Abstract:
Calibration targets for use with digital cameras comprise black and white target areas to which digital cameras are exposed simultaneously to provide proper settings for highlight and shadow sensitivity prior to making a digital photograph. In addition a white or gray neutral target area, without color bias, is provided, to which the digital cameras are exposed to provide desired settings for color balance prior to making digital photographs. In one embodiment of the calibration target, the black and white target areas are on one side of a panel and the white or gray neutral target area is on the other side of the panel. In accordance with methods of calibrating digital cameras and to methods of photography, the image of the black and white target areas is adjusted by viewing a histogram display thereof and adjusting exposure settings until spikes in the histogram representing black and white detection are equally balanced between boundaries with no lateral offset.

Description:
RELATED APPLICATIONS 
   This application claims the benefit of the filing date of U.S. Provisional Application Ser. No. 60/402,603 filed Aug. 12, 2002 and incorporated herein in its entirety by reference. 

   FIELD OF THE INVENTION 
   The present invention is directed to calibration targets for digital cameras and methods of using same. More particularly, the present invention is directed to calibration targets for selecting exposure settings and achieving white balance in digital cameras and to methods for using the targets. 
   BACKGROUND OF THE INVENTION 
   In contrast with the digital photography, film photography traditionally has an exposure tolerance of two to four stops. An original scene exposed within this tolerance range will retain a good quality tonal curve due to the film, which is the recording medium, having a light sensitivity range which exceeds the tonal extremes in an average scene of two to four stops. Exposure for digital photography has a very narrow range, so that when there is over exposure or under exposure, part of the information is lost. Consequently, unless you initially achieve the desired exposure you make an inferior image file which will not be in full detail either in highlight areas or shadow areas. There is no way to retrieve these lost details for the image file. Consequently, in digital photography techniques have evolved to fix images after they have been taken. This is a time consuming and relatively expensive undertaking in which it is still very difficult to compensate for information missing from an image. 
   Inside digital cameras there is a sensor on which a latent light impression of an original scene is made. Based on latent information from the sensor, a central processing unit in the camera processes the information into a proper color spectrum and into a proper color curve that retains the detail from highlight to shadow. The quality of image produced using the latent light impression frequently has diminished quality due to exposure error. 
   All sensors inside cameras have an optimal sensitivity setting (ISO), whether it is due to the sensitivity of film or to the sensitivity of image sensors in digital cameras. Sensitivity has an optimal range where it produces the absolute best image file in terms of color fidelity with the least image defects. In producing an optimal image file, the image file will be given a rated ISO, e.g., 100. Digital cameras have the capability of shooting other ISOs, but as one deviates from the optimum image quality suffers. When deviating from the optimal ISO, noise is introduced into the image files from the CPU and arbitrary abnormalities known as artifacts become visually apparent and the quality and color degrade. Thus, producing optimal digital image files is difficult for the professional photographer and extremely difficult for the consuming public. 
   Hand held light meters do not adequately compensate for inaccuracies in exposure because tolerances are typically plus or minus a half stop of exposure. Typically light meters select a middle tone, the placement of which varies from one manufacturer to another. Since light meters peg the middle of the tonal curve, light meters select gray rather than the black and white extremes. Pegging the middle of the tonal curve can result in the photographer loosing information at one or the other extreme so that light metering does not work effectively. This forces camera manufactures to develop methods to fix latent information. In order to compensate for inadequate latent information, camera manufacturers provide you with software solutions for manipulating improperly exposed and color balanced images. But, these “back end fixes” almost invariably produce inferior image files with which before you can even start to produce a print, require very labor intensive efforts having three times the amount of work to process an image file. Accordingly, there is a need for a technique to correctly set exposure and color balance on the front end, i.e. prior to recording a image. 
   Obtaining correct exposure is part of the problem, the other part being correcting white balance. Most digital cameras provide different options for white balance correction which may be automatic or set by the photographer. Such settings are ball park settings based on daylight, flash, overcast sky, tungsten filament lighting or fluorescent lighting. Daylight varies depending on the time of day with the color temperature being different at morning, midday and afternoon. There are also differences in white balance due to brightness of tungsten light bulbs because brightness determines color temperature. Since current white balance settings for digital cameras are quite inaccurate, there is a need for improvement. 
