrast-sensitivity relationship measured by Barten and referenced in ITU work. The goal was to place digital steps efficiently over a very large luminance range. ^[7]

This is sometimes shown as a Barten ramp or just-noticeable-difference curve.

That illustration is useful, but it needs limits.

The threshold for seeing a luminance difference depends on:

• adaptation luminance
• spatial frequency
• angular size
• viewing distance
• duration
• surrounding light
• noise
• observer
• color
• image structure

There is not one universal visible-step threshold that applies to every pixel in every moving image.

The careful claim is:

PQ was designed using a perceptual model so its code distribution is much better suited to a wide absolute luminance range than a simple SDR power curve.

How PQ works

PQ stands for Perceptual Quantizer.

SMPTE standardized its reference EOTF as SMPTE ST 2084:2014. ITU-R BT.2100 incorporates PQ as one of the two HDR television systems. ^[1][2]

Under the reference EOTF:

• normalized PQ signal E' = 0 maps to 0 cd/m²
• normalized PQ signal E' = 1 maps to 10,000 cd/m²

Values between them map to defined luminance levels according to the ST 2084 equation.

This is an absolute relationship.

A normalized PQ value has the same reference-EOTF luminance target regardless of whether the target display can reproduce it directly.

The display's limitation does not change the standard curve.

It creates the need for display mapping.

Normalized PQ values are not the same as raw digital codes

The phrase PQ code value can be ambiguous.

ST 2084 defines the EOTF using a normalized nonlinear value.

A digital file or interface then maps that normalized value into a code range.

Possible representations include:

• full range
• narrow video range
• SDI range
• reserved endpoint codes
• ten-bit values
• twelve-bit values
• integer or floating-point production formats

In a full-range ten-bit representation, the endpoints may correspond to code 0 and code 1023.

In a narrow-range ten-bit representation, reference endpoints use a different code interval.

Therefore, this sentence is too broad:

Code 1023 always means 10,000 nits.

The more accurate statement is:

A normalized PQ signal of 1 maps to 10,000 cd/m² on the reference EOTF. The raw digital code used to represent that value depends on the signal-range specification.

This distinction also affects any example that assigns one luminance to a numbered ten-bit code.

PQ does not make every image extremely bright

ST 2084 says the EOTF is intended to enable a wider luminance range, not to make every image brighter overall. It expects average picture levels and mid-range scene exposures to remain appropriate to the program and viewing application. ^[1]

HDR can keep much of an indoor scene at familiar luminance while using the extended range for:

• specular reflections
• lamps
• windows
• fire
• metallic glints
• bright outdoor regions
• selected visual effects

The 10,000 cd/m² endpoint is the system limit.

It is not a full-screen target and not a requirement that every title use the entire range.

Ten-bit PQ and twelve-bit PQ

Ten bits provide 1,024 mathematical code values.

Twelve bits provide 4,096.

More code values provide finer quantization, but the visible result depends on the rest of the pipeline.

Dolby's 2014 technical paper reported that, in its illustrated Barten-based monochrome analysis:

• twelve-bit PQ remained below the modeled contouring threshold
• ten-bit PQ could rise above that threshold in parts of the range ^[7]

The paper also stated that ten-bit PQ could be adequate for captured imagery containing noise, while noise-free graphics and computer-generated material required more care.

This is more precise than saying that ten-bit PQ is always perceptually lossless.

Visible banding can also be caused or exposed by:

• compression
• repeated grading transforms
• limited intermediate precision
• chroma subsampling
• poor dithering
• panel bit depth
• temporal processing
• incorrect range conversion
• near-black display behavior

The transfer function is one part of the result.

PQ is an absolute target, not a guarantee

SMPTE ST 2084 describes the ideal EOTF as a nominal target for real displays.

Actual displays can deviate because of output limitations and non-reference viewing conditions. ^[1]

A consumer television may be unable to reproduce:

• the encoded peak luminance
• the mastering display's black level
• the mastering color volume
• sustained bright areas
• highly saturated bright colors

The television must then decide how to fit the program into its capabilities.

That process is often called:

• display mapping
• tone mapping
• color-volume mapping

PQ makes the requested reference luminance clear.

It does not force every display to produce an impossible output.

Why display mapping changes the result

ITU-R BT.2390-12 explains that PQ content can be optimized for a reference monitor and reference environment, while actual displays may have different black levels, peak luminance, color gamut, and room conditions. In those cases the display characteristic needs to be changed through display mapping. ^[3]

A mapping can prioritize different goals:

• preserve midtone luminance
• preserve diffuse-white relationships
• preserve highlight detail
• preserve highlight intensity
• prevent hard clipping
• maintain colorfulness
• protect skin tones
• adapt to a bright room

No limited display can preserve every original luminance and color coordinate when the source exceeds its color volume.

