Abstract:
Apparatus and methods for wavelength-dependent detection are provided. A detector includes a hybrid filter having unpatterned and patterned filter layers and at least one light-detecting sensor that detects light in first and second wavelength bands from the patterned filter layer of the hybrid filter. The unpatterned filter layer passes light in two nonoverlapping wavelength bands relative to light in wavelength bands between or among the nonoverlapping wavelength bands. The patterned filter layer includes first and second regions configured respectively to pass light in the first and second wavelength bands of the nonoverlapping wavelength bands passed by the unpatterned filter layer.

Description:
This application is a Continuation of U.S. application Ser. No. 10/739,831, filed Dec. 18, 2003 now Pat. No. 7,217,91. 

   TECHNICAL FIELD 
   Embodiments in accordance with the invention relate generally to the field of imaging. More particularly, embodiments in accordance with the invention relate to methods and systems for wavelength-dependent imaging and detection using a hybrid filter. 
   BACKGROUND 
   There are a number of applications in which it is of interest to detect or image an object. Detecting an object determines the absence or presence of the object, while imaging results in a representation of the object. The object may be imaged or detected in daylight and/or in darkness, depending on the application. 
   Wavelength-dependent imaging is one technique for imaging or detecting an object, and typically involves detecting one or more particular wavelengths that reflect off, or transmit through, the object. In some applications, only solar or ambient illumination is required, while in other applications additional illumination is needed. Light is transmitted through the atmosphere at many different wavelengths, including visible and non-visible wavelengths. Thus, the wavelengths of interest may not be visible. 
     FIG. 1  is a diagram of the spectra of solar emission, a light-emitting diode, and a laser. As can be seen, the spectrum  100  of a laser is very narrow, while the spectrum  102  of a light-emitting diode (LED) is broader in comparison to the spectrum of the laser. And solar emission has a very broad spectrum  104  in comparison to both the LED and laser. The simultaneous presence of broad-spectrum solar radiation can make detecting light emitted from an eyesafe LED or laser and reflected off an object quite challenging during the day. Solar radiation can dominate the detection system and render the relatively weak scatter from the eyesafe light source small by comparison. 
   Additionally, the object being detected may not remain stationary during successive measurements. For example, if a human being is the object, the person may shift position or move during the time the measurements are taken. If measurements made at different wavelengths are made at different times, movement of the object during successive measurements can distort the measurements and render them useless. 
   SUMMARY 
   In accordance with the invention, a method and system for wavelength-dependent imaging and detection using a hybrid filter is provided. An object to be imaged or detected is illuminated by a single broadband light source or multiple light sources emitting light at different wavelengths. The light is detected by a detector, which includes a light-detecting sensor covered by a hybrid filter. The hybrid filter includes a multi-band narrowband filter mounted over a patterned filter layer. The light strikes the narrowband filter, which passes light at or near the multiple wavelengths of interest while blocking light at all other wavelengths. The patterned filter layer alternately passes the light at one particular wavelength while blocking light at the other wavelengths of interest. This allows the sensor to determine either simultaneously or alternately the intensity of the light at the wavelengths of interest. Filters may also be mounted over the light sources to narrow the spectra of the light sources. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will best be understood by reference to the following detailed description of embodiments in accordance with the invention when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a diagram of the spectra for solar emission, a light-emitting diode, and a laser; 
       FIG. 2  is a diagram of a system for pupil detection that utilizes a hybrid filter in an embodiment in accordance with the invention; 
       FIG. 3   a  illustrates an image generated with an on-axis light source in accordance with the system shown in  FIG. 2 ; 
       FIG. 3   b  depicts an image generated with an off-axis light source in accordance with the system shown in  FIG. 2 ; 
       FIG. 3   c  illustrates an image resulting from the difference between the  FIG. 3   a  image and the  FIG. 3   b  image; 
       FIG. 4  depicts a sensor in one embodiment in accordance with the invention; 
