Transparency (projection)
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A transparency is a thin sheet of transparent flexible material, typically celluloid, onto which figures can be drawn. These are then placed on an overhead projector for display to an audience. This system is still found in schools, but is being largely replaced by LCD projector.
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. A LCD is a transmissive SLM, whereas other technologies such as Texas Instrument's DLP are reflective SLMs. Not all projectors (e.g. some use devices that produce their own light) rather than function as transparencies. An example of non-SLM system are OLEDs.