Patent Publication Number: US-2021172796-A1

Title: Spatial light modulator spectroscopy

Description:
TECHNICAL FIELD 
     This application is a divisional application of U.S. patent application Ser. No. 15/824,868 filed on Nov. 28, 2017, entitled “Spatial Light Modulator Spectroscopy,” which application is hereby incorporated herein by reference in its entirety. 
    
    
     This relates generally to spectroscopy, and more particularly to spatial light modulator spectroscopy. 
     BACKGROUND 
     Spectroscopy is a technique used by science and industry to determine the composition of materials. A spectroscope shines a broad spectrum of electromagnetic energy, such as light, on or through a test object. After passing through or reflecting off the test object, a diffraction grating or prism divides the light into its constituent frequencies. The spectroscope measures the intensity of these constituent frequencies. The test object will absorb or resonate certain frequencies of the electromagnetic energy due to the physical structure of the elements and molecules in the test object. Thus, a graph of the measured energy versus the frequency of the measured energy will have peaks and valleys that are characteristic of materials in the test object. Therefore, such a graph indicates the composition of the test object. 
     One type of spectroscope uses a spatial light modulator (SLM). One type of SLM is a digital micromirror device (DMD). DMDs can have thousands or millions of addressable mirrors in a planar array. To measure a constituent frequency, the prism or diffraction grating directs the spectrum onto the DMD. The different mirrors on the DMD array will receive the constituent frequencies of the spectrum. The DMD addresses the mirrors to direct light of a selected frequency onto a detector, such as a photodiode. The detector receives and measures the light. This method repeats for each frequency of interest. However, to accomplish this process, the entire DMD loads data to direct the mirrors of the DMD to turn on (reflect to the detector) or turn off (reflect away from the detector), even though a small portion of the mirrors are involved for each frequency measurement. 
     SUMMARY 
     In described examples, a spatial light modulator includes groups of pixels. Each group is arranged to transmit only a respective portion of a light spectrum. The respective portion has a respective dominant color. The respective portions of the light spectrum are distinct from one another, according to their respective dominant colors. Each group is controlled by a respective reset signal. The spatial light modulator is coupled to receive a selection from the integrated circuit and in response to the selection: cause a selected one of the groups to transmit its respective portion of the light spectrum; and cause an unselected one of the groups to block transmission of its respective portion of the light spectrum. A photodetector is coupled to: receive the respective portion of the light spectrum transmitted by the selected group; and output a signal indicating an intensity thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an SLM-based spectroscope. 
         FIG. 2  is a schematic diagram of a portion of an SLM based spectroscope like that of  FIG. 1 . 
         FIG. 3  is a view of one mirror of a DMD as seen from the reflecting surface of the DMD. 
         FIG. 4  is a side view of a mirror like that of  FIG. 3 . 
         FIG. 5  is a diagram of a spectrum projected onto an SLM and a diagram of an example mirror configuration of the SLM. 
         FIG. 6  is a diagram of a series of SLM patterns for measuring the spectrum of light on the SLM. 
         FIG. 7  is a diagram of a spectrum illuminating an SLM that has reset zones. 
         FIG. 8  is a timing diagram of the operation of a spectroscope using the configuration of  FIG. 7 . 
         FIG. 9  is a timing chart showing the method described in  FIG. 8  in more detail. 
         FIG. 10  is a diagram of an example SLM configuration. 
         FIG. 11  is a timing diagram of an example method using the configuration of  FIG. 10 . 
         FIG. 12  is a diagram illustrating a method for extracting the light intensity of specific colors. 
         FIG. 13  is a schematic diagram of an example spectroscope including control circuitry. 
         FIG. 14  is a flow diagram of an example method. 
     
    
    
     DETAILED DESCRIPTION 
     Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The drawings are not necessarily drawn to scale. 
     The term “coupled” may include connections made with intervening elements, and additional elements and various connections may exist between any elements that are “coupled.” 
