A computer-controlled spectrometer has been built for measuring high-reflectance mirrors in the visible region (380 800 nm). The instrument is designed to scan high-reflectance mirrors (greater than 50%) on a surface up to 32 x 50 in. Plots of complete reflectance scans are available in minutes and are used for process control. The reflectometer can be set to any desired incidence angle in the range of 15 to 60° and four different polarization modes can be selected: P, S, P&S, and No Polarization.The instrument incorporates several techniques in order to perform as an absolute system with an overall accuracy of better than 1% . The optical system features double-bounce reflection, a special integrating-sphere light source, a double-stage monochromator, and a high-uniformity, 9-stage, side-on photomultiplier tube. This paper will focus on the electronics, controls and computer system used in the reflectometer.
Multilayer-overcoated, high-reflector mirrors in sizes up to 30" x 50" areproduced on a continuous coating line. A fully-automatic, highly-accurate scanning reflectometer has been built as part of the process-control system for the continuous coating line. The reflectometer covers a spectral range of 380 to 800 nm and a range of incidence angles of 15° to 6O. The s-and the p-polariation components of reflectance are scanned separately.The reflectance measurements are absolute and are not referenced to a calibrated standard reflector. The absolute measurement accuracy is This paper describes the optical system used in the reflectometer.
REFLECTANCE MEASUREMENTS. GENERALReflectance measurements are frequently made by a substitution method where the reflectance off the test part is compared with the reflectance off a calibrated standard reflector, as shown in Fig. la. The standard reflector must be calibrated for all wavelengths, polarizations and angles of incidence where measurements are to be made. Such a calibrated standard reflector is difficult to obtain and even more difficult to maintain.Absolute reflectance measurements can be made without the use of a calibrated standard reflector, as shown in Fig. lb. A 100% reference reading is obtained by placing the detector in line with the beam from the light source. The reflectance measurement is made by inserting the test part in the beam and moving the detector to a position where it receives the beam reflected off the test part. The reflectance is found by comparing the intensity of the reflected beam to the direct (100%) beam.There are problems with this method:. It is necessary to establish a separate stable reference lightpath between the source and the detector (not shown) to obtain the necessary long-term stability of the instrument. It is not possible to do this when the detector has to move between two positions.. The beam that reaches the detector after reflectance off the test part is not the same as the beam directly from the source; it is its mirror image. This can cause errors when the beam intensity and the detector sensitivity are not completely uniform.Another absolute reflectance measurement method that overcomes these problems is known as the "v-w,, method shown in Fig. 2. The 100% reference path between the source and the detector without the test part forms a V. The test part is inserted in the beam, the retro-mirror is moved to position 2, the detector is fixed in position.
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