A window can show a video image on one side but remain transparent from the other side using technology patented this week by San Francisco-based Emiscape. One use of the technology is to project advertisements onto the side of buildings without disturbing the occupants.
As described in the patent (8,123,365) and the company's Web site, a full-color image is displayed on a transparent screen using only particular wavelengths of red, green and blue light. The image can be projected onto the screen, or the screen itself can include transparent light-emitting diodes. The color image is visible from only one side of the screen (from the right, in the illustration).
A filter on the other side of the screen blocks out precisely those same wavelengths of light, so a viewer on the filter side (the left) can't see the displayed image. But the filter is transparent to most of the remaining red, green and blue light. So a viewer can still see other objects through the screen without seeing the image.
The filter technology is similar to that of the Dolby 3D system. In that system, 3D glasses use filters that pass only particular wavelengths of light to each eye while blocking the wavelengths destined for the other eye, but both eyes still see a full-color image.
Emiscape suggests that the screen can be used between the front and back seat of a car, where it would appear to be a full-sized video screen to backseat passengers but would remain transparent to the driver in front. It's a great idea, and maybe it would work at night, but Emiscape's conceptual video highlights one serious challenge: how do you project black?
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