Symmetry X system and method for absolute measurements of reflectance and transmittance of specular samples

A symmetry X system that has been constructed for the absolute measurements of reflectance and transmittance of specular samples in the infrared region is described. The system has been designed so that it can be incorporated into commercial Fourier-transform infrared spectrometers. Although ten mirrors were used in this system, it is disclosed that the geometric mean of two reflectance values is independent of the reflectance difference of the individual mirrors and the optical loss at each mirror. This system achieves spectral measurements with high accuracy and within a short period of time. In particular, the system affords us the self-diagnostic ability for measured spectra, and the simultaneous measurements of reflectance and transmittance under the same geometry enable us to evaluate measurement uncertainties. Although the symmetry X system is used for infrared spectral measurements, the measurement method, design principles, and features are generally applicable to other wavelengths as well.     

Sample holder and methodology for measuring the reflectance and transmittance of narrow-leaf samples

Measuring the reflectance and transmittance of narrow samples can be difficult, as the width of the illuminating beam may be greater than the width of the sample. The small sample area can also compound the already time-consuming process of reconfiguring the instrument between reflectance and transmittance measurements by introducing additional alignment problems. A method of measuring the reflectance and transmittance properties of narrow-leaf samples using reflectance configurations only is developed and tested. The method uses a mask and mask correction and relationships between reflectance measurements against contrasting backgrounds to determine sample reflectance and transmittance. The design of the accompanying sample-holding apparatus is also described. In testing, the mean error was less than 1% reflectance/transmittance, and standard deviation of the error was approximately 1% reflectance and 2% transmittance as compared to samples measured using conventional measurement configurations.     

Optical transmittance measurement system for coated elements with low transmittance

The low-transmittance elements required for a high power laser facility have stringent specifications about transmittance. To validate optic performance against specifications, a metrology system for transmittance is proposed. The system is composed of a laser source, an integrating sphere, a power meter, and four uncoated wedged glasses. A relative measurement method is adopted, which is that the surface reflectivity of uncoated glass is used as the reference to compare with the sample's transmittance. The systematic feature is that uncoated wedged glasses are applied to split and reflect beams, which not only avoid coating errors, but also make the two beam powers attenuate properly. Measurement results for a sample's transmittance are presented. Experiment and analysis show that the relative standard uncertainty (σ(T)/T) of measurement is 0.424%. This system is available for large-aperture elements.     

Simple high-precision method for measuring the specular reflectance of optical components

We present a simple method to determine precisely the specular reflectance of optical components. The absence of transmissive elements in this method makes a wide spectral range available. High accuracy and precision are achieved with a fast, periodic change between the reference beam and the probe beam. Special efforts were made to eliminate inhomogeneities of beam intensity and detector sensitivity. With our experimental setup we obtain a precision of ±3 × 10(-4) at the wavelength of 10.6 μm and ±3 × 10(-5) at 1.06 μm for a single-bounce-measuring setup.     

Spectrophotometer for measuring spectral reflectance and transmittance

The development of a computer-controlled spectrophotometer that enables measurement of spectral transmittance, reflectance, and optical loss of thin-film specimens is discussed. We also describe the design and testing procedure of the spectrophotometer, incorporating test sample performance data. In the visible region the overall photometric accuracy is verified to be 0.1% and 0.2% for transmittance and reflectance, respectively. The wavelength scale is accurate to within 0.5 nm with a reproducibility of 0.1 nm.     

Optical monitoring of thin film properties using in-situ measurement of transmittance and reflectance

Having a good reproducibility and uniformity of the coating properties is a mutual challenge for all coating processes. To face this challenge adequately, it is not only necessary to have accurate control of the coating process but also to have the capability to monitor the optical properties of the coating layers during or directly after deposition. Especially in sensitive multi-layer products produced by large area coating technology, small uniformity variations may give rise to a variation in the visual appearance or other deviations from the product requirements. It becomes necessary to monitor the individual layer thicknesses, requiring sensitive and accurate optical measurement techniques that offer nanoscale precision over large areas. This demand for sensitivity and accuracy puts a strain on the limits of existing in-line measurement capability. The objective of this paper is to discuss some of the measurement problems and give practical solutions to improve accuracy and repeatability of in-situ transmittance and reflectance measurements for optical monitoring of thin film properties.     