       FIG. 1  is a schematic diagram of an SLM-based spectroscope. Spectroscope  100  includes light sources  102  or  104  that shine a broad spectrum of electromagnetic energy through or onto test object  106 . Light sources  102  and  104  can provide visible, ultraviolet, or infrared light. In addition, the spectrum of light provided by light sources  102  or  104  can bridge these types of light. After the light reflects from or traverses the test object  106 , lens  108  collimates the light from test object  106 . When figures of this application depict one lens, that lens can be implemented by a system of lenses. For example,  FIG. 1  depicts lens  108  as one lens. However, lens  108  can include several lenses. After collimation, the light passes through slit  112  in plate  110 . Slit  112  orients the light in one direction. Example widths of slit  112  are 5μ to 100μ. Slit  112  also causes the light to diverge, so it is necessary for lens  114  to re-collimate the light. Diffraction grating  116  then divides the light from slit  112  spectrally, as described more fully hereinbelow. Diffraction grating  116  directs the spectrum from diffraction grating  116  to spatial light modulator (SLM)  118 . In this example, SLM  118  is a digital micromirror device (DMD). SLM  118  selectively reflects a portion of the spectrum to photodetector  122  via lens  120 . The selection of a portion of the spectrum is more fully explained hereinbelow. 
       FIG. 2  is a schematic diagram of a portion of an SLM based spectroscope like that of  FIG. 1 . Sub-unit  200  includes diffraction grating  216  and SLM  218 . Light from a slit, like slit  112  ( FIG. 1 ), reflects off diffraction grating  216 . Different frequencies of light reflect at different angles off diffraction grating  216 . The angle of reflection follows Equation (1). 
       sin(α)+sin(β)= mλD 10 −6   (1)
 
     where α is the angle of incidence, β is the angle of reflection, m is a diffraction order, the wavelength λ is in nanometers and the groove density D is in grooves/mm. The result is a spectrum where diffraction grating  216  spreads the light  220  into a spectrum where the angle of reflectance depends on the angle of incidence and the wavelength of the light. The groove density D and the distance from diffraction grating  216  and SLM  218  determines how the spectrum  222  impacts SLM  218 . As further explained hereinbelow, by selecting specific micromirrors (“mirrors”) on SLM  218 , SLM  218  directs specific frequencies of light to photodetector  122  ( FIG. 1 ). This allows for the measurement of specific frequencies of light. 
       FIG. 3  is a view of one mirror of a DMD as seen from the reflecting surface of the DMD. View  300  shows mirror  302 . Mirror  302 , in this example, has a square configuration with pivot points  304  at opposing corners of mirror  302 . 
       FIG. 4  is a side view  400  of a mirror  402 , such as the mirror  302  of  FIG. 3 . The corner of mirror  402  has pivot points  404 , such as the pivot points  304  of  FIG. 3 . Pivot connections (not shown) suspend mirror  402  from substrate  406  by pivot points  404 . To change the state of mirror  402 , a memory cell (not shown) associated with mirror  402  receives and stores a data bit of one or zero. After loading the data bit into the memory cell, a reset signal is applied to all of the DMD&#39;s mirrors (or to a subset thereof including the mirror  402 ), thereby causing mirror  402  to have a “zero” position  410  or a “one” position  412 . In absence of the reset signal, mirror  402  is parallel to the surface of substrate  406 , as shown by a “no reset signal” position  408 . In one of these three positions, such as the one position  412 , photodetector  122  ( FIG. 1 ) receives the light reflected off mirror  402 . In some examples, light traps capture the light reflected off the mirror in the other positions to avoid corrupting measurement of the desired light signal. A DMD can contain many mirrors like mirror  402 . For example, a DMD configured for high definition (HD) television includes 1920×1080 or over two million mirrors. Appropriate selection of mirrors allows for reflection of a very narrow band of light frequencies, as explained further below. However, if a small number of mirrors reflects light to photodetector  122 , the low level of light can be difficult to detect. Therefore, the configuration of the number of mirrors for each frequency is a tradeoff between precision and detectability. 
       FIG. 5  is a diagram of a spectrum projected onto SLM  218  ( FIG. 2 ), and a diagram of an example mirror configuration of the SLM. Because the orientation of slit  112  ( FIG. 1 ) is vertical relative to spectrum  500 , the constituent colors of spectrum  500  have a vertical orientation. Spectrum  500  includes red band  510 , orange band  512 , yellow band  514 , green band  516 , blue band  518 , indigo band  520  and violet band  522 .  FIG. 5  includes seven bands for simplicity of explanation. An actual spectrum is continuous. In addition, spectrum  500  shows visible light colors. However, example arrangements can use infrared or ultraviolet spectra. SLM pattern  502  shows dark regions  530 ,  532 ,  534 ,  538 ,  540  and  542 . These regions correspond to red band  510 , orange band  512 , yellow band  514 , blue band  518 , indigo band  520  and violet band  522 , respectively. The dark regions correspond to an SLM configuration that does not reflect the light to photodetector  122  ( FIG. 1 ). In this configuration, light region  536  reflects the light to photodetector  122  ( FIG. 1 ). Accordingly, for example, the memory cells (not shown) of region  536  store a one while SLM  118  ( FIG. 1 ) is receiving a reset signal. Conversely, the memory cells of regions  530 ,  532 ,  534 ,  538 ,  540  and  542  store a zero. Therefore, SLM pattern  502  is for measuring the light of green band  516 . 
