Abstract:
An optical assembly comprises an optical device ( 103 ); an enclosure ( 101 ) for containing the optical device ( 103 ), the enclosure ( 101 ) including a transparent section ( 107 ) to allow passage of a light beam ( 104 ) to or from the optical device ( 103 ); and means for measuring the attenuation of a test light beam ( 212 ) through the transparent section( 107 ). Preferably, the optical assembly also comprises means for compensating for the measured amount of attenuation by adjusting a measurement or a characteristic of the optical device ( 103 ) or a related optical device to compensate for the measured amount of attenuation.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an optical assembly, particularly for use in an industrial environment. The optical assembly preferably includes an enclosure for containing an optical device, the optical device typically being either a light source or a detector, and the enclosure includes a transparent window to allow light to enter or leave the enclosure to or from the optical device.  
         [0002]     Industrial environments are often characterised by noise, vibration, temperature, and humidity, and devices used in industrial environments are exposed to fluids, solvents, airborne dust and vapour. It is sometimes desirable to install sensitive equipment in such environments, for example, for the purposes of process monitoring. Depending on the mode of measurement, some or all of the industrial environmental factors may easily be screened or eliminated from interfering with the sensitive equipment.  
         [0003]     In particular, optical sensors are commonly used for process monitoring. For instance, in one application disclosed in WO 01/66352 a light source is used to illuminate a test surface, and characteristics of the behaviour of the light resulting from interaction with the surface are measured and quantified by means of optical detectors.  
         [0004]     In this situation of optical sensing, elimination of interference from the industrial environment presents some particular challenges. Location of an optical device within an enclosure will prevent ingress of contaminants into the device, but the optical signal or beam must pass through some part of the enclosure in order to interact with the device. A transparent section must therefore be included in the enclosure. However, contamination still builds up on the outside of the transparent section, and may impede the passage of light into or out of the enclosure and adversely effect the measurement. Particularly in unattended operation, there will always be uncertainty regarding the presence and/or the extent of contamination present and this may completely negate any benefit of using the instrument.  
       SUMMARY OF THE INVENTION  
       [0005]     According to the present invention, an optical assembly comprises:  
         [0006]     an optical device;  
         [0007]     an enclosure for containing the optical device, the enclosure including a transparent section to allow passage of a light beam to or from the optical device; and  
         [0008]     means for measuring the attenuation of a test light beam through the transparent section.  
         [0009]     The optical assembly of the present invention has the capacity to measure and thereby monitor the attenuation caused by any contamination that has built up on the transparent section of the enclosure.  
         [0010]     The measurement may be used to merely alert an operator when the attenuation reaches a threshold value, indicating that the contamination build up has reached a level where it may be adversely effecting the operation of the optical device. However, preferably the optical assembly includes means for compensating for the measured amount of attenuation. Preferably, a measurement or a characteristic of the optical device or a related optical device is adjusted to compensate for the measured amount of attenuation. If the optical device is a detector, the sensitivity of the detector may be adjusted or the signal from the detector may be adjusted by an appropriate factor. If the optical device is a light source, the brightness of the light source may be adjusted. Rather than adjusting a characteristic of the optical device itself, a characteristic or measurement of an associated device may be adjusted. If the device is part of a source-detector pair, it may be that the other device is adjusted. For example, if the optical device is a source, the signal from the associated detector may be adjusted to compensate for the attenuation of the light beam from the source.  
         [0011]     Preferably, the light path to or from the optical device and the test light path intersect substantially at the transparent section. This ensures that the contamination of the transparent section is measured at the same position as the position where the beam from or to the optical device passes through the transparent section.  
         [0012]     Preferably, the optical assembly includes a light source and a detector for generating and detecting the test light beam respectively, the light source and the detector being located on opposite sides of the transparent section. Preferably, the light source is located externally to the enclosure and the detector is located inside the enclosure.  