   In view of the aforementioned considerations, in digital photography there is need for improvement in the ability to select correct or desired exposure settings, as well as a need for improving the ability to correct or select desired white balance settings. 
   SUMMARY OF THE INVENTION 
   The present invention is related to a calibration target and to methods of calibrating digital cameras prior to exposing an image of an original scene to an image sensor within the camera by exposing the image sensor to the calibration target located at the original scene. 
   The calibration target has a substantially white target area which reflects substantially all wavelengths of visible light and a substantially black target area which absorbs substantially all wavelengths of visible light, which calibration target when focused upon achieves an exposure setting for the digital camera. The exposure setting is then adjusted for the target until an adjusted exposure setting for black and white detection is substantially balanced within the camera for a selected intensity and distribution of light at the original scene. The adjusted exposure setting is then applied to at least one image photographed at the scene. 
   In a further aspect of the invention, at least one substantially gray target area is provided and exposed simultaneously with exposure to the black and white target areas in order to assist the photographer in centering black and white spikes on a histogram display on the camera. 
   In still a further aspect of the invention, the adjusted exposure setting for an exposure balanced image is exposed to a substantially neutral, white or gray target area without color bias that reflects equally substantially all wavelengths of light to produce settings for a color balanced, calibrated image having a selected white balance for the original scene. 
   In still another aspect of the invention, adjusting the exposure is performed by viewing a histogram display within the digital camera and adjusting the exposure settings until spikes representing black and white detection are not offset laterally in the histogram display. 
   In still another aspect of the invention, an initial exposure setting is obtained of the location prior to exposing the image sensor to the calibration target. The initial exposure setting is then adjusted with subsequent exposures to the calibration target to achieve correct or desired exposure settings. 
   The present invention is also directed to a target configured for calibrating digital cameras prior to exposing images of original scenes to image sensors within the cameras. The target configuration comprises at least a white target area reflecting substantially all wavelengths of visible light incident thereon and at least a black target area absorbing substantially all wavelengths of visible light incident thereon. The white and black target areas have substantially equal areas, wherein simultaneous exposure of the image sensors to the target at locations of original scenes determines desired exposure settings for images positioned at the scenes. 
   In a further aspect of the invention, the target further includes at least one middle gray target area which reflects substantially all wavelengths of light without color bias. 
   In still a further aspect of the invention, the target includes a neutral white or gray target area without color bias, which neutral area when positioned at locations of scenes prior to recording images of the scenes, determines settings for desired color balances for the images. 
   In a further aspect of the invention, the target configuration is a panel having black and white target areas on a first side and the neutral target area on a second side of the panel. 
   The invention further relates to a method of digital photography utilizing the aforedescribed calibration methods and calibration targets. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various other features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views, and wherein: 
       FIG. 1  is a perspective view of a scene to be photographed; 
       FIG. 2  is a perspective view of a portion of the scene of  FIG. 1  showing a light meter being employed to select exposure settings; 
       FIG. 3  is a perspective view showing a first embodiment of a calibration target configured in accordance with the present invention; 
       FIG. 4  is an uncorrected image of the calibration target of  FIG. 3  in a view finder of the camera; 
       FIG. 5  is a histogram display of the uncorrected image of  FIG. 4 ; 
       FIG. 6  is a corrected image of the calibration target of  FIG. 3  in the view finder of the camera; 
       FIG. 7  is a histogram display of the corrected image of  FIG. 6 ; 
       FIG. 8  is a perspective view showing a second embodiment of a calibration target configured in accordance with the present invention; 
       FIG. 9  is an uncorrected image of the second embodiment of the calibration target in the view finder of the camera; 
       FIG. 10  is a histogram display of the uncorrected image of  FIG. 9 ; 
       FIG. 11  is a corrected image of the second embodiment of the calibration target in the view finder of the camera; 
       FIG. 12  is a corrected histogram display for the corrected image of  FIG. 11 ; 
       FIG. 13  is a perspective view of a white or gray neutral target for achieving proper white balance at the image scene of  FIG. 1 ; 
       FIG. 14  is a display of custom white balance function determined by exposure to the white or gray neutral target of  FIG. 13 ; 
       FIG. 15  is a view finder image of the white balance target of  FIG. 13 ; 
       FIG. 16  is a tone selector display of an image taken using the white balance target of  FIG. 13  for color balance; 
       FIG. 17  is a planar view of the calibration target of  FIGS. 3 ,  8  and  13  being folded, and 
       FIG. 18  is a view of the calibration target folded in accordance with  FIG. 17  being inserted into a carrying pouch. 
