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More accurately however, the images that are commonly seen and referred to as HDR or HDRI images are tone-mapped.
Tone mapping is a technique used in image processing and computer graphics to map a set of colours to another; often to approximate the appearance of
high dynamic range images in media with a more limited dynamic range. Print-outs, CRT or LCD monitors, and projectors all have a limited dynamic range which is inadequate to reproduce the full range of light intensities present in natural scenes. Essentially, tone mapping addresses the problem of strong contrast reduction from the scene values (
radiance) to the displayable range while preserving the image details and color appearance important to appreciate the original scene content.
Find more information
here and
here along with tutorials to create your own HDR photographs using Photoshop.
From the Estetica Design & Photography Forum