Abstract:
A detector calibration reference is disposed along a path of travel for radiation that extends from a radiation source to a radiation detector. The detector calibration reference has mutually exclusive first and second portions that are offset in a direction transverse to the path of travel, the first portion being substantially opaque to radiation from the source, and the second portion being substantially transmissive to radiation from the source. The detector calibration reference is moved relative to the path of travel in a manner so that the first and second portions become successively aligned with the path of travel.

Description:
FIELD OF THE INVENTION 
   This invention relates in general to calibration of optical measurement systems and, more particularly, to optical references for calibration, and calibration techniques that use optical references. 
   BACKGROUND 
   Optical systems have been developed that are used to make optical measurements. For example, a spectrophotometer is an optical system than can be used to measure the level of transmission or absorption of a sample material with respect to a number of different wavelengths of radiation. A spectrophotometer has a radiation source that transmits radiation along a path of travel to a radiation detector. During operational use, the sample under test is positioned optically between the source and the detector, along the path of travel. Radiation from the source that is traveling along the path of travel must pass through the sample, and the detector measures the intensity of received radiation, which represents the amount of radiation that is able to pass through the sample. The accuracy of optical measurements provided by such a system depends on the accuracy of the calibration of the system. 
   It is relatively simple to calibrate a spectrophometer for a transmissivity of 0% and/or a transmissivity of 100%. In particular, it is easy to completely block the radiation beam, or to leave it completely unblocked. However, radiation detectors are typically nonlinear, and in fact there may be differences in the nonlinearity of equivalent detectors that in theory should be identical. Consequently, calibrating for only 0% and/or 100% is not sufficient. It is desirable to perform calibration for one or more different levels of transmissivity that are between 0% and 100%. This can improve the accuracy of the calibration, for example by an average of a factor of ten. 
   A related consideration is that radiation detectors are not always spatially uniform. For example radiation impinging on one portion of the detector may produce a different measurement than if that same radiation were to impinge on a different portion of the same detector. 
   To calibrate for a level of transmissivity between 0% and 100%, a traditional approach is to insert a stationary optical reference (or several successive stationary references) between the source and detector. Each such optical reference has a known transmissivity. One known type of optical reference is a filter with a known transmissivity, typically a neutral density filter. However, filters of this type work only for particular wavelength ranges. Further, materials in the filter may gradually deteriorate and change performance, due to handling, exposure and/or aging. Care must be taken to avoid abrading, scratching or otherwise altering the filter. Moreover, contaminates from the air can accumulate on the filter, altering performance. Cleaning the surface of the filter to remove contaminates may alter the performance of the filter. 
   A different type of known optical reference is made from a material that is well characterized. For example, the optical reference may be a piece of calcium fluoride (CaF 2 ). This type of reference can be more stable than a neutral density filter, but is still subject to some of the same problems. Further, only a limited selection of transmissivity levels may be available. For example, in the visible spectrum, there are very few materials having a transmissivity in the 0% to 70% range. 
   Thus, although existing optical references and calibration techniques have been generally adequate for their intended purposes, they have not been satisfactory in all respects. For example, existing optical references used for calibration are not always durable, stable and highly accurate, and cannot always be obtained for every desired level of transmissivity. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention will be realized from the detailed description that follows, taken in conjunction with the accompanying drawing, in which: 
       FIG. 1  is diagrammatic view of an apparatus that is a spectrophotometer embodying aspects of the invention, and that includes a detector calibration reference. 
       FIG. 2  is a diagrammatic sectional view, taken along the section line  2 - 2  in  FIG. 1 . 
       FIG. 3  is a diagrammatic fragmentary sectional view, taken along the section line  3 - 3  in  FIG. 2 . 
       FIG. 4  is a diagrammatic fragmentary sectional view similar to  FIG. 3 , but showing part of a detector calibration reference that is an alternative embodiment of the detector calibration reference in the embodiment of  FIGS. 1-3 . 
