Abstract:
An optical system comprising an optical instrument and a processing unit. The optical instrument may comprise an illumination source and a sensor. The processing unit may comprise a data storage having stored thereon a characterization of the illumination source and a characterization of the sensor. The processing unit may also comprise a computer configured to calculate a system response of the illumination source and the receiving element considering the characterization of the illumination source and the characterization of the receiving element.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/710,920 filed on Aug. 24, 2005, and U.S. Provisional Application No. 60/708,222, filed on Aug. 15, 2005, which are incorporated herein by reference. This application is also related to a concurrently filed United States patent application entitled, “IMPROVED OPTICAL INSTRUMENT AND COMPONENTS THEREOF,” by Jon Nisper, Mike Mater and Bernie Berg, which is herein incorporated by reference. 
    
    
     BACKGROUND 
     Spectrometers, colorimeters, and other optical instruments have been used for years to measure various properties of materials (e.g., hue, lightness, chemical composition, etc.) by illuminating a material sample and analyzing light or other radiation that is either reflected by or transmitted through the sample. Due to the large range of perceivable differences in reflected light, it is desirable for these instruments to have a high degree of accuracy, repeatability, and inter-instrument agreement. 
     Some existing methods for manufacturing optical instruments meet these goals by driving the hardware output of the optical instruments toward an accepted standard. For example, instruments may be constructed with tight-tolerance components and then mechanically tuned and adjusted to the accepted standard during the manufacturing process. These methods, however, do not adequately account for changes in the instrument in the field due to temperature, age, environmental conditions, etc. This is left to simple calibration procedures, which are often inadequate. Also, these methods are limited in the types of components that they can use. For example, low cost, efficient illumination sources, such as light emitting diodes (LED&#39;s) cannot be easily used because they are not currently available with sufficiently tight tolerances, and because their spectral output varies with temperature. 
     Other existing methods for manufacturing optical instruments attempt to use looser tolerance components, such as LED&#39;s, by developing instrument-level correction factors that are applied to the hardware output in an attempt to bring it into conformance with the accepted standard. The correction factors are developed based on an extensive and often expensive, characterization of the instrument as a whole. Instrument level characterizations, though, are often not adequate to compensate for complex non-linear changes in the instruments due to changes in temperature and other environmental changes that affect the individual instrument components (such as LED&#39;s) in the field. 
     Still other attempts have been made to address the shortcomings of LED&#39;s, however, these also leave room for improvement. For example, various methods have been developed that attempt to stabilize the output of an LED by manipulating its current and voltage drop. Also, some known methods involve heating an LED in an attempt to make its output constant. All of these methods, however, add additional cost and complexity to optical instruments, and still fail to give the optical instrument a desired level of accuracy. 
     SUMMARY 
     In one general aspect, the invention is directed to an optical system comprising an optical instrument and a processing unit. The optical instrument may comprise an illumination source and a sensor. The processing unit may comprise a data storage having stored thereon a characterization of the illumination source and a characterization of the sensor. The processing unit may also comprise a computer configured to calculate a system response of the illumination source and the receiving element considering the characterization of the illumination source and the characterization of the receiving element. 
     In another general aspect, the invention is directed to methods of operating an optical instrument. The optical instrument may comprise an illumination source and a sensor. The methods may comprise the steps of measuring a temperature of the illumination source, and calculating a system response of the optical instrument considering the temperature. Calculating the system response may involve considering a response of the illumination source and a response of the at least one sensor. The methods may also comprise the steps of performing a measurement with the optical instrument, and normalizing the measurement considering the system response. 
