This time we address the UHD’s flagship theme, HDR, and the amalgamation of standards that all this brings us
HDR stands for High Dynamic Range. The dynamic range of an image is the difference between the darkest and the brightest parts of the scene. Dynamic range is measured in diaphragm stops and it gives us an idea of the difference between the darkest and brightest areas of an image. The human eye has a dynamic range of 14 stops, which means that we can see a broad range of luminosities in the same scene. The adaptability of the human eye is also very wide and fast between scenes that fall outside of those 14 stops.
Until now, the dynamic range covered six stops within the BT.709 standard, something really limited in comparison with our eyes. This was due to the former limitations of the standard definition, which led to the definition of a limited dynamic range. In fact, beyond colourimetry or definition, dynamic range is perhaps the most limiting part of technical equipment when it comes to capturing and reproducing images.
The new HDR standards that we will see later gives us up to 17 stops, way beyond the human visual capacity. This does not mean that current equipment can actually work with such a wide dynamic range, but in anticipation of this happening in the future, standards already cover these capabilities.
The Barten limit or Barten ramp was defined to delimit how much we must increase the dynamic range of an image for it to be perceptible.
This graph confronts the luminance present in an image concerning the contrast breaks necessary for this difference in luminance to be visible. The more contrast, the easier it is to distinguish between the differences of luminance in the scene. However, there is a limit above which the increase does not bring any change in visual perception, and this occurs in high luminance of the image.
Below the Barten ramp, increasing the contrast implies a visual change in luminance, while above this ramp providing more contrast does not necessarily imply that we can see a change in luminance.
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