Optics has been an important part of the research programs at the National Bureau of Standards since its first laboratories began operating in 1904, and we appreciate the opportunity to present to the readers of Optical Engineering some samples of our current work in optics.

The range of NBS radiometric calibration services has been extended into the far ultraviolet region of the spectrum where a dc high power hydrogen wall-stabilized arc is used as a primary standard of spectral radiance. A capability in the range 130 nm to 360 nm (overlapping conventional tungsten strip lamp radiometry) is presently available with estimated uncertainties between 5 and 10 percent depending upon wavelength. The status of radiometric source standards in the far ultraviolet is briefly reviewed and the hydrogen arc and NBS calibration facility are described in detail. The use of commercially available mercury Krefft-Rossler lamps and deuterium arc lamps as transfer or secondary standards is discussed and the spectra of these lamps calibrated with the hydrogen arc standard are presented.

A non-contacting length comparator utilizing two specially designed photo-electric micro-scopes has been constructed. Performance tests of this comparator, using lapped and polished steel surfaces demonstrate a resolution of -1 nanometer, a precision of--10 nanometers, and a linear range in excess of 50 micrometers.

A high precision photoelectric azimuthal polarimeter has been designed and constructed. The instrument is designed to determine the angle of rotation with an accuracy (3a) of better than 1 part in 104. The instrument is of a relatively compact design and quite simple in construction.

A wavefront shearing interferometer for testing lenses has been developed at the National Bureau of Standards. In contrast to most interferometric test systems, the wavefront shearing interferometer is inexpensive, portable, relatively insensitive to vibration, does not need laser illumination, and requires only a minimum of experimental time and operational expertise. Reading of the interferograms and subsequent data reduction require the major effort in testing with the wavefront shearing interferometer. However, with automatic scanning of the interferograms and a high-speed electronic computer to perform the analysis, the data reduction may be completely automated.

A 24-inch (60.96 cm) paraboloidal mirror having a vertex radius of curvature of 32 feet (9.75 m) was tested on-axis with a laser interferometer in a normal atmosphere. A compensating null lens corrected the spherical aberration. Simultaneous observation of the fringe pattern and point spread function produced consistent, high quality interferograms. The residual opti-cal path differences obtained from ten interferograms were averaged. Analytically removing astigmatism present in the system produced a residual root-mean-square optical path difference of .014 of the wavelength of the light. The coma amplitude measured was .04 of a wavelength. The computed modulation transfer function at the mid-spatial frequency with astigmatism analytically removed is .002 below that of a diffraction limited system.