Optical scanning method and apparatus

Optical scanning method and apparatus suitable for use with a video player for scanning an information bearing medium to produce a standard television signal is disclosed. The scanning apparatus includes a movable mirror that deflects an optical path to effect a scan. The mirror is angularly displaced, back and forth, between first and second positions at a substantially constant rate in a "back-to-back sawtooth" fashion, such that one scan is made as the mirror is moved from the first position to the second position, and the next scan is made as the mirror is returned from its second position to its first position. The scanning apparatus inverts an image transmitted along the optical path between each successive scan to provide scanning in a "sawtooth" fashion, e.g. scanning an image from top to bottom--quickly retracing to the top of the image--and repeating the scanning process from top to bottom, as is required in the standard television scanning format.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to optical scanning and more particularly to such 
scanning effected by mirrors. 
2. Discussion Related to the Problem 
Optical scanning apparatus such as that used in a video player for scanning 
still pictures (e.g. photographic prints or slides) to produce a 
television signal, is well known. In a video player a picture is scanned 
in a horizontal direction at a standard television line rate, and the line 
scan is displaced in a vertical direction at the standard television field 
rate. In the NTSC standard television signal format, the picture is 
scanned from top to bottom 60 times per second. Approximately 1 
millisecond is available between successive scans for retracing from the 
bottom of the picture back to the top. Typical prior art optical scanners 
employ a CRT to effect the horizontal and vertical scans. Achieving 
vertical retrace in less than 1 millisecond in such so-called 
"flying-spot" scanners is no problem. 
Recent developments in solid-state line sensing arrays make them 
particularly attractive from cost, size and energy consumption 
standpoints, for use as an alternative to a CRT in a video player. A 
solid-state line sensing array can easily scan an image in the horizontal 
direction, and it remains to provide the required periodic vertical 
displacement of the horizontal scan with respect to the scanned image. The 
use of a pivoting mirror to periodically deflect an optical path in a 
scanner is well known in the art (see U.S. Pat. No. 2,590,281, issued Mar. 
25, 1952 to G. C. Sziklai et al). In the standard NTSC television signal 
format, the vertical scan of the image takes about 16 milliseconds and as 
stated above, about 1 millisecond is allowed for retracing between 
vertical scans. To accomplish such a scan with a mirror, the mirror would 
be moved smoothly through some angular displacement in about 16 
milliseconds then quickly returned to the initial starting position in 
about 1 millisecond. A plot of the mirror angular position versus time 
would resemble a "sawtooth" waveform. Unfortunately, because of the 
inertia of the mirror, practical pivoting mirror systems having inadequate 
high frequency performance to accomplish the vertical retrace in the 
approximately 1 millisecond time period available. One possible solution 
to this problem is to employ two pivoting mirrors, the angular 
displacement of each, versus time, resembling "back-to-back sawtooth" 
waveforms, which are 180.degree. out of phase. Scanning apparatus 
employing two mirrors shifts the optical path back and forth from one 
mirror to the other so that while one mirror is scanning a picture from 
top to bottom, the other mirror is pivoting back to its starting position 
(see U.S. Pat. No. 2,590,281 above). This arrangement greatly relaxes the 
high frequency response required of the mirror pivoting system, however, 
the complexity of two mirrors increases the expense of the apparatus and 
leads to added complication in aligning and matching the response 
characteristics of the two pivoting mirrors. Another way to accomplish 
quick retrace in a mirror scan system is to employ a rotating optical 
polygon as the deflecting element (see U.S. Pat. No. 4,148,071 issued Apr. 
3, 1979 to Zinchuk). Unfortunately, optical polygons in themselves are 
relatively expensive optical elements. 
The problem faced by the inventor, therefore, was to provide a scanning 
method and apparatus employing a pivoting mirror for deflecting an optical 
path, that (a) quickly retraced between successive vertical scans and (b) 
avoided the complexity and expense of prior art multiple mirror and 
rotating polygon solutions. 
SOLUTION--SUMMARY OF THE INVENTION 
The scanning method and apparatus according to the present invention solves 
the problems noted above by including a movable mirror that deflects an 
optical path to effect a scan. The mirror is angularly displaced, back and 
forth, between first and second positions at a substantially constant rate 
in a "back-to-back sawtooth" fashion, thereby relaxing the high frequency 
performance requirements of the scanning mirror. To provide "sawtooth" 
type scanning with quick retrace, an image transmitted along the optical 
path is periodically inverted in the direction of scanning. Means are 
provided for synchronizing the motion of the scanning mirror with the 
inversion of the image so that the image is repetitively scanned in a 
sawtooth fashion. In a preferred embodiment of the apparatus, two 
alternative optical paths to the scanning mirror are provided. The first 
alternative optical path to the scanning mirror includes an even number of 
reflections in the plane of scanning, and the second alternative optical 
path includes an odd number of reflections in the plane of scanning. In 
this way the image transmitted along the first path is inverted in the 
scanning direction with respect to the image transmitted along the second 
path. A light valve activates the first alternative optical path when the 
scanning mirror is scanning in one direction, and activates the second 
alternative optical path when the scanning mirror is scanning in the other 
direction, thereby providing sawtooth type scanning with quick retrace. 
The scanning apparatus according to the present invention is combined with 
a self-scanning linear image sensing array and an image projector to 
provide a video player. The linear image sensing array scans the image 
line-by-line in a horizontal direction, and the scanning apparatus 
according to the present invention displaces the line scan with respect to 
the image in the vertical direction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A video player, generally designated 10, including optical scanning 
apparatus 11, according to a preferred embodiment of the present 
invention, is shown schematically in FIG. 1. The video player includes a 
light source 12 and a condensing lens 13 for illuminating an information 
bearing medium (e.g. a slide) 14 in a film gate 16. Light from the film 
gate is transmitted along either one of two optical paths, labelled A and 
B, to a scanning mirror 20. Light reflected by the scanning mirror 20 is 
imaged on a solid-state linear image sensing array (such as a charge 
coupled device image sensor) 22 by a scan lens 24. Scanning mirror 20 is 
pivotally mounted on a flexure hinge 26 and is connected by a link 28 to a 
cantilever spring 30. The distal end of cantilever spring 30 carries a cam 
follower 32, that is biased into contact with a cordiform cam 34 by 
cantilever spring 30. Cam 34 is attached to a shaft 36 that is driven by a 
synchronous motor 38. As shown in FIG. 2, each revolution of cordiform cam 
34 causes mirror 20 to be rotated about a virtual pivot defined by the 
flexure hinge 26, via cam follower 32 and linkage 28, in a "back-to-back 
sawtooth" fashion through an angular displacement 2.alpha.. As described 
thus far, each revolution of cam 34 would cause mirror 20 to scan the 
projected image of slide 14 from top to bottom, then from bottom to top. 
This, of course, is not the desired scanning sequence, since the standard 
TV signal is generated by repeatedly scanning an image, line by line, from 
top to bottom--quickly retracing to the top of the image between 
successive scans--and repeating the scan from top to bottom. The 
"back-to-back sawtooth" scanning sequence shown in FIG. 2 is converted, 
according to the present invention, into the desired "sawtooth" scanning 
sequence by alternately switching between the two optical paths A and B. 
One optical path has an odd number of reflections, and the other has an 
even number of reflections in the scanning plane to cause an inversion of 
the image from top to bottom between successive scans. 
Light is switched from one optical path to the other by a light valve 
comprising a rotating disc 40 (see FIG. 1) having a reflecting portion 42 
and a transmitting portion (e.g. a cut-out portion) 44. When the 
transmitting portion of disc 40 is disposed in the optical path of the 
scanner (as shown in FIG. 1), light from gate 16, following optical path 
A, is reflected first from a mirror 46 and then from a beam splitter 48 
prior to reaching scanning mirror 20. When the reflecting portion of disc 
40 is disposed in the optical path of the scanning apparatus, light from 
film gate 16, following optical path B is first reflected from the disc 
40, then from a mirror 50, then from a mirror 52, and finally through beam 
splitter 48 before reaching scanning mirror 20. Because path A has an even 
number of reflections (i.e. 2) prior to mirror 20, and path B has an odd 
number of reflections (i.e. 3), the image transmitted via path A will 
appear inverted top to bottom with respect to the image transmitted via 
path B. The relative vertical positions of the film image on the line 
imager for paths A and B, as a function of the changing angular position 
of scanning mirror 20 with time, are shown in FIG. 3. The light valve is 
employed to switch back and forth between paths A and B at the appropriate 
times to obtain a "sawtooth" type scan as shown by solid lines in FIG. 3. 