       FIG. 6  is a diagram of a series of SLM patterns for measuring the spectrum of light on the SLM. Pattern group  600  includes seven patterns. SLM pattern  602  allows for measurement of red band  510  ( FIG. 5 ). SLM pattern  604  allows for measurement of orange band  512  ( FIG. 5 ). SLM pattern  606  allows for measurement of yellow band  514  ( FIG. 5 ). SLM pattern  608  allows for measurement of green band  516  ( FIG. 5 ), like SLM pattern  502  ( FIG. 5 ). SLM pattern  610  allows for measurement of blue band  518  ( FIG. 5 ). SLM pattern  612  allows for measurement of indigo band  520  ( FIG. 5 ). SLM pattern  614  allows for measurement of violet band  522  ( FIG. 5 ). Therefore, cycling through each of these SLM patterns allows for measurement of the entire spectrum  500  ( FIG. 5 ). The seven color bands of  FIGS. 5 and 6  is a simplification for explanation. More useful measurement uses more precise spectral data, which uses many more bands. 
       FIG. 7  is a diagram of a spectrum illuminating an SLM that has reset zones. A reset zone (or group) is a subset of the SLM&#39;s pixels (e.g., mirrors). In some examples, the SLM is divided into several reset zones, such as eight or sixteen. SLM  700  includes eight reset groups  702 ,  704 ,  706 ,  708 ,  710 ,  712 ,  714  and  716 . Each reset group is controlled by its own respective reset signal, separately from the other reset groups. Accordingly, all of a reset group&#39;s mirrors are activated by that reset group&#39;s respective reset signal. For example, in response to the reset group&#39;s respective reset signal, each mirror in such reset group has either the “zero” position or the “one” position, according to the data bit stored in such mirror&#39;s respective associated memory cell. In this example, the spectrum extends from blue to green in the long direction of the SLM. The long direction includes more columns, and thus more pixels per color along the spectrum. For example, an HD-configured SLM is 1920 columns by 1080 rows. 
       FIG. 8  is a timing diagram  800  of the operation of a spectroscope using the configuration of  FIG. 7 . Time  802  loads a pattern, for example SLM pattern  602  ( FIG. 6 ), on to the SLM, for example SLM  700  ( FIG. 7 ). The loading of all data for the mirrors occurs during time  802 . Therefore, this loading requires a relatively long time. After time  802 , SLM  700  ( FIG. 7 ) receives a reset signal during time  804 . During this time, the pattern of mirrors SLM  700  ( FIG. 7 ) enables measurement of one frequency band in the spectrum illuminating SLM  700  ( FIG. 7 ). Thus, a measurement of the light impacting photodetector  122  ( FIG. 1 ) taken during time  804  is a measure of the intensity of that one frequency band. Subsequently, time  804  loads a pattern, such as SLM pattern  604  ( FIG. 6 ), onto the SLM, such as SLM  700  ( FIG. 7 ). Thus, photodetector  122  ( FIG. 1 ) measures a different frequency band during time  808 . Repeating the method of loading patterns followed by a reset signal provides measurement of the full spectrum or, if desired, a portion of interest. 
       FIG. 9  is a timing chart showing the method described in  FIG. 8  in more detail. The vertical axis of chart  900  separates the reset groups on example SLM  700  ( FIG. 7 ). For example, a first reset group loads data  902 . The second through eighth reset groups subsequently load data  904 ,  906 ,  908 ,  910 ,  912 ,  914  and  916 , respectively. The time for loading all of data  902 ,  904 ,  906 ,  908 ,  910 ,  912 ,  914  and  916  corresponds to time  802  ( FIG. 8 ). After loading all of data  902 ,  904 ,  906 ,  908 ,  910 ,  912 ,  914  and  916 , a reset signal  918  applies the loaded data across all of SLM  700  ( FIG. 7 ). Light measurement occurs during reset signal  918 . After reset signal  918 , the first through eighth reset groups subsequently load data  920 ,  922 ,  924 ,  926 ,  928 ,  930 ,  932  and  934 , respectively. The time for loading all of data  920 ,  922 ,  924 ,  926 ,  928 ,  930 ,  932  and  934  corresponds to time  806  ( FIG. 8 ). After loading all of data  920 ,  922 ,  924 ,  926 ,  928 ,  930 ,  932  and  934 , a reset signal  936  applies the loaded data across SLM  700  ( FIG. 7 ). 
