Response of an HDR camera (left) and a conventional camera (right).
Photometry is a term used in physics, optics, and illuminating engineering for the measurement of quantities associated with light. The process of photometric calibration of a camera allows to find a function which converts the pixel values produced by a given digital camera to photometric quantities such as SI light units.
The current trends in computer graphics focus on acquiring, processing and reproducing real world scenes, often with a quite high dynamic range (HDR). Many algorithms require such HDR image or video data as an input and the increasing availability of HDR sensors provides a natural source of them. But at the same time the diversity of HDR sensor technology leads to a strong need for photometric calibration of such HDR camera systems in order to obtain a physically correct final result.
On the other hand, for conventional low dynamic range (LDR) digital cameras such a calibration allows to merge several differently exposed images to create a High Dynamic Range image.
We present a calibration approach based on an earlier HDR recovery algorithm for standard low dynamic range (LDR) cameras. We focus here on the specific challenges posed by cameras with a dynamic range of more than six orders of magnitude such as complex camera response curves or selection of appropriate calibration targets. We perform an absolute calibration of two HDR and one LDR camera systems to allow for recovery of real-world luminance values. To validate our approach, we compare these luminance values to measurements performed with a luminance meter for grayscale patches covering a dynamic range of six orders of magnitude. The achieved accuracy of the photometric calibration is sufficient for many measurement and image-based acquisition applications.
Further details can be found in the MPI Research Report: MPI-I-2005-4-005.
Images with different exposure settings used to create an HDR image.
A photographic camera with a standard CCD sensor is able to acquire an image with simultaneous dynamic range of not more than 1:1000. The basic idea to create an image with a higher dynamic range is to combine multiple images with different exposure settings, thus making use of available sequential dynamic range.
For a detailed description of the method see the tutorial for PFScalibration package.
We provide a software package PFScalibration which contains a set of command line tools to recover a camera response of both HDR and LDR cameras. Using this software it is also possible to create an HDR image from a set of LDR exposures.
For more information see the PFScalibration project page.
Several external resources related to HDR imaging and calibration: