Exposure level correction for film-to-video conversion

The average video level of a video camera is corrected for film-to-video conversion by having a film aperture plate with a light-passing opening that is slightly enlarged with respect to the image area of the film. Non-image light passes through a peripheral region of the film, as defined by the enlarged opening, and strikes a part of the camera sensor not receiving image-wise illumination. By enlarging the aperture plate opening just enough that the video signal from the non-imaged part of the sensor approximates a desired average video level, the automatic video level correction performed by the camera will pertain only to the image. The aperture plate is part of a film gate that is rotatably mounted so that the film may be supported in either a "horizontal" or "vertical" orientation, depending on how the film image was initially exposed.

BACKGROUND OF THE INVENTION 1. Field of the Invention 
This invention relates to apparatus for establishing an average level for a 
video signal generated by a video camera.

Description Relative to the Prior Art 
A typical video camera includes an automatic exposure control system for 
maintaining the video signal produced by the camera within an acceptable 
range . . . acceptable in the sense that a normal change in the brightness 
of a sensed scene will not cause a video reproduction to be excessively 
over- or under-exposed. Traditional cameras rely on an average video level 
of the whole scene to control an aperture of a lens system, and thereby to 
control the exposure level. The video signal derived from the scene is 
averaged and compared to a reference video level that experience shows 
will allow enough latitude for the full range of expected scene 
brightness. If the average video level is too high, the lens aperture is 
closed down until the averaged video signal reaches the reference level. 
If too small, the aperture is opened up until the reference level is 
obtained. 
There are times when a conventional video camera is used to photograph a 
framed image on a film transparency. If the frame is rectangular, as with 
a 35 mm film transparency, there are commonly two ways in which the 
subject matter of the image is oriented with respect to the longer axis of 
the film frame: the subject matter can generally align with the longer 
axis (the film image is then said to be oriented "horizontally") or it can 
be rotated approximately 90.degree. relative to the longer axis (the film 
image is then said to be oriented "vertically"). It all depends on how the 
35 mm camera was held at the time of exposure. The "vertical" orientation 
presents a problem in transferring the photographic image to a video form 
since a "vertical" image will appear tipped on its side on a video display 
unless something is done to "right" it at the transfer stage. For example, 
in U.S. Pat. No. 4,485,406, the image sensor is carried on a rotary stage 
that can rotate 90.degree. about its optical axis to accommodate a 
"vertically" oriented picture. Alternatively, a rotatable dove prism may 
be disposed in the optical path to rotate the image of the film with 
respect to the sensor. Either approach, however, requires a specially 
designed video camera. 
Another problem with "vertically" oriented pictures concerns the active 
image area of the sensor: if the full area of the sensor is used for one 
film orientation . . . say, horizontal, which occurs most frequently . . . 
then, by rotating the sensor (or the film image), the other orientation 
will cast an image that occupies a lesser area of the sensor. This will 
affect any procedure that depends on the whole image area, such as the 
determination of the average video level. FIGS 1A and 1B illustrate the 
problem. In FIG. 1A, an aperture plate 2 overlays a filmstrip 4 such that 
an opening 6 in the aperture plate 2 generally coincides with edges of a 
negative film frame 8. The frame 8 is imaged upon an image sensor 10 
(shown by broken line) by an optical system (not shown). As shown by FIG. 
1A, the projected image of the frame 8 is shown in relation to the sensor 
area which receives light coming through the actual film frame 8 (i.e., 
the sensor is much smaller than the actual frame 8). The length l.sub.1, 
and the width w.sub.1, of the imaged film frame 8 generally correspond to 
the length l.sub.2 and width w.sub.2 of the sensor 10. The active image 
area of the sensor 10 is thus practically its full area. 
If the film frame 8 is rotated 90.degree. to accommodate a "vertically" 
oriented exposure, then the condition in FIG. 1B is obtained, which shows 
the relative position of the rotated, imaged film frame 8 on the sensor 
10. The optical system has "shrunk" the imaged film frame 8 so that the 
length l.sub.1 fills the width w.sub.2 of the sensor 10; now, however, the 
width w.sub.1 of the imaged film frame 8 falls short of occupying the 
length l.sub.2 of the sensor 10. In this orientation the sensor 10 
includes outlying areas 11 and 12 which are shadowed by the aperture plate 
2 and thus receive no illumination. This will cause borders to appear on 
the reproduced picture. More importantly, the average video level for the 
picture will be incorrect. The conventional camera circuit takes into 
account the full area of the sensor in calculating an average video level 
for the scene. The outlying areas 11 and 12 of the image sensor in FIG. 
