A novel birefringence measurement with double rotating polarization elements is proposed. This system includes rotating components of linear polarizer and quarter-wave plate with different rotating speeds. The birefringence is calculated by analyzing the Fourier components for rotation frequency in a detected signal. In this paper, the basic principle and experimental results of the birefringence measurement are described.

Optical lithography continues its transition to shorter wavelengths to support the semiconductor industry’s production of faster microchips to meet evolving market demands. The next step for optical lithography is likely to use the F₂ excimer laser at 157.63 nm (157 nm,according to the industry’ s naming convention).At 157 nm, among the limited number of fluoride crystals with acceptable optical properties calcium fluoride is the only practical lens material for step and scan systems due to its readiness for mass production. Since the discovery of intrinsic birefringence in CaF₂ at deep ultraviolet (DUV)wavelengths,the optical lithography industry has developed a critical interest in measuring birefringence at 157 nm. In response to this need, we have developed a DUV birefringence measurement system. In this article,we describe the working principle, system construction, technical performance and selected applications for measuring lithography grade calcium fluoride lens blanks at DUV wavelengths.

We report on an all-optical method to determine the photo-elastic constants of optical materials. The two-laser technique is based on a combination of the acousto-optic effect and the laser generation of ultrasound. The laser generated ultrasonic wave produces a Raman-Nath type diffraction. The diffraction effciency is proportional to the photo-elastic constants of the material investigated. The constants are determined by careful analysis of the diffraction signals obtained from a stack comprising an unknown material and a reference material.

To meet demands for increased traceability of fluorescence measurement a new two-monochromator reference spectrofluorimeter has been designed and built at NPL. This new instrument is capable of measuring the radiance factor and associated colorimetry of solid and liquid fluorescent materials at virtually any angle to the incident light beam. A 1 kW xenon light source and the use of reflective optics enable the device to make bispectral measurements from the ultraviolet to the near infrared. This paper describes the optical arrangement of the new system..

Traditional spectrophotometry, including regular and diffuse scattering measurements of reflected and transmitted light, employs lamp-based sources of radiation which are spectrally filtered by a spectrometer or by individual optical filters. Lasers offer distinct advantages to the traditional sources, most notably high spectral power and the possibility of very small irradiated area. NPL has established a new laser-based facility for reflectance and transmittance measurements which utilises NPL’s National Laser Radiation Facility, a suite of lasers covering a broad spectral range from the UV to the IR. This paper will highlight the capabilities of this new facility, discuss several of its unique design features, and present an analysis of the factors which influence its ultimate accuracy.

Precise in-situ transmission testing is necessary for evaluation of materials to be used in lithography systems, which use light source at the wavelength of 157 nm. Fluorine (F2) excimer lasers have pulse-to-pulse energy variation (< 9 %, 3 sigma, from manufacturer's specification), and pulse energy monitoring is required for precise evaluation. Due to the uncontrolled fluctuations of the polarization and high pulse energy of the laser, it is difficult to monitor the laser pulse energy and to achieve precision measurement. We have built a precise in-situ transmittance measurement system, employing polarization insensitive beam splitters for precision pulse energy monitoring. The beam splitter consists of three parallel plates. This scheme eliminates the effects of polarization fluctuation and decreases the energy to the detector. We have obtained 0.1 % (3 sigma) stability in our transmission measurement using this assembly.

Integrating spheres are appropriate tools for the measurement of the near-normal/hemispherical reflectance and transmittance. The sphere walls have to be highly reflecting and should have a Lambertian BRDF in the interesting wavelength range. Diffusely reflecting gold-coatings are used in the mid infrared, which have a suitable surface roughness. Their surface properties influence the scattering behavior wavelength-dependent resulting usually in decreasing total integrating scattering with increasing wavelength and changing BRDF. We investigated two different spheres with the same dimension (200mm diameter) and design, but with different coating, which were attached to a Fourier-transform spectrometer (Bruker IFS 66) for measurements between 1.7μm and 17μm. The surfaces of the spheres were investigated by Scanning Electron Microscopy and angular-dependent reflectance measurements in the relevant spectral range. Samples with different surface roughness, but with the same coating were measured with both spheres. The correction of the results using formula derived by Hanssen yielded a very good agreement of the spectral reflectance.

Incomplete hemispherical irradiation of the sample (or collection of radiation, in the reverse geometry) in an integrating sphere reflectometer is unavoidable due to sample viewing (or illumination) requirements. This means that one measures with nearly hemispherical irradiation and obtains a quantity that is not identical to the hemispherical/directional (or directional/hemispherical) reflectance factor (HDR) of the sample. The assumption that this quantity is identical to the HDR can lead to error in the measurement result, which is not generally corrected for. The error, a.k.a. "port loss error", is minimal for comparative measurements of samples with similar reflective properties, but can easily amount to several percent for a diffuse vs. specular comparison and could be even higher for samples that exhibit strong forward scattering. Few papers in the literature have dealt with this issue, perhaps due to the existence of other larger sources of error. Our approach to this problem has three elements: (1) establish a definition and set of equations that quantify the error due to incomplete irradiation of the sample due to the presence of the viewing port ("port loss" uncertainty); (2) develop algorithms and computer models to predict port loss effects for a specular-diffuse sphere coating and specular-diffuse (or more general bi-directional reflectance distribution function (BRDF)) sample; (3) design a technique and instrumentation to allow routine direct measurements of port loss to correct the error; using reference standards such as an Al mirror and a polytetraflouroethylene (PTFE).

