Scientists at the National Institute of Standards and Technology's (NIST) Optoelectronics Div. have developed a stable linear optical retarder for use as a calibration reference. Optical retarders, known as wave plates, are critical for polarization control in optics and are found in an array of applications such as optical sensors and military systems. Traditionally, wave plate manufacturers measure retardance using laboratory benchtop systems with varying levels of uncertainty. NIST's device is a quarter-wave retarder at 1.3 µm, chosen to meet fiber optic telecommunications needs. The retarder is stable within 0.1° over a wavelength range greater than 50 nm.