       FIG. 5  is a cross-sectional diagram of a detector in accordance with one embodiment in accordance with the invention; 
       FIG. 6  depicts spectra for the polymer filters and the narrowband filter shown in  FIG. 5 ;  FIG. 7   a  illustrates a first method for fabricating a dual spike filter in an embodiment in accordance with the invention; 
       FIG. 7   b  depicts the spectrum for the dual spike filter shown in  FIG. 7   a;    
       FIG. 8   a  illustrates a second method for fabricating a dual spike filter in an embodiment in accordance with the invention; 
       FIG. 8   b  depicts the spectrum for the dual spike filter shown in  FIG. 8   a;    
       FIG. 9  is a diagram of a light source and a narrowband filter in an embodiment in accordance with the invention; 
       FIG. 10  illustrates the spectra of the light source and the combination of the light source and the narrowband filter, shown in  FIG. 9 ; 
       FIG. 11  is a diagram of a second system for pupil detection that utilizes a narrowband filter with each light source and a hybrid filter with a sensor in an embodiment in accordance with the invention; 
       FIG. 12  is a diagram of a device that can be used for pupil detection in one embodiment in accordance with the invention; 
       FIG. 13  is a diagram of a system for detecting transmission through an object that utilizes a hybrid filter in another embodiment in accordance with the invention; 
       FIG. 14  depicts spectra for polymer filters and a tri-band narrowband filter in an embodiment in accordance with the invention; 
       FIG. 15  depicts a sensor in accordance with the embodiment shown in  FIG. 14 ; 
       FIG. 16  depicts spectra for polymer filters and a quad-band narrowband filter in an embodiment in accordance with the invention; and 
       FIG. 17  depicts a sensor in accordance with the embodiment shown in  FIG. 16 . 
   

   DETAILED DESCRIPTION 
   The following description is presented to enable one skilled in the art to make and use the invention, and is provided in the context of a patent application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments. Thus, the invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the appended claims and with the principles and features described herein. It should be understood that the drawings referred to in this description are not drawn to scale. 
   Embodiments in accordance with the invention relate to methods and systems for wavelength-dependent imaging and detection using a hybrid filter. A technique for pupil detection is included in the detailed description as one such system that utilizes a hybrid filter in accordance with the invention. Hybrid filters in accordance with the invention, however, can be used in a variety of applications where wavelength-dependent detection and/or imaging of an object or scene is desired. For example, a hybrid filter in accordance with the invention may be used to detect movement along an earthquake fault, or to detect the presence, attentiveness, or location of a person or subject. Additionally, a hybrid filter in accordance with the invention may be used in biometric applications, such as, for example, systems that identify individuals using their eyes or facial features. 
   With reference now to the figures and in particular with reference to  FIG. 2 , there is shown a diagram of a system for pupil detection that utilizes a hybrid filter in an embodiment in accordance with the invention. The system includes a detector  200  and two light sources  202 ,  204 . The system may also optionally incorporate a controller or processor (not shown) in other embodiments in accordance with the invention. 
   Light sources  202 ,  204  are shown on opposite sides of detector  200  in the  FIG. 2  embodiment. In other embodiments in accordance with the invention, light sources  202 ,  204 , may be located on the same side of detector  200 . Light sources  202 ,  204  may also be replaced by a single broadband light source emitting light at two or more different wavelengths in other embodiments in accordance with the invention. One example of such a broadband light source is the sun. 
   In this embodiment for pupil detection, two images are taken of a subject&#39;s  206  face and/or eyes using the detector  200 . One of the images is taken using light source  202 , which is close to or on the axis  208  of the detector  200  (“on-axis”). The second image is taken using light source  204  that is located at a larger angle away from the axis  208  of the detector  200  (“off-axis”). When the subject&#39;s  206  eyes are open, the difference between the images will highlight the pupils of the eyes. This is because the specular reflection from the retinas is detected only in the on-axis image. The diffuse reflections from other facial and environmental features are largely cancelled out, leaving the pupils as the dominant feature in the differential image. This can be used to infer the subject&#39;s  206  eyes are closed when the pupils are not detectable in the differential image. 