       FIG. 5  is a diagrammatic exploded perspective view of a detector calibration reference that is an alternative embodiment of, and can be substituted for, the detector calibration reference in the embodiment of  FIGS. 1-3 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a diagrammatic view of an apparatus that is a spectrophotometer  10 , and that embodies aspects of the invention. The spectrophotometer  10  includes a base  12 . A radiation source  16  of a known type is fixedly supported on the base  12 , and emits a beam of radiation that propagates along a path of travel  18 . The beam includes radiation having a range of different wavelengths. 
   A radiation detector  26  of a known type is fixedly supported on the base  12 , at a location that is spaced optically from the source  16 , and that is at an end of the path of travel  18  remote from the source  16 . A control unit  28  controls the source  16 , and receives signals from the detector  26 . 
   A support  31  is fixedly provided on the base  12 . During normal operation, a sample  33  can be removably and stationarily supported on the support  31 . The sample  33  is shown in broken lines in  FIG. 1 , because the focus of the present discussion is calibration of the spectrophotometer  10 , and the sample  33  is not present during calibration. During normal operation, radiation from the source  16  propagates along the path of travel  18  to the sample  33 . A portion of that radiation will be absorbed and/or reflected by the sample. The rest of the radiation will pass through the sample  33 , and continue along the path of travel  18  to the detector  26 . For each of a number of different wavelengths, the detector  26  measures the amount of radiation at that wavelength arriving at the detector, which represents the level of transmissivity of the sample  33  for that particular wavelength. 
   In order to ensure that measurements taken with the spectrophotometer  10  are accurate, the spectrophotometer must be periodically calibrated in relation to a known reference. It is relatively straightforward to calibrate for transmissivity levels of 0% and 100%. For 100%, radiation is allowed to travel from the source  16  along the path of travel  18  to the detector  26 , without encountering or passing through any physical structure. For 0%, the source  16  can be turned off, or a not-illustrated part that is completely non-transmissive can be provided along the path of travel, for example in place of the sample  33 . But it is desirable to calibrate for more than just a transmissivity of 0% and/or a transmissivity of 100%. This is because the detector  26  is nonlinear, and in fact the nonlinearity may differ from one detector  26  to another detector that in theory should be identical to the detector  26 . As explained earlier, the traditional calibration approaches for transmissivities between 0% and 100% have been adequate for their intended purposes, but have not been completely satisfactory. The spectrophotometer  10  therefore includes some additional structure that is provided for the purpose of calibration. 
   In more detail, a motor  51  of a known type is fixedly supported on the base  12 . In the disclosed embodiment, the motor  51  is a stepper motor, but it could alternatively be any other suitable type of motor. The motor is controlled by the control unit  28 . The motor  51  has a shaft  52  that rotates about an axis  53 . The axis  53  extends approximately parallel to the path of travel  18 . A detector calibration reference  61  is fixedly mounted on the shaft  52 , for rotation therewith.  FIG. 2  is a diagrammatic sectional view of the shaft  52  and the calibration reference  61 , taken along the section line  2 - 2  in  FIG. 1 . 
   As discussed above, the axis  53  in the disclosed embodiment extends approximately parallel to the path of travel  18 . however, it would alternatively be possible for the axis  53  to extend at an angle to the path of travel  18 . For example, the detector  26  may emit a small amount of heat, and where the detector  26  is used to measure infrared radiation, it is desirable that the calibration reference  61  not take heat emitted by the detector  26  and reflect that heat directly back to the detector  26 . If the axis  53  is oriented at an angle to the path of travel  18 , so that side surfaces of the calibration reference  61  are not perpendicular to the path of travel  18 , then the calibration reference  61  will reflect heat from the detector  26  in a direction other then directly back to the detector  26 . 
