A spatial light modulator (SLM) is an object that imposes some form of spatially-varying modulation on a beam of light. A simple example is an overhead projector transparency. Usually when the phrase SLM is used, it means that the transparency can be controlled by a computer. In the 1980s large SLM's were placed on overhead projectors to project computer monitor contents to the screen. Since then more modern projectors have been developed where the SLM is built inside the projector. These are commonly used in meetings of all kinds for presentations.

Usually, an SLM modulates the intensity of the light beam, however it is also possible to produce devices that modulate the phase of the beam or both the intensity and the phase simultaneously.

SLM's are used extensively in holographic data storage setups to encode information into a laser beam in exactly the same way as a transparency does for an overhead projector.

An example of an SLM is the Digital Micromirror Device at the heart of DLP displays.