   The amount of time the subject&#39;s  206  eyes are open or closed can be monitored against a threshold in this embodiment in accordance with the invention. Should the threshold not be satisfied (e.g. the percentage of time the eyes are open fall below the threshold), an alarm or some other action can be taken to alert the subject  206 . Other metrics, such as the frequency or duration of blinking, may be used in other embodiments in accordance with the invention. 
   Differential reflectivity off a retina of the subject  206  is dependent upon the angle  210  between light source  202  and the axis  208  of the detector  200 , and the angle  212  between the light source  204  and the axis  208 . In general, a smaller angle  210  will increase the retinal return. As used herein, “retinal return” refers to the intensity (brightness) that is reflected off the back of the subject&#39;s  206  eye and detected at detector  200 . “Retinal return” is also used to include reflection off other tissue at the back of the eye (other than or in addition to the retina). Accordingly, angle  210  is selected such that light source  202  is on or close to axis  208 . In this embodiment in accordance with the invention, angle  210  is in the range of approximately zero to two degrees. 
   In general, the size of angle  212  is chosen so that only low retinal return from light source  204  will be detected at detector  200 . The iris (surrounding the pupil) blocks this signal, and so pupil size under different lighting conditions should be considered when selecting the size of angle  212 . In this embodiment in accordance with the invention, angle  210  is in the range of approximately three to fifteen degrees. In other embodiments in accordance with the invention, the size of angles  210 ,  212  may be different. For example, the characteristics of a particular subject may determine the size of the angles  210 ,  212 . 
   Light sources  202 ,  204  emit light that yields substantially equal image intensity (brightness) in this embodiment in accordance with the invention. Light sources  202 ,  204  emit light of different wavelengths in this embodiment in accordance with the invention. Even though light sources  202 ,  204  can be of any wavelength, the wavelengths are selected in this embodiment so that the light will not distract the subject and the iris of the eye will not contract in response to the light. The selected wavelengths should be in a range that allows the detector  200  to respond. In this embodiment in accordance with the invention, light sources  202 ,  204  are implemented as light-emitting diodes (LEDs) or multi-mode lasers having infrared or near-infrared wavelengths. Each light source  202 ,  204  may be implemented as one, or multiple, sources, where each such device is located at substantially the same angle from the axis  208 . 
     FIG. 3   a  illustrates an image generated with an on-axis light source in accordance with the system shown in  FIG. 2 . The image shows an eye that is open. The eye has a bright pupil due to a strong retinal return created by the on-axis light source  202 . If the eye had been closed, or nearly closed, the bright pupil would not be detected and imaged. 
     FIG. 3   b  depicts an image generated with an off-axis light source in accordance with the system shown in  FIG. 2 . The image in  FIG. 3   b  may be taken at the same time as the image in  FIG. 3   a , or it may be taken in an alternate frame (successively or non-successively) to the image of  FIG. 3   a . The image of  FIG. 3   b  illustrates a normal, dark pupil. If the eye had been closed or nearly closed, the normal pupil would not be detected and imaged. 
     FIG. 3   c  illustrates an image resulting from the difference between the  FIG. 3   a  image and the  FIG. 3   b  image. By taking the difference between the images of  FIGS. 3   a  and  3   b , a relatively bright spot  300  remains against the relatively dark background  302  when the eye is open. There may be vestiges of other features of the eye remaining in the background  302 . However, in general, the bright spot  300  will stand out in comparison to the background  302 . When the eye is closed or nearly closed, there will not be a bright spot  300  in the differential image. 
     FIGS. 3   a - 3   c  illustrate one eye of the subject  206 . Those skilled in the art will appreciate that both eyes may be monitored as well. It will also be understood that a similar effect will be achieved if the images include other features of the subject  206  (e.g. other facial features), as well as features of the subject&#39;s  206  environment. These features will largely cancel out in a manner similar to that just described, leaving either a bright spot  300  when the eye is open (or two bright spots, one for each eye), or no spot(s) when the eye is closed or nearly closed. 
