Abstract:
The present disclosure describes optical radiation sensors and detection techniques that facilitate assigning a specific wavelength to a measured photocurrent. The techniques can be used to determine the spectral emission characteristics of a radiation source. In one aspect, a method of determining spectral emission characteristics of incident radiation includes sensing at least some of the incident radiation using a light detector having first and second photosensitive regions whose optical responsivity characteristics differ from one another. The method further includes identifying a wavelength of the incident radiation based on a ratio of a photocurrent from the first region and a photocurrent from the second region.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to measuring characteristics of optical radiation. 
       BACKGROUND 
       [0002]    Various manufacturing and other processes involve measuring and collecting optical radiation data, in particular spectral emission characteristics, of a source of optical radiation. The manufacturing or other process may be a process for which optical radiation is used as part of performing the process or in which optical radiation is generated by the process. In some instances, such as imaging applications, it may be desirable to measure and determine the spectral emission characteristics of the ambient light. 
         [0003]    While various techniques are available, some known techniques provide only global or averaged measurements for a region of the process. For example, an array of photodiodes can be provided to measure incident light of various wavelengths. The response of the photodiodes, however, generally depends on the intensity of the incident radiation as well as on the wavelength of the radiation. As a result, it may not be possible to assign a particular photocurrent value to a unique wavelength. For example, if ambient light is being detected, it may not be possible to correlate a particular value of photocurrent with a specific wavelength of specific part of the spectrum. 
         [0004]    Further, some light sources have multiple emission peaks. For example, sodium vapor lamps generate two emission peaks near 589 nm Those peaks, however, are outside typical red (R), blue (B) and green (G) filters. Thus, a sensor configured only with these filters will be unable to detect the sodium-vapor lines. 
       SUMMARY 
       [0005]    The present disclosure describes optical radiation sensors and detection techniques that facilitate assigning a specific wavelength to a measured photocurrent. The techniques can be used to determine the spectral emission characteristics of a radiation source. 
         [0006]    For example, in one aspect, a method of determining spectral emission characteristics of incident radiation includes sensing at least some of the incident radiation using a light detector having first and second photosensitive regions whose optical responsivity characteristics differ from one another. The method further includes identifying a wavelength of the incident radiation based on a ratio of a photocurrent from the first region and a photocurrent from the second region. 
         [0007]    Some implementations include one or more of the following features. For example, in some cases, incident radiation sensed by the light detector passes through an optical filter before being sensed by the light detector. In some instances, the first and second regions of the light detector are arranged as stacked photodiodes. The method can include sensing at least some of the incident radiation using a plurality of light detectors, each of which has respective first and second photosensitive regions wherein an optical responsivity characteristics of the first region differs from an optical responsivity characteristics of the second region, and wherein each light detector is configured to sense a different respective part of the optical spectrum. The method may further include identifying one or more wavelengths of the incident radiation, wherein each wavelength is identified based on a ratio of a photocurrent from a first one of the photosensitive regions and a photocurrent from the second photosensitive region in the same light detector as the first region. In some implementations, incident radiation sensed by each particular light detector passes through a respective optical filter before being sensed by the particular light detector, wherein each optical filter passes a respective wavelength or band of wavelengths that differs from at least some of the other optical filters. In some cases, the method can include comparing the ratio of photocurrents to values stored in a look-up table, and assigning a wavelength associated with a closest matched value in the look-up table to the incident radiation. The method may include controlling a component of a host device based on an identification of the wavelength of the incident radiation. 
         [0008]    In another aspect, a system for determining spectral emission characteristics of incident radiation includes a light detector including first and second photosensitive regions whose optical responsivity characteristics differ from one another. The system also includes processing circuitry coupled to the light detector and configured to receive respective photocurrents from the first and second photosensitive regions, to calculate a ratio of the photocurrents from the first and second photosensitive regions, and to assign a wavelength to the incident light based on the ratio of the photocurrents. 
         [0009]    In some implementations, the system includes one or more of the following features. For example, the light detector can include a stacked photodiode structure. The system can include an optical filter that limits wavelengths of light incident on the photosensitive regions of the light detector. The optical filter may allow only a single wavelength or a narrow wavelength band to pass through. In some instances, the system includes a plurality of light detectors, each of which includes respective first and second photosensitive regions, wherein optical responsivity characteristics of the first photosensitive region differ from optical responsivity characteristics of the second photosensitive region. Each light detector can have a respective optical filter that allows the light detector to sense a wavelength or narrow wavelength band different from the other light detectors. The processing circuitry can be configured to identify one or more wavelengths of the incident radiation, wherein each wavelength is identified based on a ratio of a photocurrent from a first one of the photosensitive regions and a photocurrent from the second photosensitive region of the same light detector as the first photosensitive region. 
