Video copying apparatus spectrally-responsive to slides or negatives

Video copying apparatus generates a color sequential video signal from originals having different spectral characteristics, such as photographic slides and negatives. The apparatus includes a color filter wheel with a plurality of color filters that are sequentially interposed into the optical path between a camera and the original. By providing separate red filters in the filter wheel, the spectral pass bands of the red filters can be matched to the separate and distinctive red spectral responses of a slide or a negative. The output color video signal is obtained from a progression of filtered image signals generated by the camera according to the type of original. In one embodiment, a succession of video fields including red image signals from both red filters is continuously applied to a framestore and the selection of the appropriate red signal is made by storing only the color video signals corresponding to the desired type of original.

BACKGROUND OF THE INVENTION 
Field of the Invention 
This invention relates to a color video imaging system, and in particular 
to a color video system in which spectrally-distinct color information is 
sequentially imaged upon a single image sensing device. 
Brief Description of the First Drawing 
The prior art will be described in relation to FIG. 1, which illustrates 
the spectral response of a photographic slide and a photographic negative 
and helps to show conventional color filtration for either or both types 
of objects. 
Description Relative to the Prior Art 
Color video imaging systems, specifically color video cameras, require at 
least three independent color records, such as red, green and blue, for 
conversion of a color scene into a video signal. Field sequential color 
imaging systems, which are generally well known, use a single image 
sensing device for sequentially receiving the independent color 
information at a relatively high cycle rate. Such systems typically 
separate the overall color information into red, green and blue spectral 
regions before it reaches the image sensing device. The resulting 
sequential color signals are stored line-by-line and processed together to 
form a video signal for color recording, transmission, and the like. 
Many sequential systems utilize color filter wheels with three color filter 
segments for separating the light from the scene into the three 
independent spectral regions. The color wheel is rotated at a relatively 
high rate so that each filter segment is sequentially moved into the 
optical path between the image sensing device and the scene. The image 
sensing device is electrically driven at a corresponding rate so that a 
stream of sequential color signals is produced for processing into a 
combined or composite color signal. A representative sequential system is 
shown in U.S. Pat. No. 4,404,585. This patent describes a three color 
wheel (red, green and blue filter segments) and a four color wheel (red, 
blue, yellow and cyan filter segments.). The four color wheel has a 
primary color followed two scans later by its complementary color so that 
chroma information is produced by adding and subtracting the resulting 
color signals. 
The color characteristics of the filters in the color wheel are selected in 
conjunction with the spectral characteristics of the image sensing device 
to yield color signal components of such character as to be reproduced 
with the proper color quality at a receiver. In addition, as described in 
U.S. Pat. No. 3,506,775, the color characteristics are sometimes selected 
so that signal voltages of equal amplitude are obtained for the several 
color components when a camera is scanning a specified white object field. 
The nature of the object field is sometimes predetermined so that the 
filter segments can be tailored to the spectral response of the object. 
FIG. 1 illustrates the spectral response of two common static objects--a 
photographic slide and a photographic negative. Both slides and negatives 
generally have, as shown in FIG. 1, the same response in the blue and 
green spectral regions. The two objects, however, have markedly different 
responses in the red spectral region. This happens because a negative is 
printed under safelight illumination in a darkroom while a slide is 
processed in darkness. The negative is therefore virtually opaque--i.e., 
no response--in the safelight spectral region A while the slide is at 
least partly transmissive in this region. 
European Patent Publication No. 126,597 describes a sequential video system 
for display of a static object, such as a photographic negative in the 
disc format. Although such film is only in a negative format, this 
publication supposes that the filter segments of the color wheel may be 
suited for use with negative film or with transparency film or with a 
combination of both. Having a color wheel for only one type of object is a 
problem, of course, for the other object, that is, when both slides and 
negatives are to be copied. Changing filter wheels is an obvious, albeit 
cumbersome, solution. The other option discussed in Publication No. 
126,597, that of suiting the filter segments to a combination of both 
slides and negatives, is illustrated in FIG. 1 by the red response curve 
shown in broken line covering both objects. The problem then is that the 
red filter sensitivity takes in too much, desaturating the red and green 
hues in the case of negatives. This ordinarily requires, for quality 
results, extensive electronic correction (matrixing) later in the signal 
processing chain. 
