What is HDR?

The human eye is capable of adapting to luminances as high as 1,000,000 cd/m² and as low as 0.000,000,1 cd/m². Once adapted, the eye can cope with a luminance range of 1:1000, but for a part of the scene, this can be as high as 1:10000.

Most digital image formats have been designed with the capabilities of computer graphics displays in mind. A typical 24bit image with 8bit each for the red, green and blue channel can store 256 different values for each channel, resulting in a total range of 16 million colours (2563). A value of zero is displayed as black, while 255 is interpreted as white. With current display technology, the typical contrast for a TFT screen is about 300:1.

The problem should be clear by now. Not only is computer display technology still very far from delivering images that have a luminous range even close to what the human eye can process. The information stored in traditional computer image files is also not expressed in photometric terms. Instead of describing the luminance of a pixel in cd/m², the value is just 'darker' or 'not quite as dark'.

High dynamic range (HDR) images store the information in a format that has a range of many orders of magnitude. Additionally, they may be photometrically correct. With this additional information, image processing may be done such as advanced tone mapping or calibrated false colour luminance images.

The dynamic range of the image below has been reduced. This is a process called tonemapping resulting in an image that may be displayed on non-HDR output devices:

Tone-mapped synthetic HDR image

If viewed with an appropriate image viewer, it is possible to re-adjust the image exposure of HDR images on the computer screen. The images below have an exposure of -12, +/-0 and +10 EV. 1 EV is the equivalent to halfing or doubling the exposure time on a camera.

HDR image with different exposure adjustements applied.

If you would like to experiment with the original HDR image, please feel free to to download it here:

This image is © Greg Ward and is available on the CD-ROM accompanying the High Dynamic Range Imaging book which he co-authored. Greg kindly agreed to have it included on this site. The link is listed on the References page.