   In the disclosed embodiment, the calibration reference  61  is made of a material that fully blocks radiation from the source  16 . In the disclosed embodiment, the calibration reference  61  is made from a material that is non-transmissive to radiation (0% transmissive), and in particular is made from a metal such as steel. However, it could alternatively be made from any other suitable material. As evident from  FIGS. 1 and 2 , the calibration reference  61  is a platelike circular disk. The calibration reference  61  has two openings  71  and  72  extending axially therethrough, on diametrically opposite sides of the shaft  52 . In the disclosed embodiment, the calibration reference  61  has two openings  71  and  72 . However, it would alternatively be possible to have only one opening, or to have more than two openings. In  FIG. 2 , the opening  72  has edges  76  and  77  on opposite sides thereof, and the edges  76  and  77  each extend radially with respect to the shaft  52 . In addition, the opening  72  has inner and outer edges  78  and  79 , each of which is an arc concentric to the shaft  52 . The distance between the edges  78  and  79  is greater than the width of the beam of radiation produced by the source  16 . The opening  71  has a configuration that is identical to that of opening  72 , and the opening  71  is therefore not separately described here in detail. 
     FIG. 3  is a diagrammatic fragmentary sectional view taken along the section line  3 - 3  in  FIG. 2 . As shown in  FIG. 3 , an optional anti-reflection coating of a known type is provided on the edges of the opening  72 , and on adjacent portions of the calibration reference  61 . For simplicity and clarity, the coating  86  has been omitted in  FIGS. 1 and 2 . The coating  86  is made of a known material, and a similar coating would be provided in the region of the opening  71 . In fact, the entire calibration reference  61  could be coated. During calibration of the system  10  of  FIG. 1 , the coating  86  prevents the edges  76  and  77  of the openings from reflecting light into the detector  26 . 
     FIG. 4  is a diagrammatic fragmentary sectional view similar to  FIG. 3 , but showing part of a detector calibration reference  161  that is an alternative embodiment of the detector calibration reference  61  of  FIGS. 1-3 . The calibration reference  161  is generally identical to the calibration reference  61 , except for differences that are discussed below. The calibration reference  161  has an opening  172  that is generally equivalent to the opening  72  except that, adjacent each of the radially extending edges  176  and  177 , the calibration reference  161  tapers in thickness in a direction toward the opening  172 . The edges  176  and  177  each have a shape that is referred to figuratively as a knife edge, although of course neither edge is actually as sharp as a knife. The tapering thickness adjacent these knife edges is an alternative technique for minimizing undesired reflections from the regions adjacent the edges  176  and  177 . 
   With reference to  FIGS. 1 and 2 , during calibration the motor  51  effects rotation of the calibration reference  61 . When the path of travel  18  is aligned with either one of the openings  71  or  72 , radiation from the source  16  will travel through that opening and reach the detector  26 . When neither of the openings  71  and  72  is aligned with the path of travel  18 , the opaque material of the calibration reference  61  will completely block the radiation from the source  16 , so that none of the radiation reaches the detector  26 . 
   With reference to  FIG. 2 , it can be seen that radiation from the beam will be blocked during about 90% of the angular movement of the calibration reference  61 , and will be passing through one or the other of openings  71  and  72  during the other 10% of angular movement. With reference to  FIGS. 1 and 2 , the motor  51  rotates the calibration reference  61  at a sufficiently high speed so that the radiation beam is chopped or interrupted at a frequency significantly higher than the sampling frequency of the detector  26 , for example an order of magnitude higher. Stated differently, the radiation beam is chopped or interrupted with a frequency having a period that is much shorter than the sampling interval or response time of the detector  26 . To avoid a beating effect, the calibration reference  61  should not be rotated at a speed that interrupts the beam at a direct multiple of the measurement frequency of the detector  26 . But if the speed of rotation of the calibration reference  61  is sufficiently high, the likelihood of a beating effect becomes negligible. 
