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A transparency, also known in industrial settings as a "viewfoil" or "foil", is a thin sheet of transparent flexible material, typically cellulose acetate, onto which figures can be drawn. These are then placed on an overhead projector for display to an audience. Many companies and small organizations use a system of projectors and transparencies in meetings and other groupings of people, though this system is being largely replaced by video projectors and interactive whiteboards.
Printing on transparencies
Transparencies can be printed on in one of two ways.
First, a laser printer can be connected to a computer, which is loaded with a cartridge of cellulose acetate sheets. An image or document is then created within the computer, though images are prevalent due to text being illegible at certain distances from a projection, and then formatted to be printed. From there, the printer is engaged and the document printed onto the sheet of acetate. Some people prefer to test the transparency before sending it out to the person requesting it.
Secondly, some black and white or color copiers are able to print onto, or can be configured to handle, acetate sheets. A person may place a document or a book on the surface of the copying machine and make one or several copies onto acetate, if there are simultaneous meetings or if another company or organization wants a copy.
Specialist transperencies are available for use with laser printers that are better able to handle the high temperatures present in the fuser unit. For inkjet printers, coated transparencies are available that can absorb and hold the liquid ink - although care must be taken to avoid excessive exposure to moisture, which can cause the transparency to become cloudy; they must also be loaded correctly into the printer as they are only usually coated on one side.
Uses for transparencies
Uses for transparencies are as varied as the organizations that use them.
Certain classes, such as those associated with Mathematics or History, used transparencies to illustrate a point or problem. Math classes in particular use a roll of acetate to illustrate sufficiently long problems and to create illustrations a computer cannot, due to a lack of math symbols on a standard computer keyboard. This problem is typically limited to High School and College-level mathematics, because of the inclusion of Algebra and Calculus courses, respectively. In recent years, more and more colleges are switching to digital projectors and Powerpoint presentations.
Aerospace companies, like Boeing and Beechcraft, used transparencies for years in management meetings in order to brief engineers and relevant personnel about new aircraft designs and changes to existing designs, as well as bring up illustrated problems.
Some churches and other religious organizations used them to show sermon outlines and illustrate certain topics such as Old Testament battles and Jewish artifacts during worship services, as well as outline business meetings.
Spatial light modulators (SLMs)
Many overhead projectors are used with a flat panel LCD which, when used this way is referred to as a spatial light modulator or SLM. Data projectors are often based on some form of SLM in a projection path. An LCD is a transmissive SLM, whereas other technologies such as Texas Instrument's DLP are reflective SLMs. Not all projectors use SLMs (e.g., some use devices that produce their own light rather than function as transparencies). An example of non-SLM system are OLEDs.