   

   DETAILED DESCRIPTION 
   Referring now to  FIG. 1  there is shown a location  10  at which an original scene  11  is disposed, which scene may include a subject  12  to be photographed and a background  13 . The location  10  may be any location and the scene  11  may be any scene with any particular object  12 , or no particular object. The background  13  may be any background. The scene  11  is illuminated by a light source  16 , which may be any light source, for example, a lamp, the sun, a photo flash or indirect light, which illuminates the location  10  of the scene  11 . 
   The scene  11  has shadows  18  and highlights  20 . The shadows  18  and highlights  20  may be definite and readily apparent, or may be subtle. For any subject  12 , the shadows and highlight are usually intermingled, varied and complex. 
   The subject  12  and background  11  also has colors and tones, which may vary widely and interact with highlights and shadows to produce a visual image of the original scene  11 . A digital camera  24  having a lens, focuses the scene  11  on image sensors in the camera that record light electronically. Typically, in digital cameras the image sensors are charged coupled devices (CCD) or complimentary metal oxide semiconductor (CMOS) devices. A computer device in the form of a central processing unit (CPU) converts electrical charges from the image sensors into digital data which is then stored as digitalized image information in the camera&#39;s memory. 
   As is seen in  FIG. 2  a light meter  22  takes a reading proximate the subject  12  to determine initial exposure settings for the camera  24 . 
   As is seen in  FIG. 3 , a calibration target  40 , configured in accordance with the principles of the present invention, is inserted into the scene  11  at the approximate location of the subject  12  (if there is a subject). The calibration target  40  has a black target area  42  which absorbs substantially all wavelengths of visible light incident thereon from the light source  16  and a white target area  44  which reflects substantially all wave lengths of visible light incident thereon from the light source  16 . The black and white target areas  42  and  44  cover substantially equal portions of the target  40 . In the illustrated example, the target areas  42  and  44  are approximately rectangular with curved sides and separate from one another at line  45 , however it is only necessary that the black and white target areas have the same amount of area. Accordingly, the black and white areas may have a checker board configuration or may be in the form of writing or a logo. For example, the white area  44  may form a background for the black area  42  which is in the form of writing with perhaps a logo or other design also in black. It is emphasized that the black and white target areas  42  and  44  need only be equal in area, but may have any desired configuration. While the black and white target areas  42  and  44  are illustrated as being on a panel, these areas may be on separate cards which are juxtaposed with one another, may be on adjacent sides of a carton, on a poster, on a screen or on a folded insert packaged with a digital camera. 
   The black and white target areas  42  an  44  are neutral in that they contain no color cast, the black target area  42  being substantially pure black and the white target area  44  being substantially pure white. The target  40  is not limited to specific size constraints or materials provided that it&#39;s reflective qualities are pure black and pure white. 
   Referring now to  FIGS. 4-7  in conjunction with  FIG. 3 , the method of the present invention is practiced by focusing the camera  24  on the calibration target  40  and setting the camera to a “image and histogram” or “histogram only” display  50  (see  FIGS. 5 and 7 ) on the LCD display panel of the camera  24 . Care is taken to ensure the light falling on the calibration target  40  is of the same intensity as the light which will be illuminating the subject  12  ( FIG. 1 ). In otherwords, if the source  16  of light is the sun providing full illumination, then the illumination falling on the target should be full sunlight and not sunlight obscured by clouds or haze. If the light falling on subject  12  is from a cloudy sky, then the light on the calibration target  40  should be from the same cloudy sky. If the light source is a tungsten lamp, then the same lamp at the same position is used for the calibration target  40  as for the subject  12 . 
