Abstract:
A fluorescence standard for identifying variations in illumination during imaging has a composite fluorescent laminar structure, which fluoresces in response to light in the approximate wavelength range of 375 nm to 430 nm. The fluorescent object has at least two areas with different fluorescent response, e.g., a first made from a strongly luminescing material, such as GG420 filter glass. A portion of the GG420 glass is covered by filter glass having an attenuating effect on the fluorescent response. In accordance with a method of the present invention, variations in illumination during imaging with a camera are detected by placing the standard before the camera during imaging. Each captured image may contain the image of the standard and the fluorescent response of the standard in different images can be compared to identify any response changes due to variations in illumination. The variations in illumination can then be remediated by adjusting the source of illumination, the camera or ambient lighting. Alternatively, the images can be normalized through digital image processing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/848,707 filed Oct. 2, 2006, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an apparatus and method for calibrating skin imaging systems, and more particularly, for calibrating skin imaging systems in which the skin is photographed under UV and/or blue light resulting in a fluorescent image. 
     BACKGROUND OF THE INVENTION 
     Various imaging systems have been proposed that photographically capture images of a person&#39;s face for analysis of the health and aesthetic appearance of the skin. Different images, captured at different times or under different lighting conditions can be used and/or compared to one another to gain insight into the condition of the skin and its response to treatment. This was typically done by human operators inspecting the photographs to identify certain visual indicators of skin condition and to ascertain changes between photographs. When the skin is photographed under an illuminating light, such as a flash or strobe light, the light intensity and wavelength of the light can vary from one photograph to another. Environmental lighting conditions can also lead to variations in illuminating light. Variations in illuminating light can result in variations in the digital images captured which are not attributable to skin condition changes, thereby lessening the probative value of digital imaging analysis. 
     SUMMARY OF THE INVENTION 
     The problems and disadvantages associated with conventional apparatus used in digital skin imaging are overcome by a fluorescence standard for identifying variations in illumination during imaging conducted at a plurality of times, which includes a fluorescent object that fluoresces in response to light in the approximate wavelength range of 375 nm to 430 nm. The fluorescent object has two areas with different fluorescent response. In accordance with a method of the present invention, variations in illumination during imaging with a camera are detected by placing a fluorescent object, which fluoresces in response to light in the approximate wavelength range of 375 nm to 430 nm before the camera. A first image of the fluorescent object is captured with light in the approximate wavelength range of 375 nm to 430 nm. A second image of the fluorescent object is captured with light in the approximate wavelength range of 375 nm to 430 nm. The fluorescent response of the fluorescent object in the first image is then compared to the fluorescent response of the fluorescent object in the second image. 
     Other aspects, features and advantages of the present invention will be apparent from the detailed description of the invention that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic view of a person having their skin photographed in a skin imaging station which incorporates the calibration apparatus of the present invention; 
         FIG. 2  is a perspective view of the calibration apparatus shown in  FIG. 1 , wherein the calibration apparatus is shown separate from the skin imaging system to facilitate consideration and discussion; 
         FIG. 3A  is front view of a first photographic image of a subject and the calibration apparatus, as taken at time T 1 ; and 
         FIG. 3B  is front view of a second photographic image of a subject and the calibration apparatus, as taken at time T 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention includes an apparatus and method for calibration of a skin imaging station. The calibration apparatus includes a fluorescence standard with a first layer made of material having fluorescent properties similar to that of skin when exposed to UV and/or blue light. Additional layers made of translucent material that partially overlay the first layer attenuate the fluorescence intensity of the first layer producing a multi-step calibration standard. 
     In accordance with a method of the present invention, the calibration standard is positioned proximate the subject&#39;s face, is photographed with the subject and appears in the same photographic image. On taking a UV or blue fluorescence photograph, the different portions of the calibration standard having different numbers of layers absorb the UV and/or blue light and fluoresce at different, known levels, providing multiple fluorescence standards for calibration. A plurality of digital images are recorded for comparison to each other, each recording the fluorescence intensity for the subject&#39;s face and for the standard. The fluorescence values attributable to the standard are compared in subsequent digital images to determine if a variation in intensity has occurred, signaling a variation in illumination brightness. A software routine determines the location of the fluorescence standards in the image. If the light intensity of the illuminating light is determined to have varied, the image may be recaptured by taking another photograph. The illumination intensity may be adjusted prior to taking the replacement image or the photographer may correct environmental factors that led to the variation. Alternatively, the software may adjust the display intensity of the image by adjusting the pixel intensity values to compensate for the variation in illumination intensity. 