     In yet another general aspect, the invention is directed to methods of characterizing an optical instrument. The optical instrument may comprise a light emitting diode (LED), a sensor, and a spectral filter positioned to filter light incident on the sensor. The methods may comprise the steps of calculating a first response of the LED, calculating a second response of the sensor, and calculating a third response of the spectral filter. The methods may also comprise the step of calculating a system response of the optical instrument. Calculating the system response of the optical instrument may comprise mathematically combining the first, second and third characterizations. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Embodiments of the present invention are described herein, by way of example, in conjunction with the following figures, wherein: 
         FIG. 1  shows a diagram of an optical instrument according to various embodiments; 
         FIG. 2  shows a flow chart of a process flow for operating an optical instrument according to various embodiments; 
         FIG. 3  shows a flowchart of a process flow for characterizing an optical instrument according to various embodiments; 
         FIG. 3A  shows a chart of various response curves of an LED at different temperatures; 
         FIGS. 4-6  show plots of the spectral outputs of various LED&#39;s and sensor active areas according to various embodiments; 
         FIG. 7  shows a flowchart of a process flow for calibrating an temperature measurement system according to various embodiments; and 
         FIG. 8  shows a flowchart of a process flow for measuring a temperature of an LED according to various embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Various embodiments of the present invention are directed to systems and methods for taking optical measurements that involve characterizing the illumination optics and the receiving optics of an optical instrument to generate a system response. The system response may be used, in various embodiments, to normalize readings from the optical instrument to a generally accepted standard scale or colorspace. In this way, it may be possible to construct optical instruments with less expensive components having less stringent tolerances. It may also be possible to construct optical instruments with components, such as light emitting diodes (LED&#39;s) that may not be currently manufactured to tight enough tolerances for use otherwise. 
       FIG. 1  shows a block diagram, according to various embodiments, of an optical instrument  100 . The optical instrument  100  may be a spectrometer, or any other optical instrument having illumination optics  104  and detection sensors  106  as shown. For example, in various embodiments, the optical instrument may be a densitometer, a sensitometer, a photometer, etc. In various embodiments, the instrument  100  may include a processing unit  110  or other suitable computing component or device. The processing unit  110  may also have associated memory (not shown), for example, to store component characterizations described below. Also, in various embodiments, some or all of the processing performed for the instrument  100  may be performed by an external computer, processor, etc. (not shown) in communication with the instrument  100  by any suitable wired or wireless data link. 
     The illumination optics  104  may include any suitable type of illumination source. In various embodiments, the illumination optics  104  may include one or more LED&#39;s  105 . In various embodiments, the LED&#39;s  105  may include individual dies having different nominal peak wavelengths. For example, various LED&#39;s  105  may have nominal wavelengths of 405 nm, 430 nm, 470 nm, 505 nm, 527 nm, 570 nm, 590 nm, 630 nm, 660 nm, etc. In various embodiments, the illumination optics may  104  may comprise one or more substrates with the individual LED  105  dies mounted on the substrate. The substrate may be mounted to a circuit board or other component of the instrument  100 . 
     The receiving optics of the instrument  100  may include detection sensors  106  as well as one or more monitor sensors  108 . The detection sensors  106  may be positioned to receive light emitted by the illumination optics  104  and reflected by, or transmitted through, sample material  102 . Two detection sensors  106  are shown in  FIG. 1 , although, it will be appreciated that more or fewer could be used. The monitor sensor  108  may be positioned to receive light directly from the illumination optics  104 . The signal from the monitor sensor  108  may be used, as described below, to monitor the output of the illumination optics  104 . In various embodiments, the monitor sensor  108  may be a dual-beam reference sensor  108  having the capability to discern information about the wavelength of received light (e.g., color). The sensors  106 ,  108  may be any suitable type of sensors. In various embodiments, for example, the sensors  106 ,  108  may include TSL230 surface mount sensors available from TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS (TAOS). Also, in various embodiments, the sensors  106 ,  108  may include any kind of imaging chip or even RGB camera modules. 