   Preferably in practicing the method of the invention, an initial exposure is made using the camera&#39;s internal light meter or a handheld light meter  22  to provide initial exposure settings (see  FIG. 2 ). As is seen in  FIG. 5  this results in a histogram display  50  with two distinctive spikes  52  and  54  representing an initial exposure settings. The spike  52  represents black and the outside spike  54  represents white. Typically, the first exposure using the cameras internal light meter or an external light meter  22  will result in a histogram similar to  FIG. 5  in which the black and white spikes  52  and  54  are shifted with respect to a left side boundary line  56  or a right side boundary line  58 . The exposure settings of the camera  24  are then adjusted to move the spikes  52  and  54  to be equally contained in the area  60 ; equally balanced between the boundaries  56  and  58 , and not offset to either the right or left side. 
   A second exposure ( FIG. 3 ) is made to see if the spikes  52  and  54  remain in the area  60  and equally balanced between the left and right boundaries  56  and  58 , respectively and not offset to either the right or left side. If the spikes  52  and  54  both remain in the area  60  and are equally balanced between the boundaries  56  and  58  with no lateral offset, the exposure settings are correct and the digital camera  24  is properly calibrated to photograph the subject  12 , once the subject is returned to the scene  11  (if there is a subject  12 ) (see  FIG. 1 ). If the black and white spikes  52  and  54  are still not both in the area  60  and equally balanced between boundaries  56  and  58  with no lateral offset, then the exposure settings of the camera are again adjusted and another exposure made to see if the spikes  52  and  54  are equally balanced within the left and right boundaries  56  and  58 . The process of adjusting settings and then exposing image sensors in the camera to the calibration target  40 , with the adjusted settings, and then viewing the additionally adjusted histogram display is continued until the spikes  52  and  54  are in the area  60  and equally balanced between the left and right boundaries  56  and  58  with no lateral offset. This may take a single adjustment or several adjustments to achieve. In any event, the subject  12  may now be digitally photographed with exposure settings properly set so that highlights and shadows of the photographed image of the original scene  11  will correspond to the original scene. 
   While a histogram display  50  is used in accordance with one embodiment of the invention, the characteristics of the histogram display are sensed electronically in another embodiment of the invention (not illustrated) and centered automatically by the CPU of the camera  24 . In this alternative embodiment values assigned to the locations of black and white spikes  52  and  54  are compared to values assigned to the boundaries  56  and  58 . After exposing the image sensors in the camera  24  initially to the camera&#39;s internal light meter or a hand held light meter  22  to obtain values for initial exposure settings corresponding to the positions of values for spikes  52  and  54 , the camera is focused on the calibration target  40  and another exposure made. The value for the settings for this exposure are then compared to the values for the boundaries  56  and  58 . If the calibration target values fall between the values for the boundaries  56  and  58 , the exposure settings are calibrated. If not, an additional exposure is made or several additional exposures are made, until the values for the black and white spikes  52  and  54  do fall between the values for the boundaries  56  and  58 . 
   The additional exposures may be done manually or automatically using a sequence of exposures with the histogram display simply informing the photographer visually or audibly that the exposure settings are correct. 
   While an initial exposure based on light meter readings would appear desirable, when using an automatic sequence of exposures this step could be either dispensed with or incorporated into the sequence, relying preferably on he camera&#39;s internal light meter. 
   Referring now to  FIGS. 8-12 , a second embodiment  70  of the calibration target is disclosed wherein the calibration target includes a black target area  72 , a white target area  74  and a middle gray target area  76 . The middle gray target area  76  is 50% gray, i.e. halfway between the black target area  72  and the white target  74 . As is seen in  FIGS. 10 and 12 , there is a middle target area spike  53  between the black target area spike  52  and the white target area  54 . The middle spike  53  represents the gray area and is used to indicate whether the spikes  52  and  54  are shifted left or right. 
   In  FIG. 10  the spikes  52  and  54  are shifted to the left with part of spike  52  being sheared off. This indicates that an under exposed image of the original scene with the subject  12  will occur if the setting obtained from the light meter  22  are used. The exposure settings of the camera  24  are then corrected to shift the spikes  52 ,  53  and  54  to the right as is shown in  FIG. 12 . The entire spike  52  is then inside the boundary  56  with the spike remaining inside boundary  58 . 