   Referring now to  FIG. 4 , there is shown a sensor in one embodiment in accordance with the invention. In this embodiment, a sensor  400  is incorporated into detector  200  ( FIG. 2 ), and is configured as a complementary metal-oxide semiconductor (CMOS) imaging sensor. Sensor  400 , however, may be implemented with other types of imaging devices in other embodiments in accordance with the invention, such as, for example, a charge-coupled device (CCD) imager. 
   A patterned filter layer is formed on sensor  400  using two different filters shaped into a checkerboard pattern. The two filters are determined by the wavelengths being used by light sources  202 ,  204 . For example, in this embodiment in accordance with the invention, sensor  400  includes regions (identified as  1 ) that include a filter material for selecting the wavelength used by light source  202 , while other regions (identified as  2 ) include a filter material for selecting the wavelength used by light source  204 . 
   In the  FIG. 4  embodiment, the patterned filter layer is deposited as a separate layer of sensor  400 , such as, for example, on top of an underlying layer, using conventional deposition and photolithography processes while still in wafer form. In another embodiment in accordance with the invention, the patterned filter layer can be can be created as a separate element between sensor  400  and incident light. Additionally, the filter pattern can be configured in a pattern other than a checkerboard pattern. For example, the patterned filter layer can be formed into an interlaced striped or a non-symmetrical configuration (e.g. a 3-pixel by 2-pixel shape). The patterned filter layer may also be incorporated with other functions, such as color imagers. 
   Various types of filter materials can be used in the patterned filter layer. In this embodiment in accordance with the invention, the filter materials include polymers doped with pigments or dyes. In other embodiments in accordance with the invention, the filter materials may include interference filters, reflective filters, and absorbing filters made of semiconductors, other inorganic materials, or organic materials. 
     FIG. 5  is a cross-sectional diagram of a detector in accordance with one embodiment in accordance with the invention. Only a portion of the detector is shown in this figure. Detector  200  includes a sensor  400  comprised of pixels  500 ,  502 ,  504 ,  506 , a patterned filter layer  508  including two alternating filter regions  510 ,  512 , a glass cover  514 , and a dual-band narrowband filter  516 . Sensor  400  is configured as a CMOS imager and the patterned filter layer  508  as two polymers  510 , 512  doped with pigments or dyes in this embodiment in accordance with the invention. Each region in the patterned filter layer  508  (e.g. a square in the checkerboard pattern) overlies a pixel in the CMOS imager. 
   When light strikes the upper surface of narrowband filter  516 , the light at wavelengths other than the wavelengths of light source  202  (λ 1 ) and light source  204  (λ 2 ) are filtered out, or blocked, from passing through the narrowband filter  516 . Thus, the light at visible wavelengths λ VIS  and the light at wavelengths (λ n ) other than λ 1  and λ 2  are filtered out in this embodiment, while the light at or near the wavelengths λ 1  and λ 2  transmit through the narrowband filter  516 . Thus, only light at or near the wavelengths λ 1  and λ 2  pass through glass cover  514 . Thereafter, polymer  510  transmits the light at wavelength λ 1  while blocking the light at wavelength λ 2 . Consequently, pixels  500  and  504  receive only the light at wavelength λ 1 , thereby generating the image taken with the on-axis light source  202 . 
   Polymer  512  transmits the light at wavelength λ 2  while blocking the light at wavelength λ 1 , so that pixels  502  and  506  receive only the light at wavelength λ 2 . In this manner, the image taken with the off-axis light source  204  is generated. The shorter wavelength λ 1  is associated with the on-axis light source  202 , and the longer wavelength λ 2  with the off-axis light source  204 , in this embodiment in accordance with the invention. The shorter wavelength λ 1 , however, may be associated with the off-axis light source  204  and the longer wavelength λ 2  with the on-axis light source  202  in other embodiments in accordance with the invention. 
   Narrowband filter  516  is a thin-film bulk dielectric stack filter in this embodiment in accordance with the invention. Dielectric stack filters are designed to have particular spectral properties. In this embodiment in accordance with the invention, the dielectric stack filter is formed as a dual spike filter. The narrowband filter  516  (i.e., dielectric stack filter) is designed to have one peak at λ 1  and another peak at λ 2 . 
