GB2350201A

GB2350201A - Actuating a camera according to flickering ambient light

Info

Publication number: GB2350201A
Application number: GB9911248A
Authority: GB
Inventors: David Neil Slatter
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1999-05-15
Filing date: 1999-05-15
Publication date: 2000-11-22
Anticipated expiration: 2019-05-15
Also published as: GB2350201B; GB9911248D0

Abstract

A camera system (10) is arranged to acquire an image of a subject (32) which is illuminated, at least partly, by cyclically flickering ambient light (34), for example from one or more fluorescent tubes (34). The system comprises: a sensor (20) for sensing a cyclically varying parameter related to the intensity of the flickering light and producing a cyclically varying sensing signal (18); and a control unit (14) responsive to the sensing signal for actuating the camera (12) at a particular instant in the phase thereof. The particular instant may be arranged to correspond generally to a minimum in the intensity of the flickering light so as to minimise glare caused by the ambient light, or it may be arranged to correspond generally to a maximum in the intensity of the flickering light as to maximise exposure of the image.

Description

TITLE Cameras, Control Systems therefor and Methods of Acquiring Images

DESCRIPTION

This invention relates to a system for controlling a camera which is arranged to acquire an image of a subject, to a camera having such a control system, and to a method of acquiring an image of a subject.

The invention was conceived in connection with the acquisition of images of 5 documents, photographs, etc., but it is more widely applicable.

Photocopiers and scanners are well known. Typically a document to be copied or scanned is placed on a platen under a cover, is illuminated by a light under the cover, and an electronic or optical image of the document is acquired. Such systems will be referred to as 'closed' systems. Consistent illumination of the document can be achieved, provided that the cover is properly closed over the document and platen so as to exclude ambient light.

There is a desire to produce 'open' systems. One form of an open system might employ a digital camera which is mounted above a desk so as to acquire an image of a document placed on the desk. In a development of this, the camera might be mounted on a pan-and-tilt head so as to acquire a succession of images (or 'tiles') of different portions of the document which are subsequently electronically 'stitched' together. Another form of an open system might employ a hand-held digital camera which is pointed by an operator at the document to be scanned or copied. In any of these open systems, reliance might be made solely on ambient light in order to illuminate the document, or additional lighting might be provided, for example by a strobe light or flash-gun. Such systems would typically be used indoors where the ambient lighting would typically be artificial and powered by mains electricity, rather than sunlight.

It has been noted that in such open systems, image quality is not consistent. Glare caused by ambient light sources can vary from one image to another, as too can the exposures of different images. This is particularly noticeable in the case where tiles are ed and subsequently stitched together; exposure inconsistency can result in a patchwork Jilt!ftect in the final image.

It has been realised by the inventor that these problems with image quality consistency are, in part, caused by the intensity of the ambient light flickering in synchronism with the AC mains waveform. One cycle of the voltage waveform of a typical single-phase mains electricity supply is shown in Figure 1A; as is well-known, it is sinusoidal and typically has a frequency of 50 Hz (for example in Europe) or 60 Hz (for example in North America). Some mains powered light sources produce light having an intensity which is, to a significant extent, a linear function on the square of the applied voltage. Other light sources may have an intensity which is, to a significant extent, a linear function on the absolute value of the applied voltage. The square and the absolute of the voltage waveform of Figure IA are shown (normalised) in Figures 113 and 1C, respectively. It will be appreciated that both of these waveforms have a frequency which is twice that of the applied voltage (i.e. typically 100 Hz or 120 HZ), and it will therefore be appreciated that the light sources will flicker at this double-frequency. In the case of fluorescent tubes which are typically found as light sources in office and factory environments, the constant in the linear function referred to above may be small, and the flicker may therefore be pronounced. In the case of tungsten filament lamps, the constant in the linear function may be significant (due to the specific heat capacity of the filament) so that the flicker is not so highly pronounced, but this does vary with different filament designs. In either case, the flicker is not necessarily in phase with the mains voltage, for example due to chokes and capacitors used in fluorescent lamp circuits and persistence of the fluorescent material, or due to a non-uniform temperature distribution across the cross-section of a tungsten filament and also its inductance.

