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A spatial light modulator (SLM) is a special device that can manipulate light by modulating the amplitude, phase or polarization of the light waves in the two dimensions of space and time. This means that light is manipulated in order to obtain a desired output, and SLM is commonly used in overhead projectors such as those used in schools and office conference rooms.
A spatial light modulator is an electronically programmable device that can modulate light output based on a specific fixed spatial pattern (pixel), essentially projecting light that is controlled in either amplitude only, phase only or both (phase-amplitude). This device makes use of liquid crystals to modulate the light, which is why overhead projectors are called LCD projectors.
There are many types of SLMs, and one common type is the electrically addressed SLM (EASLM), wherein the image is created and changed electronically just like in most electronic displays, and which usually receives input via conventional digital interfaces such as VGA or DVI. Another type is the optically addressed SLM (OASLM), which requires a separate light input encoded with an image that it can then project on its surface, again using liquid crystals. This means that an OASLM is a secondary display that takes input from an EASLM. In a process called image tiling, the images produced with an EASLM are then sequentially transferred to different parts of an OASLM before the whole image is displayed for the viewers. This can result in high-resolution images above 100 megapixels.