Abstract:
An improved light source for use in an opacity monitor (transmissometer) that reduces the variation in light intensity across a projected light beam to enable a more accurate and stable method for measuring the opacity of gases in a stack/duct, especially at low values (e.g., &lt;10%) of opacity while operating within specific performance requirements.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention pertains to a light source used for open-path gas monitoring, particularly for the measurement of the smoke and dust content of stack gases, but also applicable to the measurement of particulates in the atmosphere.  
         BACKGROUND OF INVENTION  
         [0002]    The standard method for continuous emissions measurement of particulates in stacks and ducts is optical transmissometry. The measured quantity is opacity, defined as the fraction of transmitted light which is lost in transmission through a medium.  
           [0003]    One example of a device that measures opacity, known as a transmissometer, is the Land Combustion Model 4500mkII opacity monitor which has been used for a number of years to measure the opacity of gases in stacks and ducts. A functional diagram of the Model 4500mkII is shown in FIG. 1 wherein the Model 4500mkII consists of two main units: a transceiver  20  mounted on one side of a stack/duct  22  and a passive retroreflector  24  mounted on the other side. A light source LS in the transceiver  20  projects a beam of light  26  along the transceiver&#39;s optical axis  27  across the duct  22 , through the dust/smoke in the open path  28  of the gas/smoke  29  (FIG. 2) to the retroreflector  24  which returns a reflected light beam  30  to an analyzer A in the transceiver  20 . The analyzer A then compares the intensity of the returned radiation with that measured under clear-stack conditions in order to calculate the opacity and then displays this opacity value at a remote location (e.g., a data recorder, not shown). Also see U.S. Pat. No. 5,617,212 (Stuart), whose entire disclosure is incorporated by reference herein, for a detailed description of how the analyzer A calculates the opacity.  
           [0004]    [0004]FIG. 2 shows the Model 4500mkII mounted to the stack/duct  22  and depicts the internals of the transceiver  20 . In particular, the light source LS of the transceiver  20  comprises an LED (light emitting diode)  32 . The transceiver  20  also comprises a beamsplitter  34 , a collimating lens  36 , a folding mirror  38 , and the analyzer A which comprises a measurement detector  40 , a reference detector  42  and a processor  43  (e.g., Hitachi H8/500 microprocessor). Additional components include a flood LED  44  for drift correction, an automatic zero  46  and span  48  devices and a fail-safe shutter  50 . It should be understood that the transmissometer is autocollimated meaning that the return light  30  from the retroreflector  24  is along the same path as the projected beam  26 . External electrical power (e.g., 110VAC @ 60 Hz), not shown, is provided to the transceiver  20  for energizing the electrical components.  
           [0005]    The divergence  52  of the projected light beam  26  means that the retroreflector  24  returns only a portion of the projected light  26 . Any change in alignment, (e.g., because of temperature changes, wind, settling, etc.) in the stack/duct  22  walls, results in a different portion of the projected beam  26  falling on the retroreflector  24 . Moreover, because the projected beam  26  is not perfectly homogeneous, i.e., the light intensity varies across the projected beam (see line 54), a change in alignment results in a change in light intensity. This change is wrongly interpreted by the analyzer A as a change in the opacity of the stack/duct  22  gases.  
           [0006]    Errors are also introduced where an opacity monitor (transmissometer) with an inhomogeneous light beam is calibrated in the laboratory and then installed on the stack/duct  22 . In this case, failure to perfectly reproduce the device&#39;s optical alignment between the laboratory and the duct results in a signal offset. This offset is, in many cases, the dominant source of error in the measurement. As a consequence, the detection limit of the opacity monitor may be set by this offset.  
           [0007]    A number of factors affect the homogeneity of the projected beam  26 , including the precision and cleanliness of the optical components used. However, the principal factor is usually the inhomogeneity of the light source LS. There are a number of factors which make the pattern of light from an LED inhomogeneous. Some of these are symmetrical about the optical axis of the LED and some are not. This is especially so when a LED source is used, since the electrical contact to the center of the die results in a dark spot in the middle of the beam  26 .  
