Patent GB-731865-A (GB)
Title: Improvements in or relating to optical interference layers
IPC: G02B5/28
Filed: 1953-03-23
Abstract: 731,865. Coating by vapour deposition. TECHNICOLOR MOTION PICTURE CORPORATION. March 23, 1953 [April 25, 1952], No. 7920/53. Class 82 (2). [Also in Groups XX and XL (b)] In a method of applying interference coatings to optical apparatus, a control layer is simultaneously deposited on a monitor surface, photo-electric means being provided for continuously indicating the thickness of this surface so as to cease the coating operation when specimen thickness is such as to produce in the specimen required interference maxima and minimum for the incident light angle at which the apparatus is to be used. The Specification explains that where the shape of the specimen and the monitor plate are not the same the wavelengths at which interference occurs will differ. Hence the thickness of the monitor coating necessary to produce interference for a wavelength other than the monitoring wavelength and with the light incident at a different angle must be calculated and the Specification shows with reference to Fig. 6 how this is achieved. In the left-hand side of the Figure the interference pattern for the monitor is given, the thickness of the layer being plotted as ordinate with the arbitrary meter readings as abscissae, the wavelength of the control light being constant throughout the readings at # 0 . In the righthand side of the diagram are plotted against optical thickness as ordinate the wavelengths as abscissae at which interference of the various orders occurs. The lines at 45 and 90 degrees indicate the specimen and monitor respectively. Then, if it is desired to produce half-wave interference band on the specimen at a wavelength 500 an ordinate is drawn from abscissa 500 to point C, a horizontal line connects this point to point b on the monitor interference curve and point a represents the corresponding reading of the meter indicating monitor thickness. Construction of coating chamber (Fig. 15).- As shown, the coating materials which may be zinc sulphide and lead fluoride are placed in crucibles 61, 62 which are mounted on a plate 55 which may be so rotated by means of a lever 215 that either of the two crucibles may be situated beneath a central aperture (not shown). A detailed description of the crucible arrangement is contained in Specification 731,909. The crucibles are heated to vaporize the coating material by means of filaments suspended within them. Between the crucibles and the specimens to be coated is situated a rotating attenuator disc the shape of which is shown in Fig. 18 and which has the effect of making the deposit of uniform thickness. The attenuator is rotated by a motor outside the apparatus and a driven supporting roller coupled through the casing by means of a magnetic clutch. The specimen S is supported in a holder 270 (see below) in a plate A. The plate also carries the monitor plate M (Fig. 16) which is supported beneath plate A by a bearing 263 and is arranged to be rotatable by a knob 126 outside the apparatus and a magnetic clutch 124. Thus, the separate layers to be coated on the specimen may be applied to different parts of the monitor plate enabling each layer to be controlled separately. The apparatus is enclosed in a casing 51 which is exhausted and finally scavenged by means of a spark discharge between the specimen supporting plate A and the mask support 251 which is connected to the outside of the housing through terminals 219. The specimen support ((Fig. 21) consists of a plate 283 pivotally mounted by pins not shown at each end, the specimen being held in place by clamping pieces 285. To permit adjustment of the angular position of the specimen a slotted lug carried by the pivoted base engages a thumb screw (not shown) and the specimen is clamped in position by means of set-screws sr, sb. This enables wedge-shaped coatings to be deposited. The theory of these coatings is discussed in the Specification. Photo-electric thickness monitoring apparatus (Fig. 13).-Collimated light from a source 130 passes through a rotating chopper plate 141 and is reflected by a mirror 134 coated with aluminium strips on to the monitor plate M. The beam reflected from the plate passes through the mirror and is received on a photo-electric cell 135. A second beam of light is reflected by mirror 131 passed through an adjustable optical wedge and the chopping plate and is finally received on photo-cell 135, the arrangement of apertures in the chopper plate being such that one beam is broken while the other is transmitted. The output of the photo-cell is fed to an amplifier which in turn feeds an indicator or a computer which translates photocell output into layer thickness. The output of the amplifier is measured in a null circuit, the meter being arranged to read zero when the beams are initially balanced in the absence of a coating. A high index coating is then applied and the instrument calibrated. Specification 731,910 also is referred to.