ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
The double-aperture method has been used to determine the nonlinearity correction for a new spectrophotometer having a precision of +/-4 x 10(-5) transmittance units. The random and systematic errors of the method are discussed, and techniques are described that yield the additive nonlinearity correction to the high level of precision required for this spectrophotometer. The correction was found to be independent of source polarization, free from interference errors, but slightly dependent on wavelength.