SUMMARY OF THE INVENTION 
To accommodate the different spectral responses of different static 
objects, such as positive (e.g., a slide) and negative originals, it is 
desirable to operate the video imaging system according to the invention 
in relation to a plurality of spectral responses appropriate to the 
particular originals the system is designed to utilize. The video imaging 
system includes image sensing means responsive to image light from the 
original for producing a plurality of image signals, a signal processing 
section for generating a color video signal from the image signals, and 
color filter means comprising a plurality of color filters for modifying 
the image light, including one combination of filters with spectral 
characteristics appropriate for a first type of original and a second 
combination of filters with spectral characteristics appropriate for a 
second type of original. In response to an input mode signal identifying 
the type of original, the video imaging system generates a color video 
signal from image signals corresponding to the combination of filters 
appropriate for the original, be it slide or negative. 
In the preferred embodiment, the color filter means is a color filter wheel 
for supporting the plurality of filters. The wheel is rotated so that the 
filters are sequentially positioned over the image sensor. The color 
filter wheel houses two red filters, a green and a blue filter. The filter 
combination suitable for a negative original includes the first red and 
the green and blue filters. The filter combination suitable for a slide 
original includes the second red, and the same green and blue filters. In 
operation, the image sensor provides a sequence of four color signals and 
the progression of these signals through the signal processing section is 
controlled so that the color video signal is processed from the 
combination of signals derived from the combination of filters appropriate 
to the identified type of input. For instance, the signal processing 
section may include a framestore and only the appropriate combination of 
signals is entered into the framestore.

DETAILED DESCRIPTION OF THE INVENTION 
The video apparatus shown in FIG. 2 is sometimes referred to as a video 
transfer or copy stand, which includes a self-contained camera supported 
in spaced relationship with an illuminated platen on which a static 
original is placed. The camera is frequently mounted on an adjustable, 
telescoping support for movement toward and away from the original, and/or 
utilizing a zoom lens, as desired, for proper framing. Since the 
mechanical details of a transfer stand are well known, they will not be 
further addressed in this patent disclosure. 
The preferred embodiment of the video copy apparatus shown in FIG. 5 
employs a self-contained camera head 10. Although the selection of a 
particular camera head is not critical, a solid-state CCD camera 
manufactured by Cohu, Inc. (and referred to as a 4810 series camera) was 
used. The camera head 10 includes a frame transfer CCD image sensor 12 
having an active imaging area of 754 horizontal by 488 vertical picture 
elements (pixels). The camera head 10 includes its own scanning system 
(not separately shown) for producing a video output signal. In addition, 
the camera head 10 produces a pixel clock signal and receives horizontal 
(H.sub.D) and vertical (V.sub.D) drive signals from an external source, 
such as the gen-lock circuit 14 shown in FIG. 2. (Gen-lock is a well-known 
method for locking the vertical and horizontal drive signals at their 
correct frequencies by means of digital techniques.) Other electrical 
connections are made to the camera head 10, but are not shown as they are 
unessential for describing the invention. The camera head 10 includes a 
C-mount adapter for accepting a variety of fixed focus and automatic 
lenses for focusing an image of a static original 16 upon the image sensor 
12. A lens 18 is shown in FIG. 2, including an externally adjustable iris 
20 for controlling the intensity of the image light striking the image 
sensor 12. 
A color filter wheel 22 is supported for rotation about an axis 24 by 
mechanical connection with a stepper motor 26. The color filter wheel 22 
supports four filter segments, which are shown in the plan view of FIG. 4 
to include a first red segment R1, a second red segment R2, a green 
segment G and a blue segment B. The filter wheel 22 is rotated at 3.75 
revolutions per second; this, as will be shown later, corresponds to a 
field scan rate of 60 fields per second. FIG. 2 shows a cross-section of 
the color filter wheel 22 taken along the line A--A in FIG. 4, therefore 
showing the first red filter segment R1 and the green filter segment G. 