       FIG. 10  is a diagram of an example SLM configuration. The reset groups of SLM  1000  run perpendicular to the spectrum illuminating SLM  1000 , such that each color of the spectrum runs from top to bottom of a reset group (as oriented in  FIG. 10 ). Accordingly, in this example, no color runs across reset groups  1002 ,  1004 ,  1006 ,  1008 ,  1010 ,  1012 ,  1014  and  1016 . The colors only occur in one reset group. 
       FIG. 11  is a timing diagram  1100  of an example method using the configuration of  FIG. 10 . An initialization signal  1101  including a reset signal and a clear signal clears the entire SLM array. A first reset group loads data  1102  at a first time. After loading data  1102 , a compound reset signal  1104  that includes a reset signal followed by a clear signal followed by another reset signal activates the first reset group. The compound reset signal clears the memory for the subsequent data by using a clear signal to quickly write zeros to all the memory cells followed by a reset signal. The clear signal is relatively fast, so it does not require considerable time. A photodetector, such as photodetector  122  ( FIG. 1 ), measures light (impacting the photodetector) corresponding to data  1102  during the first reset signal of compound reset signal  1104 , because all pixels of a frequency of interest are within one reset group. While applying compound reset signal  1104  to the first group, the second reset group loads data  1106  followed by compound reset signal  1108 . For the third through eighth reset groups, compound reset signals  1112 ,  1116 ,  1120 ,  1124 ,  1128  and  1132  follow loading of data  1110 ,  1114 ,  1118 ,  1122 ,  1126  and  1130 , respectively. Also, as shown in  FIG. 11 , the loading of data into a subsequent reset group coincides with the compound reset signal of the previous group. Photodetector  122  takes light intensity measurements during the first reset signal of compound reset signals  1104 ,  1108 ,  1112 ,  1116 ,  1120 ,  1124 ,  1128  and  1132 . The time from compound reset signal  1104  to compound reset signal  1132  is on the order of the time of loading data  902  to loading data  916  ( FIG. 9 ). However, during time from compound reset signal  1104  to compound reset signal  1132 , the configuration of  FIG. 10  takes eight light measurements. The configuration of  FIG. 9  takes one light measurement during a similar time. Therefore, because SLM  1000  has eight reset groups, the configuration of  FIGS. 10 and 11  is approximately eight times faster than the configuration of  FIGS. 7-9 . Available SLM configurations include as many as 16 reset groups. The configuration of  FIGS. 10 and 11  takes the number of measurements equal to the number of reset groups in the time that the configuration of  FIGS. 7-9  takes one measurement. 
       FIG. 11  also shows the next cycle where compound reset signals  1136 ,  1140 ,  1144 ,  1148 ,  1152 ,  1156 ,  1160  and  1164  follow loading data  1134 ,  1138 ,  1142 ,  1146 ,  1150 ,  1154 ,  1158  and  1162 , respectively. In addition, although there is a one in both data  1102  and  1134 , data  1102  can be different from data  1134  in most configurations. The frequencies of interest often illuminate a narrower part of SLM  1000  than an entire reset group. Therefore, data  1102  can target a subgroup of the reset group directed to a subportion of the light spectrum having light frequency with a dominant color in one portion of the first reset group and data  1134  can target another subgroup of the reset group directed to a subportion of the light spectrum having frequency with another dominant color in another part of the first reset group. However, to provide the same resolution, the configuration of  FIGS. 7-9  writes the entire SLM with a pattern directed to the band addressed by data  1102 . Then the configuration of  FIGS. 7-9  writes the entire SLM with a pattern directed to the band addressed by data  1134 . Therefore, a higher resolution increases the number of patterns written in the configuration of  FIGS. 7-9  by the same factor that it increases the number of reset group patterns in the configuration of  FIGS. 10 and 11 . Thus, for any given frequency resolution, the configuration of  FIGS. 10 and 11  takes measurements approximately n times faster than the configuration of  FIGS. 7-9 , where n is the number of reset groups in the SLM. 
