Patent Publication Number: US-2004047491-A1

Title: Image capturing device with reflex reduction

Description:
TECHNICAL FIELD  
       [0001] In its most general form, the present invention relates to an image capturing device with reflex reduction, comprising at least one wave emitter for emitting electromagnetic waves towards an object and at least one image capturing device, directed towards the object.  
       [0002] More specifically, the invention relates to a device for eye detection of a person (for example relative position of eyelids, the position of the eyes, iris and eye corners, the bright eye effect etc), also comprising a computer processor, for processing images from said image capturing device of the face of the person and determining the required information from said images.  
       TECHNICAL BACKGROUND  
       [0003] When capturing images, especially images intended for automatic analysis, several problems related to the light conditions are present, making computer treatment of the images difficult. To begin with, in some cases the light conditions are difficult or transient, resulting in the images of an object constantly changing. For instance, the surrounding light, such as sunlight from a window or light from a computer screen, may vary, resulting in consecutive images with very different appearances. In other cases it might be dark, making it necessary to illuminate the object in order to acquire any picture at all. Further, parts of the object may be hidden behind obstacles which affect the light. For example, shaded glass may effectively hide an object. These and other problems can in some cases lead to difficulties in acquiring high quality images or performing a desired analysis of the images. Under these and other circumstances, it is desirable to improve the information in the images by actively exposing the object to electromagnetic radiation (hereafter referred to as active radiation), and then capturing the image with the camera. The active radiation can be of a certain wavelength or in a certain wavelength interval, in which case the camera preferably is arranged to only capture incident waves of the determined wavelength, so that variations in the surrounding light conditions therefore interfere less with the image capturing process. The procedure also has the advantage that the camera can “see through” sunglasses, visors and the like. In practice, electromagnetic waves in the IR-region has shown to be quite advantageous, as it is easy to provide and does not disturb the vision of a user.  
       [0004] A problem with this technique occurs, however, when areas of enhanced reflection are present in the object, this often being the case when the object has a varying surface structure and reflection coefficient (e.g. a face). This problem is present also when using conventional video cameras, as light from e.g. a window may be reflected in the object, and thereby cause difficulties in the detection. However, this problem is further emphasized when exposing the object with active radiation, as use of reflected waves is the basic principle underlying the whole idea. When the reflection is disturbed, or varies over the object, the processing of images is affected negatively.  
       [0005] Also, one of the purposes with introducing reflection based detection is to enable different types of eye detection through sunglasses. Unfortunately, any type of glasses or visors present one of the most significant sources of disturbing reflexes, thus making successful detection through sunglasses difficult still.  
       [0006] Eye detection is one field of technology, in which the problem with disturbing reflexes is present. This technology is widely explored in different situations, including detection of gaze direction for vision controlled computer applications. In order to enable a computer processor to automatically detect different characteristicts of the eyes of a user, image capturing devices are sometimes used to provide a flow of digital images to the processor. To determine the required information, such as the gaze direction, advanced image processing is required, and high quality images, with little or no disturbing features, is of course desirable.  
       [0007] The above problems are especially troublesome in this field of technology, as the requirements on the image quality in some cases are very high. Also, the image capturing is often taking place close to a window or in other situations with varying light conditions, or in darkness. Further, the face of a user can present extremely varying reflective qualities, due to topology and complexion. Finally, users are often wearing glasses, and even more disturbing, sunglasses, hiding the eyes. If the purpose of the image capturing is to detect information about the eyes of a user, such as gaze direction, based on images of his/her face, these problems can be serious.  
       SUMMARY OF THE INVENTION  
       [0008] In the following description some terms are used with special meanings, defined as follows:  
       [0009] Object: any object of which an image is desired. The object may be a human being, but the invention may also be applied to other “dead” objects.  
       [0010] Image: Electronic (digital or analog) representation of an object. Note that an image in this sense not necessarily is intended to be seen by a human, but might only be used to acquire certain information, such as a coordinate point.  
