Abstract:
Heavy metals such as cadmium are not employed in optical bandpass filters employed in electro-optical readers to comply with governmental regulations concerning endangering the environment.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to electro-optical readers, such as scanners and imagers, for reading codes such as bar code symbols and, more particularly, to an optical bandpass filter for filtering light traveling along a path to a photodetector in such readers, and especially to rendering the filter of non-heavy metals. 
   2. Description of the Related Art 
   Electro-optical readers, such as bar code symbol scanners and solid state imagers, have found wide acceptance in retail, wholesale, industrial and military applications. The scanner typically illuminates a symbol comprised of regions of different light reflectivity, senses light of different intensity scattered from the symbol regions with a photodetector such as a photodiode, and determines widths and spacings of the symbol regions to derive information encoded in the symbol. The imager determines the widths and spacings of the symbol regions by capturing and processing an image of the symbol, the image capture being performed by a photodetector such as an array of photocells. 
   In both types of readers, the photodetector senses the light incident thereon. The light includes the desired light scattered from the symbol regions, as well as undesired light, such as ambient light, sunlight, and light reflected off other objects in the field of view of the reader. An optical bandpass filter is commonly employed in front of the photodetector to allow only the desired light to pass through to the photodetector, and to reject the undesired light. Detection of the undesired light compromises reader performance and can even lead to failure to read the symbol. 
   It was known in the prior art to make an optical bandpass filter using a clear glass member with an infrared-blocking coating and a Wratten filter, which consists of a brittle cellophane material. It was expensive and difficult to cut to shape the Wratten filter without waste since the Wratten filter tended to fall apart during cutting or punching. 
   In the case of a moving beam reader where a red laser beam having a wavelength of about 650 nm is swept across the symbol to be read, it was known in the prior art to make the exit window of the reader of a red-colored, plastic material, such that the window itself formed part of the bandpass filter. However, the use of plastic for the exit window made the reader susceptible to scratching and color fading due to exposure to sunlight or chemicals. 
   It was also known to position the optical bandpass filter inside the reader in front of the photodetector. In the case of the red laser beam, the filter included a red-colored glass member having a dielectric coating to set the higher passband value. The red color of the glass member is caused by a coloring, one of whose components is cadmium which is operative to set the lower passband value. The cadmium is introduced while the glass member is in a molten state. 
   The use of cadmium, however, is considered by some governmental authorities to pose a potential hazard to the environment. Cadmium is a known eye and skin irritant and has been linked to lung and kidney disease, as well as cancer. Cadmium, like other heavy metals, can be a source of pollution if it is present in high concentrations, or if it is extracted from the glass member by either some natural process, or during the disposal process, for example, by grinding or pulverization. 
   SUMMARY OF THE INVENTION 
   Objects of the Invention 
   Accordingly, it is a general object of this invention to make an optical bandpass filter devoid of heavy metals, especially cadmium, considered by some authorities to be hazardous. 
   Features of the Invention 
   In keeping with the above object and others which will become apparent hereinafter, one feature of the present invention resides, briefly stated, in an optical bandpass filter and a method of making and using the filter without heavy metals. The filter is operative for filtering light traveling along a path to a photodetector in an electro-optical reader for reading indicia such as bar code symbols. 
   In accordance with this invention, a plastic member is located in the path, and a dye comprised of elements not considered by authorities as being hazardous is distributed throughout the plastic member. For example, as noted above, cadmium is considered by some authorities as a hazardous element, and the dye is devoid of cadmium. The dye is operative for absorbing the light having wavelengths shorter than a lower passband value. A dielectric coating is also located in the path. The coating is operative for reflecting the light having wavelengths longer than a higher passband value. 
   By way of example, if the reader sweeps a red laser beam having a wavelength on the order of 650 nm, then the lower passband value at 50% transmission is about 625 nm, whereas the higher passband value at 50% transmission is about 690 nm. The filter substantially allows most of the light having wavelengths between 625 nm and 690 nm to reach the photodetector while substantially blocking most of the light having wavelengths outside of these wavelengths, all without using cadmium or other heavy metal elements. 
   The dielectric coating may be applied to an outer surface of the plastic member or, as is preferred, the coating is applied to a glass plate located in the path. The glass plate is not only an ultraviolet (UV) filter, but is a convenient support for the coating. In a typical application, high temperatures are needed to deposit the coating, and such high temperatures can soften or melt many plastic materials. By depositing the coating on glass, the glass will not deform under the high temperatures normally encountered during deposition of the coating. 
