What Is High Dynamic Range?

High dynamic range, also called wide dynamic range or extended dynamic range, describes a system whose usable measurement span from its smallest resolvable signal to its largest undistorted signal is much wider than conventional practice allows. Dynamic range is the ratio between those two limits, and it is quoted as a ratio, in decibels, or in photographic stops, where each stop is a factor of two. In imaging, where the term is most familiar, the goal is to capture and display a scene containing both deep shadow and direct sunlight without crushing one end into black or clipping the other into white. The same idea applies wherever a sensor or converter must span orders of magnitude, including audio recording, radio receivers, radiation detection, and analog-to-digital conversion.

The motivation is physiological as much as technical. Human vision spans roughly fourteen orders of magnitude of luminance through adaptation and can resolve several orders simultaneously within a single scene, far more than the range that conventional cameras and displays were engineered to reproduce.

Measuring and Extending Dynamic Range

In an image sensor, the ceiling is set by the full well capacity of a pixel, the number of electrons it can hold before saturating, and the floor by the read noise of the pixel and its readout chain, with dark current contributing at long exposures. Their ratio gives the intrinsic dynamic range, and in an analog-to-digital converter the analogous figure is the signal-to-noise and distortion ratio, related to the effective number of bits. Engineers extend the range with several strategies. Exposure bracketing captures the same scene at multiple integration times and merges the frames, which works well for static subjects but produces ghosting when anything moves. Dual conversion gain switches a pixel between a high-gain low-noise mode and a low-gain high-capacity mode. Logarithmic and lateral overflow pixel designs compress the response curve in the analog domain, and split-pixel architectures place large and small photodiodes side by side, an approach now common in automotive sensors that must handle a tunnel exit and oncoming headlights in the same frame.

Representation, Transfer Functions, and Display

Captured high dynamic range values are scene-referred and roughly linear in radiance, so they need floating point or extended integer formats rather than the eight-bit encodings built for standard dynamic range. Displaying them requires a transfer function that spends the available code values where the eye can see differences. Two are standardized in Recommendation ITU-R BT.2100, which defines image parameter values for high dynamic range television: the perceptual quantizer, published as SMPTE ST 2084, whose absolute encoding covers luminance from near zero up to 10,000 candelas per square meter, and hybrid log-gamma, whose relative encoding remains compatible with legacy displays and suits live production. Both are normally paired with the wide color gamut primaries of Recommendation ITU-R BT.2020, since expanding luminance without expanding chromaticity produces unnatural highlights. Because no display reaches the full encoded range, content must be tone mapped to the panel's actual peak luminance and black level, and objective quality models for compressed and tone-mapped high dynamic range video exist to predict how much of the original impression survives that conversion together with compression.

Applications

High dynamic range techniques have applications in a wide range of fields, including:

• Broadcast television, streaming, and cinema production and mastering
• Consumer and professional photography, including computational imaging on mobile devices
• Automotive vision systems and driver assistance cameras
• Machine vision and industrial inspection under uncontrolled lighting
• Astronomical and scientific imaging, where sources differ by many orders of magnitude
• Audio recording and radio receiver design, where strong and weak signals coexist
• Instrumentation and analog-to-digital conversion for wide-span measurement