       FIG. 1  shows a skin imaging station  10  having the features and functionality described in applicants&#39; co-pending U.S. patent application Ser. No. 10/008,753, entitled, “Method of Taking Inages of the Skin Using Blue Light and the Use Thereof”, which was published as United States Application Publication No. US 2004/0146290 A1, U.S. patent application Ser. No. 10/978,284 entitled “Apparatus for and Method of Taking and Viewing Images of the Skin,” which was published as United States Patent Application Publication No. US 2005/0195316 A1 (“U.S. Publication No. 2005/0195316”), application Ser. No. 11/169,813 entitled “Skin Imaging System with Probe”, which was published as United States Application Publication No. US 2006/0092315 A1 (“U.S. Publication No. 2006/0092315”), all of which are incorporated by reference herein in their entirety. U.S. Publication Nos. 2005/0195316 and 2006/0092315 describe the use of alternative illuminating techniques to highlight and emphasize skin conditions, such as wrinkles or acne, wherein a flash unit which is capable of producing light of a particular wavelength is activated and an image captured with a camera. Various filters may also be employed in this process. 
     One technique described in the above referenced applications involves taking a blue fluorescence photograph of a subject&#39;s skin to illuminate and reveal skin conditions such as acne and “horns” (i.e., mixtures of sebaceous lipids, keratinocytes, and possibly sebocytes impacted in open comedones and blackheads on the skin) by producing bright images of the distribution of coproporphyrin, a substance associated with these conditions. By using substantially only blue light (i.e., light having a wavelength ranging from about 375 to about 430 nm), the fluorescence emission of coproporphyrin is maximized. Excitation in the blue region of the spectrum therefore yields bright fluorescence emission images of the distribution of horns. 
     Blue fluorescence photography typically uses filters having a very narrow bandwidth, and the resulting attenuation requires the use of high-intensity light sources (e.g., flashes). However, high intensity light sources are prone to fluctuations in intensity and color temperature, which may result in inconsistent images. These inconsistencies may also result from slight fluctuations of the power source or environmental factors, such as accidental light exposure from another source (e.g., outside light from opening the door of the room in which the subject is being imaged). Such inconsistencies may appear in successive photographs taken of the subject, if the light intensity of the flash varies between the taking two or more photographs. As a result, images of a subject&#39;s skin that are not taken under substantially identical lighting conditions may vary, which adversely affects the quality and/or consistency of the images obtained and compromises the information gleaned therefrom. Therefore, there is a need for a fluorescence standard to aid in interpreting each photograph, to compare light intensity levels of successively-taken photographs, to adjust for varying incident light intensity and to provide a standard reference for calibration. 
     As shall be apparent from the following, the present invention could be utilized to calibrate other imaging systems, but the referenced system may be used to illustrate the present invention. The skin imaging station  10  has a chin rest  14  for supporting a subject S&#39;s chin during the imaging process. A camera  16  is mounted in imaging station  10  across from the chin rest  14  and the subject S. The distance between chin rest  14  and the front end of the lens of camera  16  and the camera zoom setting is adjusted so that the subject S&#39;s face substantially fills the “frame” of camera  16 , the chin rest  14  positioning the subject in a consistent orientation and distance from the camera  16 . One or more blue flash units  18   a  (only one of which is shown, for the sake of clarity), which are used for blue fluorescent photography, are mounted in the imaging station  10  to illuminate the face of the subject S. A blue filter  18   b  is placed in front of each blue flash unit  18   a . A power pack (not shown) is provided to power blue flash units  18   a . Blue flash unit  18   a  is directed toward the center of the subject S&#39;s face. Other flash units, and their associated filters and power packs, may also be mounted in the imaging station  10  for standard and other types of photography (see U.S. Publication No. 2005/0195316). 
     Still referring to  FIG. 1 , skin imaging station  10  further includes a display monitor  20  operably connected to a computer (not shown) housed in imaging station  10 . More particularly, the computer runs software programs that operate monitor  20 , camera  16 , flashes, e.g.,  18   a  and a user interface. After the subject S has entered his or her relevant biographical and medical information using monitor  20  and is ready to be photographed, the operating software makes a function call to imaging acquisition and display software (“IFDL software”) (IDL Research Systems, Inc., Boulder, Colo.). The IDL software then triggers camera  16  to acquire blue fluorescence photographs (as well as other types of photographs, if desired). Flash unit  18   a  is triggered through the use of radio transceivers (see U.S. Publication No. 2005/0195316), a custom made flash sequencer, a programmable logic controller or “flash distributor”. Prior to taking the blue fluorescence photographs, the IDL software moves long pass filter  24  (Kodak Wratten No. 8 or 12, Eastman Kodak, Rochester, N.Y.) in front of the lens of camera  16 . The blue fluorescence photographs are then taken. After the photographs are taken, the IDL software triggers the servo motor  22 , solenoid or filter wheel to move long pass filter  24  away from the lens of camera  16 . 