           [0008]    One way of producing a homogeneous light source is to use an integrating sphere, such as that described in “A Guide to Integrating Sphere Theory and Applications” by Labsphere. However, an integrating sphere is both bulkier and more expensive than the present invention.  
           [0009]    The limitations of the present state of the art are reflected in ASTM (American Society for Testing and Materials) Standard Practice for Opacity Monitor Manufacturers to Certify Conformance with Design and Performance Specifications D6216-98 (1998) which is incorporated by reference into U.S. 40 C.F.R. §60, Appendix B, EPA Performance Specification 1, and which concerns the use of opacity monitors for regulatory applications at opacity levels of 10% or higher. However, where detecting opacity levels of less than 10% is important, e.g., in the steel industry, no performance specification currently exists for the use of opacity monitors to ensure compliance with opacity limits below 10%.  
           [0010]    Thus, there remains a need for a transmissometer that can minimize the inhomogeneity of the light source and can therefore detect opacity levels below 10% while operating within specific performance requirements.  
         SUMMARY OF THE INVENTION  
         [0011]    A light source for use in an opacity monitor for measuring the opacity of gases in an open path of gases wherein the light source reduces the variation in light intensity across a beam of light projected therefrom.  
           [0012]    A light source adapted for use in open path gas monitoring wherein the light source generates a homogeneous light beam.  
           [0013]    An opacity monitor for measuring the opacity of gases in an open path of gases wherein opacity is defined as the fraction of transmitted light which is lost in transmission through the open path of gases. The opacity monitor comprises: an optical transmitter for projecting a light beam across the open path of gases using a light source that reduces the variation in light intensity across the projected beam; a reflector for reflecting a portion of the projected light beam back towards the optical transmitter through the open path gas of gases; an analyzer for detecting the portion of the projected light beam and calculating the opacity of the gases; and wherein the optical monitor detects opacities less than 10 percent while operating within specific performance requirements (e.g., all the requirements of ASTM D6216-98, including amendments to specific portions of ASTM D6216-98 to ensure compliance with opacity limits below 10%, such as thermal stability, insensitivity to ambient light, zero and span calibration, measurement of output resolution, calibration error, optical alignment indicator, calibration device value and repeatability, and insensitivity to supply voltage variations).  
           [0014]    An opacity monitor for measuring the opacity of gases in an open path of gases wherein opacity is defined as the fraction of transmitted light which is lost in transmission through the open path of gases. The opacity monitor comprises: an optical transmitter having a light source that projects a homogeneous light beam across the open path of gases; a reflector for reflecting a portion of the projected homogeneous light beam back towards the optical transmitter through the open path gas of gases; an analyzer for detecting the portion of the projected homogeneous light beam and calculating the opacity of the gases; and wherein the optical monitor detects opacities less than 10 percent while operating within specific performance requirements(e.g., all the requirements of ASTM D6216-98, including amendments to specific portions of ASTM D6216-98 to ensure compliance with opacity limits below 10%, such as thermal stability, insensitivity to ambient light, zero and span calibration, measurement of output resolution, calibration error, optical alignment indicator, calibration device value and repeatability, and insensitivity to supply voltage variations).  
           [0015]    A method for reducing the variation in light intensity across a beam of light projected from a light source used in an opacity monitor. The method comprises the steps of: (a) providing a plurality of light emitting diodes (LEDs), each having a respective optical axis and each emitting respective light beams; (b) arranging the plurality of LEDs at a predetermined angular orientation with respect to each other and aligning each of the optical axes to be parallel to each other; and (c) positioning an optical diffuser at a predetermined distance away from the plurality of LEDs for mixing and reflecting the respective light beams to form the beam of light having a reduced variation in light intensity.  