Many offices and factories have a three-phase mains supply, with the three phases having voltage waveforms as shown in Figure 2A, and different lamps illuminating the same space may be powered by different phases of the supply. Similarly to Figures 113 and 1C, Figures 213 and 2C illustrate the normalised squares and the absolutes, respectively, of the three v oltage waveforms of Figure 2A. Interestingly, if a document were illuminated equally by three identical light sources powered by the three different phases, and if the light intensity of each source were solely a linear function of the square of the applied voltage, there would be no flicker, because, as shown normalised in Figure 21), the sum of the squares of the voltages of the three phases is a constant. It will be appreciated, however, that this scenario is extremely unlikely to occur. In a similar scenario, except that the light intensity of each source is solely a linear function of the absolute of the applied voltage, a flicker would arise at. six times the mains frequency (i.e. 300 Hz or 360 Hz), as will be appreciated from Figure 2E which shows the normalised sum of the absolutes of the three phases. In the case where a document is illuminated equally by two light sources powered by different phases, a flicker would be produced at twice the mains frequency if the light intensity of each source were solely a linear function of the square of the applied voltage, as will be appreciated from Figure 217 which shows the normalised sum of the squares of the voltages of two of the three phases. If, on the other hand, the light intensity of each source were solely a linear function of the absolute of the applied voltage, there would be a primary flicker at twice the mains frequency with a secondary flicker out of phase with it, as will be appreciated from Figure 2G which shows the normalised sum of the absolutes of two of the three phases. Of course, it is unlikely that a document will be equally illuminated by two or three different sources powered by different phases. Figure 2H shows the normalised sum of the squares of the three phases with weightings of 6:4: 1, and this has a flicker at twice the mains frequency. Figure 21 shows the normalised sum of the absolutes of the three phases with weightings of 6A 1, and this has a flicker which is complex, having three phase-shifted components each at twice the mains frequency.

It has also been realised by the inventor that the above-mentioned problems with image quality consistency occur if the acquisition time (or shutter period) for the image is not large compared with the flicker period, and particularly when the acquisition time is small compared with the flicker period. If the subject is illuminated solely by the mains lighting, different exposures will be obtained for image acquisitions at different times in the flicker period. If additional lighting is being used, and glare is produced by the mains lighting, then the amount of glare will differ for image acquisitions at different times in the flicker period.

One partial solution to these problems would be to increase the acquisition time. However, this is not desirable, especially if the camera is hand-held, because camera shake will produce blurred images.

By contrast, the solution to the problems mentioned above, according to a first aspect of the present invention, is to provide a system for controlling a camera which is arranged to acquire an image of a subject which is illuminated, at least partly, by cyclically flickering light, the system comprising: means for sensing a cyclically varying parameter related to the intensity of the flickering light and producing a cyclically varying sensing signal; and means responsive to the sensing signal for actuating the camera at a particular instant in the phase thereof.

By actuating the camera a particular instant in the phase of the cyclically varying sensing signal (and therefore at a particular instant in the phase of the flicker), image consistency can be achieved.

The particular instant may be arranged to correspond generally to a minimum in the intensity of the flickering light. This is particularly useful if additional lighting is being employed and mains lighting is causing glare, because the glare is minimised.

Alternatively, the particular instant may be arranged to correspond generally to a maximum in the intensity of the flickering light. This is particularly useful if flickering mains lighting is the sole or primary source of illumination, because exposure is maximised.

The actuating means may include means for adjusting the particular instant in the phase of the sensing signal. The adjustment may be manual, or it may be automatic.

The actuating means preferably includes a phase locked loop and/or a zero crossing detector for conditioning the sensing signal.

In the case where the actuating means is arranged to actuate the camera a plurality of times, it preferably does so at the same particular instant in the phase of respective different cycles of the sensing signal. Accordingly, the image consistency achieved by capturing each image at the same instant in the flicker cycle allows the images to be stitched together or combined into one larger image without any significant exposure differences in the different images so that a patchwork quilt effect can be avoided.

In the case where the source of the flickering light is at least one AC electrical light source, the sensing means may be arranged to sense the supply voltage to the light source. Alternatively, the sensing means may comprise a light sensor which can be directed towards the subject.

In accordance with a second aspect of the present invention, there is provided a camera having: an image sensor for acquiring an image of a subject; and a control system according to the first aspect of the invention.