       FIG. 12  is a diagram illustrating a method for extracting the light intensity of specific colors. Patterns  1202  are successively received by SLM  1000 . Spectrums  1204  show where the “on” pattern (which is the portion of the pattern where the micromirrors of the SLM reflect the light impacting those mirrors to output optics for measurement of the light intensity, such as via lens  120  to photodetector  122  of  FIG. 1 ). In the examples of  FIG. 12 , the first pattern transmits orange bar  1206 , which is the portion of the light spectrum in which the dominant color is orange, to the photodetector, the second pattern transmits green bar  1208 , which is the portion of the light spectrum in which the dominant color is green, to the photodetector, the third pattern transmits the blue bar  1210 , which is the portion of the light spectrum in which the dominant color is blue, to the photodetector, and the fourth pattern transmits the violet bar  1212 , which is the portion of the light spectrum in which the dominant color is violet, to the photodetector. Spectral graph  1214  shows the measurements of the bars on the color spectrum graphically. A graph like spectral graph  1214  uses many more than four measurements. For ease of explanation, patterns  1202  and spectrums  1204  are a simplification. A practical example could include hundreds of pattern bars and measurements. 
       FIG. 13  is a schematic diagram of an example spectroscope apparatus including control circuitry. Spectroscope  1300  includes light sources  1302  or  1304  that shine broad spectrum light through or off, respectively, test object  1306 . Lens  1308  collimates the light that passes through slit  1312  in plate  1310 . The light through slit  1312  is re-collimated by lens  1314  and reflected off diffraction grating  1316 , which serves as a spectrum dispersion device that divides the light into its spectral components. SLM  1318  selectively reflects portions of the light spectrum to lens  1320 , which focuses the light on photodetector  1322 . Analog-to-digital converter (ADC)  1324  receives the light intensity signal from photodetector  1322  via input  1323  and converts the output of photodetector  1322  from an analog signal to a digital signal. Processor  1326  stores and processes the output of ADC  1324  to provide an output like spectral graph  1214  ( FIG. 12 ). Processor  1326  also controls spatial light modulator (SLM) controller  1328 . SLM controller  1328  controls the operation of SLM  1318 , such as loading patterns onto SLM  1318  and providing reset signals for the appropriate reset group. In an example, SLM  1318  is a digital micromirror device (DMD) and SLM controller  1328  is a DMD controller. 
     As described hereinabove, SLM  1318  can be a digital micromirror device (DMD) or another type of spatial light modulator. ADC  1324 , processor  1326  and SLM controller  1328  can be implemented as separate integrated circuits, can be on one combined integrated circuit  1330 , or can be in a hybrid package including the three components in combinations of integrated circuits. Examples of processor  1326  include a microprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit (ASIC), field programmable gate array (FPGA) or other suitable integrated circuit (e.g., processing device). A prism is an alternative spectrum dispersion device to diffraction grating  1316  for separating the light from lens  1314  into spectral components. Photodetector  1322  can be a photodiode. Light sources  1302  and  1304  can be any number of broad spectrum light sources, and can provide visible, infrared or ultraviolet light, or combinations thereof. In addition, an ideal output of light sources provides equal light output for all relevant frequencies. However, practical light sources rarely have an ideal output. Therefore, processor  1326  adjusts the signal provided by ADC  1324  to compensate for frequency to frequency output anomalies of light sources  1302  and  1304 . Also, processor  1326  can execute instructions for periodically measuring the output light of sources  1302  and  1304  and altering the spectral anomaly adjustment procedure to address operational effects on the light sources  1302  and  1304 , such as aging. 
       FIG. 14  is a flow diagram of an example method. Method  1400  begins with step  1402 . Step  1402  loads a first pattern into a first reset group, like reset group  1002  ( FIG. 10 ). The pattern may be like patterns  1202  ( FIG. 12 ). Step  1404  loads a next pattern into the next reset group while applying a reset signal to the previous reset group, in this case the first reset group. Step  1406  measures the light reflecting from an SLM like SLM  1318  ( FIG. 13 ) in response to the reset signal applied in step  1404 . Step  1407  clears and resets the previous reset group. Step  1408  determines if step  1404  loaded the pattern for the last reset group. If not, the method returns to step  1404 . If so, step  1410  issues a reset signal for the last reset group. Step  1412  measures the light intensity reflected from the SLM by the last reset group. Step  1414  clears and resets the last reset group and returns to step  1402 . 
     Modifications are possible in the described examples, and other examples are possible, within the scope of the claims.