         [0013]     Preferably, the transparent section is recessed into the enclosure. This helps to prevent airborne dust and other contaminants from reaching and building up on the transparent section. Preferably, the transparent section is located in an enclosed passage, and the light path through the transparent section passes along the length of the passage.  
         [0014]     Preferably, when the test light beam is generated by a test source and detected by a test detector, the component which is located outside the enclosure is recessed. This prevents contamination from building up and affecting the performance of the external component. Preferably, the external component is located in a passage through which the test beam passes.  
         [0015]     In a preferred aspect of the present invention, the optical assembly includes means for directing a flow of gas onto an external surface of the transparent section, to reduce build up of contaminants on the surface. Cleaning fluid may be introduced in the flow of gas.  
         [0016]     In this preferred aspect of the present invention, the flow of gas directed at the external surface of the transparent section serves to substantially reduce build up of contaminants on the surface which would otherwise attenuate the emitted or received beam, and adding cleaning fluid to the flow cleans away any residue which has built up.  
         [0017]     Preferably, the flow of gas is compressed gas, and more preferably compressed air.  
         [0018]     Preferably, the flow of gas is directed along the passage in which the external component of the test source-detector pair is located, and the flow is directed away from the external component and is incident on the transparent section. Furthermore, the flow of gas along the passage away from the test light source or detector also prevents build up of contaminants on the test source or detector.  
         [0019]     In one embodiment, wherein the optical device is a light source, the optical assembly preferably includes a beam splitter located inside the enclosure and arranged to direct a portion of the beam into a test detector which is also arranged to detect the test beam. This enables the test detector to also be used to measure the output of the light source, to enable a final measurement result to be adjusted for variations in the light output of the source, as well as being adjusted to allow for attenuation of the beam by contamination of the transparent situation.  
         [0020]     In another embodiment, wherein the optical device is a detector, the detector can be arranged to detect both the test beam and a primary beam. When the light source for the primary beam is deactivated, and the light source for the test beam is activated, the detector receives light intensity from the primary beam, and vice versa. The advantage of this configuration is that it is simpler and requires fewer components. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, in which;  
         [0022]      FIG. 1  is a schematic drawing of a first embodiment of the present invention;  
         [0023]      FIG. 2  is a schematic drawing of a second embodiment of the present invention in which the optical device is a light source;  
         [0024]      FIG. 3  is a schematic drawing of a third embodiment of the present invention;  
         [0025]      FIG. 4  is a schematic drawing of a fourth embodiment in which the optical device is a detector; and  
         [0026]      FIG. 5  is an elevated view of an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0027]      FIG. 1  illustrates a sealed enclosure  101  containing an optical device  103 . The enclosure  101  is constructed to meet one of the most stringent requirements for electrical enclosures (e.g. IP67) including the requirement of being able to be totally immersed in water without leakage.  
         [0028]     The enclosure  101  is positioned relative to a surface  102  to be measured. Optical device  103  represents, in one embodiment, an optical source e.g. LED or laser etc, or in another embodiment an optical detector. A light pathway  104  is shown to describe the motion of light either (a) from the source  103  to the surface  102  or vice versa in the case of the device  103  being a detector. The light beam travels along a passage  105  that recesses the window  107  into the enclosure  101 . Other optical elements such as filters, polarisers, lenses etc may be located between the optical device  103  and the enclosure window  107 . Sealing of the enclosure  101  is accomplished using a window  107  that is attached into the enclosure  101  in such a manner as to exclude contamination e.g. by means of an O-ring seal. Compressed gas is directed onto the external surface of the window  107  through passage  108 . The compressed gas is supplied from an external supply at a controlled pressure  109 . The compressed gas, having flowed across the window surface, travels along the passage  105  and is exhausted to ambient  110 . Bursts of cleaning fluid may be introduced upstream in the compressed gas stream so that it travels down passage  108  and is applied to window  107  and cleans away any residue on the window  107 .  