Both FIG. 4 and the cross sectional view show a synchronization index mark 
28 at one edge of the color filter wheel 22. A photodetector 30 is mounted 
adjacent the color filter wheel 22 for detecting passage of the mark 28 as 
the wheel 22 rotates. A pulse signal is accordingly produced for each 
revolution of the wheel 22. The color filter wheel 22 is supported 
relative to an optical path delimited by the broken lines 32 such that the 
filter segments R1, R2, G, and B are sequentially interposed into the 
optical path 32 between the original 16 and the image sensor 12 as the 
wheel 22 is rotated. The occurrence of the pulse signal establishes a 
reference from which the identity of any filter segment interposed in the 
optical path is determined. 
The principal parts of the signal processing section of the video copy 
apparatus include an input processing circuit 34, a look-up table 36 for 
gamma and tone scale correction, an application-specific integrated 
circuit (ASIC) 38 for controlling the look-up table 36, enhancing the 
video signal and making exposure measurements and adjustments, and a 
framestore 40 for storing selected color field components of the 
sequential video signal. Insofar as a large part of the signal processing 
is digital, an analog-to-digital converter 42 is connected between the 
input processing circuit 34 and the look-up table 36, and a 
digital-to-analog converter 44 is coupled to the output of the framestore 
40. The use of the look-up table 36 allows for special gamma and tone 
scale corrections optimized for the particular type of original being 
copied. 
A microprocessor 46 controls the signal processing section. More 
specifically, the microprocessor 46 sets the video gain and the black 
level of the video signal by connections to the input processing circuit 
34. The integrated circuit 38, which includes a mode control circuit 48, 
an exposure measurement circuit 50, and a video enhancement circuit 52, 
communicates with the microprocessor 46 on a serial data input line 
(SD.sub.in) and a serial data output line (SD.sub.out). For instance, a 
particular gamma or tone scale correction is desirable for a particular 
type of original 16. A signal identifying the type of correction is 
applied to the circuit 38 on the serial data input line (SD.sub.in) and 
transferred through a serial-to-parallel latch (not shown) connected to 
the mode control circuit 48. An appropriate control signal (MODE') is 
provided to the look-up table 36, which accordingly selects the look-up 
table(s) for the type of original 16. Likewise, certain image parameters 
(e.g., coring and gain levels) are latched from the serial data input line 
(SD.sub.in) and used by the video enhancement circuit 52 during processing 
of the video signal. The exposure measurement circuit 50 measures, e.g., 
maximum, minimum and average brightness levels, which are transferred 
through respective parallel-to-serial latches (not shown) to the 
microprocessor 46 on the serial data output (SD.sub.out) line. The timing 
of the respective data transfers is controlled by a system clock (SYS CLK) 
signal from the microprocessor 46 to the circuit 38. 
The microprocessor 46 also controls an iris driver 54, which is connected 
to the external part of the iris 20, and a speed controller 56, which is 
connected to the stepper motor 26. An input select switch 58 provides a 
mode signal to the microprocessor 46 depending upon the type of object 16, 
that is, one condition of the mode signal indicates a positive original 
(such as a photographic slide) and another condition of the mode signal 
indicates a negative original. (The preferred embodiment is intended for 
processing photographic positive transparencies and negative 
transparencies, but the principle of the invention can be applied to any 
types of originals having different spectral characteristics. 
The microprocessor 46 regulates the rotation speed of the color filter 
wheel 22 in relation to coincidence (or lack thereof) between the arrival 
of the pulse signal from the photodetector 30 and the vertical drive 
signal (V.sub.D) from the gen lock circuit 14. From such coincidence, the 
microprocessor 46 controls the speed waveform generated by the speed 
controller 56. Based on such information and control, the microprocessor 
46 determines which color filter segment is wholly within the optical path 
32. As this information is useful elsewhere in the signal processing, the 
color segment identification is applied as a color signal to the 
integrated circuit 38 and to an input port COL of the framestore 40. 
Because the circuit 38 operates on discrete pixels, the pixel clock signal 
and the horizontal (H.sub.D) and vertical (V.sub.D) drive signals are also 
provided to the circuit 38. 
