K. L. Eckerle

New Reference Spectrophotometer

A new single beam spectrophotometer is described in which transmittance is measured by placing samples normal to a parallel beam of light. Collimation and focusing of the main beam are achieved by means of off-axis parabolic mirrors. The wavelength at which the transmittance is to be measured is selected by a plane grating monochromator having off-axis parabolic mirrors and circular holes as entrance and exit apertures. The instrument has an inherent accuracy estimated to be 0.0001 transmittance unit. Its precision is characterized by a repeatability of 0.00004 transmittance units for neutral-density filters with transmittances between 10% and 30%. The design philosophy used to achieve these results is presented. A discussion of some systematic errors commonly neglected in routine spectrophotometric, measurements is given. Systematic errors such as detector nonlinearity and stray radiant energy are measured.

Averaging sphere for ultraviolet, visible, and near infrared wavelengths: a highly effective design.

Through the use of an efficient design and a newly available sphere coating material, a simple, passive, sturdy averaging sphere was made that operates effectively over the wavelength range from 200 nm to 2000 nm. Data are reported for a sphere of this type in which the sphere transmittance is 0.32 at 200 nm and rises rapidly to near the maximum theoretical value of 0.56 over the remainder of the wavelength range. The several orders of magnitude reduction in error due to beam displacement more than compensate the slight reduction in signal for many spectrophotometric and radiometric applications.

Spectrophotometric tests using a dye-laser-based radiometric characterization facility

A new high-accuracy dye-laser-based system useful for measuring the spectral transmittance of filters or the spectral response of optical detectors has recently been developed at NBS. This paper describes the system and discusses the results of measurements made for purposes of comparison with a high-accuracy spectrophotometer also developed at NBS.

Accuracy of Polarization Attenuators

The accuracy of various polarization attenuators is discussed, including systematic errors due to imperfections of the polarizers, setting errors of the rotating elements, alignment errors of fixed elements, and errors caused by oblique incidence and partial polarization of the incident light. Whereas the accuracy of conventional three-polarizer attenuators is limited to 0.001 transmittance unit, at least ten times more accuracy can be achieved with two new types of attenuators, which employ either a half-wave or a quarter-wave retardation plate in conjunction with two sheet polarizers.

Standards for Corrected Fluorescence Spectra

A set of four fluorescent standards has been produced and calibrated at the National Institute of Standards and Technology (NIST) for corrected relative spectral emission over the wavelength range from 400 to 740 nm. This new Standard Reference Material (SRM) 1931 has been produced in the form of sintered mixtures of inorganic phosphors and polytetrafluorethylene (PTFE) powder for spectral response calibration of spectrofluorimeters used in biology, medicine and studies of molecular structure. They provide a means for correcting spectrofluorimeter errors due to wavelength dependencies of optics, monochromators, and detectors, thus permitting meaningful intercomparison of results obtained in different laboratories.

NBS reference retroreflectometer

A long-range retroreflectance instrument has been built in the photometric range of the Radiometric Physics Division of the NBS. It is designed to measure photometric properties of retroreflectors for different geometries. It satisfies many needs of the measurement community, and it is planned to use this instrument as the basis for a Measurement Assurance Program (MAP) and for Standard Reference Materials (SRM). This paper describes the design and testing of the instrument. Some estimated uncertainties for typical samples are given.

Averaging spheres without target

The design of averaging spheres without internal target is described. The performance of such spheres is analyzed theoretically and tested experimentally. In a final design, an efficiency of 40% is achieved for visible and near-uv wavelengths. The averaging effectiveness is characterized by signal variations of the order of 1 part in 10(4) for beam displacements of several 0.1 mm.