     Now referring to  FIGS. 1 and 2 , a calibration standard  26  is mounted on chin rest  14  (e.g., in a slot provided therein) in the present embodiment such that when the subject S positions their chin in the chin rest  14 , the calibration standard  26  is positioned proximate to their face (see  FIG. 1 ). The calibration standard  26  has two or more overlapping layers, and is shown in  FIG. 2  as having three such layers  28 ,  30  and  32 , respectively. The first layer  28  is fabricated from GG420 filter glass (Schott Glass Technologies Pa., Duryea,) a material having fluorescence (excitation and emission) properties similar to that of skin when exposed to UV or blue light, i.e., light having a wavelength of about 375-430 nm. The second layer  30  has a smaller area than that of first layer  28 , and partially overlays first layer  28  (see  FIG. 2 ). The second layer  30  is fabricated from BG39 filter glass (Schott Glass Technologies Pa., Duryea,) a translucent, non-fluorescent material that acts as an attenuating layer. The third layer  32  is similar to second layer  30  in that it is also fabricated from BG39 filter glass and also acts as an attenuating layer. The third layer  32  has a smaller area than that of first and second layers  28 ,  30 , and partially overlays second layer  30  (see  FIG. 2 ). The second and third layers  30 ,  32  progressively reduce the fluorescence intensity of first layer  28 . The three layers  28 ,  30 ,  32  may be held together in a stacked configuration by a plastic housing (not shown). This layered assembly may be removeably attached to the imaging system  10  to allow removal for storage to protect the standard  26  from damage and contamination. Various standards  26  can be used with an imaging station  10  for different imaging sessions. 
       FIGS. 3A and 3B  show images  34   a  and  34   b , respectively, of the subject S and calibration standard  26 , as shown on skin imaging station monitor  20 . During the blue light imaging of the subject S, as fully described in U.S. Publication No. 2005/0195316, the three layers  28 ,  30  and  32  of calibration standard  26  receive blue light of the same intensity as that which illuminates the subject S&#39;s face. The portion of first layer  28  exposed to the blue light (i.e., the area not covered by second and third attenuating layers  30 ,  32 ), has a fluorescence response similar to skin. The second layer  30  has an attenuating effect on the fluorescence of first layer  28 , reducing the amount of fluorescence produced in response to the blue light. The third layer  32 , when combined with second layer  30 , has a greater attenuating effect on the fluorescence of first layer  28 , further reducing the amount of fluorescence produced in response to the blue light. By absorbing the blue light and fluorescing at different, consistent, known levels, the three layers  28 ,  30 ,  32  function as three fluorescence standards to provide multiple reference standards for calibration. A software routine may be used to determine the location of the fluorescence standards in images  34   a  and  34   b , analyze the returning light intensity from the standards incorporated in apparatus  26 , and calibrate the system based on this analysis, as described hereinbelow. 
     Both of the images  34   a  and  34   b  are formed by two-dimensional matrices of pixels. Every pixel occupies a unique (X,Y) location in a matrix and has an intensity value. In each of  FIGS. 3A and 3B , the locations of three sample pixels are illustrated, viz., a pixel located in the area representative of third layer  32  of the standard  26  on the images  34   a  and  34   b  with location (X 1 , Y 1 ), and two pixels at areas representative of the subject S&#39;s skin having locations (X 2 ,Y 2 ) and (X 3 , Y 3 ). Image  34   a  is taken at a time T 1 , while image  34 b is taken at time T 2 . The time each image was taken is denoted with the location coordinates in the images (e.g., (X 1 , Y 1 , T 1 ) in the image  34   a  and (X 1 , Y 1 , T 2 ) in the image  34   b ). 