PQ provides the common scale on which the mismatch can be described.

Does PQ require metadata?

No.

The PQ EOTF itself does not require dynamic metadata.

A PQ signal can be delivered:

• without dynamic metadata
• with static mastering-display metadata
• with content-level statistics
• with scene-based dynamic metadata
• with proprietary display-mapping metadata

Metadata can help a display understand the mastering environment or intended mapping.

It does not define PQ itself.

HDR10 is a PQ-based system associated with static metadata and defined signaling.

HDR10+ uses PQ with dynamic metadata. HDR10+ Technologies states that its system uses the SMPTE ST 2084 PQ transfer function. ^[8]

Many Dolby Vision mastering and delivery workflows use ST 2084 PQ. Dolby's current generic studio guidance lists PQ for common TIFF, ProRes, and IMF HDR masters. ^[9]

Dolby also documents HLG-related profiles and conversions, so it is too broad to say that every possible Dolby Vision signal is PQ.

What happens when PQ is interpreted as SDR

A PQ image interpreted through an SDR transfer function will be wrong.

It commonly appears:

• too dark
• low in contrast
• gray or washed in some regions
• incorrectly saturated
• clipped or oddly compressed

The exact appearance depends on:

• signal range
• colorimetry
• transfer function
• picture mode
• source conversion
• display processing

A dark image does not prove that the PQ equation failed.

It usually indicates that some part of the chain did not identify, preserve, or process the HDR signal correctly.

PQ compatibility can be provided outside the curve

The original article said PQ intentionally gives up compatibility with SDR.

The PQ curve itself is not directly compatible with an SDR EOTF.

A complete delivery system can still provide compatibility by:

• sending a separate SDR version
• converting PQ to SDR
• using a base layer and enhancement layer
• generating an SDR output from a PQ master
• negotiating a suitable stream in an application

Compatibility is therefore a property of the complete distribution system, not only the transfer function.

Why HLG was developed

The BBC and NHK developed Hybrid Log-Gamma, or HLG, for an evolutionary television-production and distribution path.

BBC documentation describes HLG as a relative system that adapts to the display device's peak luminance. ^[5][6]

ITU-R BT.2390-12 says HLG was designed from the outset to offer broadcasters and producers an evolutionary approach to HDR production and distribution.

Its signal characteristic resembles a conventional SDR camera signal with a knee, and the basic system requires no production metadata. ^[3]

This helps HLG fit:

• live production
• broadcast monitoring
• conventional mixing and switching infrastructure
• a range of display peaks
• changing viewing environments
• single-stream distribution in suitable receiver populations

That does not make HLG limited to broadcasting or PQ limited to prerecorded material.

It describes the design priorities.

The hybrid curve

The HLG OETF has two mathematical sections.

For normalized scene-linear signal E:

• the lower section uses a square-root relationship
• the upper section uses a logarithmic relationship

The transition occurs at:

• scene-linear E = 1/12
• nonlinear HLG signal E' = 0.5 ^[2][3]

The lower section is often described as gamma-like because the square-root form resembles familiar television camera encoding.

The upper logarithmic section compresses a much larger highlight range into the remaining signal interval.

The transition is not a fixed display luminance.

It is not the 100 cd/m² SDR white point.

Why the upper section is logarithmic

ITU-R BT.2390 explains the design using two broad visibility behaviors:

• brighter regions approximately follow Weber-like behavior, which supports logarithmic encoding
• darker regions have different visibility behavior, for which a gamma-like section is useful ^[3]

The hybrid form allows HLG to allocate code values across a larger scene dynamic range while retaining a signal characteristic that works with conventional television infrastructure.

As with PQ, quantization visibility still depends on content and conditions.

HLG is scene-referred

HLG's nonlinear signal represents relative scene light.

The HLG OETF does not assign a fixed cd/m² output to every signal value.

The display EOTF combines:

• the inverse HLG OETF
• system gamma
• nominal peak luminance
• black-level adjustment
• viewing-environment assumptions ^[2][3]

The maximum HLG signal maps to the display's nominal peak luminance.

Other values scale below that peak according to the system relationship.

A 600 cd/m² HLG display and a 1,000 cd/m² HLG display therefore do not reproduce every HLG signal value at the same cd/m².

That is a designed feature of the relative system.

HLG system gamma

The system gamma is part of the scene-light to display-light relationship.

It changes according to display peak luminance and can be adjusted for viewing-environment luminance.

Its purpose is to maintain a subjectively suitable rendering across displays and surrounds with different capabilities.

