Hiroshi Shitomi

Development of a new integrating sphere with uniform reflectance for absolute diffuse reflectance measurements

The design and concept of a new integrating sphere for absolute diffuse reflectance measurement are presented, together with an evaluation of the reflectance non-uniformity. The new integrating sphere was designed with emphasis on uniform reflectance over the sphere wall. SpectralonTM was employed as the inner material for the sphere wall. The integrating sphere with geodesic design was developed for the most appropriate specifications to realize the high reflectance uniformity. The evaluation of the reflectance non-uniformity of the sphere wall in the visible region revealed that the geodesic integrating sphere had a quite uniform reflectance with non-uniformity of about 0.16% on the average.

A new absolute diffuse reflectance measurement in the near-IR region based on the modified double-sphere method

A new method using integrating spheres for absolute reflectance measurements has been developed to establish a spectral diffuse reflectance scale in the near-IR (NIR) region. This method is a modified version of the double-sphere method developed in 1966, and the modification reduces the uncertainty due to the area measurement of the auxiliary sphere. In this new technique, the modified double-sphere method, a known absolute reflectance scale in the visible region derived from another absolute reflectance measurement is used to extend the absolute reflectance scale from the visible to the NIR region without requiring a strict area measurement. By using this modified double-sphere method, a spectral diffuse reflectance scale is established from 800 nm to 1600 nm with a relative expanded uncertainty (k = 2) of 0.48% to 0.54% depending on the wavelength. The same principle can be applied to determine a spectral diffuse reflectance standard in the ultraviolet region as well. In this paper, an outline of the modified double-sphere method is described with emphasis on its theoretical and instrumental features.

Photoluminescence from white reference materials for spectral diffuse reflectance measurements upon exposure to radiation shorter than 400 nm

Photoluminescent properties of white reference materials commonly used as spectral diffuse reflectance standards were studied in the near-UV to visible region. Spectroscopic analysis based on a two-monochromator method revealed that most of white reference materials produced photoluminescence upon exposure not only to near-UV and UV radiation but also to visible radiation shorter than 400 nm. Remarkable differences in the photoluminescence spectrum and intensity were observed between different types of white materials, which would suggest that each material has a specific origin of photoluminescence. Although most of the photoluminescence observed in this study was slight except for some kinds of opal glasses and ceramic tiles, it would cause a relatively large error in spectral diffuse reflectance measurements, depending on the measurement conditions such as the spectral responsivity of detector systems.

Optically Controlled Alignment of Liquid Crystal on Polyimide Films Exposed to Undulator Radiation

Liquid crystal (LC) alignment properties on photo-exposed polyimide films were studied in the UV and VUV regions using linearly polarized synchrotron radiation from the Onuki-type polarizing undulator installed in the electron storage ring NIJI-II in ETL. It was found that the LC alignment uniformity depends on the UV and VUV exposure conditions, especially on the wavelength of the radiation. The photoabsorption spectrum of polyimide films was also measured in both UV and VUV regions. It was concluded that at least two mechanisms exist in the photo-induced alignment of LC molecules on the polyimide films.

Experimental validation of nonlinearity suppression for an inverse-layer-type silicon photodiode and its prediction based on theoretical modeling

The spectral nonlinearity of an inverse-layer-type silicon (Si) photodiode (PD) in the visible region was investigated. As expected by theoretical calculation, supralinearity and saturation of the Si PD, which are key factors of nonlinearity, were suppressed by applying a reverse voltage above 30 V. Experimentally observed nonlinear behavior depending on the reverse bias was compared to a theoretical model describing supralinearity, including the inner parameters of the Si PD, and these comparison results were in agreement. This theoretical model enables us to quantitatively predict the behavior of the supralinearity of the inverse-layer-type Si PD in various reverse bias conditions. Accurate experimental nonlinearity supported by theoretical predictions will contribute to high-accuracy optical measurement with the Si PD over a wide range of optical power levels and various reverse-bias voltages.