Colin Lewis

The refractive index correction in luminescence spectroscopy

Abstract It is shown that the so-called refractive index or “ n 2 ” correction used when calculating luminescence quantum efficiencies is generally inappropriate. For accurate evaluations no refractive index correction is required provided that a simple modification to luminescence spectrometers is carried out.

Surface and bulk absorption in germanium at 10.6 μm

The surface and bulk optical absorption of germanium as a function of resistivity and surface condition has been measured at 10.6 microm using adiabatic laser calorimetry. It was found that with conventional optical polishing {Linde, Al(2)O(3)), up to ~50% of the total absorption in a 1-cm thick sample could be attributed to the surfaces. Subsequent polishing with colloidal silica (Syton) reduced this surface contribution to <10% of the total. The remaining surface absorption was not due to volatile impurities which could be removed by evacuation. The optimum resistivity range for germanium having the lowest bulk absorption coefficients at room temperature was found to be 10-40 ?-cm. It was also found that surface absorptions from germanium samples prepared by single-point diamond machining could be lower than those obtained after polishing with colloidal silica, although in many cases they were considerably higher.

Reply to the comment on the refractive index correction in luminescence spectroscopy

Abstract The need for a rigorous analysis in order to derive any correction factors required when measuring luminescence quantum yields is acknowledged, as is the defect in our previous method. An outline of our new full model is given and the preliminary results obtained compared with those of Ediger et al.

Interferometric Measurement of Refractive Index Variations in Infra-red Transmitting Materials at 10·6 μm

A procedure has been developed for determining the gross refractive index variations in infra-red transmitting materials. The technique uses an infra-red interferometer operating at 10·6 μm, with a pyroelectric vidicon (or any other suitable T.V. compatible) fringe visualization device, interfaced to a digital T.V. frame store. Representative examples of the results obtained are shown.

Optical Requirements For Thermal Imaging Lenses

Previously detected deviations observed when measuring the transverse ray aberrations of polycrystalline germanium lenses have been shown to be due to localised refractive index changes at the crystal grain boundaries (striae). It is suggested that these changes are due to stress introduced into the material during manufacture rather than being due to the compositional alterations which are responsible for enhanced optical absorption in these regions. These localised variations in index have been shown to adversely affect the optical performance of lenses constructed from small grain size polycrystalline germanium.

Optical Evaluation Of Thermal Imaging Lenses

The reasons why thermal imaging lenses often fail to achieve their design performance specifications are discussed. Examples of lenses having centering errors and figuring errors are shown. The effects of using material with refractive index inhomogeneities are described. Methods for reducing the discrepancies between the design and actual performances in thermal imaging lenses are briefly outlined.

Transmittance Measurements Of Thermal Imaging Lenses

Instruments for measuring the full aperture transmittance of thermal imaging lenses operating in the 8 to 14 micrometre spectral region are described, and examples of typical results obtained shown. The causes of discrepancies between the actual transmittances measured and those predicted from a combination of the anti-reflection coating performances expected and the material transmittances are discussed.

Lens Defects And Their Effect On Thermal Imager Performance

The effects of reducing the transmittance and MTF of the optics in a thermal imager on the subjective performance have been shown, as has an instrument for measuring the full aperture transmittance spectra of IR lenses. MTF degradations for large (ca. 2510mm diameter) monocrystalline germanium blanks have been described.

Comparison Of Polychromatic And Monochromatic Lens Assessment In The Far Infrared

Techniques designed for assessing the performance of lenses operating in the 8-13 micron band are described. These include interferometry and pupil scanning, both using monochromatic radiation from a CO2 laser source. In addition a nominally polychromatic line spread function apparatus is shown. Results are presented of MTF using these techniques, along with those from a commercially available polychromatic grating scanning instrument. Measurements have been made on standard audit and operational lenses. Comparisons are made and differences attributable to radiation source emphasised.