POST-IMAGE ACQUISITION EXPOSURE CONTROL SYSTEM FOR ELECTRONIC IMAGING CAMERAS

Abstract: A post-image acquisition exposure control system is disclosed for use with an electronic imaging camera (10) where the analog values from a sensor (20) within the camera (10) are adjusted to extend over a predetermined range, thus optimizing image exposure in the analog domain and automatically compensating for overexposure or underexposure within the camera.

Description: Post-Image Acquisition Exposure Control System for Electronic Imaging Cameras Background of the Invention The present invention relates generally to electronic imaging camera systems, and, more particularly, to electronic still cameras utilizing an electronic sensor to capture an image and having a post-exposure control system. In a conventional camera utilizing chemical -based film, exposure of an image onto the film substantially completes the image acquisition process. Because the film is chemical based, post-image acquisition exposure control by the camera is impractical. For such conventional imaging, adjustments to color balance, improvements in image quality, and exposure adjustments can be done only during subsequent film processing, where subjective decisions are made by an operator of film processing equipment in an attempt to improve the quality of the image. One can understand that this method is a two-step process: the image is obtained, and then the post-exposure processing operator adjusts for image quality. Moreover, where instant photography is used, there is essentially no means for image quality adjustment after film exposure. In electronic imaging camera systems, the image is typically acquired by aii electronic sensor, such as a charge coupled device (CCD). After image acquisition, the image data is transmitted to a video signal processor (VSP) and an analog-to- digital (A/D) converter. The VSP gain is normally preset before image acquisition occurs. When underexposure or overexposure occurs, the image is respectively processed below or above an optimal A/D conversion point. This results in inefficient use of the digital information available. Conventional post digitization methods of correcting the underexposure or overexposure often result in "contouring" because there are insufficient data bits available to appropriately represent the image after the acquired data bits have been shifted and stretched into the optimal range. The results of this method are illustrated in Fig. 1 where actual test image data is shown after digitization. Noise can be added to break up or dither the image. Where underexposure or overexposure occurs, there is a resultant decrease image quality. When a mechanical shutter is used to control the exposure of an electronic still camera, exposure error can occur because of mechanical limitations of the shutter. Mass, friction and tolerance buildup during fabrication all contribute to imprecise control of the opening and closing of the mechanical shutter. Additionally, motor drive or spring force limitations on the mass of the mechanical shutter limit the speed with which the mechanical shutter can operate. As a result, underexposure or overexposure of the image occurs in the electronic still camera. Summary The aforementioned shortcoming of conventional electronic camera systems are achieved by the invention which provides, in one embodiment, a post image- acquisition exposure control system. The post image-acquisition exposure control system measures, prior to exposure, ambient brightness and subject distance and makes use of these measurements to select a nominal blade trajectory for the shutter blades. If artificial illumination is required, a strobe fire f-stop is also selected. Subsequent to the exposure operation, voltage levels of ambient and infrared (IR) integrators are obtained so as to estimate the actual exposure received by the image sensor. An optimum A/D conversion point is determined prior to the exposure operation. The image sensor output is modified by a factor which varies directly as the optimum exposure and inversely as the estimate of actual exposure, with the result that the actual exposure is processed at the optimal A/D conversion point. That is, the actual exposure of the image sensor is adjusted by a gain factor such that it substantially equals the predetermined optimum A/D conversion point of the camera and thus compensates for inadequate or excessive exposure of the image. The output of the VSP complies with a predetermined voltage range of the camera from which the A/D convertor converts the voltage range across the widest spectrum available to the camera. Thus, all digital values or bits available to the electronic camera are used. Brief Description of the Drawings The foregoing and other features of this invention, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings in which: Fig. 1 is a histogram of the results of conventional image digitization; Fig. 2 is a block diagram of an exposure