Patent Application
20110243550
The present invention relates to shutter systems and methods, and more particularly to systems and methods for high-speed shuttering of rolling-mode integration focal plane arrays.
Complementary metal-oxide-semiconductor (CMOS) sensors are becoming a popular alternative to charge-coupled devices (CCDs) as electronic imaging devices due to their relatively low cost, low power consumption, and overall reduced system size and complexity. Also, the CMOS process allows for integration of digital circuitry (including image processing functions) on the same piece of silicon as the imaging circuitry. CMOS focal plane arrays, unlike CCDs, take advantage of current integrated circuit fabrication processes to provide much smaller pixel pitches than average CCDs.
The chart below sets out a comparison between the characteristics of a CCD focal plane array and a CMOS focal plane array.
Unfortunately, CMOS sensors have several drawbacks that limit their use for certain applications. For example, less sophisticated (and hence less expensive) CMOS focal plane arrays use rolling-mode integration. Such focal plane arrays are generally unable to capture imagery in a single snapshot with variable exposure times.
A dual-shutter system for a rolling-mode integration image sensor comprises two rotating shutter members each having a shutter aperture, with rotation of the shutter members carrying the shutter apertures past a system aperture position that is in registration with the image sensor while moving the shutter members into and out of registration with each other. Radiation can only reach the image sensor when the shutter apertures are in registration with each other and are in further registration with the system aperture position and hence also in registration with the image sensor. This “gates” the light reaching the image sensor, allowing a rolling-mode integration imaging device to effectively operate in a snap-shot mode.
In one aspect, the present invention is directed to a shutter system for a rolling-mode integration imaging device. The shutter system comprises a support structure and first and second shutter members rotatably mounted to the support structure so as to have respective first and second axes of rotation. The first and second shutter members have respective first and second shutter apertures defined therethrough, which have respective first and second circular orbits about the first and second axes of rotation. The first and second shutter members at least partially overlap each other, and the first and second circular orbits of the shutter apertures also at least partially overlap so that rotation of the first shutter member and second shutter member at different frequencies cycles the first shutter aperture and the second shutter aperture into and out of registration with each other.
The shutter system may further comprise at least one actuator drivingly coupled to the first shutter member and the second shutter member to rotate the first shutter member and the second shutter member relative to the support structure. A single actuator may be drivingly coupled to both the first shutter member and the second shutter member, or a first actuator may be drivingly coupled to the first shutter member and a second actuator may be drivingly coupled to the second shutter member.
The shutter members may be disc-shaped shutters rotatably mounted to the support structure at their respective centers, and the shutter apertures may be respective truncated sector-shaped gaps in the first shutter member and the second shutter member. Optically clear material may extend across at least one of the first shutter aperture and the second shutter aperture.
In one embodiment, the first axis of rotation and the second axis of rotation are co-located so that the first shutter member and the second shutter member are rotatable about a single, common axis of rotation. In another embodiment, the first axis of rotation and the second axis of rotation are spaced from each other.
In another aspect, the present invention is directed to a rolling-mode integration imaging system. The imaging system comprises a rolling-mode integration focal plane array, and a shutter system as described above. The focal plane array is carried by the support structure, and a controller is coupled to the at least one actuator to control rotation of the shutter members so that rotation of the first shutter member and the second shutter member at different frequencies cycles the first shutter aperture and the second shutter aperture into and out of registration with each other in further registration with the focal plane array. The focal plane array may be a CMOS focal plane array.
In a further aspect, the present invention is directed to a method for operating a rolling-mode integration imaging device. The method comprises rotating first and second overlapping shutter members at different rotational frequencies, with the first and second shutter members having respective first and second shutter apertures defined therethough, so that radiation is permitted to reach an image sensor of the imaging device only when the first and second shutter apertures are in registration with each other in further registration with a focal plane array of the imaging device. The difference between the rotational frequency of the first shutter member and the rotational frequency of the second shutter member is selected according to a desired frame rate for the imaging device.