         [0010]    In a further aspect, a system for determining spectral emission characteristics of incident light includes an array of light sensitive elements composed at least in part of stacked first and second photosensitive regions whose optical responsivity characteristics differ from one another. Optical filters are disposed over the array of light sensitive elements, wherein the optical filters are configured to allow only respective narrow parts of the optical spectrum to pass to different ones of the light sensitive elements such that different ones of the light sensitive elements or sub-groups of the light sensitive elements are operable to sense light in a part of the optical spectrum that differs from other ones of the light sensitive elements or sub-groups of the light sensitive elements. Processing circuitry coupled to the light sensitive elements and configured to receive respective photocurrents from the first and second photosensitive regions for each light sensitive element, to calculate respective ratios of the photocurrents from the first and second photosensitive regions for at least some of the light sensitive elements, and to assign respective wavelengths to the incident light based on the calculated ratios. 
         [0011]    In some implementations of the system, the optical filters form a continuous or semi-continuous spectrum of optical filters. The optical filters, in some instances, collectively allow substantially the entire visible portion of the optical spectrum to be sensed by the array of light sensitive elements at a resolution in a range of 2 nm-4 nm The array of light sensitive elements can be, for example, a CMOS sensor. 
         [0012]    The present disclosure can be used for a wide range of applications that involve measuring the spectral emission characteristics of optical radiation. 
         [0013]    Other aspects, features and advantages will be readily apparent from the following detailed description, the accompanying drawings, and the claim. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  illustrates an arrangement for detecting monochromatic incident light. 
           [0015]      FIG. 2  is an example of optical responsivity curves. 
           [0016]      FIG. 3  is a flow chart showing a method of determining the wavelength(s) of incident light. 
           [0017]      FIG. 4  is another example of optical responsivity curves. 
           [0018]      FIG. 5  illustrates an arrangement for detecting non-monochromatic incident light. 
           [0019]      FIGS. 6A, 6B and 6C  are further examples of optical responsivity curves. 
           [0020]      FIG. 7  illustrates another arrangement for detecting non-monochromatic light. 
           [0021]      FIG. 8  is an example of an arrangement for detecting broadband or full-spectrum incident light. 
           [0022]      FIG. 9  is a block diagram of a system for detecting incident light and determining the wavelength(s) of the incident light. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    As illustrated in  FIG. 1 , optical radiation  20  is incident on light detectors  22 , each of which has multiple spectral sensitivity. In particular, each light detector  22  can be implemented, for example, as stacked semiconductor (e.g., silicon) photodiodes  22 A,  22 B. The stacked photodiode structure has multiple (e.g., two) junctions whose optical responsivities (i.e., spectral characteristics) differ from one another. In this example, it is assumed that the incident light  20  is substantially monochromatic (e.g., having a single wavelength or a very narrow spectrum about a center peak wavelength). As shown in  FIG. 1 , a wide field-of-view (FOY) or other optical system  24  including one or more beam shaping elements such as lenses can focus the incident radiation  20  onto the light detectors  22 . In some cases, an optical filter  26  is provided over each light detector  22 . For example, the optical filters  26  may be clear so as to allow only visible light to pass through for detection by the light detectors  22 . Outputs  28 A,  28 B from the light detectors  22  can be provided to processing circuitry configured to process the photocurrents and assign a corresponding wavelength to the incident light  20  based on the photocurrents. 
         [0024]      FIG. 2  illustrates an example of two curves A 1 , A 2 , each of which represents the optical responsivity of one of the photodiodes  22 A,  22 B versus wavelength. In the illustrated example, curve A 1  represents the optical responsivity of a particular junction  22 A in one of the light detectors  22 , and curve A 2  represents the optical responsivity of the other junction  22 B in the same light detector  22 . Optical responsivity (also referred to as photoresponsivity) generally is a function of the wavelength of the incident light. As is apparent from the illustrated example, the two curves A 1  and A 2  differ from one another. Thus, assuming the wavelength of the incident light is about 600 nm, the responsivity for junction  22 A (using curve A 1 ) will be detected as I A1,1 , whereas the responsivity for junction  22 B (using curve A 2 ) will be detected as I A2,1 . 
         [0025]    The processing circuitry is operable to calculate the ratio of the photocurrents (i.e., optical responsivities) I A1,1 /I A2,1  ( 102  in  FIG. 3 ). Assuming that the incident light is substantially monochromatic, the ratio of the photocurrents will be substantially the same for a given wavelength of incident light, regardless of the intensity of the incident light. Thus, the ratio of the photocurrents can be compared, for example, to ratio values in a look-up table stored in memory associated with the processing circuitry ( 104 ) and, based on the comparison, the processing circuitry can assign a wavelength to the incident light ( 106 ). In particular, the processing circuitry assigns the wavelength that is associated with the closest match to the calculated ratio. The values stored in the look-up table may be determined experimentally. In some cases, instead of comparing the calculated ratio to values in a look-up table, the processing circuitry can determine the corresponding wavelength of the incident light using a predetermined equation. 
