The 621 laser wavelength meter introduced by Bristol Instruments Inc. uses optical interferometry to measure wavelength with high accuracy, reliability, versatility and convenience. Model 621A measures absolute wavelength with ±0.2-ppm accuracy, and the 621B delivers an accuracy of ±1.0 ppm. The built-in wavelength standard performs continuous calibration. Various versions operate from 350 nm to 4.0 µm. The devices are suitable for high-resolution laser spectroscopy, photochemistry and optical remote sensing.