       [0011] Reflex: Unless otherwise stated, the term reflex is used for high intensity reflections from areas with enhanced reflection coefficient on an object, that disturb the image quality. Examples are reflections from glasses, from teeth, polished surfaces, etc. This undesired reflection should not be confused with the “diffuse” reflection of incident light used in all image capturing. Most often in the following description, the disturbing reflexes arise from the active radiation.  
       [0012] Eye detection: Determination of different characteristics of the eyes of a user, such as eye position, direction of gaze, eyelid closure factor or bright eye effect. In some cases the eye detection comprises detection of features neighboring the eyes, such as eyebrows or eye corners.  
       [0013] The object of the present invention is to overcome the described problems in image capturing, and to succeed in acquiring an image of satisfactory quality even in difficult light conditions or darkness, and even when reflexes from e.g. glasses are present in the face of the user.  
       [0014] A second object of the invention is to enable acquiring an image of the eyes of a user that is wearing sunglasses, a visor, or the like.  
       [0015] These objects are achieved with a device and a method of the type given by way of introduction, wherein said wave emitter is arranged to emit said waves towards the object alternately from at least two points, said points being separated such a distance (B) that disturbing reflexes, resulting from waves emitted from different points and appearing in an image captured with said image capturing device, are seperable.  
       [0016] According to the present invention, the wave emitter provides alternating illumination of the object, and the image capturing device can thus acquire images based on reflections of waves emitted from different angles. These images will present different patterns of enhanced reflections, or reflexes, caused by the waves from the emitter. For example, reflexes from glasses a person is wearing, caused by the emitted waves, will be located in different places in images of the face acquired when the waves are emitted from different points. This fact can be used advantageously in the subsequent treatment, where the disturbing reflexes from the emitted waves can be eliminated or reduced, for example by comparing the images and eliminating areas with increased intensity.  
       [0017] The distance between said points is preferably 2-6 times greater than the cross measure of the aperture(s) of said wave emitter. However, the distance is dependant upon the optics of the image capturing device, and with the development of this technology, other distances might be found advantageous.  
       [0018] According to a first embodiment of the present invention, the wave emitter comprises at least two wave sources located in different points of said at least two points, and arranged to be activated alternately. This eliminates the need for moving parts, but of course requires several wave sources. If the waves sources are inexpensive and easy to activate/deactivate, this embodiment is normally preferred, as the stationary wave sources secure consistent images.  
       [0019] According to a further embodiment of the invention, the wave emitter comprises one wave source and wave guiding means to alternately direct waves from said wave source along different paths. This reduces the number of required wave sources, to the cost of wave guiding means such as mirrors and shutters.  
       [0020] According to a further embodiment of the invention, the wave emitter includes only one wave source, which is arranged to be movable between said at least two points. A possible way to achieve this is to arrange a wave source continuously movable on a closed trajectory, e.g. a circle. The wave source can be arranged to be activated in suitable, different points or, alternatively, the source is activated continuously, and said image capturing device is arranged to capture images when the wave source is located in suitable points. In any case, the intended alternating illumination of the object is accomplished.  
       [0021] Preferably, the device comprises first processing means, adapted to receive at least two images, acquired with active radiation from different points, and to generate a reflex reduced image. In a first process, it is reflexes from the active radiation that are reduced or eliminated, but by further processing, also other reflexes can be reduced. Thereby a complete system for generating reflex reduced images is realized. This system can be used in a wide variety of applications.  
       [0022] The wave emitter is normally synchronized with the image capturing device, and timing may be non-periodical. However, often a periodic image capturing is implemented, with an alternating frequency dependent upon the application, for example type of video format. In some cases, a frequency of 10-70 Hz may be preferred. In other cases, a higher frequency of more than 100 Hz may be preferred.  
       [0023] The wave emitter is preferably arranged to emit IR waves, and can then be of LED type. An advantage with IR waves is that they do not blind the user nor affect his night vision, and also that they consist of waves with wave lengths outside the visible range, and thus are less affected by other light sources, especially light sources with wave lengths restricted to the visible field, such as fluorescent lamps.  
