  We present a cryogenic wafer mapper based on light emitting diodes (LEDs) for
spatial mapping of a large microwave kinetic inductance detector (MKID) array.
In this scheme, an array of LEDs, addressed by DC wires and collimated through
horns onto the detectors, is mounted in front of the detector wafer. By
illuminating each LED individually and sweeping the frequency response of all
the resonators, we can unambiguously correspond a detector pixel to its
measured resonance frequency. We have demonstrated mapping a 76.2 mm 90-pixel
MKID array using a mapper containing 126 LEDs with 16 DC bias wires. With the
frequency to pixel-position correspondence data obtained by the LED mapper, we
have found a radially position-dependent frequency non-uniformity < 1.6% over
the 76.2 mm wafer. Our LED wafer mapper has no moving parts and is easy to
implement. It may find broad applications in superconducting detector and
quantum computing/information experiments.
