Patent Application
20090159685
The present invention relates to a filter for an imaging-based bar code reader.
Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a series of bars and spaces having differing light reflecting characteristics. The pattern of the bars and spaces encode information. In certain bar codes, there is a single row of bars and spaces, typically of varying widths. Such bar codes are referred to as one dimensional bar codes. Other bar codes include multiple rows of bars and spaces, each typically having the same width. Such bar codes are referred to as two dimensional bar codes. Devices that read and decode one and two dimensional bar codes utilizing imaging systems that image and decode imaged bar codes are typically referred to as imaging-based bar code readers or bar code scanners.
Imaging systems include charge coupled device (CCD) arrays, complementary metal oxide semiconductor (CMOS) arrays, or other imaging pixel arrays having a plurality of photosensitive elements or pixels. An illumination system comprising light emitting diodes (LEDs) or other light source directs illumination toward a target object, e.g., a target bar code. Light reflected from the target bar code is focused through a lens of the imaging system onto the pixel array. Thus, an image of a field of view of the focusing lens is focused on the pixel array. Periodically, the pixels of the array are sequentially read out generating an analog signal representative of a captured image frame. The analog signal is amplified by a gain factor and the amplified analog signal is digitized by an analog-to-digital converter. Decoding circuitry of the imaging system processes the digitized signals and attempts to decode the imaged bar code.
United States published patent application entitled “Ambient Light Shield and Color Filter for Imaging-Based Bar Code Reader”, publication no 2007/0199996 describes an ambient illumination shielding apparatus. That application also describes a filter disposed in proximity to an imaging system that passes illumination within a predetermined wavelength range to a sensor pixel array. This published patent application is incorporated herein by reference.
Many CMOS sensors are sensitive to deep red and infrared wavelengths. This high sensitivity is undesired because imaging lens are optimized for visible light of shorter wavelength. Since infrared light has longer wavelength than visible light, the lenses tend to focus at greater lengths resulting in color separating and bigger diffraction spot size. In addition, longer wavelengths deplete farther in the pixel substrate, resulting in more leakage current, and thus reduce effective pixel resolution per sensor module. Moreover, longer wavelengths result in pixel cross talk since they are focused at a greater distance. The net result is reduced image contrast making the resultant image less sharp.
An exemplary barcode reader includes an imaging system that includes a light monitoring pixel array for converting light reflected from a target into electrical signals, and an optical system having one or more focusing lenses positioned with respect to the pixel array to transmit an image of a target object toward said pixel array. An illumination system comprising a light source for illuminating a the target within a field of view defined by the optical system.
A filter is disposed in proximity to the imaging system for passing illumination within a predetermined wavelength range to the pixel array and impeding the passage of illumination outside of the predetermined wavelength range. The exemplary filter passes light in the visible range and impedes or blocks the passage of light in the infrared range.
These and other objects, advantages, and features of the exemplary embodiment of the invention are described in detail in conjunction with the accompanying drawings.
An imaging-based reader, such as an imaging-based bar code reader, is shown schematically at 10 in
The imaging system 20 has an imaging camera assembly 22 and associated imaging circuitry 24. The imaging camera 22 includes a housing 25 supporting a focusing lens 26 and an imager 27 comprising a pixel array 28. The imager 27 is enabled during an exposure period to capture an image of the field of view FV of the focusing lens 26.
In one preferred embodiment of the present invention, the bar code reader 10 is a hand held portable reader encased in the pistol-shaped housing 11 adapted to be carried and maneuvered by a user. As is best seen in
The imaging system 20 includes the imaging circuitry 24 and decoding circuitry 40 for decoding the imaged target bar code 14′ (shown schematically in
The imaging system 20 includes the imager 27 of the imaging camera assembly 22. The imager 27 comprises a charged coupled device (CCD), a complementary metal oxide semiconductor (CMOS), or other imaging pixel array, operating under the control of the imaging circuitry 24. In one exemplary embodiment, the pixel array 28 of the CCD imager 27 comprises a two dimensional (2D) mega pixel array with a typical size of the pixel array being on the order of 1280×1024 pixels. The pixel array 28 is secured to the printed circuit board 25b, in parallel direction for stability.
As is best seen in
The analog image signal 46 represents a sequence of photosensor voltage values, the magnitude of each value representing an intensity of the reflected light received by a photosensor/pixel during an exposure period. The analog signal 46 is amplified by a gain factor, generating an amplified analog signal 48. The imaging circuitry 24 further includes an analog-to-digital (A/D) converter 50. The amplified analog signal 48 is digitized by the A/D converter 50 generating a digitized signal 52. The digitized signal 52 comprises a sequence of digital gray scale values 53 typically ranging from 0-255 (for an eight bit processor, i.e., 28=256), where a 0 gray scale value would represent an absence of any reflected light received by a pixel (characterized as low pixel brightness) and a 255 gray scale value would represent a very intense level of reflected light received by a pixel during an integration period (characterized as high pixel brightness).
The digitized gray scale values 53 of the digitized signal 52 are stored in the memory 44. The digital values 53 corresponding to a read out of the pixel array 28 constitute the image frame 42, which is representative of the image projected by the focusing lens 26 onto the pixel array 28 during an exposure period. If the field of view FV of the focusing lens 26 includes the target bar code 14, then a digital gray scale value image 14′ of the target bar code 14 would be present in the image frame 42.