   The narrowband filter  516  and the patterned filter layer  508  form a hybrid filter in this embodiment in accordance with the invention.  FIG. 6  depicts spectra for the polymer filters and the narrowband filter shown in  FIG. 5 . As shown in  FIG. 6 , the hybrid filter (combination of the polymer filters  510 ,  512  and the narrowband filter  516 ) effectively filters out all light except for the light at or near the wavelengths of the light sources (λ 1  and λ 2 ). The narrowband filter  516  transmits a narrow amount of light at or near the wavelengths of interest, λ 1  and λ 2 , while blocking the transmission of light at other wavelengths. Therefore, only the light at or near wavelengths λ 1  and λ 2  strike polymer filters  510 ,  512  in the patterned filter layer  508 . The patterned filter layer  508  is then used to discriminate between λ 1  and λ 2 . Wavelength λ 1  is transmitted through filter  510  (and not through filter  512 ), while wavelength λ 2  is transmitted through filter  512  (and not through filter  510 ). 
   Those skilled in the art will appreciate the patterned filter layer  508  provides a mechanism for selecting channels at pixel spatial resolution. In this embodiment in accordance with the invention, channel one is associated with the on-axis image and channel two with the off-axis image. In other embodiments in accordance with the invention, channel one may be associated with the off-axis image and channel two with the on-axis image. 
   Sensor  400  sits in a carrier (not shown) in this embodiment in accordance with the invention. The glass cover  514  typically protects the sensor  400  from damage and particle contamination (e.g. dust). In another embodiment in accordance with the invention, the hybrid filter includes the patterned filter layer  508 , the glass cover  514 , and the narrowband filter  516 . The glass cover  514  in this embodiment is formed as a colored glass filter, and is included as the substrate of the dielectric stack filter (i.e., the narrowband filter  516 ). The colored glass filter is designed to have certain spectral properties, and is doped with pigments or dyes. Schott Optical Glass Inc., a company located in Mainz, Germany, is one company that manufactures colored glass that can be used in colored glass filters. 
   Referring now to  FIG. 7   a , there is shown a first method for fabricating a dual spike filter in an embodiment in accordance with the invention. As discussed in conjunction with the  FIG. 5  embodiment, the narrowband filter  516  is a dielectric stack filter that is formed as a dual spike filter. Dielectric stack filters can include any combination of filter types. The desired spectral properties of the completed dielectric stack filter determine which types of filters are included in the layers of the stack. 
   For example, a dual-spike filter can be fabricated by combining two filters  700 ,  702 . A band-blocking filter  700  filters out the light at wavelengths between the regions around wavelengths λ 1  and λ 2 , while a bandpass filter  702  transmits light near and between wavelengths λ 1  and λ 2 . The combination of the two filters  700 ,  702  transmits light in the hatched areas, while blocking light at all other wavelengths.  FIG. 7   b  depicts the spectrum for the dual spike filter shown in  FIG. 7   a . As can be seen, light transmits through the combined filters only at or near the wavelengths of interest, λ 1  (peak  704 ) and λ 2  (peak  706 ). 
   Referring now to  FIG. 8   a , there is shown a second method for fabricating a dual spike filter in an embodiment in accordance with the invention. A dual-spike filter can be fabricated in this embodiment by combining a cut-off filter  800 , a cut-on filter  802 , and a band-blocking filter  804 . The combination of the three filters transmits light in the hatched areas, while blocking light at all other wavelengths.  FIG. 8   b  depicts the spectrum for the dual spike filter shown in  FIG. 8   a . As can be seen, light transmits through the combined filters only at or near the wavelengths of interest, λ 1  (peak  806 ) and λ 2  (peak  808 ). 
   In some applications, such as in pupil detection, it is more desirable to use LEDs rather than lasers as light sources. LEDs are typically safer to use in eye detection compared with lasers. LEDs also have lower coherence than lasers, which eliminates speckle. Additionally, more of the object is illuminated by LEDs because LEDs have broader divergence. And finally, LEDs are usually less expensive than lasers. 