In order that the subject matter may be readily understood, embodiments are illustrated by way of examples in the accompanying drawings, in which:
The term “focal plane array”, as used herein, refers to a sensor that is sensitive to optical radiation and which is spatially discrete so as to be capable of forming an image, and is not limited to CMOS devices. As used herein, the term “rolling-mode integration” as applied to an image sensor refers to a situation in which all of the photodetectors on the focal plane array are essentially integrating, that is, recording received light, all of the time. Not all CMOS devices operate with rolling-mode-integration, and CMOS or other imaging devices that do not use rolling-mode integration are not appropriate for the systems and methods of the present invention. The present invention is intended for use with rolling-mode integration image sensors, that is, image sensors for which the detection process is active all of the time. As such, the term “image sensor” is not intended to be limited to electronic or digital image sensors, and may also include, for example, conventional film. As such, a rolling-mode integration imaging device may be, for example, a CMOS camera system or a conventional film camera. In the context of a conventional camera, the portion of film being exposed may be considered to fall within the scope of the term “focal plane array”.
For a rolling-mode focal plane array to capture instantaneous (i.e. “snap-shot”) images, some external mechanism is required. Ideally, an electronic shutter could be employed. Unfortunately, such devices (e.g. normal or ferro-electric LCDs) typically suffer from two disadvantages: at least one half of all incoming light is lost, which exacerbates the comparatively poor low-light performance of CMOS imaging devices, and the extinction-ratio is rarely better than 1000:1.
This leaves mechanical shutters as the primary method for controlling the light input to the focal plane array. Several mechanisms exist to provide high-speed shuttering. Familiar examples are the moving slot used in film and digital SLR cameras, and the leaved-iris used in medium/large format camera lenses; both of these work well for still cameras, but will eventually fail mechanically when used repetitively on motion video.
High-end film movie cameras shutter light via a wheel rotating at the film's frame rate with a variable aperture width specified in degrees; typical ranges are 10-180°, resulting in 1.16-13.89 ms exposure. The length of the exposure is approximately tframe×θ/π, where tframe is the frame rate.
It is difficult for such single-wheel shutter systems to provide controlled exposures at the low-end (e.g. the difference between 1-2 ms for a 1 Hz frame rate represents an aperture width difference of 0.36°, which would be about 300 μm on a 100 mm diameter wheel).
According to an aspect of the present invention, a dual-shutter system is provided which comprises two rotating shutter members, each having a shutter aperture. Rotation of the shutter members will repeatedly move the shutter apertures past a system aperture position that is in registration with the image sensor while also moving the shutter members into and out of registration with each other. Radiation can pass through the shutter members and reach the image sensor only when the shutter apertures are in registration with each other at the system aperture position.
Referring now to
The first and second shutter members 104, 106 have respective first and second shutter apertures 114, 116 defined therethrough. In the illustrated embodiment, the first and second shutter apertures 114, 116 are circular. Optically clear material 108 extends across the first shutter aperture 104 and the second shutter aperture 106; optionally the optically clear material 108 may be omitted. The optically clear material 108 is “optically clear” in the sense that it does not distort radiation that it permits to pass through the respective aperture 104, 106; the optically clear material 108 may be truly transparent and thus permit virtually all radiation to pass through the apertures 104, 106, or may provide suitable spectral and/or polarization filtering. Optically clear material may extend across one or both of the first shutter aperture 104 and the second shutter aperture 106 to provide desired optical effects during exposure of the image sensor (not shown in
The first and second shutter members 104, 106 are mounted to the support structure 102 so that they partially overlap each other and, as can be seen, the first circular orbit O1 and the second circular orbit O2 partially overlap each other. In the particular exemplary shutter system 100 shown in
When incorporated into an imaging device, such as a CMOS imaging device, one or more actuators, such as motors of sufficient precision, are drivingly coupled to the first shutter member 104 and the second shutter member 106 to rotate the first shutter member 104 and the second shutter member 106 relative to the support structure 102.
