Photochromic Image-Plane Filter Extends Dynamic Range of CCD

A simple technique for extending the dynamic range of a charge-coupled-device (CCD) video camera involves the use of photochromic material — the same material used in self-adjusting sunglasses. The dynamic range of an image is the ratio between the maximum and minimum brightness levels in the image. The dynamic range of a CCD is the ratio between an overexposure brightness level (above which the image becomes saturated or "washed out") and an underexposure level (below which details disappear into the darkness). The dynamic range of a CCD is less than that of the human eye; for example, a human observer can often see both shadowed and unshadowed features in a scene illuminated by sunlight, whereas a CCD cannot capture details simultaneously in both the brightest and darkest parts of the scene (see Figure 1).