       [0024] The object of which images are captured can be a human face, and the device according to the invention can then be used in the process of eye detection of a person. Such a device further comprises second processing means, adapted for processing images from said image capturing device and determining required characteristics from said images. This is a field of technology in which actively radiated, reflex reduced images can be of great use, and significantly improve the quality of detection. One type of eye detection of special interest is determination of direction of gaze, but other eye characteristics are also possible. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0025] A currently preferred embodiment of the present invention will now be described in more detail, with reference to the accompanying drawings.  
     [0026]FIG. 1 is a schematic view of an arrangement of a device according to a first embodiment of the invention.  
     [0027]FIGS. 2 a - e  is a schematic view of a wave source in FIG. 1.  
     [0028]FIG. 3 is a schematic view of a wave emitter according to another embodiment.  
     [0029]FIG. 4 is a schematic view of a wave emitter according to another embodiment.  
     [0030]FIG. 5 is a schematic view of a wave emitter according to another embodiment.  
     [0031]FIG. 6 is a schematic view of a camera provided with a wave emitter according of the invention.  
     [0032]FIG. 7 is a flow chart of a general image acquiring process according to the invention.  
     [0033]FIG. 8 is a flow chart of an image acquiring process according to an embodiment of the invention.  
     [0034]FIG. 9 is a flow chart of an image acquiring process according to another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT  
     [0035]FIG. 1 illustrates an arrangement for capturing images of a face  5 , with a device according to a first embodiment of the invention, especially adapted for detection of the bright eye effect of the eyes. In this case, a camera  1  with an electronic image sensor is used as image capturing device, and the wave emitter  2  comprises two separate wave sources  3 .  
     [0036] The camera is preferably a conventional electronic image sensor camera, either of snapshot type or delivering a stream of consecutive images (i.e. a video camera). The images can be in a digital format, e.g. a bitmap format, or in analog form which then can be converted to a digital format, e.g. using a framegrabber circuit.  
     [0037] The electromagnetic waves emitted by the wave sources can be of different types, including IR radiation. In some cases it is preferred that the waves are within a relatively narrow wave length range outside the range of visible light, and that the camera is provided with a band pass filter  4  corresponding to this range. The influence from the surrounding light is thereby further reduced, as many light sources (computer screens, fluorescent lamps, etc) practically only emit waves in the visible light range. The influence from other sources, e.g. the sun, can be reduced if the total radiated energy in the wave band from the wave emitter is at least a significant fraction of the total sun radiation in the same band.  
     [0038] In conventional arrangements with illumination of an object, quite large wave emitting areas are used, in order to accomplish active radiation with high intensity evenly distributed over the object. In the device according to the invention, however, each wave source has as small aperture as possible, as this requires less separation of the sources for achieving an illumination of the object from one source which is distinguishable from illumination from another source. In conventional arrangements where LEDs are employed for illuminating an object with IR-radiation, normally more than 20 LEDs are arranged in a rectangular pattern. In an arrangement according to the invention, less LEDs  10  can be enough in each wave source  3 . FIGS. 2 a - e  illustrates some different suitable LED patterns, with the number of LEDs ranging from one (FIG. 2 a ) to  19  (FIG. 2 e ).  
     [0039] In order to achieve a satisfying result, it is important that reflexes arising from illumination from different points are seperably distinguishable by the image capturing device. Apart from the rather small wave emitting area discussed above, the separation is dependent upon the ability of the camera to distinguish separate high intensity “point” sources of radiation.  
     [0040] Returning to FIG. 1, the wave sources each consist of three LEDs (according to FIG. 2 b ) and are arranged at a distance A of about 50 cm from the face, spaced apart a distance B 4-5 cm. This distance B roughly corresponds to 3 times the cross measure of each wave source area, and has been found to satisfy the requirement of reflex separation, still being close enough to the camera axis to result in the bright eye effect. If the bright eye effect is not required, the two sources may be separated a greater distance.  