   Since the calibration reference  61  is rotated at relatively high speed, the detector  26  effectively sees an average of all the radiation passing through the rotating calibration reference  61 , rather than alternating bursts of 0% and 100% radiation. Stated differently, the level of the average depends on the relative circumferential lengths of the openings  71  and  72  and the solid regions between these openings. In the case of the calibration reference  61 , approximately 90% of the radiation emitted by the source  16  will be blocked by the calibration reference  61 , while the other 10% will pass through the openings  71  and  72 , and ultimately reach the detector  26 . By altering the size of the openings and/or the number of openings in the calibration reference  61 , the calibration reference  61  can be set to provide any desired transmissivity between 0% and 100%. At the completion of the calibration process, the motor  51  is stopped in a position where the shaft  52  is stationary, and holds the calibration reference  61  in a position where radiation from the source  16  passes through one of the two openings  71  and  72 , without contacting any portion of the calibration reference  61 . Alternatively, the calibration reference  61  could be removed from the shaft  52 . 
   The calibration reference  61  shown in  FIGS. 1-3  provides an optical reference for a selected but fixed level of transmissivity, such as 10%. In order to provide a different level of transmissivity, the calibration reference  61  would be detached from the shaft  52  of the motor  51 , and replaced with a different calibration reference that is effectively identical to the calibration reference  61 , except that it would have openings with a configuration and/or size different from the openings  71  and  72 . 
     FIG. 5  is a diagrammatic exploded perspective view of a detector calibration reference  261  that is an alternative embodiment of, and can be substituted for, the detector calibration reference  61  of  FIGS. 1-3 . The calibration reference  261  includes two circular plates  263  and  264 . The plate  263  is fixedly secured to the motor shaft  52 , and the plate  264  is rotatably supported on the shaft  52 , so that it can be pivoted in relation to the plate  263 . The plate  263  has two openings  271  and  272  that are generally similar to the openings  71  and  72  in  FIG. 2 , except that the openings  271  and  272  each have a circumferential length that is significantly longer than the circumferential length of the openings  71  and  72 . The plate  264  has similar openings  273  and  274 . 
   The plate  264  has an arcuate slot  282  that is concentric to the axis  53  of the motor shaft  52 , and that has an angular length of approximately 90°. A screw  281  has a threaded shank that is slidably received within the slot  282 , and that engages a threaded opening  283  provided in the calibration reference  263 . If the screw  281  is tightened, the plate  264  is forced against the plate  263 , so that friction prevents relative rotation of the plates  263  and  264 . If the screw is  281  is loosened slightly, the plate  264  can be rotated with respect to the plate  263 , while the shank of the screw slides within the slot  282 . This permits variation of the amount of overlap between the openings  271  and  273 , and the amount of overlap between the openings  272  and  274 . This has the effect of varying the effective size of the openings through the overall calibration reference  261 . 
   Not-illustrated indicia can be provided along the circumferential edges of the two plates  263  and  264 . The indicia on one plate can be selectively aligned with indicia on the other plate to identify relative rotational positions of the plates  263  and  264  that would, for example, provide 5% transmissivity, 10% transmissivity, 15% percent transmissivity, and so forth. After the plates have been positioned so as to provide a desired level of transmissivity, the screw  281  can be tightened in order to releasably hold the two plates in that position. 
   The disclosed calibration references each limit the beam of radiation mechanically, such that calibration is not based on a sample that is referenced to a measurement previously made by a different optical device. The disclosed calibration references can be manufactured to great accuracy, thereby providing much more accurate reference values. Further, The disclosed calibration references can be readily manufactured to provide any desired level of transmissivity from 1% to 99%. In addition, the disclosed calibration references are not limited to particular wavelength ranges, but can be used for virtually any wavelength ranges of interest. Also, the disclosed calibration references are each made of metal, and are thus more durable than existing references. Scratches and/or contamination do not affect the performance of the disclosed calibration references, and the disclosed calibration references are not affected by temperature variations. Although the disclosed calibration references are discussed in association with a spectrophotometer, they can alternatively be used for calibrating other types of optical instruments. 
   Although selected embodiments have been illustrated and described in detail, it should be understood that a variety of substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the claims that follow.