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high dynamic range (HDR)

The human eye has an extremely high dynamic range and can distinguish light conditions of 10,000,000:1 and more, between darkest night and brightest sunlight. Such a dynamic range cannot be achieved with a printer, photography, a conventional display or a projector.

Color capture, color display and printing of color prints and photographs are limited in dynamic range. All recording and reproduction processes are subject to certain technical limitations in their contrast ratios. These limits must be extended to match the dynamic range of the human eye as far as possible.

Perceptible dynamic range of the eye and extended dynamic range in HDR display

Perceptible dynamic range of the eye and extended dynamic range in HDR display

To improve the contrast ratio in recording and playback technology, the ratio between the brightest and darkest image detail must be increased, since every recording is limited by the contrast between the brightest and darkest image detail. This is particularly evident when partially bright image details are to be captured in a dark environment. For example, an object illuminated by the sun in a dark room. The contrast of the CCD sensors is not sufficient to make details in the dark area clearly visible. Such a contrast ratio can be 10,000:1 and higher. However, digital cameras only have a resolution of 8 bits, or a contrast ratio of 256:1, and paper printouts have an even lower resolution.

Increasing the dynamic range with High Dynamic Range

High Dynamic Range(HDR) is an approach to increasing dynamic range that is applied equally to image capture, HDR photos, HDR videos, HDR displays and HDR printers in different ways. The HDR technique works with considerably more brightness levels, namely with 10 bits, HDR10, and 12 bits, HDR12, than the classic technique with Standard Dynamic Range( SDR), which works with 8 bits and accordingly can only represent 256 brightness differences. The 10 bits or 12 bits of the HDR method correspond to 1,024 or 4,096 brightness levels. The dynamic range of the images is compressed to a feasible level during the recording technique and the display. This is also referred to as dynamic compression or tone mapping.

High Dynamic Range starts with the recording technology. To circumvent the limited dynamic range of digital cameras, the images can be underexposed or a gradation filter can be used. The disadvantage of underexposure is that noise is particularly emphasized in the already dark image details. In contrast, neutral gradation filters have a smooth transition to neutral gray in the upper area. There are also digital cameras with optimized image sensors that cover a wider contrast range. In addition, there is Binary Pixel, a technique developed by Rambus that can display brightness differences in saturation.

HDR display with 32 bit and 64 bit, photo: Codinghorror

HDR display with 32 bit and 64 bit, photo: Codinghorror

Another shooting technique relies on multiple shots of the same subject taken blur-free from the same position with different apertures. The individual shots, which should already be taken in RAW format because of the contrast, are then calculated using an appropriate program in the HDR process with a contrast of 32 bits. As far as the rendering technique is concerned, in HDR displays the dynamic range is significantly increased by backlighting.

Instead of multiple shots with different apertures, modern smartphones rely on triple cameras, where three lenses take pictures with different apertures.

Englisch: high dynamic range - HDR
Updated at: 22.03.2021
#Words: 525
Links: dynamic range, light, printer, display, projector
Translations: DE

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