     [0041] The wave emitter  2  further comprises a control unit to alternately illuminate the face  5 , whereby any disturbing reflexes from the emitted waves in the face appear at slightly different places. The wave sources  3  is synchronized with the camera  1 , preferably by the same control unit  6 , and acquires one image frame for each illumination. These images can then be treated by processing means  7 , such as a computer processor or especially designed hardware, for reducing the reflexes.  
     [0042] The control of the wave sources can be performed by alternately turning the sources on and off, easily accomplished when using LEDs and schematically illustrated in FIG. 1. Alternatively, as shown schematically in FIG. 3, the sources  3  are constantly activated, and mechanical or optoelectrical shutters  12  are arranged in front of the sources and controlled by a controller  13 , in the simplest case to alternately open and close the shutters  12 .  
     [0043] According to a further embodiment of the invention, illustrated in FIG. 4, only one wave source  3  is used, and two different paths  14   a ,  14   b  for the waves are provided, for example by using mirrors  15  and/or beam splitters. Shutters  16  are arranged in front of each path  14 , and are controlled in the same way as in the case with two wave sources.  
     [0044] Another possibility, illustrated in FIG. 5, is to arrange a wave source  3  movable between two points  17   a ,  17   b , so that this single source can illuminate the object from different angles. In the illustrated example, which is showed as viewed from the user, the wave source  3  is arranged to be movable along a trajectory surrounding the camera  1 . The camera can then be controlled to acquire images every time the wave source is located in e.g. one of the points  17   a ,  17   b.    
     [0045] As mentioned above, the images captured by the camera are in the illustrated example fed to a processing device  7 , adapted for treating the images in order to generate a reflex reduced image.  
     [0046] The image acquiring process is illustrated in FIG. 7. First, in step  21 , the object is illuminated with electromagnetic waves emitted from a first angle, and a first image is acquired, step  22 . Then, in step  23 , the illumination is altered in any of the ways described above, resulting in waves emitted towards the object from a different angle, and a second image is acquired in step  24 . The two images are processed in step  25 , resulting in an image C in which reflexes from the active radiation have been reduced.  
     [0047] The processing of the images can be of different kinds. Basically, two images, acquired with illumination from different angles, are compared to each other to result in an improved image where disturbing reflexes from the active radiation have been reduced. The reduction can be accomplished by building the improved image of information from either the first or second images, systematically selecting the information that has least intensity. The processing device  7  preferably comprises a computer processor, programmed to perform a pixel by pixel comparison between different images. Note that the word pixel is used as “picture element” not necessarily implying a digital image format. Both analog and digital signals may be used, and the comparison can be performed serially or in parallel, in hardware or in software.  
     [0048] A simple formula implementing this method could read as follows:  
       I ( C   n )=MIN( I ( A   n+c ),  I ( B   n+d )  
     [0049] where I(x) is the intensity in pixel x, A n  is pixel n in the first acquired image A, B n  is pixel n in the second acquired image B, C n  is pixel n in the improved image C, and c and d are offset factors for adjustments between the different images, e.g. if cameras with different characteristics are used for different images.  
     [0050] To achieve an improved result, and especially to reduce also other disturbing reflexes, it can sometimes be advantageous to acquire additional images in an additional step  26 . Two different implementations of this method will be described in more detail with reference to the FIGS. 7 and 8.  
     [0051] In the first case, illustrated in FIG. 8, the purpose of the image acquiring is detection of bright eye effect, and two wave sources  3   a ,  3   b  are arranged as shown in FIG. 1, separated approx. 4 cm, with the camera in the middle. Additionally, two wave sources  3   c ,  3   d  are provided at a greater distance from the camera, for example 20 cm away. In the illustrated example, these sources  3   c  and  3   d  are located on the same side of the camera, and are separated 5 cm. In FIG. 7, the first sources  3   a ,  3   b  are arranged in the horizontal plane and the additional sources  3   c ,  3   d  in the vertical plane, but this is not necessary.  