   The photodiode is typically encapsulated in a plastic housing. Hence, in another embodiment, the dye is preferably distributed in the plastic housing and, hence, a separate plastic member is not necessary. 
   In other embodiments, a collection lens is sometimes used to collect and direct the light to the photodetector. If the lens is made of plastic, then the dye could be distributed throughout the plastic lens. If the lens is made of glass, then the dielectric coating could conveniently be deposited thereon. 
   The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic view of an electro-optical reader in which an optical bandpass filter in accordance with this invention is used; 
       FIG. 2  is a bandpass characteristic of the filter of  FIG. 1 ; 
       FIGS. 3–6  are diagrammatic views of different embodiments of the filter of  FIG. 1 ; 
       FIGS. 7–8  are diagrammatic views of other embodiments of the filter of  FIG. 1 ; and 
       FIG. 9  is a graph comparing the bandpass characteristics of the embodiments of  FIGS. 7–8 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the drawings, reference numeral  10  in  FIG. 1  generally identifies a portable reader for electro-optically reading indicia such as bar code symbols. The reader  10  is preferably implemented as a gun shaped device, having a pistol-grip handle  53 . A lightweight plastic housing  55  contains a light source  46 , a detector  58 , optics  57 , signal processing circuitry  63 , a programmed microprocessor  40 , and a power source or battery pack  62 . An exit window  56  at a front end of the housing  55  allows an outgoing light beam  51  to exit and incoming reflected light  52  to enter. An operator aims the reader at a bar code symbol  70  from a position in which the reader  10  is spaced from the symbol, i.e., not touching the symbol or moving across the symbol. 
   The optics  57  may include a suitable lens (or multiple lens system) to focus the light beam  51  into a scanning spot at an appropriate reference plane. The light source  46 , such as a semiconductor laser diode, introduces a light beam into an optical axis of the lens  57 , and other lenses or beam shaping structures as needed. The beam is reflected from an oscillating mirror  59  which is coupled to a scanning drive motor  60  energized when a trigger  54  is manually pulled. The oscillation of the mirror  59  causes the outgoing beam  51  to scan back and forth in a desired pattern, such as a scan line or a raster pattern of scan lines, across the symbol. 
   The light  52  reflected or scattered back by the symbol  70  passes back through the window  56  for transmission to the detector  58 . In the exemplary reader shown in  FIG. 1 , the reflected light reflects off the mirror  59 , passes through an optical bandpass filter  47  and impinges on the light sensitive detector  58 . As described in detail below, the filter  47  is typically designed to have a bandpass characteristic in order to pass the reflected (return) laser light and block the light coming from other optical sources. The detector  58  produces an analog signal proportional to the intensity of the reflected light  52 . 
   The signal processing circuitry includes a digitizer  63  mounted on a printed circuit board  61 . The digitizer processes the analog signal from detector  58  to produce a pulse signal where the widths and spacings between the pulses correspond to the widths of the bars and the spacings between the bars of the symbol. The digitizer serves as an edge detector or wave shaper circuit, and a threshold value set by the digitizer determines what points of the analog signal represent bar edges. The pulse signal from the digitizer  63  is applied to a decoder, typically incorporated in the programmed microprocessor  40  which will also have associated program memory and random access data memory. The microprocessor decoder  40  first determines the pulse widths and spacings of the signal from the digitizer. The decoder then analyzes the widths and spacings to find and decode a legitimate bar code message. This includes analysis to recognize legitimate characters and sequences, as defined by the appropriate code standard. This may also include an initial recognition of the particular standard to which the scanned symbol conforms. This recognition of the standard is typically referred to as autodiscrimination. 
   To scan the symbol  70 , the operator aims the bar code reader  10  and operates the movable trigger switch  54  to activate the light source  46 , the scanning motor  60  and the signal processing circuitry. If the scanning light beam  51  is visible, the operator can see a scan pattern on the surface on which the symbol appears and adjust aiming of the reader  10  accordingly. If the light beam  51  produced by the source  46  is marginally visible, an aiming light may be included. The aiming light, if needed, produces a visible light spot which may be fixed, or scanned just like the laser beam  51 . The operator employs this visible light to aim the reader at the symbol before pulling the trigger. 
   The reader  10  may also function as a portable data collection terminal. If so, the reader would include a keyboard  48  and a display  49 . 