Dedicated spectrophotometer for localized transmittance and reflectance measurements

A dedicated spectrophotometer is built to achieve localized transmittance and reflectance measurements. The spatial resolution can be chosen from 100 microm to 2 mm, the spectral resolution from 0.5 to 5 nm, and the spectral range from 400 to 1700 nm. This apparatus can be used to study the index and thickness uniformity on single layers to determine and optimize the characteristics of the deposition chamber. It can also be used to measure the spatial variations of optical properties of intended nonuniform coatings such as linear variable filters.     

Real-time method for fitting time-resolved reflectance and transmittance measurements with a monte carlo model

An efficient method is proposed for the evaluation of theabsorption and the transport scattering coefficients from atime-resolved reflectance or transmittance distribution. Theprocedure is based on a library of Monte Carlo simulations and is fastenough to be used in a nonlinear fitting algorithm. Tests performedagainst both Monte Carlo simulations and experimental measurements ontissue phantoms show that the results are significantly better thanthose obtained by fitting the data with the diffusion approximation, especially for low values of the scattering coefficient. The methodrequires an a priori assumption on the value of theanisotropy factor g. Nonetheless, the transportscattering coefficient is rather independent of the exact knowledge ofthe g value within the range 0.7 < g < 0.9.     

Diffuse and specular reflectance from rough surfaces

We present a reflection model for isotropic rough surfaces that have both specular and diffuse components. The surface is assumed to have a normal distribution of heights. Parameters of the model are the surface roughness given by the rms slope, the albedo, and the balance between diffuse and specular reflection. The effect of roughness on diffuse reflection is taken into account, instead of our modeling this component as a constant Lambertian term. The model includes geometrical effects such as masking and shadowing. The model is compared with experimental data obtained from goniophotometric measurements on samples of tiles and bricks. The model fits well to samples with very different reflection properties. Measurements of the sample profiles performed with a laser profilometer to determine the rms slope show that the assumed surface model is realistic. The model could therefore be used in machine vision and computer graphics to approximate reflection characteristics of surfaces. It could also be used to predict the texture of surfaces as a function of illumination and viewing angles.     

Procedures and standards for accurate spectrophotometric measurements of specular reflectance

Procedures and standards that have been developed at the National Research Council of Canada for the accurate measurement of specular reflectance are discussed for both absolute and relative methods over the spectral range 250 to 2500 nm. There has been an increasing demand for these types of measurements, particularly for coated samples approaching the extremes of 0% reflectance and 100% reflectance. In some applications of these coatings, such as energy conservation and control, conventional methods of measuring specular reflectance give insufficient accuracies for the prediction of optical performance. Details of alignment procedures for both absolute and relative reflectance methods, the preparation and application of several candidate reflectance standards, and the compensation, attenuation, and verification procedures that have been developed to improve the precision and accuracy of specular reflectance measurements are described. Using these various techniques, one can routinely achieve accuracies of 0.3% reflectance at reflectance values as high as 97% and as low as 4%.     

Measuring the absolute absorptance of optical laser components

The precise determination of the absolute absorptance of a laser component is of high scientific and commercial importance. Our intention is to demonstrate that laser calorimetry can be a reliable and sensitive characterization tool for this purpose. Furthermore, the limitations of laser calorimetry are discussed and suggestions for possible revisions of the ISO 11551 (International Organization for Standardization, Geneva, Switzerland) standard are made. Finally, laser calorimetry is compared with photothermal deflection methods with respect to their practicability in different fields of laser optic characterization.     