Hemispherical Directional Reflectometer (HDR) measurements provide broadband IR data for oblique polarized reflection as well as normal-incidence transmission. Tests on thin polymer films in low-loss wavelength ranges typically show fringes conforming to Fresnel reflection/transmission. Hence, HDR experiments are a promising approach to determining optical constants for organic materials. The same experiments also quantify operational features of the HDR for applications to inorganic materials. Exploration of the HDR as a device for determining the optical constants of thin films shows that accuracy of data-reduction algorithms for n and k can be improved by simulating a feature that arises from limits on angular resolution achievable in the HDR configuration. In particular, an overhead mirror used to collect IR radiation scattered from the sample film subtends a non-negligible angle. This effect causes measured reflection extrema to be "damped" relative to rigorous calculations assuming incidence at a discrete angle. The transmission and polarized reflection actually observed are well simulated by a simple algorithm that averages power scattering over an angular spread corresponding to the size of the mirror. An algorithm incorporating corrections for angular spread has been developed to determine film thickness as well as optical constants on low-loss ranges, and has been validated by application to a polyethylene film.

When a beam of light is incident on a translucent sample, a significant fraction of the light is scattered at high angles. Some of this scattered light may be trapped inside the substrate through multiple reflections and total internal reflection, similar to light coupling into an optical fiber. The trapping depends on factors such as the surface roughness of the external surfaces and/or the size and distribution of scattering particles inside the sample. The scattered light may thus escape out of the sample at a shifted position relative to the incident beam. This leads to port losses in an integrating sphere. The detected signal from the light entering the sphere then underestimates the hemispherical transmittance. In this paper the signal versus lateral position has been measured in an attempt to estimate the error and to find an extrapolation procedure for the correct transmittance value. The lateral measurements were carried out by moving a detector behind the sample, a procedure carried out at several angles of incidence. Different illumination methods have also been studied both theoretically and experimentally to further investigate what effect light trapping can have when characterising scattering samples.

Baffles are often placed in integrating spheres to accommodate the non-ideal aspects of other sphere components. These include detectors, sources, sphere wall surface shape and coatings. Baffles intentionally prevent light interchange between these and other important sphere components and regions such as entrance/exit ports, sample, reference and detector field-of-view. The challenge for an integrating sphere designer is to position and construct baffles that achieve the primary goal of shadowing specific elements from each other, while at the same time minimizing all other “side” effects that the baffles may have. Perhaps the most important side effect is the additional signal loss for light arriving at or leaving the sample from or to the baffle due to its absorptance. This is especially true for coatings and spectral ranges where the wall reflectance is relatively low such as for BaSO4 above 1.5 mm and diffuse gold. A potential improvement that we have investigated in an infrared reflectometer sphere is the use of a specular coating that has significantly higher reflectance than any other available diffuse coating. In our case we have used specular gold versus the diffuse gold-coated plasma-sprayed metal coating that is on the sphere wall. Although this provides for lower loss of light reflected from the sample onto the baffle, the side effects must also be considered and reduced in the design. Specifically one needs to consider the mirroring that will take place in the sphere. In this paper we discuss the important design issues along with some integrating sphere characterization results that demonstrate improved sphere performance by use of specular baffles.

The National Physical Laboratory (NPL) realises and disseminates the UK spectrometric scales from 200 nm to 56 μm wavelength. Its mid-infrared (MIR) regular and hemispherical reflectance and transmittance scales, and transfer standards for the wavenumber and ordinate calibration of MIR spectrometers are realised from 2.5 μm to 56 μm using specially modified grating spectrometers. This paper will discuss a technique, recently developed at NPL, for the direct absolute realization of the hemispherical reflectance scale. This scale was previously based on a relative method using an absolutely calibrated mirror and published data for BaSO₄. The new method has given an improved value for the pure BaSO₄ to underpin the relative method. It opens up new applications as it can be applied to types of sample not previously measurable e.g. foil-covered insulation, and this aspect is discussed. The various sources of uncertainties are considered and the existing standards and services are also described briefly to place the technique in context. Although developed on grating instruments, this new measurement capability can be realized on Fourier transform (FT) spectrometers. Progress is being made at NPL in transferring its other MIR measurement capabilities to FT instrumentation. This will safeguard NPL’s future ability to provide these services to customers.

