The adoption in 1979 of a new definition of the candela, which permitted a detector-based approach to the realization of the unit, has emphasized the importance of high-accuracy absolute radiation detectors. This paper describes a new electrical-substitution absolute radiometer operating at 5 K, based on a standard commercial helium cryostat, which has been developed at NPL for optical radiant-power measurements. The principal advantages of operating the radiometer at liquid helium temperatures are that the detector can be a large, highly absorbing cavity (absorptivity 99.998%) and that exact equivalence of electrical and radiant heating can be clearly demonstrated. It will be the primary absolute detector for a new realization of the candela at NPL and will provide the basis for the development of a wider range of radiometric and spectroradiometric standards. The use of the radiometer to measure the radiant power of an intensity-stabilized laser source at the milliwatt level with an uncertainty of 4 parts in 105 is also described, together with its application in the determination of the responsivity of transfer-standard detectors, such as silicon photodiodes.

http://iopscience.iop.org/article/10.1088/0026-1394/21/3/007/pdf

A Cryogenic Radiometer for Absolute Radiometric Measurements

The values of the optical constants of magnesium fluoride (MgF2) and zinc sulfide (ZnS) thin films are obtained using a classical oscillator model and the experimental values of their spectral transmittance. Auger electron spectroscopy was performed on the samples to determine the chemical composition of the films. These materials are important in the design of filters, mirrors, and antireflection coatings for optical instrumentation. Unfortunately their properties strongly depend on evaporation conditions. The procedure described here allowsdirect measurement of the dispersive refractive index of the film after deposition.

/pdf/determination-of-the-optical-constants-of-mgf-2-and-zns-from-65ssduhokh.pdf

Determination of the optical constants of MgF(2) and ZnS from spectrophotometric measurements and the classical oscillator method.

Shadowing causes form birefringence in optical thin films that are deposited at oblique incidence. For light incident in the plane containing the direction of deposition and the substrate normal, the TE and TM polarizations in the film are decoupled, and field transfer can be described using 2×2 matrices. The three principal indices of birefringent films are deduced from measurements made on narrow-band interference filters. In one example refractive indices of 2.688, 2.429, and 2.452 were computed for TiO2 deposited at 27° to the substrate normal. Both TiO2 and ZrO2 behave as positive biaxial media.

Measurement of the principal refractive indices of thin films deposited at oblique incidence

The design of a common-path polarimetric interferometer system, developed primarily for measuring the profiles of ‘super-smooth’ surfaces, is described and measured comparisons with a mechanical stylus technique are discussed. The instrument has a sensitivity to changes in surface height of better than 0.1 nm and is capable of a lateral resolution of 1 μm with a helium-neon laser light source.

Optical system for measuring the profiles of super-smooth surfaces

The optical properties of thin dielectric films prepared by vacuum deposition and r.f. sputtering have been measured using guided waves, ellipsometry and reflectometry to assess the relative merits and disadvantages of the three techniques. Particular attention has been given to the effect on these measurements of typical film imperfections, for example refractive index inhomogeneity and anisotropy. The guided wave technique is shown to possess the highest precision of measurement, to be relatively insensitive to film inhomogeneity and to provide the most suitable method for studying film birefringence. Ellipsometry has a high sensitivity of measurement that can be usefully applied to thinner films than those required by the other techniques. The method is well suited for studying inhomogeneity, but because of its sensitivity to this film defect, ellipsometric measurements must always be interpreted with care. Reflectometry, although only of limited accuracy in measuring film properties, is widely applica...

A Comparison of Thin Film Measurement by Guided Waves, Ellipsometry and Reflectometry

A new method of measuring the refractive index of a prism using the critical angle is described. The accuracy of the technique and tolerances on angle measurements are discussed. Two standard prisms were measured using this method and the values agreed with those measured by the minimum deviation method to five units in the fifth decimal place, consistent with the error analysis given.

Measurement of the Refractive Index of a Prism by a Critical Angle Method

Very clean surfaces are required for successful vacuum deposition of optical thin films. This paper describes a simple technique which has been developed at NPL for cleaning substrates and discusses its effectiveness. Cleaning procedures used by some other workers are reviewed.

S.P. Talim

Papers

A Comparison of Thin Film Measurement by Guided Waves, Ellipsometry and Reflectometry

Measurement of the Refractive Index of a Prism by a Critical Angle Method

The Preparation of Optical Surfaces for Thin Film Deposition