This does not mean every HLG television automatically implements the recommended behavior accurately.

A real implementation can be affected by:

• incorrect peak-luminance assumptions
• ambient-light settings
• panel limits
• manufacturer processing
• mode selection
• black-level setup
• signal conversion
• calibration error

HLG reference white

HLG operational practice commonly places HDR Reference White at 75 percent of the HLG signal.

On a reference HLG display with nominal peak luminance of 1,000 cd/m² and the reference surround, that level is nominally 203 cd/m². ^[2][4][5]

Because HLG is relative, the displayed luminance changes with the display peak.

BBC's 2026 explanatory table gives these examples at a 5 cd/m² surround:

These are calculated examples under the stated system formula.

They are not universal measurements of every television.

PQ reference white behaves differently. On a reference PQ display, the nominal 203 cd/m² reference level remains absolute.

What HLG compatibility really means

HLG is often called backward compatible.

That phrase needs a condition.

ITU-R BT.2390-12 says HLG provides a significant degree of direct compatibility on SDR displays designed to accept BT.2020 color signals. It describes a possible single HEVC Main 10 stream serving HDR and suitable SDR receivers. ^[3]

The same report states:

Both PQ and HLG provide limited compatibility when directly connected to legacy SDR displays with BT.709 colorimetry.

HLG's advantage is therefore not that any old television will show a perfect SDR image.

The advantage is that the HLG signal characteristic can work through conventional production infrastructure and can provide a usable direct image on compatible SDR UHD receivers under defined conditions.

Why BT.709 still needs conversion

HLG uses BT.2100 colorimetry based on BT.2020 primaries and matrix behavior.

A legacy BT.709 television expects different colorimetry.

BBC's HDR FAQ states that HLG does not provide direct backward compatibility with BT.709 HD displays and that a color-space conversion is required for an SDR simulcast. It notes that an unconverted picture can appear desaturated. ^[12]

A broadcaster serving conventional HD SDR viewers may therefore create:

• a converted BT.709 SDR service
• a simultaneous SDR grade
• a downmapped output from an HDR production
• receiver-specific conversion

The phrase single signal for everyone is valid only for the receiver and infrastructure conditions that support it.

HLG does not merely clip the upper half on SDR

The original article described the HLG upper section as being clipped by an SDR television.

That is too simple.

An unconverted HLG signal displayed through an SDR-like response can resemble a camera signal with a highlight knee. That is part of HLG's compatibility design. ^[3]

The actual image depends on:

• whether the display accepts BT.2020
• how it applies the transfer response
• its black and peak luminance
• whether color conversion occurs
• signal range
• picture-mode processing
• any receiver mapping

The result may be usable for monitoring or distribution in a compatible system.

It is not guaranteed to match a separately approved BT.709 SDR grade.

Does HLG use tone mapping?

HLG does not use PQ's fixed absolute-luminance target.

It still adapts scene-relative signal values into the target display's luminance range.

That adaptation is built into the HLG EOTF and system gamma.

A manufacturer may also add other processing.

It is therefore misleading to say:

PQ needs tone mapping, while HLG needs none.

A more accurate distinction is:

• PQ often needs display mapping when the absolute source range exceeds the target color volume.
• HLG inherently renders the relative signal according to target peak, black, and system gamma.
• Either system can receive additional manufacturer processing.

Metadata and HLG

HLG's basic display relationship does not require mastering metadata.

That is valuable for live production because the system does not need scene-by-scene mastering instructions to establish the EOTF.

An HLG service still needs signaling so equipment knows:

• that the signal is HLG
• the colorimetry
• bit depth
• signal range
• codec profile
• frame rate
• resolution

A service may also carry unrelated metadata for audio, captions, accessibility, mastering, or distribution management.

“No production metadata required” does not mean “no metadata exists anywhere in the service.”

Where PQ is used

PQ is used in several major HDR systems and workflows.

Examples include:

• HDR10
• HDR10+
• many Dolby Vision mastering and mezzanine workflows
• PQ-based broadcast profiles
• games and applications that output PQ HDR

HDR10+ Technologies identifies ST 2084 PQ as its transfer function. ^[8]

Dolby's current generic studio guidance lists ST 2084 PQ for common Dolby Vision HDR masters and deliveries. ^[9]

The transfer function does not tell you which metadata system, codec, container, or tone-mapping instructions are present.

Where HLG is used

HLG was designed for broadcast and live production and is supported in broadcast standards.

