SHEILA TERRY / SCIENCE PHOTO LIBRARY SHEILA TERRY / SCIENCE PHOTO LIBRARY
The imaging characteristics of a digital camera can be assessed by using the sensitometric methods developed for silver-based photographic systems. The DSLR camera has been set up to photograph a calibrated transmission step wedge consisting of a series of neutral densities ranging, on a logarithmic scale, from a minimum density of 0.08 to a maximum of 3.08 in 21 steps each increasing by 0.15. This represents a luminance range of 1000 to 1. A characteristic curve can be constructed by plotting the pixel values of the incremental steps obtained from imaging software against the relative luminance of the grey scale. From this the dynamic range, or useful exposure range, can be determined,. The speed or sensitivity of the camera can also be measured, as well as the effects of photo-imaging software manipulation. For digital scanners a reflection step wedge is used.
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