High-accuracy measurements of the normal specular reflectance

The French Laser Megajoule (LMJ) is designed and constructed by the French Commissariat à l'Energie Atomique (CEA). Its amplifying section needs highly reflective multilayer mirrors for the flash lamps. To monitor and improve the coating process, the reflectors have to be characterized to high accuracy. The described spectrophotometer is designed to measure normal specular reflectance with high repeatability by using a small spot size of 100 mum. Results are compared with ellipsometric measurements. The instrument can also perform spatial characterization to detect coating nonuniformity.     

An apparatus for infrared transmittance and reflectance measurements at cryogenic temperatures

A facility for measuring the optical properties of solid materials at cryogenic temperatures is being developed at the National Institute of Standards and Technology. A cryostat that houses four ur bolometric detectors and a six-position sample holder was designed and built. The bolometers operate near 5 K, and the sample temperature can he varied from 6 to 100 K. The beam from a Fourier transform spectrometer is directed to the cryostat by reflective optical components. The measurable wavelengths extend from 1m to 1 mm, with appropriate sources and beamsplitters in the spectrometer as well as windows and detectors in the cryostat. The angle of incidence on the sample ranges from 7.5 to 60. The mechanical electrical, and optical designs are described in this paper. Initial measurement results at wavelengths from 2 to 30m and a sample temperature of 10 K are presented.     

Generalized matrix method for analysis of coherent and incoherent reflectance and transmittance of multilayer structures with rough surfaces, interfaces, and finite substrates

A generalized matrix method is presented for calculating the optical reflectance and transmittance of an arbitrary thin-solid-film multilayer structure on very thick substrates with rough surfaces and interfaces. We show that the effect of roughness and the influence of incoherently reflected light on the back side of a thick layer can be accounted for with a more general transfer matrix that enables the inclusion of modified complex Fresnel coefficients. Coherent, partially coherent, and incoherent multiply reflected light inside the multilayer structure is treated in the same way. We demonstrate the method by applying it to simulated and experimental reflectance spectra of thin epitaxial Si overlayers on very thick SiO(2) substrates and on a separation by ion implantation of oxygen structure with a SiO(2) buried layer exhibiting substantial roughness on both of its interfaces (Si/SiO(2) and SiO(2)/Si).     

High-accuracy spectrometer for measurement of regular spectral transmittance

A high-accuracy spectrometer has been developed for measuring regular spectral transmittance. The spectrometer is an automated, single-beam instrument that is based on a grating monochromator, reflecting optics, and an averaging sphere detector unit with a silicon photodiode. The uncertainties related to wavelength calibration, detector nonlinearity, system instability, beam displacement, polarization, stray light, interreflections, and beam uniformity are determined for the visible spectral range from 380 to 780 nm. A total uncertainty of 3 × 10(-4) (1σ) is estimated for transmittance measurements of homogeneous neutral-density filters. The uncertainty of the wavelength scale is 0.06 nm. As a specific application, calibration of V(λ)-correction filters is studied. To verify the accuracy of the transmittance measurements, a comparison of the measured and predicted transmittances of a sample of high-purity fused silica is made, revealing agreement at the 5 × 10(-4) level.     

Bidirectional reflectance distribution function of specular surfaces with hemispherical pits

We derive the bidirectional reflectance distribution function for a class of opaque surfaces that are rough on a macroscale and smooth on a microscale. We model this type of surface as a distribution of spherical mirrors. Since our study concerns geometrical optics, it is only the aperture of the concavities that is relevant, not the dimension. The three-dimensional problem is effectively transformed into a much simpler two-dimensional one involving the possibly infinitely many reflections in a spherical mirror. We find that these types of surface show very strong backscattering when the pits are deep but forward scattering when the pits are shallow. Such surfaces also show spectral effects as a result of multiple reflections and polarization effects that are due to the orientation of the effective surface. Both this model and the locally diffuse thoroughly pitted surface model [Int. J. Comput. Vision 31, 129 (1999)] are superior to other models in that they allow for an exact treatment for physically realizable surface geometries.