The dynamic range of an image is the ratio between the brightest and darkest representable pixel values. In the case of a typical 8-bit digital image, this is 255:1, or a dynamic range of about 8 stops. Several units are used to express dynamic range. In photography, the stop (also called f-stop or EV) is commonly used. One whole stop (1 EV) is a factor of two in exposure.
The HDReye™ is capable of capturing images with a total dynamic range of about 26 stops, achieved with a 10-bit CMOS sensor combined with 16 stops of exposure time control. The captured image is stored in a floating-point representation so that no dynamic range is lost due to the storage format.
This figure shows several exposures extracted from a single HDReye image. The exposures span a range of 12 stops, and are shown in two-stop steps. The larger images show portions of the field of view at two distinct exposures showing that usable image detail is available over a wide range of exposures.
Notice in particular that in the darkest exposures that the light bulb becomes well-exposed, revealing its structure to be a twisty compact fluorescent. A small Macbeth chart can be seen in the background behind the bulb.
In the brighter exposures, the bulb detail is no longer visible, but the rest of the scene is well-exposed. Details on the album cover, book cover and candy tin are will lit.
When the HDReye's shutter release is activated it automatically takes a sequence of individual exposures. The actual sequence used for the image shown above had seventeen exposures ranging in one-stop steps from 1/16000 s to 4 s. None of those source exposures are shown in the figure.
The HDReye software on the PC computes a weighted geometric mean of the estimated irradiance at each pixel in each exposure. A weighting function emphasises well-exposed pixels, and excludes fully over-exposed or fully under-exposed pixels from the average. To extend the range into the saturation region of the sensor, the weights are modified in the shortest exposure.
Cropped Sample (3.65 MB RGB floating point TIFF, 637 x 502 pixels)
This is the HDR TIFF post processed to RGB pixels with 32-bit floating point channels, from which the exposure sequence shown above was extracted. It has been cropped from the view of a single facet of the HDReye model 1. A Macbeth mini color checker is visible behind the compact fluorescent bulb, and the Altoids tin is sitting on an 18% grey card. The other objects provide a range of typical colors, including some patches of northern sky visible between leaves of the tree outside the window.
Copyright © 2004 Cheshire Engineering Corporation. All Rights Reserved. HDReye™ is a Trademark of Cheshire Engineering Corporation.
HDRExplained.html last revised August 12, 2004 by Ross Berteig