Dual Slope "Super-Dynamic" Range Expansion

The SI-1280F and SI-640F can be used in a dual slope bi-linear mode, extending their useful dynamic range in scenes where detail in bright areas are to be preserved at the same time as maintaining details in darker regions.

       

 Short Exposure                                   Long Exposure                                          Dual-Slope

The image on the right has been captured in dual slope mode. Observe the detail visible both indoors (ie. under the table, Macbeth chart) and through the window (trees and houses).

In normal linear response a camera requires a short exposure to keep the bright areas from saturation.  However, the darker regions of the image would not have enough time to integrate charge (yellow curve).  If the exposure is set longer the detail in the dark areas will become visible by the brighter areas will become saturated (pink curve).

The dual-slope operation (blue curve) combines the transfer of nominal integration time (steep slope, high sensitivity) with the transfer curve obtained from a short electronic shutter (shorter exposure time, lower sensitivity), into a single exposing operation (i.e. without the need to combine two different images).  This Super-dynamic function is accomplished by clipping (or reseting) the brighter image data part way through the exposure at a “kneepoint” and then allowing the image to continue exposing.  This allows a longer time for the darker regions to exposuse and a short re-exposure of the bright areas to be combined in the single image.    The 'kneepoint” in the resulting bi-linear electro-optical transfer can be positioned by adjusting the reset level and the time of reset during exposure.