A grism (also called a grating prism) is a combination of a prism and grating arranged so that light at a chosen central wavelength passes straight through. The advantage of this arrangement is that one and the same camera can be used both for imaging (without the grism) and spectroscopy (with the grism) without having to be moved. Grisms are inserted into a camera beam that is already collimated. They then create a dispersed spectrum centered on the object's location in the camera's field of view.

The resolution of a grism is proportional to the tangent of the wedge angle of the prism in much the same way as the resolutions of gratings are proportional to the angle between the input and the normal to the grating.

Grisms are being used in the NIRCam instrument on the James Webb Space Telescope for wavefront sensing.[1]

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  1. ^ Greene, Thomas P.; et al. (2016). "Slitless spectroscopy with the James Webb Space Telescope Near-Infrared Camera (JWST NIRCam)". Space Telescopes and Instrumentation 2016: Optical, Infrared, and Millimeter Wave. Proceedings of the SPIE. 9904. pp. 99040E. arXiv:1606.04161. Bibcode:2016SPIE.9904E..0EG. doi:10.1117/12.2231347. ISBN 9781510601871.