     In various embodiments, it may be desirable for the sensors  106 ,  108  to discern color. Accordingly, various spectral filters may be positioned on or near the sensors  106 ,  108  to filter incoming light. For example, the sensors  106 ,  108  may have active areas  112 ,  114 ,  116 ,  118  having red ( 112 ), green ( 114 ), blue ( 116 ) and/or clear ( 118 ) spectral filters. In various embodiments, each of the active areas  112 ,  114 ,  116 ,  118  may include a separate sensor device with a single spectral filter. In other various embodiments, some or all of the sensors  106 ,  108  may include an array of sensor pixels. The active areas  112 ,  114 ,  116 ,  118  may include a single pixel or group of pixels included in sensors  106 ,  108  having the appropriate spectral filter positioned to filter incoming light. It will be appreciated that not all of the sensors  106 ,  108  need to include all of active areas  112 ,  114 ,  116 ,  118 . Also, some sensors  106 ,  108  may include active areas having other types of spectral filters (e.g., other than red, green, blue or clear) in addition to, or instead of one or more of active areas  112 ,  114 ,  116 ,  118 . 
       FIG. 2  shows a process flow  200 , according to various embodiments, for using the instrument  100 . At step  202  of the process flow  200 , the system response of the instrument  100  may be found. The system response may describe how the instrument  100  generally responds to a given input. In various embodiments, the system response may consider the response of the electrical-to-optical components of the system (e.g., the illumination optics  104 , LED&#39;s  105 , etc.) as well as the optical-to-electrical components (e.g. the sensors  106 ,  108 ). It will be appreciated that the individual responses of the illumination optics  104  and its LED&#39;s  105 , as well as the responses of the sensors  106 ,  108 , may be non-linear. Accordingly, it may be desirable to consider the responses of each individually when determining the overall system response. Additional details of determining the system response are discussed below with respect to  FIG. 3 . 
     At step  204 , a temperature of the illumination optics  104  and LED&#39;s  105  may be sensed. It will be appreciated that LED&#39;s  105  have a spectral output that changes with temperature. As the spectral output of the LED&#39;s  105  change, the response of the illumination optics  104 , as well as the overall system response, also changes. Accordingly, after the temperature of the illumination optics  104  is taken, the system response may be modified, at step  206 , to take this into account. Additional details of determining the temperature of the illumination optics  104  and LED&#39;s  105 , according to various embodiments, are discussed below with reference to  FIGS. 6-8 . 
     At step  208 , a measurement may be performed using the instrument  100 . For example, the illumination optics  104  may illuminate the sample material  102 . Detection sensors  106  may sense light that is either reflected from or transmitted by the sample material  102 . The result may be a reflectance or transmittance of the sample material  102  at the various wavelengths produced by the illumination optics  104  and sensed by the detection sensors  106 . At step  210 , the result of the measurement taken at step  208  may be normalized based on the system response of the instrument  100 . For example, the result may be normalized to a known colorspace. In this way, the instrument may generate an accurate result that can be related to results from other instruments with different system responses. 
       FIG. 3  shows a process flow  300  for developing a system response of the instrument  100  according to various embodiments. At step  302 , the LED&#39;s  105  of the illumination optics  104  may be characterized. Characterizing the LED&#39;s  105  may involve observing one or more optical criteria of the LED&#39;s  105  and using the observed criteria to fit the LED&#39;s  105  to a model or models of the LED&#39;s  105  response. In various embodiments, each of the LED&#39;s  105  may be characterized separately to account for variations in behavior due to slight differences in material or manufacture. 
     The optical criteria of the LED&#39;s  105  may be found by illuminating the LED&#39;s  105  over a given amount of time and observing them with a spectrometer. In various embodiments, the spectrometer used to characterize the LED&#39;s  105  may be the instrument  100  itself. It will be appreciated that illuminating the LED&#39;s  105  over a given amount of time may allow them to heat up, providing readings over a range of operating temperatures. Example optical criteria that may be measured include: the spectral output, spectral optical power, spectral optical linearity, degree of collimation, illuminated spot size, spot intensity profile, illumination uniformity, spatial and temporal phase/coherence, temporal modulation, etc. Various optical criteria describing the illumination optics  104  as a whole may also be found. 