         [0029]      FIG. 1  illustrates an arrangement for the diagnosis of window contamination. Gas tube  108  accommodates a test light source  211  that projects a test beam along gas tube  108  onto and through window  107 . The transmitted light continues to follow path  212  and finally falls onto test detector  213  where it is converted into an electrical signal. Light path  212  is configured to intersect light path  104  substantially at the window  107 .  
         [0030]     The method of contamination diagnosis involves initial calibration of the signal from test detector  213  when test light source  211  is activated and when window  107  is in a contamination-free state. This is preferably conducted at or shortly after manufacture, but before installation in the contaminating environment. The signal from test detector  213  in the contamination-free state is measured and recorded by electronic systems associated with the optical device  103 —this value is called ID 0 . Following the installation of the optical device  103  in the potentially contaminating environment, a similar operation of measuring the signal from test detector  213  is conducted to give a value called ID t . Test light source  211  is only activated when test detector  213  is being measured; for the rest of the time it is off. If contamination has been able to attach to the external surface of window  107 , such that it impedes primary light path  104 , then it will also impede test light path  212  which will result in less light intensity falling on test detector  213  and give rise to less electrical signal ID t . A comparison of ID t  with the stored value ID 0  will determine whether ID t  has decreased as a result of contamination on window  107 .  
         [0031]     In order to accommodate test light source  214 , the gas flow is introduced into tube  108  via a T-junction  214 . The advantage of this configuration is that test light source  211  is located at a position separated from the contaminating environment by a tortuous path as well as being protected by a gas flow opposing the ingress of contamination. Positioning light source  211  at this position significantly reduces the probability of contamination.  
         [0032]     In a further refinement of the invention, the sensor system may be made to be substantially tolerant to moderate levels of contamination on window  107 . Assuming the properties of light paths  212  and  104  are attenuated by contamination in substantially similar extents, then the contamination induced attenuation in test light path  212  may be used as an estimate for the attenuation in primary light path  104  caused by contamination on window  107 . Attenuation in test light path  212  can be enumerated as:  
       Attenuation   =         ID   t       ID   0       .         
 
         [0033]     Using this value as an estimate for the attenuation in primary light path  104 , a correction can be applied to the measured signal from detector  103 . Thus:  
         IS   Ct     =         IS   t     ×     ID   0         ID   t           
 
         [0034]     Where IS t  is the measured signal from detector  103  at or after measurement ID t , and IS Ct  is a corrected value for detector  103 , based on the estimated effect of contamination.  
         [0035]     If optical device  103  is a source, the measured signal from an associated detector may be adjusted, or the brightness of the source may be adjusted.  
         [0036]     In a preferred method, ID t  will be measured frequently perhaps daily, hourly or even more frequently depending on the probability of contamination and its impact on the measurement efficacy or downstream use.  
         [0037]     The embodiment shown in  FIG. 1  depicts a configuration in which the measurement light path 104 makes an angle of approximately 60 degrees with the normal of the measurement surface  102 . It will be recognised by a person skilled in the art that the principles described are not limited to this configuration and that any angle can be accommodated. It will also be recognised that a multiplicity of angles and configurations may be combined in a single instrument or enclosure.  
         [0038]     As illustrated in  FIG. 2 , the addition of a beam splitter  315  into the configuration already shown in  FIG. 1  enables the output of a primary light source  103  to be measured. This maybe important in applications where measurement accuracy requirements are beyond the stability specifications of the light source output.  FIG. 2  shows a beam splitter  315  inserted into the same configuration as  FIG. 1 . Much of the common labelling has been omitted for clarity. In the instance in which  103  is a light source, the output light path  104  travels from the source  103  towards the measurement surface  102 . When it passes the beam splitter  315  a small proportion of the light intensity is reflected along path  316  and impinges on test detector  213  where it is converted into an electrical signal. The electrical signal may be used as a diagnostic measure by comparing periodic measurements with a stored initial value such that gradual or sudden changes in light output may be identified and an error signal or alarm triggered. Alternatively (or additionally) the electrical signal may be used to normalise the results of any subsequent optical measurement for which the light source  103  supplies light intensity e.g. the measurement of an associated detector. Small variations in light output of source  103  may, in this way, be corrected and their effects substantially eliminated from a final measurement result.  