1B, however, do not have any scene information to process. The camera 
circuit will "see" no exposure for these areas and determine that the 
average video level is too low (i.e., the overall scene is over exposed 
and the negative is too dark). Then the diaphragm will be opened to let in 
more light. The result: the "vertically" oriented 35 mm negative will be 
over-exposed; thus the reproduction will be excessively dark. 
In a typical video camera, the only way to correct the video level for 
"problem" exposures is to modify the internal circuitry of the camera. In 
other words, a special camera must be designed to address this problem. It 
would be more attractive . . . from the standpoint of cost and complexity 
. . . to have some way of accommodating a "vertically" oriented exposure 
without necessitating extensive camera modification. 
SUMMARY OF THE INVENTION 
The invention treats the problem of exposure level adjustment in a 
"vertical" orientation by providing a specially designed aperture plate 
that "tricks" the camera into making the necessary video level 
accommodation for the unexposed outlying areas. By locating the 
problem-solving improvement in the external aperture plate that supports 
the film, a conventional video camera can be used for "vertically" 
oriented exposures without necessitating internal modification of the 
camera. 
In practicing the invention, an original, such as a film transparency, is 
so supported in a path of illumination that image-wise illumination 
reaches a partial area of an image sensor in the camera. A further 
(outlying) area of the sensor is thus unexposed to image-wise 
illumination. Instead of blocking all non-image illumination from striking 
the outlying area, the improvement is obtained by allowing some 
illumination to get through. For this purpose, only so much of this area 
is illuminated with non-image illumination as will approximate a 
predetermined average video level from the whole outlying area. Exposure 
level adjustment then becomes mainly a function of the imaged area of the 
sensor alone. 
A specially-provided aperture plate is the preferred way of controlling the 
illumination. The aperture plate has an opening according to the invention 
that restricts the passing illumination to two parts: A first part is 
modulated image-wise by passage through the supported original and strikes 
the partial area of the sensor. A second part passes outboard of the 
original without receiving image-wise modulation and strikes part of the 
outlying area of the sensor. The expanded aperture plate opening is 
especially sized so that it passes non-image illumination typically 
through the unexposed border area of the film and its perforations. 
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Since video cameras are well known, the present description will be 
directed in particular to elements forming part of, or cooperating more 
directly with, the present invention. Elements of a video camera, 
including optical and circuit elements, not specifically shown or 
described herein may be selected from those known in the art. 
FIG. 2 shows the basic aspects of a film-to-video transfer station 
affording improved control of the average video level according to the 
invention. A conventional video camera 14 is mounted, for example, by its 
tripod socket to an adjustable mounting plate 16. The mounting plate 16 is 
attached to a frame member 18, which in turn is mounted on a base plate 
20. A section of the base plate 20 is removed so as to receive a rotatable 
turret 22 centered along the optical axis of the camera 14 and its lens 
15. An aperture plate 24 holds the filmstrip 4 in place relative to the 
optical axis. Light from an illuminator 26 passes through an aperture (not 
shown) in the turret 22 and then through the filmstrip 4 and the lens 15 
to a photosensor in the camera 14. The turret 22, which is supported for 
rotation relative to the base plate 20, is manually movable from a radial 
position A to a radial position B by sliding a turret handle 28 between 
the two positions. As shown by FIG. 2, the filmstrip 4 is in a 
"horizontal" orientation in which the turret handle 28 is aligned with the 
position A. By rotating the turret 22 by 90.degree. and aligning the 
turret handle with the position B, the filmstrip 4 is placed in a 
"vertical" orientation. 
As better seen by FIGS. 3A and 3B, the aperture plate 24 has an aperture 
opening 29 of width w.sub.3 that is greater than the corresponding width 
w.sub.1 of the image frame 8 on the filmstrip 4. The enlarged width 
w.sub.3 includes two like regions 30 and 30' which include transparent 
border areas 32 and perforations 34 on the filmstrip 4. In the 
"horizontal" orientation depicted in FIG. 3A, the area of the image frame 
8 is slightly larger than the projected area of the sensor 10. (The 
respective reference characters 8 and 10 are connected by lead lines to 
the respective borders of the film frame and the sensor). All photosites 
of the sensor 10 are therefore exposed to image-wise illumination and 
light passing through the outlying border regions 30 and 30' do not reach, 
and therefore do not affect, the sensor 10. 
When the turret 22 is rotated to its "vertical" orientation, as depicted in 
FIG. 3B, the image frame 8 no longer occupies the full area of the sensor 
10. (The aperture plate 24 and the filmstrip 4 are seen in FIG. 3B to 
appear smaller than their representation in FIG. 3A; this happens because 
the area of the sensor 10 is kept the same in both illustrations but the 
magnification ratio has been changed in FIG. 3B in order that the length 
l.sub.1 of the frame 8 corresponds to the width w.sub.2 of the sensor 10). 