     The observed criteria may then be used to fit the LED&#39;s  105  to one or more models. The one or more models may take into account changes in the response of the LED&#39;s  105  based on operating conditions. For example, the materials used to produce LED&#39;s  105 , such as GaAs, GaN, etc., have inherent dispersive properties (e.g., dielectric constant, complex refractive index, etc.) that vary with both wavelength and temperature (dn/dT and dn/dλ). As a result, the LED&#39;s  105  may exhibit behaviors that vary proportionately to λ 2 /Δλ as the temperature, forward current and/or forward voltage change. Also, as temperature increases, the physical dimensions of LED&#39;s  105  dies may change. This, together with the changes in dispersion, may result in a net shift of the peak wavelength, λ, output, a decrease in light output, and a change in the bandwidth, Δλ, as the temperature of the LED&#39;s  105  change. 
     For example,  FIG. 3A  shows a general shape of a model  350  of the spectral output of an LED, according to various embodiments. Three curves  356 ,  358 ,  360  are shown representing the output of the LED at three different temperatures. The curve  356  shows the spectral output of the LED at a first temperature. The curve  358  shows the spectral output of the LED at a second temperature higher than the first. Finally, the curve  360  shows the LED at a third temperature lower than the first temperature. It can be seen that, generally, as temperature increases, the LED&#39;s spectral output may generally increase in bandwidth and decrease in intensity. 
     It will be appreciated that, despite tolerance-related variations from LED to LED, most LED&#39;s  105  of a similar make-up and structure may conform to the same general model. Accordingly, it may not be necessary to observe the LED&#39;s  105  optical criteria over the entire range of operating conditions. Instead, the LED&#39;s  105  optical criteria may be observed over a suitable range of known operating conditions, for example, as described above. The observed optical criteria may then be fitted to the appropriate model, for example, by solving for a coefficient or coefficients of the model. Which model or models apply to a particular LED  105  may depend on various factors, including the material from which the LED′  105  are made, the type of dopants used in the LED&#39;s  105 , their geometric configuration, etc. 
     Referring back to the process flow  300 , at step  304  the sensors  106 ,  108  may be characterized by measuring one or more optical criteria of the sensors  106 ,  108 . Like LED&#39;s  105 , one or more physical models may exist describing the response of sensors  106 ,  108  and their respective active areas  112 ,  114 ,  116 ,  118 . Accordingly, characterizing the sensors  106 ,  108  may involve observing the response of the sensors  106 ,  108  to various input conditions, and fitting the model or models to the observed results (e.g., by finding a coefficient or coefficients for the model). It will be appreciated that characterizing the sensors  106 ,  108  may also involve characterizing the spectral filters of the active areas  112 ,  114 ,  116 ,  118 . In various embodiments, the respective active areas  112 ,  114 ,  116 ,  118  may be characterized together based on their spectral filter type. For example, all of the red active areas  112  may be characterized together, etc. In this way the sensors  106 ,  108  and the various spectral filters may be characterized together. Example optical criteria that may be found for each sensor  106 ,  108  or sensor pixel include: spectral responsivity, spatial and temporal responsivity, field of view, degree of specular rejection, degree of stray light rejection, dynamic range of response, etc. 
     At step  306 , a response may be found for each spectral channel of the instrument  100 . A channel of the instrument  100  may be the combination of a particular wavelength of LED  105  and all of the sensor  106  active areas of a given type. For example, the combination of a 660 nm LED  105  and the red active areas  112  would be one channel.  FIG. 4  shows a chart  400  of a channel comprising a 470 nm LED and the clear active areas  118 . Curve  402  represents the spectral output of the LED  105  and curve  404  represents the spectral responsivity of the active areas  118 .  FIG. 5  shows a second example chart  500  of a second channel comprising a 590 nm LED and the green active areas  114 . Curve  502  represents the spectral output of the LED  105  and curve  504  represents the spectral responsivity of active areas  114 . 
     Finding the response of a channel may involve mathematically combining an optical criterion of the LED  105  a corresponding optical criterion of the appropriate active areas  112 ,  114 ,  116 ,  118 . For example, finding a spectral response of the channel shown by chart  500  may involve mathematically combining the spectral response  502  of the LED  105  with the spectral responsivity  504  of the sensors. The particular type of mathematical combination used may depend various factors including, the form of the various optical criterion, etc. For example, in various embodiments, the mathematical combination may include one or more of convolution, vector summation, arithmetic summation, etc. 
