GB861724A - Method and apparatus for facsimile telegnosis - Google Patents

Abstract

861,724. Copying telegraphy. TIMES FACSIMILE CORPORATION July 4, 1958, No. 21500/58. Class 40(3) Apparatus for the reproduction of a picture having a high degree of contrast, such as an X-ray film, employing a light spot moved in a scanning pattern over the picture to scan it in successive elemental areas to derive electrical signals representative of the shades of the areas includes an arrangement to vary the intensity of the light spot automatically and concurrently with the degree of shade of the area being scanned in such a way that the range of the output signals is reduced whilst maintaining a substantially linear relation between incremental changes in shade density and incremental changes in the said output signals. A transparent drum 10 carrying the picture 11 is driven through gearing 16 from a motor 15 and carries a half nut 18 engaging a threaded shaft 12 to traverse the drum 10 relative to a scanning head comprising an encased photo tube 23 and a light source 20 in a casing 19 with an apertured face 24. The picture signals are passed to a bridge modulator 25 fed with an audio frequency carrier from a source 26, and the modulated carrier is fed back through an amplifier 41, demodulator 42 and filter 43 to an amplifier 44 by which the intensity of the lamp 20 is varied inversely with respect to the input to the amplifier 44. The falling characteristic impressed on the lamp 20 is arranged to be linear as indicated by the line A, Fig. 2, and by the arrangement shown the range of output current passed over the output lines 28 to the receiver station 29 in respect of a high density D5 and a low density D1, corresponding respectively to nearly opaque and almost transparent areas of the original, is reduced from a-x to a-b. The picture is reproduced by a modulated discharge lamp 32 in association with a sensitive sheet 34 on a drum 35 moved synchronously with the transmitter drum 10.