The Cosmic Background Imager (or CBI) was a 13-element astronomical interferometer perched at an elevation of 5,080 metres (16,700 feet) at Llano de Chajnantor Observatory in the Chilean Andes.

(In comparison, the pioneering COBE satellite, which produced the first detection of fluctuations in the microwave background in 1992, had a resolution of about 7 degrees.)

More technically, CBI was the first experiment to detect intrinsic anisotropy in the microwave background on mass scales of galaxy clusters; it provided the first detection of the Silk damping tail; it found a hint of excess power at high-l multipoles (CBI-excess) than expected from the ΛCDM model; and it detected fluctuations in the polarization of the microwave background obtaining the first detailed E-mode polarization spectrum providing evidence that it is out of phase with the total intensity mode spectrum.

Another experiment operated from Antarctica, the Arcminute Cosmology Bolometer Array Receiver, used total power (bolometric) detection and a single antenna at higher frequency and similar angular resolution to obtain results comparable to the CBI.

The confluence of these and other CMB experiments employing different measurement techniques in recent years is a great triumph of observational cosmology.