     In various embodiments, the responses of the LED&#39;s  105 , the sensors  106 ,  108  and/or various channels may be normalized to a common value according to any suitable weighting method. For example, the responses of each of the LED&#39;s  105  and each of the sensors  106 ,  108  may be normalized to a single value (e.g., 65,535). Channel responses may also be normalized. For example, channel responses may be found as a weighted integral response, with each channel response being weighted to the same area (e.g., 512) under the curve. Normalizing the respective responses may be done to conform to total energy conservation, or alternatively, to a normalized object reflection. Also, in various embodiments, the responses may be normalized to represent that portion of the Bi-Directional Reflectance Distribution Function (BRDF) represented by the optical sampling position of the sensor channel. It will be appreciated that, in various embodiments where the LED&#39;s  105  are illuminated sequentially during measurement, each may be normalized separately. Referring back to the process flow  300 , at step  308 , the illumination optics  104  and detection sensors  106  may be normalized to white and black samples. In this way, the channel responses calculated above can be related to appropriate the colorspace. 
     As discussed above, the temperature of the illumination optics  104  may be sensed, for example, at step  204  of the process flow  200 . In various embodiments, this temperature may be taken by observing the spectral shift of one or more of the LED&#39;s  105 , for example, using the monitor sensor  108 . As described above, the monitor sensor  108  may be positioned to directly, or indirectly, observe the illumination optics  104 . The spectral shift of one or more of the LED&#39;s  105  may be found by observing the LED&#39;s through active areas  112 ,  114 ,  116 ,  118  that are spectrally adjacent to each other. 
     For example,  FIG. 6  shows a diagram  600  of the spectral responses of an LED  105 , the green active areas  114  and red active areas  112  of the sensor  108 . Curve  602  represents the spectral output of a 590 nm LED  105 , while curves  604  and  606  represent the spectral responses of green  114  and red  112  active areas respectively. The respective green and red spectral filters used by green  114  and red  112  active areas may be chosen to have adjacent or roughly overlapping attenuation bands at about the peak wavelength of the LED  105 , shown by LED curve  602 . Because the example LED  105  shown has a nominal peak wavelength of 590 nm, its spectral response may fit roughly between the pass bands of the green  114  and red  112  active areas. Although the present example is presented with a 590 nm LED and with green and red spectral filters, it will be appreciated, however, that any suitable LED and filter combination may be used. For example, a 505 nm LED and/or a 525 nm LED could be used in conjunction with blue and green filters. 
     Referring again to  FIG. 6 , it will be appreciated that as the temperature of the LED  105  changes, the position and/or shape of the curve  602  will also change in a predictable way, for example, as shown above by  FIG. 3A . As the temperature of the LED  105  increases, the LED spectral output  602  may be shifted to a longer wavelength (to the right in the diagram  600 ). When this occurs, more of the LED&#39;s total output may be attenuated by green active area  114  (curve  204 ), and more of the LED&#39;s total output may be passed by the red active area  112  (curve  206 ). As the temperature of the LED  105  decreases, the opposite may occur. Accordingly, the peak wavelength of the LED  105 , and therefore its temperature, may be sensed by comparing the intensity of the LED  105  as viewed by active area  114  to the intensity of the LED  105  as viewed through active area  112 . In various embodiments, where the LED&#39;s  105  are mounted in close proximity, it may be assumed that the temperature of one LED  105  is the temperature of some or all of the LED&#39;s  105  in the illumination optics. 
       FIG. 7  shows a process flow  700 , according to various embodiments, illustrating a method for calibrating the instrument  100  for sensing the temperature of an LED  105 . It will be appreciated that the steps of the process flow  700  may be performed in any suitable order, and that some or all of the steps may be performed simultaneously. At step  702 , the LED  105  may be activated at a first known temperature. The intensity of the LED  105  as viewed by the green active area  114  may be measured at step  704 . The intensity of the LED  105  as viewed by the red active area  112  may be measured at step  706 . At step  708 , the processing unit  110  may create or supplement a model relating the temperature of the LED  105  and the intensity of the LED  105  as viewed through the active areas  112 ,  114  (e.g., the model shown above at  FIG. 3A ). In various embodiments, creating or supplementing the model may involve calculating one or more coefficients matching the observed intensities to the model. In various embodiments, the processing unit  110  may also explicitly solve for the peak wavelength of the LED  105 . 
