High Dynamic Range Imaging (HDRI or HDR)

High-dynamic-range imaging (HDRI or HDR) is a set of techniques used in imaging and photography to reproduce a greater dynamic range of luminosity than possible using standard digital imaging or photographic techniques. The purpose of this is to attempt to present the human eye with a similar range of luminance as that which, through the eye-brain system, we are familiar with in everyday life. The human eye, through adaptation of the iris (and other methods) is able to constantly adjust to the huge dynamic changes ubiquitous in our environment and the brain is able to continuously interpret this information so that most of us are able to see in a wide range of light conditions; most cameras on the other hand are not.

Additionally, in view of the limitations of printing and monitors, acquiring an HDR image is only half the story, one must also develop methods of observing the results. Consequently we have firstly, the methods of obtaining the information in the first place and secondly, the methods of rendering that information to be available to a standard monitor or printing device- this is tone mapping.

HDR images can represent a greater range of luminance levels than can be achieved using more 'traditional' methods. Images such as those containing many real-world scenes, from very bright direct sunlight to extreme shade or very faint nebulae. It is often achieved by capturing and then combining several different exposures of the same subject matter. Non-HDR cameras take photographs with a limited exposure range, resulting in the loss of detail in bright or dark areas.

The two primary types of HDR images are computer renderings and images resulting from merging multiple low-dynamic-range (LDR) or standard-dynamic-range (SDR) photographs. HDR images can also be acquired using special image sensors, like oversampled binary image sensor. Tone mapping methods, which reduce overall contrast to facilitate display of HDR images on devices with lower dynamic range, can be applied to produce images with preserved or exaggerated local contrast for artistic effect.