   A narrowband light source can be created with an LED by placing a narrowband filter in front of, or on top of, the LED light source to narrow the spectrum of the LED.  FIG. 9  is a diagram of a light source and a narrowband filter in an embodiment in accordance with the invention. Light source  900  is a light-emitting diode (LED) and narrowband filter  902  is a single spike dielectric stack filter in the  FIG. 9  embodiment. In other embodiments in accordance with the invention, however, narrowband filter  902  may be configured as other types of filters. Furthermore, the dielectric stack filter may be fabricated as, or including, a colored glass filter. And other light sources, such as white light sources, may be used in other embodiments in accordance with the invention. 
   Referring now to  FIG. 10 , there is shown an illustration of the spectra of the light source and the combination of the light source and the narrowband filter, as shown in  FIG. 9 . As can be seen, the spectrum ( 1000 ) of the light source  900  by itself is broader than the spectrum ( 1002 ) of the combination of the light source  900  and the narrowband filter  902 . As discussed earlier, a narrowband light source can be constructed with a broader spectrum light source  900  by forming or placing a narrowband filter  902  on top of, or in front of, the broader spectrum light source. 
     FIG. 11  is a diagram of a second system for pupil detection that utilizes a narrowband filter with each light source and a hybrid filter with a sensor in an embodiment in accordance with the invention. Similar reference numbers have been used for those elements that function as described in conjunction with earlier figures. A detector  200  includes a sensor  400 , a patterned filter layer  508 , a glass cover  514 , and a narrowband filter  516 . A lens  1100  captures the light reflected off subject  206  and focuses it onto the top surface of the narrowband filter  516  in detector  200 . 
   The light source  202  includes a narrowband filter  902   a , while the light source  204  includes a narrowband filter  902   b . Narrowband filters  902   a ,  902   b  have been fabricated to create filters having appropriate spectral properties for the light sources  202 ,  204 , respectively. As discussed earlier, the narrowband filters  902   a ,  902   b  allow the system to utilize light sources that have broader spectra but may be safer and less expensive to use than narrower spectrum light sources. In other embodiments in accordance with the invention, different types of filters may be used with light sources  202 ,  204 . Examples of different filter types include, but are not limited to, cut-on filters and cut-off filters. 
   The on-axis image is captured by detector  200  using light source  202 , and the off-axis image is captured by detector  200  using light source  204 . The hybrid filter includes the patterned filter layer  508  and the narrowband filter  516  in this embodiment in accordance with the invention. The hybrid filter blocks out light at all wavelengths other than the wavelengths of the light sources  202 ,  204 . Therefore, sensor  400  detects only the light at the wavelengths of the light sources  202 ,  204 . 
   Referring now to  FIG. 12 , there is shown a diagram of a device that can be used for pupil detection in one embodiment in accordance with the invention. Device  1200  includes the detector  200 , a number of on-axis light sources  202 , and a number of off-axis light sources  204 . Each on-axis light source  202  is located at substantially the same angle from the axis of the detector  200  in this embodiment in accordance with the invention. Similarly, each off-axis light source  204  is located at substantially the same angle from the axis of the detector  200 . 
   The on-axis image is captured by detector  200  using light sources  202 , and the off-axis image is captured by detector  200  using light sources  204 . The light sources  202 ,  204  are shown as being housed in the same device  1200  as the detector  200  in this embodiment in accordance with the invention. In other embodiments in accordance with the invention, light sources  204  may be located in a housing separate from light sources  202  and detector  200 . Furthermore, light sources  202  may be located in a housing separate from detector  200  by placing a beam splitter between the detector and the object. 
     FIG. 13  is a diagram of a system for detecting transmission through an object that utilizes a hybrid filter in another embodiment in accordance with the invention. Similar reference numbers have been used for those elements that function as described in conjunction with earlier figures. A detector  200  includes a sensor  400 , a patterned filter layer  508 , a glass cover  514 , and a narrowband filter  516 . 
   A broadband light source  1300  transmits light toward a transparent object  1302 . The broadband light source  1300  emits light at multiple wavelengths, two of which will be detected by detector  200 . In other embodiments in accordance with the invention, the broadband light source  1300  may be replaced by two light sources transmitting light at different wavelengths. 