control system in an electronic still camera in accordance with the invention; Fig. 3 is a more detailed block diagram of the system of Fig. 2; Fig. 4 is a histogram of voltage levels obtained from an analog signal processor showing both before and after post exposure corrections in the system of Fig. 2; Fig. 5 is a histogram of pixels vs. digital value in the system of Fig. 2 for actual test data prior to post-exposure correction; and Fig. 6 is a histogram of pixels vs. digital value in the system of Fig. 2 for actual test data subsequent to post-exposure correction. Detailed Description While the present invention retains utility within a wide variety of photographic devices for both still and video photography and may be embodied in several different forms, it is advantageously employed in connection with an electronic still camera. Though this is the form of the preferred embodiment and will be described as such, this embodiment should be considered illustrative and not restrictive. Fig. 2 shows an electronic still camera 10 which captures image-bearing light 12 reflected from a subject. Image-bearing light 12 is selectively allowed to pass into electronic still camera 10 by a set of shutter blades 14. Image-bearing light 12 then impinges upon a beam splitter 16 which allows a large portion of image-bearing light 12 to be transmitted therethrough as image light 18. Image light 18 impinges upon an imager 20. A remaining portion of image bearing light 12 is reflected from beam splitter 16 as reflected light 22 onto both an ambient photodiode 24 and an IR photodiode 26. Reflected light 22 is used to determine the ambient light level by means of ambient photodiode 24 and the infrared light level by means of IR diode 26. In an alternative embodiment, tracking apertures are used to measure ambient light. Prior to exposure, shutter blades 14 open to allow ambient light to pass therethrough such that ambient photodiode 24 measures reflected light 22 and passes an electronic signal representative of the ambient light signal so obtained to an ambient exposure integrator 28. Ambient exposure integrator 28 serves as a summing node for ambient light to determine a value representative of the intensity of ambient light. The intensity value is transmitted to an exposure microprocessor 30 which uses this value to select a nominal blade trajectory for shutter blades 14 and. if the intensity is less than adequate or fill flash is required, then a strobe fire f-stop number is chosen to determine the point at which a strobe should be fired. When that the ambient brightness level has been determined, an exposure microprocessor 30 causes shutter blades 14 to open and, if necessary, signals strobe control 29 to fire strobe 25. This calculation can also incorporate subject distance as determined by object ranging systems well known in the art. Image bearing light 12 then passes through shutter blades 14 and transmitted light 18 presents an image to imager 20. In the preferred embodiment, imager 20 comprises a CCD, but one skilled in the art will appreciate that imager 20 may be a charge injection device (CID) or other such electronic sensor. During exposure, as imager 20 is converting transmitted image bearing light 18 into electrical signals, photodiodes 24 and 26 are monitoring the exposure. Strobe exposure integrator 27 sums the current from IR photodiode 26 to determine when strobe 30 is to be shut off. Once a predetermined value has been reached, strobe control 29 is shut off to avoid overexposure. Once exposure is complete, exposure microprocessor 30 closes shutter blades 14. Exposure microprocessor 30, by using preferred exposure date from ambient exposure integrator 28 and strobe exposure integrator 27, and having controlled the actual exposure by controlling shutter blades 14, has the means to calculate a difference between the preferred exposure and the actual exposure. Further, exposure microprocessor 30 examines the actual voltage values of ambient exposure integr JΓ 28 and strobe exposure integrator 27 from ambient photodiode 24 and IR photodiode 26 to determine if the voltage values lie within a predetermined range indicative of proper exposure. Exposure microprocessor 30, having information related to the exposure and having prior knowledge of the preferred exposure in which the image signals should pass, can now calculate a gain factor necessary to achieve that exposure value. Thus, if the image is underexposed, analog signal 34 can be amplified in the analog signal processor to achieve the proper exposure. The proper exposure as herein discussed is one which places an analog value representative of eighteen percent (18%) grey in the image at a predetermined optimal point of the analog voltage scale. In the preferred embodiment, the analog voltage scale is from zero to two volts as an expected voltage range to A/D convenor 40. In this instance, a nonlinear A/D convertor is presumed, although in the actual embodiment, a gamma curve is