     When a series of successive photographic images such as  34   a  and  34   b  is taken of a subject S, fluctuations in illumination (flash) light intensity described above may occur between the times T 1  and T 2 , resulting in different light intensity values for the pixels in the areas representative of the standard  26 , e.g., at (X 1 , Y 1 ), as well as the subject S&#39;s skin, e.g., at (X 2 Y 2 ). Varying light intensity of pixels representative of the standard  26  is an indicator that the illumination light has varied. Accordingly, one of the aspects of the present invention is to identify the situation where the illumination light intensity has varied between at least two digital images taken in such varying illumination light. Without the use of the standard, it would not be possible to attribute the difference in light intensity values between one or more pixels, e.g., at (X 2 , Y 2 ) in successive images of the skin (e.g.,  34   a  and  34   b ) to such illuminating light fluctuations, or to varying skin conditions exhibited by the subject S at times T 1  and T 2 . 
     In order to discern intensity variations in the image area corresponding to the standard  26 , that area in the images, e.g.,  34   a ,  34   b  must be identified/isolated so that the intensity values of the correct pixels can be identified. This may be done by assigning a pre-determined region of the image to the standard  26 . More particularly, if the focus setting and orientation of the camera  16  remains fixed, then the standard  26  will appear in the same areas of each image taken, such that the image area corresponding to the standard  26  (and subparts  28 ,  30 ,  32 ) can be empirically determined and remains constant. Alternatively, the image can be scanned (entirely or a subset of pixels, e.g., one of every 50 pixels) to test for repeating intensity values in the form of a rectangle (having a rectangular shape). In the case of a multipart standard  26 , like that shown in  FIG. 2 , the presence of more than one adjacent rectangle (here three) each with consistent intensity values, (progressively decreasing for each area  28 ,  30 ,  32 ) is a reliable indicia of locating the standard  26 . Scanning for the standard  26  has the advantage that movement of the standard in the image, e.g., due to movement or focus change of the camera  16  will not result in erroneous readings for the standard. 
     Having located the pixels representing the standard  26  in the images  34   a ,  34   b , the light intensity values of corresponding pixels, e.g., (X 1 , Y 1 , T 1 ) and (X 1 , Y 1 , T 2 ) can be compared. Subtracting one intensity value, e.g., at (X 1 , Y 1 , T 1 ) from the other, e.g., at (X 1 , Y 1 , T 2 ) yields a number representing the quantified difference in intensity between the pixels. Alternatively, more sophisticated analyses of the intensity differences between the images can be effected that are non-linear, e.g., gamma curves or conversion into alternate colorspaces, particularly for large differentials. In conducting numerical analysis of digital images, e.g.,  34   a ,  34   b , it is frequently beneficial to convert the image from RGB format to L*a*b* format in order to simplify the mathematics and gain greater insight into the color composition and brightness of the images. 
     Given the identification (and quantification) of illumination light variation between images taken at different times, as determined by the present invention, optional remedial steps maybe taken: (i) correct the environmental conditions of the imaging, e.g., instructing an operator to eliminate extraneous environmental lighting input, e.g., from an open door or shade, repositioning the subject, etc. (ii) adjust/correct the source of illumination, e.g., the light  18   a , e.g., by repositioning it, replacing it with another or electronically adjusting its output, e.g., by adjusting the voltage input to the light; or (iii) normalizing the relevant image by adjusting the intensity values of all pixels in the image relative to the image selected as the reference image, e.g., globally adding or subtracting the quantified intensity difference identified by comparing the difference in intensity attributable to the portion of the images representing the standard  26  (and saving the normalized/corrected image for comparison). For example, if the image intensity of a second image is less than a first image by a value of “5” (due to a variation in illumination intensity as determined by the image intensity of the standard  26  appearing in each image) then the second image can be normalized to the first by adding “5” to the pixel intensity of all pixels in the second image. Alternatively, more sophisticated analyses of the intensity differences between the images can be effected that are non-linear, e.g., gamma curves or conversion into alternate colorspaces, particularly for large differentials. With respect to the first two options, i.e., adjusting the environment or the illuminating light, the image with variations is discounted and a new image is taken. With the third option of adjusting intensity values, the image need not be retaken. 
     It should be appreciated that the process of normalizing can be conducted with reference to the standard  26  image intensity values taken from any arbitrary image, e.g.,  34   a  or  34   b , since the process of adjustment is relative, and that the process of normalization can be conducted for any number of images ranging from 1 to any number N. The normalized image(s) may then be displayed or stored with other images in the computer memory or a file. 
     It should be understood that the embodiment of  FIGS. 1-3B  is merely exemplary, and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. For instance, more or fewer attenuating layers may be included in the calibration apparatus  26 . While the present invention has been explained in the terms of adjusting for variations in blue illumination light, the present invention could also be utilized to identify and compensate for variations in illuminating light of other wavelengths. All such variations and modifications are intended to be included within the scope of the invention.