         [0026]    An advantage of using the ratio of the detected photocurrents to assign a wavelength to the incident light can be appreciated from the example of  FIG. 4 , which shows the same two responsivity curves A 1 , A 2  as  FIG. 2 . As indicated by the values I A2,1  and I A2,2  in  FIG. 4 , the first junction  22 A of one of the light detectors  22  generates the same photocurrent at two different wavelengths (i.e., 700 nm and 970 nm). The second junction  22 B, however, generates a different photocurrent for incident light at those same wavelengths. Thus, for incident light having a wavelength of 700 nm, the second junction  22 B generates a photocurrent I A1,1 , whereas for incident light having a wavelength of 970 nm, the second junction  22 B generates a smaller photocurrent I A1,2 . Here too, the processing circuitry can calculate the ratio of the actual photocurrents and, based on the ratio, can assign a wavelength to the incident light. As in the example of  FIG. 2 , it is assumed in the example of  FIG. 4  that the incident light is substantially monochromatic. 
         [0027]    In some implementations, even if the incident light is non-monochromatic and consists of multiple narrow bands in different parts of the spectrum, it is possible to determine the wavelengths of the incident light with reasonable accuracy by providing narrow band pass filters.  FIG. 5  illustrates an example in which it is assumed that the incident light  20  consists of discrete wavelengths in three different narrow bands  20 A,  20 B,  20 C (e.g., red, green and blue). Each of the light detectors  22  can be similar to those described with respect to  FIG. 1 . In addition, some of the light detectors  22  have a respective optical filter  26 A- 26 C that allows light of only a single part of the spectrum to pass through to the underlying stacked photodiodes  22 A,  22 B. For example, as shown in  FIG. 5 , one of the light detectors  22  has a filter  26 A that allows only red light in the visible part of the spectrum to pass, another light detector  22  has a filter  26 B that allows only green light to pass, and a third light detector  22  has a filter  26 C that allows only blue light in the visible part of the spectrum to pass. 
         [0028]    Some light sources are monochromatic. For example, a sodium vapor lamp may emit light only at 589 nm If the emitted wavelength (or narrow band of wavelengths) is outside the ranges of wavelengths passed by the filters  26 A- 26 C, then the emitted light will not be detected by any of the light detectors  22  having those filters. To address such situations, an additional light detector  22  can be provided, for example, with a clear filter  26 D that allows visible light of all colors to pass. Assuming that the incident (visible) light is monochromatic, the processing circuitry can determine the wavelength of the light in the manner described above by using the ratio of the photocurrent outputs from the light detector having the clear filter. 
         [0029]    In the foregoing example, it is assumed that the clear filter allows only visible light to pass. In other cases, the clear filter may also allow light in other parts of the spectrum (e.g., IR, near-IR or UV) to pass. 
         [0030]    Each light detector  22  thus can detect only light within a specified narrow band (see  FIGS. 6A, 6B and 6C  in which the dashed lines indicate, respectively, the blue, green and red parts of the visible spectrum that can be detected by the individual detectors  22 . Thus,  FIG. 6A  indicates the blue part of visible spectrum detectable by the detector  22  having the blue band pass filter  26 C,  FIG. 6B  indicates the green part of visible spectrum detectable by the detector  22  having the green band pass filter  26 B, and  FIG. 6C  indicates the red part of visible spectrum detectable by the detector  22  having the red band pass filter  26 A. Each pair of photocurrent outputs  28 A,  28 B from a given one of the detectors  22  can be used by the processing circuitry to determine a ratio and to assign a wavelength to the incident light within that part of the spectrum, as described above in connection with  FIGS. 2-4 . 
         [0031]    In some instances, instead of assigning a calculated photocurrent ratio to a particular wavelength, the processing circuitry may simply determine whether the calculated wavelength is within a specified tolerance of a predetermined ratio. If the calculated ratio is outside the specified tolerance, the processing circuitry can cause an alarm or message to be generated to indicate that the incident light differs from the expected wavelength. 
         [0032]    Although the filters in the particular example of  FIG. 5  are designed to pass red, green and blue light, respectively, in other cases, filters designed to pass different parts of the spectrum may be provided (e.g., infra-red or ultra-violet). Further, a different number of light detectors, each corresponding to a respective wavelength band, may be provided. In general, as long as each wavelength (or narrow band) in the incident light falls within one of the defined regions of the optical spectrum, the processing circuitry can determine the wavelengths of the incident light. 
