Patent GB-1447832-A (GB)
Title: Focus detecting apparatus
IPC: G02B27/10
Filed: 1974-07-08
Abstract: 1447832 Automatic focussing HONEYWELL Inc [9 July 1973] 30126/74 Heading H4D In an automatic focusing system, the image of the scene 9 being focused, which ideally is one having a strong contrast change boundary 11, is divided into two sections in dependence on which half of the focusing lens 16 is being used therefor, and the sections compared by respective parallel arrays of detectors 17, 19, the boundary dividing off the lens into the two halves being parallel with said contrast boundary. In the arrangement of Fig. 1, with the two boundaries being horizontal, the focusing lens 14 has a polarizing filter 13 in juxtaposition therewith formed of upper and lower halves P1, P2 with respective crossed polarizations. The image produced by the lens splitting mirror 41 directing half the beam of an upper array 17 of four photodetectors 21, 23, 25, 27 and half to a lower array 19. Analysing filters in front of the detectors, ensure that array 17 only receives the image from the upper half (P1) of the lens/filter assembly 16 and that 19 only receives the image from the lower half (P2). The outputs from corresponding detectors in the two arrays are subtracted (compared) and the sum of the determined differences used as a measure of the focus (minimum difference= in focus) to control the movement of the lens assembly 16. In the in focus condition Figs. 2A, 2B, 2C the image detected by the upper array, Fig. 2B, is substantially that detected by the lower array Fig. 2C. In the "inside focus" condition Figs. 3A, 3B, 3C the image detected by the upper array Fig. 3B includes that of the boundary 11 "spread" from ray 1R to ray 3R, so that the image of the bright area "overlaps" with the image of the dark area. The image detected by the lower array, Fig. 3C has the image of the dark area "overlapping" into the image of the bright area. This causes the difference signals (and their sum) be significant and of a sign causing the repositioning of the lens assembly 16 by the control signal in the correct sense. In the "outside focus" condition, Figs. 4A, 4B, 4C (not shown) the effects on the upper and lower arrays are interchanged, yielding a control signal of the correct sign. The detector assembly 15 may be changed Fig. 5A for one in which the two arrays 82, 84 with their analysing filters are co-planar and aligned parallel and at right-angles to the boundary of the scene 75 and polarizing filter (P2/P1). As before, the two arrays 82, 84 receive their images from respective halves (P1) (P2) of the lens assembly, yielding out of focus conditions shown in Figs. 5B, 5C. The polarizing filter 77 may be replaced by two prisms, side by side and relatively inverted, refracting the images through them to the respective arrays (Fig. 6, not shown), or by two mirrors, side by side and inclined at slightly different angles (Fig. 7, not shown). In the arrangement of Fig. 8, the image, limited in area to a narrow band by stop 137 is focused by split lens 139, the resulting two images being detected by repective photo-resistive arrays 141, 143. Stop 137 may be dispensed with if a "horizontal" light shield (174, Fig. 10, not shown) extends from the lens 139 to the line between the detectors, and only the top half of half lens 140 and the bottom half of half lens 142 are used.