           [0016]    A method for reducing the variation in light intensity across a beam of light projected from a light source used in an opacity monitor. The method comprises the steps of: (a) providing a plurality of light emitting diodes (LEDs), each having a respective optical axis and each having symmetrical and asymmetrical inhomogeneities in respective light beams emanating from each LED; (b) minimizing the symmetrical and asymmetrical inhomogeneities in the respective light beams by: (1) orienting the plurality of LEDs within in a common plane; and (2) positioning an optical diffuser at a predetermined distance away from the plurality of LEDs to mix and reflect the respective light beams to form the beam of light having the reduced variation in light intensity across the beam of light.  
           [0017]    A method for measuring the opacity of gases in an open path of gases wherein opacity is defined as the fraction of transmitted light which is lost in transmission through the open path of gases. The method comprises the steps of: (a) projecting a light beam across the open path of gases using a light source that reduces the variation in light intensity across the projected beam; (b) reflecting a portion of the projected light beam; (c) detecting and analyzing the portion of the portion of the projected light beam; (d) detecting opacities less than 10 percent while operating within specific performance requirements (e.g., all the requirements of ASTM D6216-98, including amendments to specific portions of ASTM D6216-98 to ensure compliance with opacity limits below 10%, such as thermal stability, insensitivity to ambient light, zero and span calibration, measurement of output resolution, calibration error, optical alignment indicator, calibration device value and repeatability, and insensitivity to supply voltage variations).  
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a functional diagram of a prior art transmissometer coupled to a stack;  
         [0019]    [0019]FIG. 2 is a side view, shown in partial cross-section, of the prior art transmissometer of FIG. 1;  
         [0020]    [0020]FIG. 3 is an isometric view of the light source of the present invention;  
         [0021]    [0021]FIG. 4 is an exploded view of the light source of FIG. 3;  
         [0022]    [0022]FIG. 5 is side cross-sectional view of the light source taken along line  5 - 5  of FIG. 3;  
         [0023]    [0023]FIG. 6 is a view of the light-emitting diode holder taken along line  6 - 6  of FIG. 5;  
         [0024]    [0024]FIG. 7 is an exploded view showing how the light-emitting diodes are properly oriented by lead holes in a clamp plate; and  
         [0025]    [0025]FIG. 8 is a graphical depiction of the light intensity vs. distance from the optical axis of different light emitting diodes and of the light source of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    Referring now in detail to the various figures of the drawing wherein like reference characters refer to like parts, there is shown at  100  in FIG. 3, a light source which provides improved light beam homogeneity compared to other light sources used in conventional stack/duct gas analyzers. The result of utilizing this improved light source  100  is a transmissometer analyzer which is more tolerant of optical misalignment than previous designs, and is therefore able to make accurate measurements at very low levels of opacity (e.g., less than 10%).  
         [0027]    It should be understood that the light source  100  described herein, and as will be discussed in detail below, replaces the light source LS (e.g., LED  32 ) described earlier with respect to FIGS.  1 - 2 . However, in all other aspects, e.g., the beamsplitter  34 , the collimating lens  36 , etc., of the transceiver portion  20  of the transmissometer which utilizes the present invention  100  is similar and is not discussed any further.  
         [0028]    As shown most clearly in FIG. 4, the light source  100  basically comprises a plurality (e.g., three) of LEDs  102 A- 102 C (e.g., NSPG320BS LED by Nichia Corp.) positioned in a precision-drilled holder  104 , to ensure the LEDs&#39; accurate location, and an optical diffuser  106  to blend the light output of the individual LEDs  102 A- 102 C. In particular, the three LEDs  102 A- 102 C are held in a precisely-determined angular orientation and location by the precision-drilled holder  104  and a clamping plate  108 . As shown in FIGS.  5 - 7 , the precision-drilled holder  104  aligns the optical axis  103  (FIG. 5) of each LED  102 A- 102 C so that they are parallel with the optical axis  27  of the transceiver  20  and also mounts the LEDs  102 A- 102 C so that they are positioned 120° with respect to each other (tolerance on each angular position should be &lt;10°); to properly orient these LEDs  102 A- 102 C in the holder  104 , a central boss  111  of the plastic clamp plate  108  is fitted over the leads  110  of the LEDs  102 A- 102 C. Holes  107  in the central boss  111  fit tightly to the leads  110  ensuring the each LED  102 A- 102 C is held in the correct angular position around its optical axis  103 , with the respective flat sides  117 A- 117 C of collars  115 A- 115 C towards the main optical axis  27 . Indicators  109  on the face of the boss  111  ensure that, during assembly, the two leads  110  of any one LED  102 A- 102 C are inserted between two of the indicators  109  for proper LED orientation.  