The sensing means may employ the image sensor. In this case, the camera may be actuable (a) in an image acquisition mode to acquire a full frame of pixel data and (b) in a sensing mode to acquire, in a cycle of the flickering light, a series of sensing frames of at least sorne of the pixel data; and the sensing means may be operable to produce, for each such sensing frame, a value dependent on the sum of the pixel data in that sensing frame, these values forming the sensing signal. In the case where the actuating means is arranged to actuate the camera a plurality of times, the camera may include means for adjusting the field of view of the camera in the period(s) between the actuations of the camera.

The camera may be provided in combination with means for producing a flash of light synchronised with the actuation of the camera to provide additional illumination of the subject.

In accordance with a third aspect of the present invention, there is provided a method of acquiring an image of a subject, comprising the steps of. illuminating the subject at least partly with cyclically flickering light; sensing a cyclically varying parameter related to the intensity of the flickering light and producing a cyclically varying sensing signal; and actuating a camera at a particular instant in the phase of the sensing signal.

Specific embodiments of the present invention will now be described, purely by way of example, with reference to the accompanying drawings. In the drawings:

Figures 1A-lC referred to above are various single-phase waveforms; Figures 2A-2I referred to above are various three-phase waveforms; Figure 3 is a schematic block diagram of a first embodiment of camera system; Figure 4 is a schematic block diagram of a camera control unit in the system of Figure 3; Figure 5 is a circuit diagram of an alternative way of producing a sensor signal for input to the camera control unit of Figures 3 and 4; and Figure 6 is a schematic block diagram of a second embodiment of camera system.

Referring to Figure 3, the camera system 10 shown therein comprises: a digital camera 12 having a CMOS image sensor; a camera control unit 14 which (a) receives (i) an image capture signal on line 16 from a manual shutter release switch or other input device and also (ii) a sensor signal on line 18 from a sensor unit 20 having a lens 22 and photodiode 24, and (b) produces a shutter signal on line 26 which is supplied to the camera 12, in response to which the camera acquires and outputs a frame of image data on line or bus 28. Optionally, a xenon flash gun 30 is also responsive to the shuttersignal on line 26 from the camera control unit 14.

The camera system 10 is set up so that the camera 14 is focused on a subject such as a document 32, and the sensor unit 20 and flash gun 30 are also directed at the document 32. The system 10 would typically be used in an environment having ambient lighting produced, for example, by at least one mains-powered fluorescent tube unit 34.

The camera control unit will now be described in more detail with reference to Figure 4. The sensor signal on line 18 is fed to a phase locked loop circuit (PLL) 36 having a multiplier 38, filter 40 and voltage controlled oscillator 42. The output from the PLL 36 is supplied via a variable delay circuit 44 and monostable multivibrator circuit 46 to a hybrid AND logic circuit 48, which also receives the image capture signal on line 16. The. output of the logic circuit 48 provides the shutter signal on line 26. The logic circuit 48 is constructed so that it responds to a rising edge of a pulse from the monostable multivibrator circuit 46 to pass that pulse to the shutter signal line 26 only if the image capture signal on line 16 is high, and then blocks further such pulses for a predetermined time, such as 1 second, even if the image capture signal on line 16 remains high.

The operation of the camera system of Figures 3 and 4 will now be described. Ambient flickering light from the fluorescent tube unit(s) 34 which falls on the document 32 and is reflected via the lens 22 to the photodiode 24 of the sensing unit 20 produces the sensor signal on line 18 corresponding to the intensity of the reflected light. The PLI, 36 conditions the sensor signal to produce a phase-locked pulse train having the same general frequency as the flicker frequency of the ambient light and a generally constant phase relationship relative thereto. The variable delay circuit 44 phase-shifts the output signal from the PLI, 36 and is set so that the rising edge of each pulse from the delay circuit 44 corresponds to a minimum in the intensity of the flickering ambient light. The multivibrator circuit 46 sets the length of each pulse to correspond to a predetermined or preset shutter period for the camera 12, which is only a small fraction of the period of the flickering ambient light. Thus a series of pulses is output from the multivibrator circuit 46, each rising at an instant of minimum ambient light and each lasting for the required shutter period. When an image capture pulse is applied on line 16, the next complete pulse from the multivibrator circuit 46 is passed by the logic circuit 48 as the shutter signal on line 26 and causes the flash gun 30 to fire and the camera 12 to acquire a frame of image data over the period of that pulse and to output the frame of image data on the line or bus 28, whereupon it can be printed, processed or stored. It will therefore be appreciated that the frame of image data is captured by the camera 12 at a time when the intensity of the flickering ambient light is generally at a minimum. Accordingly, any glare produced by the fluorescent tube unit 34 is also generally minimised in the acquired image.

