Page 117 - Chalcogenide Glasses for Infrared Optics
P. 117

Characterization of Glass Pr operties    95

                 Early infrared spectrophotometers were designed to be double-
              beam with one used for reference I  and the other for the sample I.
                                            0
              Optical paths were equal in length and intensity so that the detected
              output signals were always I/I . Some instruments used the ratio of
                                        0
              the infrared detected electric signals while others, termed optical null
              instruments, had a wedge or comb placed in the reference beam to
              balance the two signals during the scan while recording the I/I  ratio
                                                                  0
              continuously based on the position of the wedge or comb.
                 The transmission accuracy of commercial instruments under
              ideal conditions was usually considered 1 to 2 percent. For absorbing
              materials this may be fine. But for low-absorbing, transparent materials,
              it means thicker samples 2 cm or more are required for better accuracy.
              Unfortunately, a thick sample with a large refractive index increases
              the optical path in the sample beam, leading to loss in accuracy. The
              instruments in this generation were designed to work with organic
              compounds that were thin and had low refractive indexes.
                 The appearance of Fourier transform instruments was a great
              step forward. In this instrument, there is only one beam and it is poly-
              chromatic. The beam transmitted through the sample is made to
              interfere with itself optically by using a scan mirror producing a pattern
              that when analyzed mathematically (Fourier transforms) reveals the
              variation in transmitted energy as a function of wavelength. Multiple
              scans are used to increase signal-to-noise results. The outcome is
              compared to a previously recorded, no-sample same-number-of-scans
              reference outcome. The results are displayed in transmission or absorp-
              tion terms and printed out after desired additions to the display. Each
              scan takes only a few seconds. For poorly transmitting samples,
              increased signal-to-noise accuracy may require 50 to 100 scans and the
              results averaged, eliminating the influence of noise. The instruments
              are very versatile and useful, a real advance in the state of the art.
                 Fourier transform infrared (FTIR) instruments used at AMI are a
              Perkin Elmer Paragon 1000 and a Nicolet AVATAR 320 utilized in the
              production area. The wavelength range generally used is 2 to 14 µm
              although the scan range may be changed for slightly shorter wavelengths
              than 2 µm or longer than 14 µm, out to 20 µm. Figure 4.6 shows a Perkin
              Elmer FTIR transmission scan for an Amtir 1, 8-in-diameter 9-kg plate
              6 cm thick. The scan is used with the standard QC documentation pre-
              served for each plate produced. Notice the two narrow absorptions at 4.9
              and 4.5 µm due to dissolved H Se molecules in the glass that couple to
                                       2
              the Ge atom (4.9) and the As atom (4.5). The magnitude of absorption for
                                     −1
              the gas is low, 0.04 to 0.05 cm , of little consequence for lenses less than
              1 cm thick. The glass tested is considered low-absorbing and has flat
              parallel faces so that the full expression given below for transmission
              should be used to calculate absorption, assuming multiple reflections.
                                            2 -α
                                           )
                                   I   1 (  − Re  x
                               T =   =
                                           2 -
                                   I   1  − Re  2α x
                                    0
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