In the shutter system 100 shown in
As also shown in
In the second embodiment 200, a single actuator in the form of a motor 230 carried by the support structure 202 drives both the first shutter member 204 and the second shutter member 206. The first shutter member 204 is mounted to the drive shaft 234 of the motor 230, and the second shutter member 206 is mounted to a driven shaft 238 carried by the support structure 202. The motor 230 is drivingly coupled to the second shutter member 206 by way of a transmission assembly 240 comprising a first gear 242 on the drive shaft 234 of the motor 230, a second gear 244 on the driven shaft 238, and a gear belt 246 mounted on the first and second gears 242, 244 to transmit rotation from the first gear 242 to the second gear 244. The sizes of the first and second gears 242, 244 are selected according to the desired rotational frequency of the second shutter member 206 in view of the rotational frequency of the first shutter member 204. The transmission assembly 240 shown in
Reference is now made to
A first motor 330 is drivingly coupled to the first shutter member 304 and a second motor 332 is drivingly coupled to the second shutter member 306. In particular, the first motor 330 has a first drive shaft 334 which carries the first shutter member 304 and the second motor 332 has a second drive shaft 336 which carries the second shutter member 306. The first and second drive shafts 334, 336 are coaxial with each other and with the common axis of rotation 3ARCOMMON, with the second drive shaft 336 rotatably nested inside the first drive shaft 334. The first drive shaft 334 has a larger diameter than the second drive shaft 336, and the first motor 330 has a central passageway 348, including an axial bore through the first drive shaft 334, through which the second drive shaft 336 extends and in which the second drive shaft 336 can rotate freely and independently of the first drive shaft 334. For example, the second drive shaft 336 may be supported inside the first drive shaft 334 by lubricated bearings. Thus, the first motor 330 can rotate the first shutter member 304 independently of the second motor 332 and the second shutter member 306, and the second motor 332 can rotate the second shutter member 306 independently of the first motor 330 and the first shutter member 304.
In the third embodiment 300 of a shutter system shown in
Reference will shortly be made to
Where a coaxial shutter system, such as the shutter systems 300, 400, 500 shown in
The lens assemblies 466, 566, 1066A, 1066B shown in the Figure are not necessarily a single lens, but may be any suitable combination of optical elements.
The time history of radiation (e.g. light energy) passing through the shutter apertures is the integration of the shape(s) of the shutter apertures and the bundle of light being propagated by the optical system (similar to a convolution integral). Manipulating the aperture shapes directly impacts the light throughput function over time. In the exemplary embodiments described in respect of
Moreover, shutter members and shutter apertures within the same shutter system may have different shapes from one another.
Shutter systems according to aspects of the present invention enable a method for operating an imaging device, in particular a CMOS imaging device. As shown in
The system aperture position 720, 820, 920 is defined by the position of the image sensor 780, 880, 980 of the imaging device, and hence is shown with dashed lines. Thus, in the exemplary implementations of the method shown in
The first shutter member 704, 804, 904 is shown rotating at the same rotational frequency in each of
The length of each exposure of the image sensor will be determined by the aperture width and the rotational frequencies of the shutter members, and is given, for the case of identically sized shutter members with identical aperture widths θ, by the formula tframe×θ/2π; thus the dual-shutter system enables more precise, and hence significantly shorter, exposure lengths than with a single rotating shutter (for which the length of the exposure is approximately tframe×θ/π). Higher frequencies provide shorter exposures. The exposure length will be halved by rotating the shutter members in opposite directions, as shown in
Even though the photodetectors on the focal plane array are essentially integrating all of the time, the dual-shutter system gates the incoming radiation (e.g. visible light) so that such radiation strikes the focal plane array for a shorter duration than the frame period of the imaging system. Although the gated burst of light radiation reaches each photodetector at a slightly different time, i.e. there is a slightly different relative time offset for each pixel, as long as the shutter members are rotating sufficiently quickly the data output of the image sensor will effectively represent a single point in time, i.e. a snap-shot.
Although power sources 180, 192, 280, 292, 380, 392, 480, 482, 492, 580, 582, 592, 1080, 1082, 1092 have been shown schematically as batteries, this is merely for ease of illustration; any suitable power source may be used in accordance with aspects of the present invention. Moreover, while these power sources have been illustrated as being individual power sources, this is again merely for ease of illustration, and two or more components may share a common power source.
In the Figures, the controllers 190, 290, 390, 490, 590, 1090 have been illustrated as a microprocessor unit carried by the respective support structure 102, 202, 302, 402, 502, 1002; any suitable controller, including without limitation purpose-built controllers and suitably programmed general purpose computers, may be used in accordance with aspects of the present invention. In addition, a controller need not be a separate component, but may form part of the imaging or image processing systems associated with an imaging device.
In
One or more currently preferred embodiments have been described by way of example. It will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as defined in the claims.