         [0029]    As mentioned earlier, there are symmetrical and asymmetrical inhomogeneities that make the pattern of light from an LED uneven. Symmetrical inhomogeneities in the light emitted by each LED  102 A- 102 C are minimized by ensuring that the LEDs  102 A- 102 C point straight forward, are distributed evenly across the diffuser  106 , and placed at the correct distance from it (e.g., 12.5 mm from the front of the LED flange  141  to the inside face  143  of the diffuser  106 ; FIG. 5). Asymmetrical unevenness (e.g., light beam asymmetry that exists due to the position of the die within each LED package as well as the chip die lead) is minimized by placing each LED  102 A- 102 C at 120° rotation to its neighbor.  
         [0030]    The optical (glass) diffuser  106  is mounted in a diffuser holder  122 . The inside surface  124  of the holder  122  is polished to so as to reflect any scattered light. The glass diffuser  106  and the polished inside surface  122  together diffuse (e.g., reflect and mix the combined light several times) the light from the three LEDs  102 A- 102 C to form an even, homogeneous, non-directional light source. The finish of the precision-drilled holder  104  and the internal surfaces of the diffuser holder  122  are left as “fine machined” as this provides an increased light output compared to anodizing. A glare shield  128  reduces the amount of scattered light reaching the optical detector (similar to the one shown in FIG. 2) in the transceiver  20 . An aperture  126  (FIG. 5) in the glare shield  128  defines the size of the light source  100 . The diffuser holder  122  is made from a low-magnesium aluminum alloy which has a low rate of oxidation and the diffuser holder  122  is sealed with silicone rubber during assembly, to prevent the ingress of any gases and therefore maintain the internal surface finish.  
         [0031]    Electrical contacts of the LEDs  102 A- 102 C are made by soldering the leads  110  (FIG. 4) of the LEDs  102 A- 102 C to a printed circuit board (PCB)  120 . An electrical connector  130  (e.g., a 3-pin Molex connector) couples to an electrical 3-way cable (not shown) that provides electrical power to the light source  100  and a DC/DC (PCB mount) converter  131  (e.g., NME1215S by Newport) is also provided to generate the proper LED excitation. Capacitors C 1  and C 2  (e.g., 10 μF, 35V, 20%, tap series) smooth out any remaining ripple from the DC/DC converter  131 ; the resistors R 1  (FIG. 4), R 4  and R 5  (all zero ohms) are links which are normally set to connect the three LEDs  102 A- 102 C in series with the option to connect them in parallel. Three screws  132 A- 132 C (e.g., M3×14 STL. slot pan/hd) are used to releasably secure the various components to the PCB  120 . Retainers  133  and  135  retain mounting screws  137  and  139 , respectively, until the light source  100  is ready for installation in the transceiver  20  at which time the retainers  133 / 135  are discarded.  
         [0032]    As mentioned earlier, the transmissometer projects a beam of light  26  across the stack/duct  22 . This beam diverges slightly so that its diameter at the plane of the retro-reflector is larger than the reflecting surface. Small movements of the stack/duct  22  structure due to thermal effects, wind, or settling, cause the relative positions of the reflector and the projected beam to move slightly. If the beam does not have precisely the same intensity at all points, there will be a consequent change in the amount of light received at the detector. This will be misinterpreted as a change in the opacity of the gases in the duct.  
