very cost-effective way to convert a theater for 3-D as all that is needed are the attachments and a non-depolarizing screen surface, rather than a conversion to digital 3-D projection. Thomson Technicolor currently produces an adapter of this type. When stereo images are to be presented to a single user, it is practical to construct an image combiner, using partially silvered mirrors and two image screens at right angles to one another. One image is seen directly through the angled mirror whilst the other is seen as a reflection. Polarized filters are attached to the image screens and appropriately angled filters are worn as glasses. A similar technique uses a single screen with an inverted upper image, viewed in a horizontal partial reflector, with an upright image presented below the reflector, again with appropriate polarizers. On TV and computer screens Polarizing techniques are easier to apply with cathode ray tube (CRT) technology than with Liquid crystal display (LCD). Ordinary LCD screens already contain polarizers for control of pixel presentation — this can interfere with these techniques. In 2003 Keigo Iizuka discovered an inexpensive implementation of this principle on laptop computer displays using cellophane sheets. One can construct a low cost polarized projection system by using a computer with two projectors and an aluminium foil screen. The dull side of aluminium foil is brighter than most silver screens. This was demonstrated at PhraJomGlao University, Nônthaburi, Thailand, September 2009. Health care In optometry and ophthalmology, polarized glasses are used for various tests of binocular depth perception (i.e. stereopsis). History Polarized 3-D projection was demonstrated experimentally in the 1890s. The projectors used Nicol Prisms for polarization. Packs of thin glass sheets, angled so as to reflect away light of the unwanted polarity, served as the viewing filters. Polarized 3-D glasses only became practical after the invention of Polaroid plastic sheet polarizers by Edwin Land, who was privately demonstrating their use for projecting and viewing 3-D images in 1934. They were first used to show a 3-D movie to the general public at "Polaroid on Parade", a New York Museum of Science and Industry exhibit that opened in December 1936. 16 mm Kodachrome color film was used. Details about the glasses are not available. At the 1939 New York World's Fair, a short polarized 3-D film was shown at the Chrysler Motors pavilion and seen by thousands of visitors daily. The hand-held cardboard viewers,