United Kingdom scales for diffuse reflectance in the ultra-violet to near-infrared region are realized at National Physical Laboratory using goniometric techniques, while the mid-infrared scale is based on measurements with a hemisphere reflectometer. Both of these scales have changed recently. In the ultra-violet to near-infrared region this took place on adoption of the goniometrically realized scale which replaced the previous scale traceable to NRC and PTB, while in the mid-infrared the scale change was based on the development of a new direct absolute technique which allowed a re-determination of the diffuse reflectance of BaSO4. Dissemination of the UV/vis/NIR scale will still in the main rely on integrating sphere techniques, and this requires the use of reference standards calibrated using the goniometric technique. The new scale is found to be 0.4 - 0.5 % higher than the previous scale in the visible region, while harmonization with the mid-infrared at 2.5 μm has made good progress, thereby ensuring an independent and continuous scale from the ultra-violet to the far-infrared.

Through the use of measurements and analysis we have devised a series of physics-based analytic models for the BRDF that describe the differential polarization of light scattered from a random rough surface. These models incorporate both intrinsic (refractive index) and extrinsic (statistical moments of the surface height variations) properties of the surface as well as wavelength dependence. Detailed surface statistics are acquired with a stylus scanner. Physical optics theory relates these statistics (and thus the complex coherence factor of the fields at the surface) to the far-field intensity of the scattered light. An outcome of these models is the ability to predict the spectrally varying differential polarization of emittance. Excellent agreement between measured and modeled BRDF’s is demonstrated.

An algorithmic model of bi-directional reflectance distribution function (BRDF) based on the ray optics approximation and microfacet model of randomly rough surface is proposed. Its central idea is that for every incident ray, the normal vector to the surface undergoes a random perturbation, and the direction of specular reflection is calculated using this perturbed normal. Such behavior of the normal can be treated within the framework of a microfacet model of randomly rough surfaces. The algorithm allows one to model reflection from both isotropic and anisotropic surfaces, with two-dimensional Gaussian and other probability density functions for the normal vector perturbations, and various geometrical attenuation functions. The proposed "perturbed normal microfacet (PNMF) model" exhibits experimentally observed effects such as increased reflectance near grazing incidence and off-specular peaks, and allows fast importance sampling. A weighted sum of Lambertian and PNMF BRDFs can be fitted to experimental data by varying the appropriate parameters. Adherence to the reciprocity principle and energy conservation law is demonstrated via results of forward and backward ray tracing. The PNMF model can be used in Monte Carlo calculations of radiative heat exchange among rough surfaces, in realistic image synthesis, lighting engineering, for modeling of such radiometric devices as blackbody radiation sources, integrating spheres in the infrared spectral range, cavity detectors of radiation, diffusely reflected panels, etc.

BSDF and BRDF measurements of randomly rough surfaces are often limited to the plane of incidence. For a surface with no change in optical properties upon rotation in the plane of the sample, this is sufficient to completely represent the BRDF or BSDF of a material at a specific frequency. We apply a simple empirical model that accurately represents the full bi-directional dependence of the scatterance or reflectance based on this limited experimental data set. From these models the total integrated reflectance, total integrated scatterance, and emittance can be obtained. Example measurements of opaque painted flat surfaces, transparent samples, and fibers are presented.

Thermal barrier coatings are widely used in heat engines for improving efficiency by allowing higher operating temperatures; yttria stablised zirconia is the most widely used material. Their use has been extended to rotating parts, in particular to gas turbine engine blades, and any loss of coating would represent a major problem. During deposition of the coating, a thin (< 1μm) alumina layer grows due to oxidation of the bondcoat, and it is this alumina layer which promotes bonding between the coating and the coated substrate. The spectral shape and position of the R-line fluorescence of Cr³⁺ ions normally present in small amounts in the alumina is sensitive to stress, temperature and other environmental effects. Stress is the key factor determining spallation, and piezospectroscopy refers to the use of spectroscopic measurement to determine stress within a material. Measurements have been carried out as a function of various ageing treatments in order to evaluate the potential of the technique to be a non-destructive probe for determining the onset of spallation. Interpreting the changes in the fluorescence spectra requires the use of sophisticated curve-fitting techniques and therefore requires reliable and accurate measurements. This paper will discuss these measurement requirements and their potential for development into a non-destructive tool for lifetime prediction of these structures.

Polarized light imaging can be used to map the borders of skin cancer that are invisible to the human eye. We designed a hand-held polarized-light camera that is sensitive to the superficial layers of the skin where cancer often arises. The camera system consists of two 8-bit CCD mounted on two sides of a polarizing beam splitter. An imaging lens mounted on the beam splitter collects light reflected from the skin. The collected light is divided into two states of polarization: parallel (PAR) and perpendicular (PER) to the incident light orientation. A custom-code combines images streaming from both cameras to yield an image based on the polarization ratio (PAR - PER)/(PAR + PER), which is sensitive to the superficial skin layer. Experiments in reflectance mode on micro-spheres solutions were conducted to test the system; Monte Carlo simulations of these experiments demonstrated excellent correlation. Early clinical work was conducted at the Oregon Health and Science University in the Dermatology department. Images of several skin lesions are presented.