         [0033]    Thus, the number of photodiodes  22  and associated filters  26  for different spectral regions can be increased so that even greater numbers of wavelengths can be identified from a multi-band light source.  FIG. 7  illustrates an example that includes seven light detectors  22 . Six of the light detectors  22  have a respective optical filter ( 26 A,  26 B,  26 C,  26 E,  26 F,  26 G) that allows a different respective spectral region to pass. Thus, the individual wavelengths of incident light including up to six discrete wavelengths or narrow wavelength bands  20 A- 20 F can be identified as long as each wavelength or narrow band falls within a different one of the non-overlapping wavelength regions encompassed by a respective one of the light detectors  22  as defined by the color filters. As described above, each light detector  22  can include a stacked photodiode structure having junctions whose optical responsivities differ from one another. The processing circuitry can calculate the ratio of the photocurrent outputs  28 A,  28 B from each particular light detector  22  and, using the ratios, can identify the wavelengths of the incident light. In some cases, another one of the light detectors  22  includes a clear filter  26 D that allows visible light of all colors to pass. The foregoing arrangement can be expanded to identify the wavelengths of other multi-band light sources by increasing the number of light detectors  22  and providing each light detector with an optical filter that allows only a different wavelength or narrow wavelength band to pass. 
         [0034]    In some instances, even if some (or all) of the wavelength bands of the incident light are somewhat wide (i.e., covering more than a single wavelength), the ratio of the optical responses from a particular photodiode can be used to identify the approximate value of the wavelength(s) in a corresponding band. Thus, although in some cases it may not be possible to identify the precise wavelengths of broadband incident light, the processing circuitry can use the ratios of the photocurrent outputs  28 A,  28 B from each particular light detector to identify the approximate position within each color-filter range so as to determine the approximate wavelengths of the incident light. 
         [0035]    In the foregoing implementations, the light detectors  22  are discrete devices each of which has multiple spectral sensitivity (e.g., a stacked photodiode structure having multiple junctions whose optical responsivity curves differ from one another).  FIG. 8  illustrates another implementation that can be particularly useful for determining the wavelengths of multi-band or full-spectrum incident light  120 . In this implementation, instead of multiple discrete devices for the light detectors  22 , an array  122  of light sensitive elements (e.g., a CMOS sensor) can be provided. The pixel array  122  includes multiple (e.g., two) vertically stacked photodiodes organized in a two-dimensional grid and having junctions  122 A,  122 B whose optical responsivity curves differ from one another. A continuous or semi-continuous spectrum of optical filters  126  can be provided over the pixel array  122 . Each filter  126 A can be configured to allow only a narrow part of the optical spectrum to pass. The number of filters  126 A can be made sufficiently large such that, collectively, the filters allow a wide range of narrow wavelength bands to pass to the underlying pixels. The filters  126 A are arranged, however, such that each pixel (or sub-group of pixels) receives light within only a narrow wavelength band. In a particular implementation, the CMOS sensor pixel array may have dimensions of 100×100 pixels and can cover substantially the entire visible spectral range from 400 nm-700 nm with resolution in the range of 2 nm-4 nm. 
         [0036]    As shown in  FIG. 9 , a system for detecting incident light and determining the wavelength(s) of the incident light can include processing circuitry  30 . The processing circuitry  30  to operable to read signals from the light detectors  22  (or  122 ) and to process the signals so as to identify one or more wavelengths in the incident light in accordance with the techniques described above. The processing circuitry  30  can be implemented, for example, as one or more integrated circuits in one or more semiconductor chips with appropriate digital logic and/or other hardware components (e.g., read-out registers; amplifiers; analog-to-digital converters; clock drivers; timing logic; signal processing circuitry; and/or microprocessor). The processing circuitry  30  is, thus, configured to implement the various functions associated with such circuitry. 
         [0037]    The foregoing techniques may be applicable in a wide range of applications, including semiconductor processing where monitoring of spectral emission characteristics of the ambient environment may be required or tuning of a radiation source may be needed. The techniques also may be useful in spectrometry application. Further, the techniques also can be advantageous in imaging applications, where it may be desirable to measure and determine the spectral emission characteristics of the ambient light. 
         [0038]    The optics assembly and light detectors  22  (or  122 ) can be incorporated into a compact module having a relatively small footprint. The module, in turn, can be integrated into a host device (e.g., a smart phone or other handheld computing device) that includes, for example, a camera. The photocurrent outputs from the light detectors  22  (or  122 ) can be provided to processing circuitry  30  residing in the host device. Further, in some cases, an output from the processing circuitry  30  can be provided to other components  32  of the host device (e.g., a camera or a display screen) to indicate ambient light information. The camera may use such information, for example, to adjust the camera aperture or to adjust the brightness of the display screen. 
         [0039]    Various modifications can be made within the spirit of the foregoing disclosure. Thus, other implementations are within the scope of the claims.