         [0033]    [0033]FIG. 7 shows the variation in light intensity across a single diameter of an opal diffuser placed in front of a conventional LED light source. The box  200  represents a mask placed in front of the opal diffuser screen. Only the portion of the projected light beam between lines  200 A and  200 B is projected, with the rest being masked off. Lines  202 ,  203  and  204  are experimental measurements obtained from three different LEDs. Large variations of light intensity are apparent with respect to the distance from the optical axis. In contrast, line  205  shows the effect of placing three LEDs in the angular orientation described above. A dramatic reduction in the variation of intensity across the projected light beam is immediately apparent.  
         [0034]    The very small variation in light intensity across the projected light beam results in a consequent small variation of opacity due to misalignment of the transmissometer and retro-reflector. As this is a major component of the total uncertainty of the displayed opacity value, the accuracy of the transmissometer is greatly improved without any reduction in the degree of misalignment which can be tolerated.  
         [0035]    Utilizing this improved light source  100  in an opacity monitor results in the following:  
         [0036]    enabling the opacity monitors to ensure compliance with opacity limits below 10% as exemplified by 40 C.F.R. §60 Paragraph 650.272 (a) (21) which requires operators of electric arc furnaces to maintain flue gas opacity at or below 3%;  
         [0037]    an opacity monitor that can tolerate small movements of the stack/duct structure due to thermal effects, wind, settling, etc. that can cause the relative positions of the retroreflector and the projected light beam to move slightly;  
         [0038]    a more evenly illuminated light source, which greatly reduces errors caused by misalignment of the transceiver and retroreflector so that stable, accurate readings can be made at opacity levels below 10%.  
         [0039]    a significantly brighter light source which leads to an improvement in the signal-to-noise ratio of the transmissometer.  
         [0040]    light source performance is highly repeatable from one opacity monitor to another.  
         [0041]    Therefore, as a result of using the light source  100  in the transceiver  20 , an opacity monitor is provided that meets what is hereinafter referred to as “specific performance requirements (SPRs)” for ensuring compliance with opacity limits below 10%. These SPRs are defined as all of the requirements of ASTM D6216-98 (a copy of which is attached as APPENDIX) except that the indicated sections of ASTM D6216-98, set forth below, have been amended to include the following changes:  
         [0042]    6.4 Insensitivity to Supply Voltage Variations  
         [0043]    Permissible drift: a change of less than or equal to 0.2 percent opacity when the main supply voltage is increased or decreased from the nominal voltage by 10 percent.  
         [0044]    6.5 Thermal Stability  
         [0045]    Permissible drift: a change of less than or equal to 0.2 percent opacity for a 40° F. (22° C.) change in ambient temperature.  
         [0046]    [0046] 6 . 6  Insensitivity to Ambient Light  
         [0047]    Permissible drift: a change of less than or equal to 0.2 percent opacity when exposed to ambient sunlight over the course of a day.  
         [0048]    6.8 Zero and Span Calibration  
         [0049]    Zero error: 0.2% or less  
         [0050]    6.12 Measurement Output Resolution  
         [0051]    Resolution of visual indication: 0.1%  
         [0052]    Resolution of analog output: 0.1%  
         [0053]    Resolution of digital output: 0.1%  
         [0054]    7.8 Calibration Error  
         [0055]    &lt;1% opacity  
         [0056]    7.9 Optical Alignment Indicator  
         [0057]    Opacity monitor, when misaligned, displays a clear indication of that misaligment if the resulting change in opacity is 0.3% or greater.  
         [0058]    7.11 Calibration Device Value and Repeatability  
         [0059]    Repeatability: 0.2% or less  
         [0060]    95% confidence limit: 0.3%  
         [0061]    Without further elaboration, the foregoing will so fully illustrate our invention that others may, by applying current or future knowledge, readily adopt the same for use under various conditions of service.