     [0052] Steps  31 - 35  are equivalent to steps  21 - 25  in FIG. 7, and results in an image C with reduced reflexes from the active radiation, as described above. As the sources  3   a  and  3   b  are so close to the camera  1 , a bright eye effect will be present in the images A, B and C. In step  36 , the object is radiated from wave source  3   c , and an image D is acquired in step  37 . Then, in step  38 , active radiation is provided from source  3   d , and an image E is acquired in step  39 . These later images D and E are then processed in step  40 , to result in a second reflex reduced image F, this time without bright eye effect. In a final step  41 , the image F is subtracted from image C.  
     [0053] The most significant difference between images C and F is the bright eye effect present in image C. The subtraction of these images will thus remove most of the image, including disturbing reflexes from the diffuse radiation, leaving only the bright eye effect. This enables a quick and precise determination of the position of the eyes.  
     [0054] In a more simple alternative method, the steps  38 - 41  are substituted with step- 42 , in which image D is subtracted from image C. In this case, the reflexes from the active radiation is not removed from the image without bright eye effect (image D). Therefore, the final image will comprise some patterns apart from the bright eye effect. In some cases, this simpler method is sufficient to locate the eyes.  
     [0055] In the second case, illustrated in FIG. 9, the purpose of the image acquiring is not to detect the eyes, but only to acquire an image of the object without disturbing reflexes from the active radiation nor fromn the ambient light. The wave sources  3   a ,  3   b  are here arranged at a slightly larger distance from the camera  1 , as no bright eye effect is required.  
     [0056] Steps  51 - 55  are equivalent to steps  31 - 35  above, resulting in an image C with reduced reflexes from the active radiation. Then in step  56 , the active radiation is deactivated, and an image D is acquired in step  57 . This image D is then subtracted from the image C to generate a final image.  
     [0057] As images C and D comprises the same image information resulting from the ambient light (sunlight, indoor illumination, etc) the contribution from these sources, including disturbing reflexes, will be eliminated in the resulting image.  
     [0058] It is not necessary that reflex reduction be performed for every image in a sequence. Depending on the field of application, it might be sufficient to apply the reflex reduction only to one image frame out of an image sequence, or on chosen still pictures.  
     [0059] In FIG. 1, the device is illustrated with the wave emitter separated from the image capturing device. Of course, it is possible to provide a conventional electronic image sensor camera  18  (video or snap shot), with a wave emitter  2  according to the invention, as illustrated in FIG. 6. The camera  18  can further be provided with a memory  19  (internal in the image capturing device, or external), for storing images. In some cases it might be advantageous to store the original pictures, acquired with different illumination according to the invention, for later processing. In other cases, it might be preferred to perform the reflex reduction and to store the reflex reduced images. In this case, the camera is also provided with processing means  7  (hardware or software), thereby providing a camera with reflex reduction capacity.  
     [0060] One field of application, particularly suitable for the device according to the invention, is the field of eye detection.  
     [0061] In this case, the reflex reduced images are further processed in a second processing means  8 , preferably a suitably programmed computer processor. The processor analyzes the images and determines the required characteristics, for example the location of the iris of the user, or to the direction of gaze by relating the location of the iris to the other points of the face. In processes such as these, the reflex reduced images acquired with the present invention improve the quality of the results.  
     [0062] Note that in figure one, the processing devices  7  and  8 , and the control unit  6  are illustrated schematically as separate units. Normally, however, these units are integrated in one processing device, such as a personal computer, illustrated by the dashed box  9  in FIG. 1.  
     [0063] In the figures, and in the description above, the number of points from which waves are emitted have been limited to two. This is not to be regarded as a limitation of the invention, and more points can very well be advantageous, further extending the reflex reduction capacity as mentioned above.  
     [0064] When using more than one camera, the minimum is still two different points for wave emitting. It may be advantageous to implement a two-camera system, with at least one wave source provided in each camera.  
     [0065] Also, it may be advantageous to allow for combinations of illumination from several wave sources. For example, the intensity of the electromagnetic waves are normally equal in the two sources. However, two sources can be controlled to illuminate with different intensities. Further, if the active radiation is restricted to a certain wave band, this band can be varied depending on the situation.