The framestore 40 is enabled to store video signal according to the 
condition of a capture signal applied by the microprocessor 46 to a 
control port CN. Video field signals are applied to an input port IN of 
the framestore 40 and stored in the framestore whenever the capture line 
is pulsed high at the beginning of a field transmission. In this way, a 
selected combination of signal segments corresponding to the selected 
combination of filter segments is stored in the framestore 40. The 
framestore 40 is operated in two modes: a live mode and a store mode. In 
the live mode, which is used for set-up and framing of the original, a 
live output signal is continuously provided by continuously storing 
fields, and outputting fields, as they are provided to the framestore 40. 
In the store mode, which is used for the final copy image, the most recent 
set of fields is output. In either case the output is taken from the 
framestore 40 at video rates although the input to the framestore may be 
at some other (non-video) rate. 
The video signals are input serially (through port IN) to the framestore 40 
and output in parallel (though output ports 01, 02, 03) to the D/A 
converter 44. The analog red, green and blue output signals are applied in 
parallel to a display controller 60, which includes conventional manual 
controls for contrast, brightness, hue, saturation, white balance, 
sharpness and the like. The display controller 60 generates a RGB output 
signal both as a direct output of the video copy apparatus and as an input 
to an encoder circuit 62, which encodes the video signal and generates 
either a composite NTSC (or , or SECAM, as the application requires) 
signal or a colorplexed Y/C (luminance, chrominance) signal. 
The color filter segments R.sub.1, R.sub.2, B and G are specially sized to 
accommodate two consecutive fields of each color. This is better seen in 
FIG. 5, in which sixteen positions of the image sensor 12 are projected 
upon the color wheel 22 for one revolution thereof. In other words, the 
beginning position of each field scan is shown relative to the filter 
segments for each sixteenth of a revolution. One consequence of this 
arrangement is that, for an angular velocity of 3.75 revolutons per 
second, exactly 60 fields will be scanned in one second, which is the 
nominal video field rate. The diagram should also be understood to show 
that the beginning position of one field scan, e.g., field scan 1, is also 
the ending position of the previous field scan, e.g., field scan 16. From 
that observation, it should be clear that some field scans are wholly 
within the area of the filter segments R.sub.1, R.sub.2, G and B while 
other field scans are partially completed over the opaque areas 22a 
between filter segments. The latter scans contain useless video 
information and are discarded during signal processing. 
For example, considering the first red filter segment R.sub.1, field scans 
1 and 2 are wholly within the red transmissive area of the filter segment 
and therefore receive valid video information--that is, the first scan 
starts at field scan 1 and ends at field scan 2, and the second scan 
starts at field scan 2 and ends at field scan 3. The next two scans, 
however, are only partly over the transmissive area of the filter segment 
and therefore receive partially useless video information--that is, the 
third field scan starts over the transmissive area at field scan 3 but 
ends over the opaque area 22a at field scan 4 while the fourth field scan 
starts over the opaque area 22a at field scan 4 and ends over the 
transmissive area shown as field scan 5. As this type of scan extends over 
a full revolution, it leads to a pattern of saving two fields, discarding 
two fields, saving two fields, discarding two fields . . . and so on. 
Keeping the sequence of field utilization in mind, the operation of the 
video copy apparatus will be described in connection with FIG. 3. Two 
revolutions of the color filter wheel 22 are shown, as indicated by a 
twice repeated sequence of sixteen field scans. Boundaries are delimited 
by broken line above the field scan designations to show the approximate 
extent of the two red filter segments R.sub.1 and R.sub.2, the green 
filter segment G, and the blue filter segment B. A hypothetical situation 
is presented in which a slide is scanned during the first revolution and a 
negative is scanned during the second revolution--obviously, in actual 
practice, since the wheel 22 is spinning at 3.75 revolutions per second, 
there will be many hundreds of revolutions while one original is being 
lined up, framed, and copied, and the transition between types of 
originals will seldom correspond exactly to the transition between field 
16 and field 1, as shown in FIG. 3. 
The mode signal from the input select switch 58 indicates the type of 
original. The switch 58 can be set by the user or it may result from 
automatic detection of either a slide holder or a negative holder in the 
system. The indication for a slide is the high condition of the mode 
signal; the indication for a negative is the low condition. The capture 
pulse provided to the control port CN of the framestore 40 determines 
which field scans are to be saved and, by the absence of pulses, which 
field scans are to be discarded. The main decision involves saving of the 
field scans corresponding to either the first red filter segment R.sub.1 
or the second red filter segment R.sub.2. The first red filter segment has 
a transmissivity corresponding to the red response curve shown in FIG. 1 
for slides; the second red filter segment has a transmissivity 
corresponding to the red response curve for negatives. 