         [0039]      FIG. 3  shows an embodiment of a single optics tube in which all of the previously described features have been combined. In order to operate the full range of features possible, the light sources  103  and  211  must be individually controlled. In order to measure window contamination test light source  211  must be activated and  103  deactivated. In order to measure the light output of primary light source  103 , primary light source  103  must be activated and test light source  211  deactivated.  
         [0040]     If the physical configuration permits, a simplified embodiment of the contamination diagnostic device may be employed as depicted in  FIG. 4 . The sensor detector  417  is used to receive the measurement signal along light path  104 , but when the light source for light path  104  is deactivated and test light source  211  is activated the detector  417  receives light intensity from the direction of test light path  212 . Both light paths travel through the optical element  107  that seals the instrument&#39;s enclosure  101 . Therefore, the use of test light path  212  can be used in order to measure the presence and degree of contamination on optical element  107 . The same method of correcting for contamination on optical element  107  can be used as previously described. Test light source  211  and test light path  212  travel along tube  108  (as described previously) and are protected from the possible ingress of contamination coming up tube  105 .  
         [0041]     The advantage of this configuration is that it is significantly simpler than the embodiments that have preceded. However, it can only be used in situations in which detector  417  is capable of measuring light intensity and also where the geometric configuration permits both light paths  212  and  104  to be incident on a single detector  417 . It will be recognised by one skilled in the art that  FIG. 4  represents only one of many geometric configurations that satisfy these criteria.  
         [0042]      FIG. 5  shows an actual embodiment, having a source assembly for directing a beam of light at a surface, and two detector assemblies arranged at different orientations to detect light scattered and reflected from the surface, respectively.  
         [0043]     The source assembly has the arrangement shown in schematic  FIG. 2 , and includes a source  103  located in an enclosure (not shown) having a transparent window  107  for allowing passage of the beam emitted by the source  103 . The beam emitted from the source  103  passes along passage  105  to be incident on a surface. Inside the enclosure (not shown), between the window  107  and the source  103  is a beam splitter  315  which directs a portion of the emitted beam on to a detector  213 . A test source  211  is located at the end of a recessed passage (not shown) to direct a test beam through the window  107  to be incident onto detector  213 . When the source  103  is switched on, detector  213  receives a portion of the emitted beam from the beam splitter  315 , and when the source  103  is switched off, the test source  211  is switched on, and the detector  213  detects the test beam. A flow of air A passes along passage  214  to join the passage in which the test source  211  is located at a T-junction. A flow of air thus is directed away from test source  211  and is incident on window  107 .  
         [0044]     Both detector assemblies have the arrangement shown in schematic  FIG. 4 , wherein the same detector  417 ,  517  is used to detect incident radiation from the surface, and the test beam is generated by a test source  311 ,  411 . Each detector  417 ,  517  is located in an enclosed chamber  201 ,  301  such that a detecting surface of the detector  417 ,  517  is adjacent a window  207 ,  307 . The window  207 ,  307  is recessed such that incoming light passes along a passage  205 ,  305  to be incident on the window  207 ,  307 . A test source  311 ,  411  directs a test beam at the window  207 ,  307  along a passageway  208 ,  308 . The test beam passes through the window  207 ,  307  and is detected by the detector  417 ,  517 . A flow of air B, C is directed into the passage  208 ,  308  from a side passage  314  such that the air flow is directed along passage  208 ,  308  away from the test source  311 ,  411  and is incident on the window  207 ,  307 .  
         [0045]     For the purposes of this specification it is to be clearly understood that the word “comprising” means “including but not limited to”, and that the word “comprises” has a corresponding meaning.  
         [0046]     Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.