introduced before the A/D convertor for the nonlinear adjustment, as described in greater detail below. In this system, eighteen percent grey is placed at approximately one volt. The remainder of the image is left intact so as to not distort the image. One skilled in the art will realize that eighteen percent grey is chosen because it is conventionally accepted as being a midpoint in the human tone perception scale. Imager 20 downloads analog signals 34 representative of the image to an analog signal processor 36. Analog signal processor 36 then passes a compensated analog signal onto A/D converter 40 which digitizes the image siiznal and passes the digitized signal 42 to a remaining portion (not shown) of camera 10. This remaining portion may include digital signal processing, image storage, transmission, and other such electronics well known in the art. Fig. 3 shows a block diagram showing additional detail in analog signal processor 36. As before, shutter blades 14 open to allow image bearing light to impinge upon imager 20 and phoiodiodes 24 and 26. Ambient and infrared exposure integrators 31 then integrate light incident upon ambient photodiode 24 and IR photodiode 26. The resulting values are passed to exposure microprocessor 30 which comprises a gain block 33. Gain block 33 calculates a gain factor necessary to increase (or decrease as the case may be) the image values such that the midpoint of the human tone perception scale corresponds to approximately half of the available analog scale. For example, if the exposure range is from zero to one volt, where one volt is the maximum value (indicating that the image has been severely underexposed), and with eighteen percent grey located at five tenths (0.5) volt, and the camera is designed for a range of zero to two volts, then gain block 33 determines that the values from sensor 20 are to be doubled. Analog signal 34 from imager 20 is transmitted to analog signal processor 36 where it is multiplied, by the value determined in gain block 33, in a multiplication block 50. The multiplication operation in multiplication block 50 is performed to create a multiplied signal 52 compensated to increase or decrease exposure of the image, as required. Multiplied signal 52 is then passed to a gamma block 54 which performs a conversion such that the tone scale spectrum of the image is somewhat altered. This conversion is explained in commonly-assigned U.S. Patent 5,539,459 "Optimal tone scale mapping in electronic cameras,' * issued to Bullitt et al. on 23 July 1996, and is incorporated herein by reference. Processed signal 38 is then passed to A/D converter 40 which digitizes processed signal 38 to create a digitized signal 42 which is passed to the remainder of camera 10. The graph of Fig. 4 is a histogram giving pixel distribution as a function of voltage level. The signals shown are referred to with numerals corresponding to those numerals in previous Figures. Signal 34', shown as a solid line, is the analog signal as it comes directly from the sensor. It can be seen that if that analog signal were immediately digitized, it would be severely underexposed because maximum value reaches only to 127 in an 8-bit, 256-value digitization system. Since the camera is expecting 256 values, black (designated as 'B' on the graph) and white (designated as 'W') are determined to be located in predetermined areas on such a graph. Where the underexposure of 34' is utilizing only half of the values available, the black area B transgresses into other areas of the image itself, thereby making the image less usable. Dashed line 52', which is shown after the multiplication but before a gamma curve is applied, shows how this curve is now spread out, making better use of the 256 digital values available to it. This curve is shown having a lower amplitude that that of signal 34' since the area under the curves remains constant (i.e., the number of pixels does not change). The next step (not shown) of applying the gamma value concentrates the image in between the B and the W points. Figs. 5 and 6 provide actual test data to illustrate use of the invention. As with the previous example, digital pixel values are plotted on the abscissas of the histograms and the number of pixels having that digital value are on the ordinates. Like numerals designate previously-described elements. Fig. 5 shows signal 134 which is an analog signal representative of an underexposed image as it comes directly from sensor 20. It can be seen that if that analog signal were immediately digitized, that it would be severely underexposed. Fig. 6 shows an analog signal 152 representative of the underexposed image 134 corrected by post-exposure, pre-digitization gain correction. Analog signal 152 is shown after multiplication but before a gamma curve is applied, and illustrates how this curve is now spread out to make better use of the available digital values. The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims: Claims