In the system 10 described above, the delay which the delay circuit 44 is required to produce is dependent predominantly on the characteristics of the PLI, 36 and may therefore be preset.

It will be appreciated that many modifications and development may be made to the embodiment of the invention described with reference to Figures 3 and 4. For example, in the case where the period of each shutter pulse produced by the multivibrator circuit 46 is not such a small fraction of the flicker period, the delay produced by the delay circuit 44 may be reduced by, for example, one half of the shutter period, so that the minimum in the intensity of the flickering ambient light corresponds to the middle of the shutter pulse.

Also, in the case where the flash gun 30 is not used and the document 32 is illuminated solely by the flickering light from the fluorescent tube unit(s) 34, the delay produced by the delay circuit 44 may be set so that the beginning or middle of each shutter pulse output from the multivibrator circuit 40 corresponds generally to a maximum in the intensity of the flickering ambient light so as to achieve maximum exposure of the document 32.

Furthermore, in the case where the system is to be used where the flickering ambient lighting is produced only by one conventional such fluorescent tube unit 34 or a plurality of them powered by the same mains phase, the sensor unit 20 may be replaced with a mains sensor unit 50 as shown in Figure 5. This comprises an isolating stepdown transformer 52 having its primary connected to the same mains phase as the fluorescent tube unit(s) 34 and its secondary connected to a zero-crossing detector circuit 54 which produces the sensor signal on line 18. In this case, it may also be possible to omit the PLI, 36 in the camera control unit 14.

In more complex ambient lighting situations, the delay produced by the delay circuit 44 may be adjustable. The adjustment may be done manually by trial and error in order to achieve the desired result (i.e. image capture at minimum ambient light intensity, or image capture at maximum ambient light intensity). Alternatively, in the case where the light sensor unit 20 is employed, the delay adjustment may be done automatically by sampling the sensor signal on line 18 with each output pulse from the multivibrator circuit 46 and nudging the delay period of the delay circuit 44 until the values of the sensor signal samples become a minimum or a maximum as required.

The second embodiment of the invention will now be described with reference to Figure 6, mainly in terms of the differences with respect to the embodiment of Figures 3 and 4.

In Figure 6, the separate ambient light sensor unit 20 is omitted, and the camera control unit is implemented by a pre-programmed microcontroller 56. Furthermore, the camera 12 which is employed has two modes of operation. In a first mode which is triggered by a ftillframe shutter signal on line 26 from the microcontroller 56, the camera 12 outputs a full frame of pixel data on the line or bus 28, similarly to the first embodiment. In the second mode which is triggered by a sub-frame shutter signal on line 58 from the microcontroller 56, the camera 12 outputs a sub-frame of pixel data on the bus 60 to the microcontroller. T he sub-frame consists of a fraction of the possible pixel data spatially distributed over the image area, and the camera is capable of acquiring such sub-frames at a substantially higher rate than the flicker frequency of the ambient light.

The microcontroller 56 is progranuned to operate as follows. In the absence of an image capture signal on line 16, the microcontroller 56 repeatedly outputs sub-frame shutter signals on line 58 and accordingly repeatedly receives sub-frames of pixel data on bus 60, the repetition frequency being, for example, twenty times the mains frequency (e.g. I kHz or 1.2 kHz) and being synchronised with the mains waveform. For each sub-frame which is received on bus 60, the microcontroller 56 averages the values of the pixel data making up that subframe in order to obtain a measure of the intensity of the ambient light when that sub-frame was acquired, and stores the average value. Once at least twenty sub-frames have been processed in this way, for each new sub-frame, the microcontroller 56 determines which of the twenty most recent sub-frames has the lowest average value (if glare is to be minimised), or has the highest average value (if exposure is to be inaximised), and stores the identity of that sub-frame. The microcontroller 56 has therefore determined which of twenty temporal positions in the mains cycle corresponds to the minimum, or maximum, intensity of the ambient lighting falling on the document 32. Then, when the image capture signal is received on line 16, the microcontroller 56 temporarily stops the sub-frame shutter signal on line 58, determines when the mains cycle next reaches the determined temporal position for minimum, or maximum, ambient light intensity, and then supplies the full- frame shutter signal on line 26 so that the flash gun 30, if provided, is fired and so that the camera 12 acquires a full-frame of pixel data which is output on the line or bus 28.

