Jean M. Bennett

Precise Method for Measuring the Absolute Phase Change on Reflection

A method is described for measuring the absolute phase change on reflection of semitransparent films which is both precise and accurate. The films are deposited on portions of two fuzed-quartz optical flats and the shift in the fringes of equal chromatic order between the coated and uncoated portions of the interferometer is measured. Since the phase change is very sensitive to small changes in the optical constants, this method is useful for studying the effects of aging, applied electromagnetic fields, oxide growth, and other factors. Also, since areas of the order of 1.3 by 0.0033 mm are sufficient for each measurement, the phase change can be used to study possible variations in the film structure over the interferometer surface. Measured values of phase change on reflection versus wavelength are smooth to ±0.1°. When systematic errors have been taken into account, the measurements are still accurate to about ±1°.

Infrared Reflectance of Evaporated Aluminum Films

The specular reflectance of evaporated aluminum films has been measured with high precision (±0.1%) from 0.55 to 32 μ. The measurements were made at normal incidence on fresh and aged films deposited on fuzed quartz substrates with rapid evaporation rates at pressures of about 1×10−5 Torr. The dependence of the reflectance on the evaporation conditions was also investigated, and it was found that the infrared reflectance of aluminum prepared under the usual high-vacuum conditions is reproducible and insensitive to the conditions of film preparation. Although the measured reflectance of freshly evaporated aluminum films differs by only 0.1% at 30 μ from the value calculated from the free-electron approximation assuming bulk parameters for aluminum, the variation of the measured reflectance with wavelength does not fit the theory in the infrared. It is felt that the validity of the free-electron approximation should be further tested by using films deposited in ultra-high vacuum.

Interferometric Wavelength Measurement in the Infrared by the Method of Exact Orders.* Precision Measurement of the Index of Refraction of Air at 1.65μ

A new technique of interference wavelength measurement for absorption lines in the near infrared is described. The spectrometer used has a resolving power of 140 000 at 1.7μ. The air pressure in a Fabry Perot etalon is adjusted until a fringe coincides with an absorption line. This “tuning” can be made to 0.001 order of interference corresponding to a precision of relative frequency measurement of 0.0004 cm−1. The plates of the etalon have been coated with dielectric films giving a reflectivity of 70 percent and high contrast fringes. Smooth scanning of a small spectral region is accomplished by translation of a wedge prism. The method of exact orders has been used to show that there is a change in the apparent etalon spacer with wavelength amounting to 10−4A/A. The index of refraction of dry CO2 free air at 1.65μ has been measured giving the result (μ−1)s106=273.18+0.07−0.15.

Distribution of Infrared Radiance over a Clear Sky

Series of measurements of the distribution of radiance over a clear sky made from locations at approximately 1000, 6000, and 14 000 ft elevation are reported. A theoretical relation is shown to correctly predict the observed variation in radiance with elevation angle in the 2–40 μ wavelength region, where radiation from the sky is mainly due to thermal emission by the atmosphere. In the 0.6–2 μ region where the radiation is mainly scattered sunlight the distribution is related to that which was observed simultaneously in the visible but forward scattering is relatively more important in the near infrared and the relative intensities in the two regions are not constant.

Comparator with Photoelectric Detection for Setting on Broad Interference Fringes with Precision

A comparator with a photoelectric detection system is described. Experimental data are reported to demonstrate that the same setting precision of ±0.2 micron is associated with the measured position of a normally exposed two-beam interference fringe, an overexposed two-beam interference fringe, a multiple-beam interference fringe, and a spectral line. The corresponding setting precisions obtained by the usual visual techniques are ±6.2, ±3.0, ±1.1, and ±0.6 microns, respectively.

Measurement of Interferometric Secondary Wavelength Standards in the Near Infrared

High-precision absolute interferometric wavelength determinations in the lead sulfide region of the infrared are reported. Very thin silver films were used to coat the etalon plates. In this way sufficiently high reflectivity in the visible region for high precision measurements without prohibitively high absorption in the infrared was obtained. In addition the correction due to phase penetration in these films is relatively small. The “Stadia Method,” which was used for all measurements, made it possible to achieve a precision in wavelength measurements using energy integrating detectors comparable to that which may be obtained photographically in the visible using high-reflectivity films. In addition a technique was developed which made it possible to measure absorption lines interferometrically relative to an emission line.Three infrared wavelengths of mercury-198 were measured in emission and the wavelengths of P(16), P(10), P(1), R(7), R(14), and R(17) of the 002 band of HCN were measured in absorption relative to mercury-198 λ5460 A. The band origin ν0 of the 002 band was determined from these measurements using values of B′ and B″ previously reported from measurements on lines of high J number. The band constants were then recomputed using the new value of ν0. Final results are:Hg198 λ10 140 A9859.4342±0.002 cm−1Hg198 λ13 570 A7366.866±0.005 cm−1Hg198 λ15 295 A6535.8855±0.002 cm−1002 band of HCN ν0=6519.6148±0.003 cm-1B′=1.457063B″=1.478225.

A Simulated Channel Spectrum

In order to obtain the highest accuracy in interferometric wavelength measurements with energy integrating devices, the theory of the Fabry-Perot interferometer must be extended. In this paper the portion of the theory dealing with the integrated energy in the part of the Fabry-Perot ring system virtually intercepted by a slit of finite length is presented. We are assuming that the interferometer is placed in parallel light and an infinitely narrow band of wavelengths is used. For this situation, it is shown how the order at the center of the ring pattern corresponding to maximum and minimum energies respectively varies with slit height. The contrast is then determined as a function of slit height. The experimental methods for measuring these quantities are described and the results compared with theory. The theory is also extended to the case of an actual channel spectrum where a finite band of wavelengths is present.

Quantitative Method for Measuring the Resolution of a Large Grating

A method capable of giving quantitative information about the resolving power of large high-quality diffraction gratings has been developed. This method is based on the ability of the grating to resolve the perpendicular components of a normal Zeeman triplet when the central parallel component is removed. Since the splitting of these components is a known function of the field strength, one has in effect two lines whose separation can be varied at will, thus providing as stringent a test for the grating as is necessary. Suitable corrections are made for the effect of Doppler breadth in the source and for the finite width of the slit.An excellent plane grating has been obtained from Bausch & Lomb Optical Company. Its resolving power has been measured by this method under various conditions and is shown to be a constant fraction of the theoretical value.The effects of error of run on resolution and focus have been reviewed and investigated experimentally. Nonplaneness of the grating blank has been measured in order that the contribution of this defect to the focal curve can be calculated and separated from the shift of focus caused by linear error of run.

Method for Determining Comparator Screw Errors with Precision

A method is described for determining with precision the relative cumulative error and the periodic error of a comparator screw. Measured curves of these errors are shown for two comparators. The method also gives the relative lengths of the intervals on the scales used in the measurements.

Reflection, scattering, and polarization from a very rough black surface

In this paper we study the effects of depolarization and scattering from a rough surface over a large range of wavelengths, additional measurements are reported that have been made on a very rough black anodized aluminum sample.