The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 1. An electronic still camera (10) for capturing image-bearing light reflected from a subject to form an electronic image, said electronic still camera (10) comprising: preferred exposure means for measuring an intensity of ambient light and calculating a preferred exposure for that intensity of ambient light; actual exposure means for generating an actual exposure value representing an amount of the image bearing light received by said electronic still camera (10); comparison means (30) in electrical communication with both said preferred exposure means and actual exposure means, for determining an exposure error which is a ratio of the preferred exposure to the actual exposure; and gain means (33) for adjusting the electronic image to compensate for said exposure error. 2. The electronic still camera (10) according to claim 1 wherein said preferred exposure means comprises: ambient light means (24) for measuring ambient light during exposure and calculating an exposure value representative of an exposure of the subject in said electronic still camera; and an exposure processor (28) which uses said exposure value to calculate a gain factor by which the electronic signal is multiplied to obtain a compensated image having the preferred exposure. 3. The electronic still camera (10) according to claim 2 further comprising a gamma block (54) for converting said compensated image such that the tone I I scale spectrum of said image is altered to optimize scene brightness over a digital scale. 4. The electronic still camera (10) according to claim 2 wherein said exposure processor (28) analyzes voltage values from said ambient light means to determine if the voltage values fall into a predetermined range indicative of proper exposure. 5. The electronic still camera (10) according to claim 1 wherein said gain means (33) adjusts the electronic image such that a tone scale of the electronic image has a center at a predetermined optimal point of the analog voltage scale. 6. The electronic still camera (10) according to claim 5 wherein said predetermined optimal point is an approximate midpoint of the human tone perception scale. 7. The electronic still camera (10) according to claim 1 wherein said gain means (33) calculates a gain factor necessary to increase or decrease image values of the electronic image such that a median of image values produced by the exposure corresponds to approximately one half of an available analog scale. 8. An electronic camera (10) comprising an electronic sensor (20) for detecting image-bearing light (12) representative of an object and converting the image-bearing light (12) into analog signals, said sensor (20) having a preferred exposure range for proper exposure of the image, said electronic camera (10) further comprising amplification means (33) in electrical communication with said electronic sensor (20) for amplifying the analog signals such that a predetermined voltage value in the analog signals is adjusted to correspond to an optimal point in the proper exposure range of said electronic still camera (10), thus compensating for inadequate or excessive exposure of the image. 9. The electronic camera (10) according to claim 8 further comprising analog- to-digital conversion means (40) for converting the analog signals into a predetermined range of digital values. 10. The electronic camera (10) according to claim 8 wherein said amplification means (33) sets the minimum voltage to approximately zero volts corresponding to black scene information. 11. The electronic camera (10) according to claim 8 wherein said amplification means (33) multiplies the analog signals by variable gain factor. 12. The electronic camera (10) according to claim 1 1 further comprising processor means (36) for determining the variable gain factor. 13. A method for adjusting exposure of an electronic image captured by an electronic camera, the method comprising the steps of: measuring an exposure of ambient light and calculating a preferred exposure for that intensity of ambient light; generating an actual exposure value representative an amount of the image bearing light received by the electronic still camera; determining a ratio between the preferred exposure and the actual exposure; and adjusting the electronic image to compensate for the ratio. 14. The method according to claim 13 wherein said step of measuring an exposure of ambient light further comprises the steps of: measuring ambient light during exposure and calculating an exposure value representative of an exposure of the ambient light; and using the exposure value to calculate a gain factor by which the electronic signal is multiplied to obtain a compensated image having the preferred exposure.

Inventors: ERNEST PAUL W, FANTONE STEPHEN D, GREEN ALLAN C, ERNEST, PAUL, W., FANTONE, STEPHEN, D., GREEN, ALLAN, C.
Applicants: POLAROID CORP, POLAROID CORPORATION
Application year: 1997
Main classification: H04N5
IPC codes: H04N5