It is important to note that, in the "vertical" orientation of FIG. 3B, 
image-wise illumination coming through the frame 8 strikes only a part of 
the full area of the sensor 10. Non-image illumination passes through the 
border areas 30 and 30' and strikes another part of the sensor 10. 
Finally, the sections 35 and 35' of the aperture plate 24 shadow further 
outlying parts of the sensor 10 and prevent any light from reaching such 
parts. 
Referring now to FIG. 4, the turret 22 is shown in a "vertical" orientation 
such that the turret handle 28 aligns with the radial position B. The 
video signal is influenced by what happens in three regions of the sensor 
10. In a central region 36a, the sensor 10 receives image-wise 
illumination from the image area of the image frame 8. In the regions 36b 
outboard of the central region 36a, the image sensor receives image-wise 
illumination from the border regions 30 and 30' on the filmstrip (see FIG. 
3B). Still further outboard from the central region 36a in the regions 
36c, the image sensor 12 is shadowed by the aperture plate 22 and receives 
no illumination. As will be described in detail, the video signal for the 
entire area of the sensor 10 is averaged and compared to a reference 
signal. If these signals differ, the light passing to the sensor 10 is 
adjusted until the averaged signal equates with the reference signal. In 
the "vertical" orientation shown by FIG. 3B, the averaged value will 
include signal contributions from the shadowed region 36 c (see FIG. 4), 
where the signal is essentially zero, the border region 36b and the 
central region 36c. Nonetheless, it is desirable that the averaged video 
signal represents mainly the average value for the central region 36a 
receiving the image-wise radiation. The signals from the other regions 36b 
and 36c should exhibit little influence on the average. This is done by 
forcing the signal components from the non-image regions 36b and 36c to 
approximate a desired video level. Enough of the combined regions 36b and 
36c is uncovered to illumination that the resulting signal from those 
regions assumes the desired video level. The enlarged width (w.sub.3 
-w.sub.1) of the opening in the aperture plate 24 is empirically selected 
to provide the desired average level. 
The video camera 14 includes a zoom lens 50 disposed with respect to the 
aperture plate 24, and the film frame 8 supported therewith, so as to 
direct an image upon the sensor 10. The zoom lens 50 includes an 
adjustable lens component 52 for varying the focal length of the zoom lens 
50 and an adjustable diaphragm 54 for regulating the amount of light that 
transmits through the zoom lens 50. Both the position of the lens 
component 52 and the opening of the diaphragm 54 are adjusted in a 
conventional manner, for example, by moving external rings on the barrel 
of the zoom lens 50. The illuminator 26 includes a light source 56 for 
generating the illuminating light. The illuminating light is concentrated 
in the direction of the aperture plate 24 by a mirror 58 and a condensor 
lens 60. 
Though it is shown as a two-dimensional solid state image sensor, the 
sensor 10 is meant to generally represent a conventional photosensor 
system, including the common three-tube system found in a conventional 
color video camera. (The regions 36a, b and c would then relate to any one 
tube, or all three tubes.) The sensor 10 generates an array of 
charge-based signals corresponding to the brightness of the light striking 
it. It also includes a register 62 for collecting the charge-based signals 
prior to their transfer off the sensor. The sensor is driven in a known 
manner by a clock signal generated by a clock generator 64. The sensor 
output is sent to a video processor 66, which produces a standard color 
video signal in a known manner and provides it to a circuit output 
terminal 68. 
The processed video signal is also supplied to an operational amplifier 70 
connected as a voltage follower. The output signal from the voltage 
follower amplifier 70 is applied to an integrator 73 comprising a resistor 
72 and a capacitor 74. The integrator 73 averages the video signal from 
the full area of the sensor 10, providing a voltage V.sub.INT. That is, 
the integrated voltage includes, for the "vertical" orientation of the 
film frame 8, the image-wise signal from the region 36a, the border signal 
from the regions 36b and the essentially zero signal from the shadowed 
regions 36c. The intergrated signal V.sub.INT is provided to an inverting 
circuit 75 including an operational amplifier 76, a feedback resistor 78, 
and an input resistor 80. An offset voltage V.sub.OFFSET is provided to 
the non-inverting input terminal of the amplifier 76 from a potentiometer 
82, which divides down a fixed positive voltage V.sub.c. The voltage 
V.sub.OFFSET constitutes a reference that determines the average video 
level for the camera. A diaphragm control voltage V.sub.D is generated at 
the output of the operational amplifier 76 as follows 
EQU V.sub.D =V.sub.OFFSET -V.sub.INT (R.sub.78 /R.sub.80) 
where R.sub.78 and R.sub.80 are resistances of the respective resistors 78 
and 80. This relationship indicates that a totally dark scene will provide 
a positive voltage V.sub.D, where 
EQU V.sub.D .perspectiveto.V.sub.OFFSET 
since V.sub.INT is substantially zero; conversely, the offset voltage 
V.sub.OFFSET and the resistors 78 and 80 are selected so that a totally 
blank scene (i.e., the film frame 8 is completely transparent) will 
provide a sizable negative voltage. 