The microcomputer 46 is programmed to provide the appropriate capture 
signals, as shown in FIG. 3, to the framestore 40 for storing either 
fields 1 and 2 from a slide original imaged through the filter segment 
R.sub.1, or fields 5 and 6 from a negative original imaged through the 
filter segment R.sub.2. In both cases, the same green fields 9 and 10 
imaged through the filter segment G, and the blue fields 13 and 14 imaged 
through the filter segment B, are saved. FIG. 3 is also useful in showing 
that the capture pulses are timed to store the fields (1, 2, 5, 6, 9, 10, 
13, 14) wholly imaged through the filter segments R.sub.1, R.sub.2, G and 
B while discarding the fields (3, 4, 7, 8, 11, 12, 15, 16) partially 
obstructed by the opaque portions 22a of the filter wheel 22. Color 
filters are well known and widely available in the art that have spectral 
passbands that at least closely approximate the spectral responses shown 
in FIG. 1 for either slides or negatives. 
While the color filter segments are preferably supported for rotary motion 
on a color filter wheel, other mechanical forms of conveyance can be used 
to insert a selected filter segment into the optical path 32 between the 
image sensor 12 and the original 16. FIG. 6 illustrates such an 
alternative arrangement. Each color filter R.sub.1, R.sub.2, G and B is 
individually and separately movable from a position wholly outside the 
optical path to a position in the optical path, where image light from the 
original transmits through the filter and strikes the image sensor 12. In 
FIG. 6, the blue filter B is shown in the latter-mentioned position over 
the sensor 12. Each filter is moved by an electromechanical device; for 
instance, the red filter R.sub.1 is shown attached by a rod 66 to the 
armature 68 of a solenoid 70. The solenoid 70 is activated at the 
appropriate time to move the red filter element R.sub.1 into the optical 
path of the image sensor 12, and deactivated to move it out of the optical 
path. Though not shown, the other filters R.sub.2, G and B would be 
similarly attached to additional solenoids. 
The microprocessor 46 activates the respective solenoids in a predetermined 
sequence to shade the image sensor 12 with the appropriate combination of 
filters for the selected input mode thereby generating a sequential image 
signal for further processing according to the invention. The separate 
filters of FIG. 6 may be inserted into the optical path in a sequence 
similar to that described in connection with FIG. 2, that is, each 
solenoid 70 is operated in a predetermined, unchanging sequence and the 
separation of the correct color signals is made later in the signal 
processing chain. On the other hand, as shown by broken line in FIG. 6, 
the input mode selection could be interlocked with the solenoids 70 and 
only the correct color filters are inserted over the image sensor 12 for a 
given input mode. In that case, the selection of the color signals 
appropriate for the input mode (slide or negative) is made at the front 
end of the system by activating the correct combination of filters. 
The embodiment of FIG. 6 is particularly useful with an image scanner of 
the type in which the image sensor 12 is a linear array with 
photosensitivity for one line. A typical scanner completely scans all 
lines in one color (by relative movement between the original and the 
linear array) before proceeding to the next color; thus each color filter 
is inserted over the linear array for the duration of a complete scan of 
the original. Depending on the nature of the original, one of the two red 
filters is used for that part of the scan. In an alternative 
configuration, each line scan is completed in all three colors before 
proceeding to the next line. In this case, the three color filters are 
sequentially inserted over the linear array for each line. 
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. For instance, while the preferred embodiment has been described 
in connection with the NTSC television standard, it should be clear that 
the invention can be utilized with any other television standard by, e.g., 
appropriate modification of the line scan and the field rate. Although the 
preferred embodiment produces an analog output subsequent to digital 
processing, the invention is equally applicable in systems directly 
producing a digital output. The invention may be employed in applications 
other than typical video copying applications; in particular, the 
embodiment of FIG. 6 in the form of an image scanner can be used in a 
graphic arts application. In such an application, the output signal may 
not be a typical (NTSC, , SECAM) video signal; the invention is 
nonetheless intended to extend to such apparatus.