Patent GB-1449255-A (GB)
Title: Automatic focussing device
IPC: G01C3/32
Filed: 1973-11-14
Abstract: 1449255 Automatic focusing ASAHI KOGAKU KOGYO KK |14 Nov 1973 [14 Nov 1972] 52835/73 Heading H4D An automatic focussing device adjusts the focus of an image forming optical system, in response to a signal dependent on the contrast of the image, and a two-dimensional diffraction grating is positioned in the optical path to the image. The diffraction grating 6 (Fig. 6) acts effectively as a band pass filter for the Optical Transfer Function of the system, i.e. for the power spectrum of the image, and is constituted by a two dimensional "checkerboard" toothed phase grating. The lower frequency spatial variations in intensity across the image tend to be rejected, and the signal so derived in the photo-electric detector 7 is closely dependent on the resolution of close detail. The Optical Transfer Function is thereby improved. In Fig. 4, curves a and b represent the O.T.F. or image power spectrum without a phase grating in position, when focused and unfocused respectively and the triangular functions A 3 H 0 A 4  and A 3 H 1 A 4 , each plus function OA 1 A 2  represent the respective spectral distribution when the grating is in position, the maximum value H 0  occurring in the in focus position. The Light Sensor 7 which may be a photo electric detector, e.g. Cd5 having dip effect characteristic as in Fig. 7, detects the changes in the image contrast and develops an output voltage which has a maximum at the in-focus position and controls the position of the lens via a servo. In one embodiment the Light Sensor may be placed in simple harmonic motion by an oscillator driven vibrator (at e.g. 100Hc), so that the optical path length is modulated. The detection signal is thus sinusoidally modulated, and amplified at 8, its level being at a maximum level when in focus, but the optical path length modulation has a phase shift of # between positive defocusing d and negative defocusing e (Fig. 8), which phase difference is detected by phase detector 11. The polarity of the D.C. output voltage from the phase detector, corresponds to whether the system is positively or negatively defocussed, which voltage is applied to a D.C. servo motor which drives lens focusing rack and pinion 16 and 15 thereby maintaining the image at maximum focus. In a further embodiment, the optical path modulation is replaced a purely electronic servo control method, Fig. 9, wherein the signal voltage from sensor 7 is amplified as a D.C. voltage at 81 and its amplitude sampled by gate 20. Oscillator 19 provides clock pulses, and level changes in input voltage are compared over a three pulse cycle. On the first clock pulse, memory 21 records voltage V 1 , on the second pulse memory 22 records the new voltage V 2 , on clock pulse 3 gate 25 enables voltage hold 24 which reads the difference value V 1 -V 2  from comparator 23, which voltage drives servomotor 14, to focus the lens system as above. The optimum position for automatic focussing is obtained by moving the grating 6 along the optical axis so that the voltage reading on volt meter 13 is a maximum. In other embodiments, amplitude or complex amplitude gratings replace phase grating 6.