The control voltage V.sub.D is provided to a driver circuit 84 for driving 
a motor 86 that gears with the external ring (not shown) controlling the 
diaphragm 54. A positive voltage V.sub.D will cause the shaft of the motor 
86 to rotate in a direction that opens the diaphragm and lets more light 
through to the sensor 10 . . . since a positive voltage V.sub.D indicates 
too little exposure. Conversely, a negative voltage V.sub.D will cause the 
motor 86 to rotate oppositely and close down the diaphragm, allowing less 
light through to the sensor 10 . . . since a negative voltage V.sub.D 
indicates too much light on the sensor 10. When the integrated voltage 
V.sub.INT, as multiplied by its gain factor, equals the offset voltage 
V.sub.OFFSET then the correct average video level has been attained and 
the diaphragm needs no further adjustment for the given frame 8. When a 
new frame is moved into the aperture gate 24, the whole procedure will be 
repeated until the correct average is again found. 
Assume now that the aperture plate 28 is positioned as shown by FIG. 3A 
since the image on the film frame 8 is "horizontally" oriented. The 
image-wise illumination from the frame 8 thus fills the whole sensor 10. 
The next frame is then positioned under the aperture plate 24. This frame, 
it is assumed, has a "vertically" oriented image. The turret 22 is 
therefore rotated 90.degree. until the turret handle 28 aligns relative to 
position B. A switch 88 (see FIG. 4) locates in a notch on the edge of the 
turret 22 and accordingly signals a zoom controller 90 that the 
magnification ratio must be changed. A motor 92 is driven to move the 
adjustable lens element 52 and thereby reduce the imaged length l.sub.1 of 
the frame 8 until it fits within the width W.sub.2 of the sensor 10 (see 
FIG. 3B). (This adjustment could also be made by manually turning the 
external ring controlling the lens component 52.) Because a significant 
portion of the sensor 20 receives no image-wise illumination, the 
light-admitting aperture 29 is just wide enough that the charge-based 
signals from the non-imaged portion (regions 36b and 36c) provide a video 
signal having the desired average video level. Then, for an average scene 
in a "vertical" orientation, the video level correction provided by the 
integrator 73 and the inverting circuit 75 will reflect the scene content 
of the film frame alone. If the scene is not representative of an average 
brightness level, then the automatic correction afforded by the widened 
aperture will bring the camera nearly to a correct average video level. 
(An additional manual adjustment of the diaphragm may be desirable in this 
case. Alternatively, the potentiometer 82 . . . which is normally preset 
and not further touched . . . could be brought out to a control panel of 
the camera as a contrast adjustment and made adjustable within an 
accepatable range.) 
FIG. 5 is an exploded view of the turret 22 showing in particular that the 
aperture plate 24 includes an upper film gate 100 and a lower film gate 
102. The channel-like lower film gate 102 includes uplifted edges 104 for 
guiding the filmstrip 4. The upper film gate 100 includes a pressure plate 
106 and a down-thrusted camming plate 108. The aforementioned enlarged 
aperture 29 is in the pressure plate 106. The camming plate 108 is 
resiliently separated from the pressure plate 106 by a set of springs 110 
coiled about a set of spacers 112. The camming plate 108 forces the 
pressure plate 106 against the lower film gate 102 by the action of a 
camming assembly 114. The assembly 114 includes a pair of cams 116 mounted 
for rotation on shafts extending from the uplifted edges 104 of the lower 
film gate 102. Each cam 116 is joined by a connecting rod 118 to a 
cross-member 120. The whole cam assembly is moved by turning a film 
release handle 122 connected to the cross-member 120. The filmstrip 4 is 
placed between the lower film gate 102 and the pressure plate 106. When 
the film release handle 122 is pressed down, the cams 116 slide across the 
top camming surface of the camming plate 108, forcing it and the 
underlying pressure plate 106 toward the lower film gate 102 until the 
filmstrip 4 is snugly in place. 
The invention has been described in detail with particular reference to a 
presently preferred embodiment, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention. It is clear, for example, that a positive transparency can be 
used in place of a negative transparency or with suitable modifications, 
especially to the illuminator, a reflection print could be used in place 
of a transparency.