   As previously mentioned, the optical bandpass filter  47  is operative for filtering the light  52  traveling along a path to the photodetector  58 . The light  52  passes through the window  56  to the mirror  59  for reflection therefrom through the filter  47  to the photodetector. An optional collection lens  45  may be used to focus the light  52  onto the photodetector. If the laser  46  emits a red beam having a wavelength on the order of 650 nm, then it is desired that the filter  47  only allows light in the vicinity of 650 nm to pass and reach the photodetector. 
     FIG. 2  depicts a typical bandpass characteristic for the filter  47 , in which a lower passband wavelength is set at about 625 nm at about 50% transmittance, and a higher passband wavelength is set at about 690 nm at about 50% transmittance. These numerical values are merely exemplary. As described above, the higher passband wavelength is conventionally established by a dielectric coating on a glass member, whereas a red coloring in the glass member established the lower passband wavelength. Yet, one of the components of the red coloring is cadmium which is regarded by some authorities as posing safety and environmental hazards. 
   In accordance with one feature of this invention, cadmium is eliminated. As shown in the  FIG. 3  embodiment, the optical filter  47  comprises a plastic member  70  throughout which a dye, as represented by stippling, is distributed. The dye is devoid of cadmium and is operative for absorbing the light having wavelengths shorter than the lower passband wavelength. A dielectric coating  72  is likewise provided in the path and is operative for reflecting the light having wavelengths longer than the higher passband wavelength. The coating  72  is actually a plurality of reflective coatings applied one on top of another, each coating being a fraction of a micron in thickness and being reflective of light of a different wavelength so as to cause interference between light of different wavelengths. In the  FIG. 3  embodiment, the coating  72  is applied directly on an outer surface of the red-colored plastic member  70 . 
   In the  FIG. 4  embodiment, the dielectric coating  72  is applied on an outer surface of a glass plate  74 . The application of the coating  72  is performed at elevated temperatures sufficient to melt or at least deform plastic, such as the plastic member  70 . Hence, to withstand such elevated temperatures, the coating  72  is performed on a glass plate.  FIG. 4  depicts that the coated glass plate  74  and the red colored plastic member  70  are discrete elements spaced apart from one another. This was done for convenience of illustration because, in practice, the coated glass plate  74  is adhered directly to the red colored plastic member  70 , typically with a light-transmissive glue. The glass plate  74  also serves as an ultraviolet light filter. 
   Rather than providing the plastic member  70  as a discrete component, the  FIG. 5  embodiment depicts that the plastic member  70  encapsulates the photodiode  58 . The photodiode  58  is often encapsulated in a plastic housing  76 , and the embodiment of  FIG. 5  proposes that the dye be distributed throughout this plastic housing. The dielectric coating  72  can then be applied directly to the colored housing  76  as shown in  FIG. 5 , or can be applied to the glass plate  74  either remote from the plastic housing  76  as shown in  FIG. 6  or adhered thereto. 
   In the event that the collection lens  45  is employed, then the lens  45  can be integrated into the filter. For example, if the lens  45  is constituted of glass, then the lens can serve as the glass plate  74  in the embodiments of  FIGS. 4 and 6 . If the lens  45  is constituted of plastic, then the lens can serve as the plastic member  70  in the embodiments of  FIGS. 3 and 4 . 
   As depicted in  FIGS. 7–8 , the plastic member  70 , together with the glass plate  74  on which the dielectric coating  72  is applied, is situated between a light collection mirror  80  and the photodiode  58 .  FIG. 7  is analogous to  FIG. 4 , except that the positions of the plastic member  70  and the glass plate have been reversed. The only difference between the embodiments of  FIGS. 7–8  is that one of the surfaces of the plastic member is planar in  FIG. 7 , whereas one of the surfaces of the plastic member is concave in  FIG. 8 . The bandpass characteristic of the optical filter of  FIG. 7  is shown by a solid line in  FIG. 9 , whereas the bandpass characteristic of the optical filter of  FIG. 8  is shown by a broken line in  FIG. 9 . 
   The incident angle of the light impinging on the glass plate  74  in  FIG. 7  is larger as compared to that of  FIG. 8 . As the incident angle is reduced, the effective bandwidth of the filter characteristic is likewise reduced. Hence, by changing the curvature of the plastic member, the bandpass characteristic can be tuned. 
   By avoiding the use of cadmium and other heavy metals, the electro-optical reader is compliant with governmental restrictions on the use of hazardous materials. 
   It will be understood that each of the elements described above, or two or more together, also may find a useful application in other types of constructions differing from the types described above. 
   While the invention has been illustrated and described as embodied in optical bandpass filters in electro-optical readers, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. 
   Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims. 
   What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.