Researchers at the National Institute of Standards and Technology (NIST) have demonstrated a prototype device capable of absolute measurements of optical power delivered through an optical fiber. The device is the world's first fiber-coupled cryogenic radiometer that links optical fiber power measurements directly to fundamental electrical units and national standards. It uses a microscopic forest of carbon nanotubes—the world's darkest material—to measure values that are about one-thousandth of the levels typically attained with a cryogenic radiometer lacking direct fiber input capability.* With improvements in temperature control and speed, the device could meet the needs for ultraprecise calibrations at ultralow power in telecommunications, medical devices and other industries. Optical power and energy are traceable to fundamental electrical units. Radiometers absorb optical…