Starting at time zero, the scanning apparatus 11 scans from the top of the 
bottom of the film image, employing optical path A. At time 1/60, the 
light valve switches from optical path A to optical path B and the image 
of the film in the gate is once again scanned from top to bottom. Rotating 
disc 40 is mechanically coupled, via an extension 36' of shaft 36 to the 
mirror driving cam 34 to synchronize the switching between optical paths A 
and B with the angular excursions of mirror 20. Of course, synchronization 
could also be achieved by other means, for example, by the use of an 
electronic servo system. 
The video player 10 generates a standard television signal, line by line, 
by clocking the solid-state line sensing array 22 in a known manner with 
polyphase clock signals .PHI.. The clock signals .PHI., and horizontal and 
vertical synchronization signals for composing a standard composite video 
signal, are generated by a control electronics and clock generator circuit 
54 in a known manner. The output of the solid-state line sensor 22, along 
with the horizontal and vertical synchronization signals produced by 
control electronics 54 are supplied as inputs to signal processing 
electronics 56, which produce a standard composite video signal from the 
inputs in a known manner. 
The angular velocity of synchronous motor 38 is determined by a control 
signal S supplied from control electronics 54 to provide the proper number 
of vertical scans per second. For example, in the NTSC standard television 
signal, 60 fields per second are scanned. Since the scanning apparatus 11 
scans two fields per revolution of cordiform cam 34, the motor is driven 
at 1800 RPM to produce the NTSC compatible television signal. The position 
of motor 38 is synchronized with the vertical sync signals supplied to the 
signal processing electronics 56 by control electronics 54. 
An alternative embodiment of scanning apparatus according to the present 
invention, and likewise employed in a video player, is shown in FIG. 4, 
where elements similar to those of the scanner shown in FIG. 1 are 
similarly numbered. In the embodiment shown in FIG. 4, both sides of the 
reflective portion 43 of disc 40 are reflective, and the disc is disposed 
to intercept the optical path from the slide gate 16 to the mirror 20 in 
two places, thereby eliminating the need for a beam splitter such as was 
used in the apparatus shown in FIG. 1. In the position shown in FIG. 4, 
optical path A is active. Light from film gate 16 reflects from the 
reflecting portion 42 of disc 40--reflects through an arrangement 60 of 
three mirrors placed 90.degree. with respect to each other to deflect the 
beam through an angle of 270.degree. via three reflections--passes through 
the transparent portion 44 of disc 40 and then reflects from mirror 52 
onto scanning mirror 20. The beam in optical path A undergoes five 
reflections in the scanning plane prior to reaching scanning mirror 20. 
When optical path B is active, as shown by the dotted line in FIG. 4, the 
light from the film gate 16 passes through the transparent portion 44 of 
disc 40--reflects through an arrangement 62 of two mirrors placed 
45.degree. with respect to each other to deflect the beam through an angle 
of 270.degree.--reflects from the backside of disc 40--and then reflects 
from mirror 52 onto scanning mirror 20, for a total of four reflections 
prior to reaching scanning mirror 20. As can be seen from FIG. 4, the 
image of the film gate 16 transmitted along optical path B will be 
inverted from top to bottom with respect to the image transmitted along 
optical path A. As motor 38 rotates disc 40 and cam 34 simultaneously, 
optical path A and optical path B are alternatively activated to provide 
the desired "sawtooth" scan of the film while the scanning mirror 20 is 
deflected in a "back-to-back sawtooth" manner. 
The invention has been described in detail with reference to particular 
embodiments, however, it is understood that variations and modifications 
may be made within the spirit and scope of the invention. For example, the 
present invention is also useful in scanners other than video players, 
such as laser scanners where a laser beam to be deflected in a "sawtooth" 
fashion may be controlled by moving a scanning mirror in a "back-to-back" 
sawtooth fashion.