In a possible modification which is shown in Figure 6 (but which call also be somewhat similarly applied to the embodiment of Figures 3 and 4), the field of view of the camera 12 covers only a portion of the document 32, and the camera 12 is mounted on a pan-and-tilt head 62 which -can move the camera 12, under control of the microcontroller 56, between different positions. Accordingly, images of different portions (or tiles) of the document 32 can be acquired in succession and can then be electronically stitched together to form a complete image of the whole document. In view of the operation of the microcontroller 56 as described above, it will be appreciated that the different tiles are acquired at the same instant in the phase of different mains cycles so that the instantaneous level of the ambient lighting is the same for each of the tiles (assuming that the overall ambient lighting has not changed).

It should be noted that the embodiments of the invention and modifications thereto have been described above purely by way of example and that many other modifications and developments may be made thereto within the scope of the present invention.

Claims

1. A system for controlling a camera (12) which is arranged to acquire an image of a subject (32) which is illuminated, at least partly, by cyclically flickering light, the system comprising: means (20;50;12) for sensing a cyclically varying parameter related to the intensity of the flickering light and producing a cyclically varying sensing signal (18;60); and means (14;56) responsive to the sensing signal for actuating the camera at a particular instant in the phase thereof.

2. A system as claimed in claim 1, wherein the particular instant is arranged to correspond generally to a minimum in the intensity of the flickering light.

3. A system as claimed in claim 1, wherein the particular instant is arranged to correspond generally to a maximum in the intensity of the flickering light.

4. A system as claimed in any preceding claim, wherein the actuating means includes means (44) for adjusting the particular instant in the phase of the sensing signal.

5. A system as claimed in any preceding claim, wherein the actuating means includes a phase locked loop (36) for conditioning the sensing signal.

6. A system as claimed in any preceding claim, and including a zero crossing detector (54) for conditioning the sensing signal.

7. A system as claimed in any preceding claim, wherein the actuating means is arranged to actuate the camera a plurality of times at the same particular instant in the phase of respective different cycles of the sensing signal. 20

8. A system as claimed in any preceding claim and for use in the case where the source of the flickering light is at least one C electrical light source (34), wherein the sensing means (50) is arranged to sense the supply voltage to the light source.

9. A system as claimed in any of claims I to 7, wherein the sensing means comprises a light sensor (20) which can be directed towards the subject.

10. A camera having: an image sensor for acquiring an image of a subject; and a control system as claimed in any preceding claim.

11. A camera as clairned in claim 10 when dependent on any of claims 1 to 7, wherein the sensing means employs the image sensor.

12. A camera as claimed in claim 11, wherein: the camera is actuable (a) in an image acquisition mode to acquire a full frame of pixel data (28) and (b) in a sensing mode to acquire, in a cycle of the flickering light, a series of sensing frames (60) of at least some of the pixel 10 data; and the sensing means is operable to produce, for each such sensing frame, a value dependent on the sum of the pixel data in that sensing frame, these values forming the sensing signal.

13. A camera as claimed in any of claims 10 to 12 when dependent directly or indirectly on claim 7, and including means (62) for adjusting the field of view of the camera in the period(s) 15 between the actuations of the camera.

14. A camera as claimed in any of claims 10 to 13, in combination with means (30) for producing a flash of light synchronised with the actuation of the camera to provide additional illumination of the subject.

15. A method of acquiring an image of a subject (32), comprising the steps of: illuminating 20 the subject at least partly with cyclically flickering light; sensing a cyclically varying parameter related to the intensity of the flickering light and producing a cyclically varying sensing signal; and actuating a camera at a particular instant in the phase of the sensing signal.