Graphic images such as those seen in advertising signs are commonly illuminated by backlighting the image. It is desirable to backlight the image with the highest possible efficiency in order to improve the dynamic range of luminance values which can be perceived by persons viewing the backlighted image.
In conventional photography, only a relatively limited range of luminance values can be faithfully reproduced. Consequently, there are many visual situations which the eye can perceive well, but which cannot be captured photographically. Examples include most situations where sources of light are in the field of view, such as sunset scenes, scenes containing highly reflective ("shiny") surfaces, or night scenes containing neon signs, lamps, etc. The ability to display a larger dynamic range of luminance values would facilitate production of more visually effective graphic images, such as scenes of the aforementioned type which contain sources of light. This would in turn have value both aesthetically and in more effective advertising.
Ultra-high dynamic range images can be recorded in a single exposure by some conventional CCD array cameras, although not all CCD cameras can do so. However, two or three different exposures of the same scene can capture all the required information. An ultra-high dynamic range image can then be constructed in a fairly straight forward manner by overlapping multiple image layers obtained through more conventional photographic means. The problem is that, in order for an observer to perceive the full dynamic range of such an image, the image must be illuminated by an extremely bright backlight (at least 10 times brighter than conventional fluorescent light boxes, for example), which is impractical.
Conventional image backlighting techniques are inefficient in two important respects. First, a conventional backlight emits light with uniform brightness in all directions. Consequently, most of the emitted light is wasted, because the backlighted image is generally viewed only by persons having a restricted field of view which is typically within about 30 degrees of perpendicular horizontally, and within about 20 degrees of perpendicular vertically. If the emitted light could be concentrated within this range of angles, then a brightness increase of a factor of 4 could be attained. Second, most regions of most images are not very bright, meaning that very little incident light needs to be transmitted through such regions to persons viewing the image. In conventional image backlighting such non-transmitted light is absorbed and hence wasted. If such non-transmitted light could be effectively recycled, then typically another brightness increase factor of 5 could be attained. In combination, these two factors could enable a 20-fold increase in backlighting efficiency. In applications involving ultra high dynamic range images, such increased efficiency could yield a 10 times brighter peak luminance in a lighting structure requiring only 50% of the input power of a conventional prior art backlighted image.
The luminance intensity of a backlighted image is directly proportional to the flux of the light which illuminates the image, and inversely proportional to both the image area and the solid angle within which light is emitted from the image toward persons viewing the image. Therefore, in order to increase the luminance intensity of the backlighted image, one must either increase the flux; or, decrease either or both of the image area or the solid angle of light emission. Since flux is determined by the light source, and since the image area is fixed, prior art efforts to increase luminous intensity have been primarily directed to decreasing the solid angle of light emission. The present invention recognizes that it is possible to reduce the effective area of the image in a manner which yields a corresponding increase in luminous intensity, without reducing the physical size of the lighting structure or its image-bearing surface.