Patent GB-1098382-A (GB)
Title: Microdensitometer
IPC: G01N21/59
Filed: 1965-05-24
Abstract: 1,098,382. Local copying by scanning. TECHNICAL OPERATIONS Inc. May 24, 1965 [June 3, 1964], No. 21994/65. Heading H4F. [Also in Division G1] A photographic record is analysed by pointby-point photo-electric scanning and is reproduced in a modified form by a pen recorder which is controlled in response to the photoelectric signals so as to effect marking in the form of spaces, dots and lines which are related to equal density steps in the photographic record in a repeating sequence as illustrated in Figs. 3B and 3C, which shows a typical density variation along a scanned line on the record. In the reproduction, Figs. 4 and 5A (not shown), small differences in density and areas of equal density in the original record are made readily apparent. Furthermore, the sequence of marking along a line, i.e. line, space, dots or line, dots, space, indicates whether the density is increasing or decreasing. The arrangement is said to be of value in the analysis of photographs of X-ray, biological and astronomical subjects. In apparatus for carrying out the invention, Fig. 1, a photograph holder 12À1 and a recording table 20À1 are linked together for scanning in one direction by a pantograph arrangement 16À1, which permits adjustable magnification, scanning in the orthogonal direction being effected by synchronized motors 14.2 and 76À1 which move the holder 12.1 and a recording pen 82. The scanning operation is controlled so as to proceed in a series of parallel lines. Light from a source 24 passes to a photomultiplier tube 36 over a first path 30.1 through the photograph record and over a second 30.2 through a variable density wedge 68. The photomultiplier is exposed to light from the two paths alternately by a shutter 34.1 to produce pulse signals which are compared in a circuit 42. The circuit controls a motor 70.1 which positions wedge 68 by means of belt 66 until the light intensities in the two paths, and hense also the pulse signals, are equal. Belt 66 also rotates a code wheel 62 which is thereby positioned continuously in accordance with the photograph density as it is analysed point-bypoint during scanning. Wedge 68 has a density distribution according to the H and D curve of the photographic material and compensates for the non-linear density relationship in the photograph. Code wheel 62 is provided with three different patterns arranged in a regularly repeating sequence around the wheel and forms part of switch system whereby signals are applied to a circuit 56 which actuates the recording pen 82. In response to the respective patterns the circuit causes the marking of spaces, dots or lines. Thus, as the analysed density varies and the code wheel rotates, markings are produced which are related to density in the manner shown in Fig. 3B. Density variations which appear in the photograph due to vignetting in the camera lens are compensated by rotating part of the switch system with respect to code wheel 62 by means of a motor 58. Current for the motor is obtained from a contact 48.1 which is attached to the recording table and moves over a series of concentric conductors 52 connected to resistors 50 which are preset in accordance with the compensation required.