The decoding circuitry 40 operates on the digitized gray scale values 53 of the image frame 42 and attempts to decode any decodable image within the image frame, e.g., the imaged target bar code 14′. If the decoding is successful, decoded data 56, representative of the data/information coded in the bar code 14 is then output via a data output port 57 and/or displayed to a user of the reader 10 via a display 58. Upon achieving a good “read” of the bar code 14, that is, the bar code 14 was successfully imaged and decoded, a speaker 59a and/or an indicator LED 59b is activated by the bar code reader circuitry 13 to indicate to the user that the target bar code 14 has successfully read, that is, the target bar code 14 has been successfully imaged and the imaged bar code 14′ has been successfully decoded.
The bar code reader 10 further includes the illumination assembly 60 for directing a beam of illumination to illuminate the target bar code 14 and the aiming apparatus 70 for generating a visible aiming pattern 72 (
The LED 62 is supported within the scanning head 11b just behind the transparent window 17 and face forwardly, that is, toward the target bar code 14. The LED 62 is positioned away from the focusing lens 26 to increase the illumination angle (shown schematically as I in
In one exemplary embodiment, the aiming apparatus 70 is a laser aiming apparatus. The aiming pattern 72 may be a pattern comprising a single dot of illumination, a plurality of dots and/or lines of illumination or overlapping groups of dots/lines of illumination (
Operating under the control of the imaging circuitry 24, when the user has properly aimed the reader 10 by directing the aiming pattern 72 onto the target bar code 14, the aiming apparatus 70 is turned off when an image of the target bar code 14 is acquired such that the aiming pattern 72 does not appear in the captured image frame 42. Intermittantly, especially when the scanner imaging circuitry 24 is transferring the captured image frame 42 to memory 44 and/or when processing the image, the aiming apparatus 70 is turned back on. If the decoding circuitry 40 cannot decode the imaged bar code 14′ and the user in the mean time has not released the trigger 12, the process of acquiring an image of the target bar code 14 set forth above is repeated.
Infrared light has longer wavelengths than visible light and less photon energy. For a fixed amount of energy there are more infrared photons than visible photons. Assuming the quantum efficiency curve is flat; this would mean higher sensor sensitivity for infrared light. The curve in
If the lens 26 is not optimized for both visible and infrared light, light with longer wavelengths will focus farther than those with short wavelengths. Thus, the effective spot size is larger and image contrast is lower. The situation is worse with higher sensor sensitivity for longer wavelength light.
Moreover, long wavelength light, particularly infrared, will deplete farther in the pixel substrate, resulting in more leakage current. This will reduce effective pixel resolution or pixel per module. Sensors have also pixel crosstalk issues as result of the use of lenslet arrays that focus steep angle light bundles and missing the proper pixel. This issue is also made worse with longer wavelength light since it focuses farther than light in the visible range. An infrared cutter or short-pass filter 34 reduces exposure to long wavelength light, in particular the infrared light.
The exemplary bar code reader 10 has a filter 34 positioned between the focusing lens 26 and the photosensor array 28. Positioning the filter 34 in space between the photosensor array 28 and the focusing lens 26 does not detrimentally affect the functioning of the focusing lens 26 (although the lens 26 may have to be positioned slightly further away from the photosensor array 28 to maintain the same focus onto the photosensor array 28). The filter 34 would most preferably block light having a wavelength of greater than 700 nm. An appropriate interference filter may be obtained from various optical suppliers such as Edmund Optics, Barrington, N.J. 08007 (www.edmundoptics.com).
The filter 34 is shown in
A presently preferred bar code reader 10 has a filter coated on to a sensor facing surface of a lens assembly 130 shown in
As seen in the Figures, the imaging lens assembly 130 includes four lenses 132, 133, 134, 135 and an intermediate front aperture stop 136 mounted in a holder 140. The aperture stop 131 defines a circular or rectangular opening, which limits the light impinging upon the sensor array 28. Additional details of the functionality of a similar lens assembly is described in the aforementioned pending United States patent application.
The four lenses 132-135 of the lens assembly 130 are supported in a generally cylindrical lens holder 140, which may be fabricated of metal or plastic. The lens holder 37, in turn is supported by the camera housing 25 which extends to the printed circuit board 25b. In addition to supporting the lens holder 37, the camera housing protects the sensor array 28 from ambient illumination. Annular seals 142-144 adhesively seal the lenses to the holder 140.
The rearmost lens includes an outer coating 150 that covers an entire generally planar rear surface of the lens assembly 130. The coating 150 is applied using the techniques of thin-film filter fabrication. A thin film filter is a multi-layer, light filtering coating that is built up layer by layer on a substrate such as clear plastic by evaporative deposition or other method. When complete, the thin film coating has appropriate wavelength blocking characteristics. Specifics on fabricating a thin film bandpass filter may be found in a book entitled Thin-Film Optical Filters, 3rd Edition, by H. Angus Macleod, Institute of Physics Publishing, Dirac House, Temple Back, Bristol, UK Bs1 6BE, copyright 2110, ISBN 0 7503 06882. The aforementioned book is incorporated in its entirety herein by reference.
While the present invention has been described with a degree of particularity, it is the intent that the invention includes all modifications and alterations from the disclosed design falling within the spirit or scope of the appended claims.
