It is important to measure both reflectance (R ) and transmittance (T ) with the same accuracy. But many commercial accessories are exchanged by themselves or a sample is replaced on the other position in one accessory, when the reflection measurement is changed from the transmission measurement, so that it is impossible to measure reflectance and transmittance with the same accuracy. Accordingly the absorptance (A=1-R-T ) of the sample is not a sufficient index to evaluate the optical properties. A new scatterometer, which overcomes the defect, has been developed in AIST. It consists mainly of two ellipsoidal mirrors and a new detection system, which is composed of a hemispherical lens, a fiber optic taper and a CCD camera. These mirrors are a belt-shape and a quarter ellipsoidal mirrors with two focal points and are combined such that each focal point is a common focal point, on which the sample is placed. A rotating mirror is set on a remaining focal point of the belt-shape mirror. Each arrangement, where the rotating mirror looks at the upper or lower arm of the belt-shape mirror, is for the transmission or reflection measurement, respectively. The center of the hemispherical lens in the detection system is set on a remaining focal point of the quarter mirror, the incident plane of the fiber optic taper coincides with the image plane of the hemispherical lens and the outgoing plane of the fiber optic taper is in contact with the CCD camera. A clear image can be obtained using this detection system. The absolute values of the reflectance and transmittance and the light distributions of the reflection and transmission of the micro-ball-lenses, whose radii were 0.75, 1, 2.5 and 4.8mm, were measured. The systematic errors of our scatterometer are briefly discussed.
Five methods for the measurement of absolute reflectance are described and compared. Four of the methods, the V-W, V-N, integrating sphere and goniometer methods, have been in use for a number of years. The fifth is a new STAR GEM method. The acronym of STAR GEM is from the capital letters of Scatter, Transmission, and Absolute Reflection measurements using a Geminated Ellipsoid Mirror. Only the goniometer and STAR GEM methods can be used to measure reflectance and transmittance at almost any angle of incidence. The STAR GEM is used in conjunction with an FTIR (Fourier-Transform Infrared) spectrophotometer and also with a grating spectrophotometer to make reflectance and transmittance measurements in the wavelength region from 0.24&mgr;m to 25&mgr;m. Ordinate errors of the FTIR spectrophotometer are estimated from measurements in the overlapping wavelength region made by both spectrophotometers. A reflectance measurement of a non-plane surface, such as a surface of micro-ball lenses, can also be made using the STAR GEM.
A STAR GEM as a scatterometer can measure diffuse reflection spectra. The acronym of STAR GEM is from the capital
letters of Scatter, Transmission, and Absolute Reflection measurements using a Geminated Ellipsoid Mirror. A biconical
accessory, such as the STAR GEM, has the advantage that it has very high collection efficiency and the ability to
measure scattered reflected light from very small samples. However, it is generally thought of as a qualitative device. It
becomes clear that the STAR GEM is superior to a goniometer on the study to measure absolute reflectance of a specular
sample. Only the goniometer and its family can quantitatively measure the bidirectional reflectance distribution function
(BRDF) of a sample. The purpose of this paper is to describe the possibilities and problems for the STAR GEM to
measure the BRDF of a sample.
A huge number of optical spectra have been measured in the ultraviolet, violet, visible, infrared and far-infrared regions. However transmittance and reflectance can only be measured by the use of a spectrometer, it is very difficult to measure the transmittance and reflectance with the same accuracy. Our STAR GEM (Scatter, Transmission and Absolute Reflection using a Geminated Ellipsoid Mirror) is the first realization not only to overcome the difficulty but also to make the absolute measurements of transmittance and reflectance at any incident angle. The STAR GEM is a new optical accessory and is designed so that it can be incorporated into commercial Fourier-transform infrared spectrometers. Although the STAR GEM is used for infrared spectral measurements, the measurement methods, design principles, and features are generally applicable to other wavelengths as well.
An adiabatic laser calorimeter has been developed with a sensitivity of the order of 10-6cm-1 with one watt of laser power using a CO2 laser (9(mu) m to 11(mu) m) in the infrared region. The heat leak by conduction and by radiation from a sample to an inner isothermal enclosure is enough small to be ignored because we succeeded in developing the temperature tracking system between the sample and the enclosure. The total uncertainty of absorption-coefficient measurements is estimated to be 5.4%. The absorption coefficient of a potassium chloride sample #2 was (3.17+/- 0.18)X10-3cm-1.
A laser calorimeter has been developed with a sensitivity of the order of 10-6cm-1 with one watt of laser power using a CO2 laser (9(mu) m to 11(mu) m) in the infrared region. In order to evaluate the accuracy of the laser calorimeter, the absorptance of a thin and semitransparent silicon sample was measured using the laser calorimeter in the infrared region and the transmittance and the reflectance of the same sample were also measured by a conventional Fourier-transform infrared spectrometer. Two absorptances obtained from two independent methods agree will within 2%, however it becomes clear that an interval of a Fabry-Perot interference fringe doesn't agree in absorptance spectra measured by these methods.
The temperature dependence of the absorption of the thick niobium films was measured using an AC far-infrared laser calorimeter. Moreover the temperature and frequency dependences of the absolute transmission and reflection of same thin niobium nitride films were measured from 5 to 20 K and from 10 to 200 cm-1 using a fourier transform infrared spectrometer. The temperature dependencies of the skin depth and the absorptance determined independently by both methods are compared. However the skin depths and the absorptances agree well each other near and above Tc, they begin to deviate from each other with decreasing temperature below Tc.
This work surveys techniques to measure the absorption coefficient of low absorption materials. A laser calorimeter is being developed with a sensitivity goal of (1 +/- 0.2)X 10-5 cm-1 with one watt of laser power using a CO2 laser (9 (mu) m to 11 (mu) m), a CO laser (5 (mu) m to 8 (mu) m), a He-Ne laser (3.39 (mu) m), and a pumped OPO tunable laser (2 (mu) m to 4 (mu) m) in the infrared region. Much attention has been given to the requirements for high sensitivity and to sources of systematic error including stray light. Our laser calorimeter is capable of absolute electrical calibration. Preliminary results for the absorption coefficient of highly transparent potassium chloride (KCl) samples are reported.
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