     At decision step  710 , the computer may determine whether additional measurements will be taken to further supplement the model. If an additional measurement is desired, the temperature of the LED  105  may be changed at step  712 . For example, the temperature of the LED  105  may be varied by allowing it to be activated for a given period of time, activating additional LED&#39;s near the LED  105 , etc. The process may then continue with step  704  as described above. It will be appreciated that one measurement may be sufficient to develop the model, however, additional measurements may improve the accuracy of the model. Also, taking measurements over a broad range of temperatures or other operating conditions may allow the model to compensate for nonlinearities in LED heating behavior, the effects of additional LED&#39;s (not shown) near the LED  105 , etc. 
       FIG. 8  shows a process flow  800 , according to various embodiments, for measuring the temperature of the LED  105  with the instrument  100 , for example, as described above at step  204  of process flow  200 . Referring back to the process flow  800 , at step  802 , the LED  105  may be activated. The intensity of the LED  105  may be measured through green active area  114  at step  804 , and the intensity of the LED  105  through red active area  112  may be measured at step  806 . It will be appreciated that the respective intensities of the LED  105  as viewed by active areas  112 ,  114  may be measured near the time that the LED  105  is activated, or at any time thereafter. At step  808 , the first and second intensities of the LED  105  may be used to calculate a temperature of the LED  105 , for example, according to a model generated as described above. The temperature of the LED  105  may then be used in any suitable way, for example, as described above. It will be appreciated that other LED&#39;s included in the illumination optics  104  may be assumed to have the same temperature as the LED  105 . This assumption is likely to be more accurate where all of the LED&#39;s in the illumination optics  104  are activated for similar amounts of time under similar conditions. 
     It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, other elements, such as, for example, details of various physical models of LED&#39;s, etc. Those of ordinary skill in the art will recognize that these and other elements may be desirable. However, because such elements are well known in the art and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein. 
     As used herein, a “processing unit,” “computer” or “computer system” may be, for example and without limitation, either alone or in combination, a personal computer (PC), server-based computer, main frame, server, microcomputer, minicomputer, laptop, personal data assistant (PDA), cellular phone, pager, processor, including wireless and/or wireline varieties thereof, and/or any other computerized device capable of configuration for processing data for standalone application and/or over a networked medium or media. Computers and computer systems disclosed herein may include operatively associated memory for storing certain software applications used in obtaining, processing, storing and/or communicating data. It can be appreciated that such memory can be internal, external, remote or local with respect to its operatively associated computer or computer system. Memory may also include any means for storing software or other instructions including, for example and without limitation, a hard disk, an optical disk, floppy disk, ROM (read only memory), RAM (random access memory), PROM (programmable ROM), EEPROM (extended erasable PROM), and/or other like computer-readable media. 
     The processing unit  110  may operate according to software code to be executed by a processor or processors of the processing unit  110  or any other computer system using any type of suitable computer instruction type. The software code may be stored as a series of instructions or commands on a computer readable medium. The term “computer-readable medium” as used herein may include, for example, magnetic and optical memory devices such as diskettes, compact discs of both read-only and writeable varieties, optical disk drives, and hard disk drives. A computer-readable medium may also include memory storage that can be physical, virtual, permanent, temporary, semi-permanent and/or semi-temporary. A computer-readable medium may further include one or more data signals transmitted on one or more carrier waves. 
     While several embodiments of the invention have been described, it should be apparent that various modifications, alterations and adaptations to those embodiments may occur to persons skilled in the art with the attainment of some or all of the advantages of the present invention. It is therefore intended to cover all such modifications, alterations and adaptations without departing from the scope and spirit of the present invention as defined by the appended claims.