A brightness measure for HDR TV

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ABSTRACT

As standards for a complete high dynamic range (HDR) television ecosystem near completion, the industry is taking its first steps in HDR production.

HDR is associated not only with a greater dynamic range, but also brighter screens than conventional television, so the potential arises for unwanted, uncomfortable brightness jumps at programme junctions and channel changes.

To ensure a degree of consistency between programmes, some production guidelines for HDR brightness are required. In this paper, we summarise tests showing that the mean displayed pixel luminance is a good predictor of subjective brightness. We then explore viewer tolerance to brightness shifts of different sizes, and propose a potential normal operating range for the mean display luminance of 10-80 cd/m2, extending to 5-160 cd/m2 for special creative effect.

INTRODUCTION

High dynamic range (HDR) television is now standardised in ITU-R BT.2100, and programme makers are starting to create HDR content.

With the brighter screens and greater dynamic range associated with HDR, there comes the potential for sudden uncomfortable brightness jumps at programme junctions, breaks for advertisements, or when switching between channels. A need for production guidelines for HDR television has been identified, just as guidelines were required for audio loudness.

The guidelines should describe suitable brightness levels for HDR programmes, to prevent unwanted, uncomfortable brightness jumps. Conventions that map specific scene luminance levels to appropriate signal levels are widely used in standard dynamic range (SDR) television, although they are not officially standardised.

They include “diffuse white’’ or an ice hockey rink at around 90% signal level, “flesh tones” at around 50-70% signal level, and grass or an 18% reflectance card at 50% signal level. Conventional SDR displays are generally too dim to allow good quality pictures to be produced using only part of the signal range, so these best-practice conventions have developed to ensure that full use is made of the available dynamic range.

This has the secondary effect of maintaining a degree of brightness consistency between programmes, which means that large jumps in brightness do not usually occur. There has not therefore been a specific need for brightness guidelines for SDR.