   While not its primary purpose or design, because the calibration target  40  or  70  contains known values for highlight, mid tone and shadow, it can be useful for back end correction when included in an original scene. While this is a desirable feature, it is primarily a convenience so that a photographer does not have to carry multiple adjustment tools in case a mistake is made and backend corrections are necessary. 
   A single gray target area  76  is shown in  FIG. 8 , which is equal in area to the target areas  72  and  74 . In accordance with a further embodiment of the invention, there may be two or more gray target areas, each having a tone different from the gray area  76 . These additional gray areas will each have an area equal to the black and white target areas  72  and  74 , and the single target area  76 . By having multiple tones for the gray scale, finer level adjustments may be made on the backend after the image of the original scene has been recorded. 
   Referring now to  FIG. 13 , in conjunction with the calibration target  40  or  70  there is shown a neutral target area  85  that is either white or neutral gray, which neutral target area substantially reflects all wave lengths of light neutrally without color bias. Subsequent to obtaining exposure settings using the calibration target  40  or  70 , the neutral calibration target area  85  is focused upon and exposure made by the digital camera  24  in order to achieve white balance for the aforementioned corrected exposure settings. Most cameras  24  provide several options for white balance by providing settings on a menu for daylight, flash, overcast skies, tungsten lighting and fluorescent lighting. These settings are generally not accurate because daylight varies depending on the time of day with the color temperature of daylight being different in the morning, midday and late afternoon. The difference in tungsten depends on how bright the tungsten light bulb really is and what the color temperature will be. All other preset settings also have variances and therefore these other settings can not be totally accurate either. 
   Prior to exposing the image of an original scene  11 , the menu on the digital camera  24  is accessed to select a custom white balance function illustrated in  FIG. 14  and the set button pressed using as the white point the neutral target area  85 . The camera  24  now has a clear white display  86  in the view finder  59  of  FIG. 15 . A tone selector display  87  of  FIG. 16  is then used to select from the previously recorded images, images  86  of the neutral calibration target  85 . The camera  24  has now been color corrected so that when the subject  12  is photographed, the colors of the image are identical thereto even though the color temperature is different. This allows one to customize the camera  24  so that the camera knows what the white point should be and by setting the white point you get perfect color images from the camera. Alternatively, white balance can be achieved prior to adjusting exposure by relying on handheld meter or the camera meter to achieve a base exposure, and then following the custom white balance procedures as outlined above. 
   As long as the settings for exposure are correct, as obtained by using the calibration targets  40  and  70  of  FIGS. 3 and 8 , and as long as the white balance is correctly set, it does not matter what light source  16  is utilized. The photographer will get consistent color, and when the subject  12  is put in front of the camera, the subject will look exactly the same to the camera. Different custom white balances may be stored in the memory of the camera  24  so that each time situations or locations  10  are changed, the appropriate white balance settings can be clipped to the situations so that they can be set up in advance. For example, a wedding photographer might set white balance at the front of the church, white balance for the back of the church and white balance for outside the church. As the photographer moves to each of these locations, the photographer picks a correct custom white balance settings for that location. 
   Referring now more specifically to  FIGS. 8 and 13 , the specific calibration target  70  used in practicing the principles of the present invention is in the form of panels  101  having a first side  102  with the black target area  72 , the white target area  74  and the gray target area  76  providing an exposure setting calibration target. On a second side  110  of the panel  101  there is the neutral target area  85  which is gray or white providing white balance as described in  FIGS. 13-16 . The panel  101  is fabricated from four separate pieces of cloth fabric, with the black, white and gray target areas  72 ,  74  and  76  of the first side  102  being stitched together to form the calibration target for determining exposure settings, and the neutral white or gray target area  85  being fabricated from a single sheet of fabric to form the second side  110  of the panel. The first and second sides  102  and  110  of the panel  101  are stitched to a cloth tube  120  which contains a flat peripheral spring which has flat sides facing radially. 
   As is seen in  FIGS. 17 and 18 , the panel  101  has relatively straight edges  130 - 133  joined by curved edge portions  135 - 139  so that the panel  101  is foldable into overlapping portions. The panel can then be inserted into a zippered pouch  140  for convenient carrying. 
   From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.