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.

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.

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.

No abstract

The work describes the methods and procedures used to determine the wavelengths of minimum transmittance of holmium oxide in perchloric acid solution. Measurements of spectral transmittance of the solutions were made by means of a high precision spectrophotometer over the wavelength range 200 nm to 680 nm. The wavelength scale accuracy of this instrument was verified by extensive measurements of mercury and deuterium emission lines. The measurements of spectral transmittance of the holmium oxide solutions were made as a function of temperature, purity, concentration, and spectral bandwidth. Analysis of the uncertainties associated with these parameters and the uncertainties associated with the calibration of the instrument wavelength scale and the data analysis have resulted in an estimated uncertainty of ±0.1 nm for the determination of the wavelengths of minimum transmittance of the holmium oxide solution.