The Low Resolution Imaging Spectrometer (LRIS) is a visible-wavelength imaging and spectroscopy instrument commissioned in 1993 and operating at the Cassegrain focus of Keck I.

LRIS was built at Caltech under the supervision of Bev Oke and Judy Cohen. Later James McCarthy and Chuck Steidel took responsibility for adding the blue side.

Reference paper: "The Keck Low Resolution Imaging Spectrometer", Oke, J.B., et al.. 1995, PASP, 107, 375

Beamsplitters separate the light between two arms, red and blue, sensitive over the 3200-10,000 Ê wavelength range.

The field of view in both modes of operation is 6×7.8 arcmin. The red camera uses a backside-illuminated Tek 2048×2048 CCD detector with a pixel scale of 0.211 "/pixel. The blue camera has a mosaic of 2 2Kx4K Marconi CCDs and the pixel scale is 0.135 "/pixel. The standard imaging filters include the UBVGRI passbands plus a small number of narrowband imaging filters.

Spectroscopy can be performed using a standard complement of longslits of various widths, or in multi-object mode, by using designed slitmasks which are milled on-site. An assortment of gratings (red side) and grisms (blue side) yield resolutions ranging from R=300-5,000, with peak system efficiencies of ~50%.

An optional polarimeter module enables spectropolarimetry.