Examples include:

• live HDR production
• broadcast distribution
• compatible UHD receiver systems
• some cameras and production monitors
• HLG-based files and web video
• HLG-related Dolby Vision profiles or conversions

ATSC A/341 permits PQ and HLG transfer characteristics in ATSC 3.0 HEVC video. ^[10]

DVB systems also support HLG and PQ profiles. DVB lists TS 101 154 V2.10.1 as a published standard dated May 22, 2026. ^[11]

The practical pattern is:

• PQ is common in mastered and metadata-oriented delivery.
• HLG is common in broadcast-oriented and live workflows.
• Either may appear outside that pattern.

Signaling the transfer system

The receiver must know whether the signal is:

• SDR
• PQ
• HLG

That information can be represented in:

• codec video usability information
• container metadata
• broadcast service signaling
• interface signaling
• application playback negotiation
• device mode selection

It is too simple to say that the HDMI handshake alone decides the curve.

Every stage in the path must preserve or translate the information correctly.

A source device can identify HDR correctly while an intermediate receiver, switch, converter, capture device, application, or display mode changes the result.

What the viewer normally should do

In a working system, the viewer should not need to choose PQ or HLG manually.

The source and display should identify the transfer system and select the appropriate HDR mode.

The viewer should verify:

• the television reports an HDR mode
• the source is set to a compatible output
• any receiver or soundbar passes the HDR format
• enhanced HDMI bandwidth mode is enabled when the model requires it
• the picture mode is appropriate for HDR
• the television is not forced into an incorrect transfer mode
• the source and display agree on video range and colorimetry

Menu names vary by manufacturer.

Use the exact manual for the model, region, firmware, and input.

When HDR looks wrong

Possible causes include:

• PQ interpreted as SDR
• HLG interpreted as SDR without suitable compatibility processing
• BT.2020 signal treated as BT.709
• full-range and narrow-range mismatch
• source output forced to the wrong format
• application conversion
• receiver or switch limitations
• disabled enhanced HDMI mode
• incorrect picture mode
• energy or ambient controls
• tone-mapping behavior
• bad metadata
• calibration error

Do not assume every dim HDR image is a handshake failure.

Begin by identifying the actual signal and the mode the television entered.

What calibration can check

For PQ, calibration can measure:

• reference PQ EOTF tracking
• black level
• peak and sustained luminance
• grayscale
• reference white
• clipping and roll-off
• color-volume behavior
• mapping options

For HLG, calibration can measure:

• HLG EOTF behavior
• assumed nominal peak
• system-gamma rendering
• black-level lift
• reference-white output
• surround or ambient behavior
• grayscale and color

The setup must identify:

• exact model and size
• firmware
• signal type
• input
• picture mode
• tone-mapping option
• meter
• profiling
• pattern source
• pattern size
• stabilization
• room condition

A preset name is not a measurement.

What calibration cannot do

Calibration cannot:

• make a limited display reproduce the complete PQ range
• make HLG absolute
• make PQ relative
• restore signal information lost during conversion
• guarantee identical mapping across manufacturers
• make one consumer television identical to a mastering display
• remove every room effect
• prove creative intent from metadata alone
• make every observer see spectrally different displays identically

Calibration can reduce measurable error relative to the selected target under stated conditions.

PQ and HLG can be converted

PQ and HLG can be converted mathematically.

BBC's 2026 explanation points to ITU operational guidance using a 1,000 cd/m² bridge display for defined conversions. ^[4][5]

Conversion still requires choices about:

• scene versus display reference
• OOTF
• reference white
• peak luminance
• black level
• color volume
• gamut mapping
• viewing environment

A mathematical conversion is not automatically lossless in creative appearance.

The safest workflow uses controlled transforms, reference monitoring, and verification.

Where this leaves us

PQ and HLG are both valid HDR television systems.

PQ uses an absolute display-luminance EOTF.

HLG uses a relative scene-light signal and adapts the display result through nominal peak luminance, black level, system gamma, and viewing conditions.

PQ's perceptual design distributes code values efficiently across a range extending to 10,000 cd/m².

Ten-bit PQ is practical and widely used, but it is not guaranteed to be free of banding in every image and workflow.

HLG combines a square-root lower OETF segment with a logarithmic upper segment.

Its transition occurs at half of the nonlinear signal, not at one fixed display luminance.

HLG offers useful compatibility with conventional television workflows and suitable BT.2020-capable SDR receivers.

It is not directly backward compatible with every BT.709 television.

PQ does not always require dynamic metadata.

HLG does not mean that no signaling or metadata exists.

PQ is common in HDR10, HDR10+, and many Dolby Vision workflows.

HLG is common in broadcast and live-production systems.

Modern standards can support both.

The practical question is not which curve is universally better.

It is:

Which system fits the production, distribution path, receiver population, target displays, viewing conditions, and creative requirements?