   A lens  1100  captures the light transmitted through the transparent object  1300  and focuses it onto the top surface of the narrowband filter  516 . One image is captured by detector  200  using light that is transmitted at one wavelength, while a second image is captured by detector  200  using light that is transmitted at the other wavelength. 
   The hybrid filter includes the patterned filter layer  508  and the narrowband filter  516  in this embodiment in accordance with the invention. The hybrid filter blocks out light at all wavelengths other than the two wavelengths of interest, allowing the sensor  400  to detect only the light at the two wavelengths. 
   Although a hybrid filter has been described with reference to detecting light at two wavelengths, λ 1  and λ 2 , hybrid filters in other embodiments in accordance with the invention may be used to detect more than two wavelengths of interest.  FIG. 14  depicts spectra for polymer filters and a tri-band narrowband filter in an embodiment in accordance with the invention. A hybrid filter in this embodiment detects light at three wavelengths of interest, λ 1 , λ 2 , and λ 3 . The spectra  1400  and  1402  at wavelengths λ 1  and λ 3 , respectively, represent two signals to be utilized by an imaging system. Light detected at wavelength λ 2  is used to determine the amount of light received by the imaging system outside the two wavelengths of interest. The amount of light detected at wavelength λ 2  may be used as a reference amount of light detectable by the imaging system. 
   A tri-band narrowband filter transmits light at or near the wavelengths of interest (λ 1  λ 2 , and λ 3 ) while blocking the transmission of light at all other wavelengths in this embodiment in accordance with the invention. Polymer filters in a patterned filter layer then discriminate between the light received at wavelengths λ 1  λ 2 , and λ 3 .  FIG. 15  depicts a sensor in accordance with the embodiment shown in  FIG. 14 . A patterned filter layer is formed on sensor  1500  using three different filters. For example, in one embodiment in accordance with the invention, sensor  1500  may include a red-green-blue color three-band filter pattern. Red corresponds to regions  1 , green to regions  2 , and blue to regions  3  in the figure. There are twice as many greens as other colors in this embodiment because human perception of brightness depends most strongly on the green range. 
   Referring now to  FIG. 16 , there is shown spectra for polymer filters and a quad-band narrowband filter in an embodiment in accordance with the invention. A hybrid filter in this embodiment detects light at four wavelengths of interest, λ 1 , λ 2 , λ 3 , and λ 4 . The spectra  1600 ,  1602  at wavelengths λ 1  and λ 3 , respectively, represent two signals to be utilized by an imaging system. Light detected at wavelengths λ 2  and λ 4  is used as a reference to determine the amount of light received by the imaging system outside the two wavelengths of interest. 
   A quad-band narrowband filter transmits light at or near the wavelengths of interest (λ 1 , λ 2 , λ 3 , and λ 4 ) while blocking the transmission of light at all other wavelengths in this embodiment in accordance with the invention. Polymer filters in a patterned filter layer then discriminate between the light received at wavelengths λ 1 , λ 2 , λ 3 , and λ 4 .  FIG. 17  depicts a sensor in accordance with the embodiment shown in  FIG. 16 . A patterned filter layer is formed on sensor  1700  using four different filters. For example, in this embodiment in accordance with the invention, sensor  1700  includes rows of filters  1  and  2  alternating with rows of filters  3  and  4 . Filters  1  and  2  detect the light at wavelengths λ 1  and λ 2 , while filters  3  and  4  detect the light at wavelengths λ 3  and λ 4 . 
   Hybrid filters in other embodiments in accordance with the invention may detect any number of wavelengths of interest. For example, an n-band narrowband filter would transmit light at or near the wavelengths of interest. Regions within a patterned filter layer would alternately block at least one (and up to (n-1)) of the wavelengths of interest. For systems that utilize a patterned filter layer having regions that transmit two or more wavelengths of interest, mathematical computations, such as linear algebra, may be performed in order to determine the amount of light received at each wavelength of interest.