Exposure control method for use with optical readers

Abstract

An exposure control apparatus for use with optical readers, such as bar code readers, which utilize photosensitive image sensors. An illumination signal generating circuit generates an illumination signal having a magnitude that varies in accordance with the illumination level at the image sensor. A window detecting circuit samples the illumination signal during a predetermined part of each scan to determine whether the illumination signal is within the window, has exited the widow, or has re-entered the window. Exposure control circuitry uses the output of the window detecting circuit to control which of a plurality of the subdivisions of the exposure control range of the image sensor will be used. Changes in exposure time are made only between adjacent subdivisions of the exposure control range. Together with a predetermined hysteresis between the exit and re-entry thresholds of the window, the latter changes stabilize the operation of the reader by reducing exposure control “hunting”.

What is claimed:  
     
         1 . A method for controlling an exposure period of an optical reader, said reader having an image sensor including a pixel array comprising a plurality of pixels, said method comprising the steps of: 
 generating an image sensor signal that varies in accordance with the intensity on light incident of said pixel array;    generating an illumination signal that varies in accordance with a time averaged value of said image sensor signal;    establishing an initial exposure control variable, said exposure control variable determining the duration of said exposure period;    detecting whether said illumination signal is inside or outside of an illumination window bounded by predetermined maximum and minimum illumination values;    increasing said exposure control variable if said illumination signal is below said predetermined minimum illumination value; and    decreasing said exposure control variable if said illumination signal is above said predetermined maximum illumination value.    
     
     
         2 . The method of  claim 1 , wherein said detecting step includes the step of detecting a plurality of times during a scan period of said reader whether said illumination signal is inside or outside of said illumination window.  
     
     
         3 . The method of  claim 1 , wherein said control variable comprises a count that determines an occurrence time of an exposure start signal.  
     
     
         4 . The method of  claim 3 , wherein said increasing step includes the step of adjusting said count in a first direction if said illumination signal exceeds said maximum illumination value.  
     
     
         5 . The method of  claim 3 , wherein said decreasing step includes the step of adjusting said count in a second direction if said illumination signal is below said minimum illumination value.  
     
     
         6 . The method of  claim 4 , wherein said adjusting step includes the step of adding to or subtracting from said count.  
     
     
         7 . The method of  claim 5 , wherein said adjusting step includes the step of adding to or subtracting from said count.  
     
     
         8 . The method of  claim 4 , wherein said adjusting step includes the step of multiplying or dividing said count by a predetermined number.  
     
     
         9 . The method of  claim 5 , wherein said adjusting step includes the step of multiplying or dividing said count by a predetermined number.  
     
     
         10 . The method of  claim 1 , wherein said establishing step includes the step of responding to a user selected option for setting an initial exposure period.  
     
     
         11 . An optical reader comprising: 
 an image sensor comprising an array of pixels;    an optical assembly for focusing an image onto said image sensor;    a circuit for controlling an exposure period of said optical reader, said circuit being adapted to: 
 (A) generate an illumination signal based on an output of said image sensor;  
 (B) establish an initial exposure control variable, said exposure control variable determining a duration of said exposure period;  
 (C) detect whether said illumination signal is within a window bounded by minimum and maximum illumination signal values;  
 (D) increase said exposure control variable if said illumination signal is below said minimum illumination value; and  
 (E) decrease said exposure control variable if said illumination signal is above said maximum value.  
   
     
     
         12 . The optical reader of  claim 11 , wherein said reader in detecting whether said illumination signal is within said window detects a plurality of times during a scan period of said reader whether said illumination signal is inside or outside said window.  
     
     
         13 . The optical reader of  claim 11 , wherein said reader in detecting whether said illumination signal is within said window detects a single time during a scan period of said reader whether said illumination signal is inside or outside said window.  
     
     
         14 . The optical reader of  claim 11 , wherein said reader in increasing said exposure control variable increases said exposure control variable by a predetermined increment.  
     
     
         15 . The optical reader of  claim 11 , wherein said reader in decreasing said exposure control variable decreases said exposure control variable by a predetermined increment.  
     
     
         16 . The optical reader of  claim 11 , wherein said reader in increasing said exposure control variable multiplies or divides said exposure control value by a constant.  
     
     
         17 . The optical reader of  claim 11 , wherein said reader in decreasing said exposure control variable multiplies or divides said exposure control value by a constant.  
     
     
         18 . The optical reader of  claim 11 , wherein said window comprises first and second component maximum illumination values that define separate window exit and re-entry thresholds.  
     
     
         19 . The optical reader of  claim 11 , wherein said reader includes an exposure control menu, and wherein said reader in establishing said initial exposure control variable establishes said initial exposure control variable in response to user-initiated control input into said reader is response to a request presented by said exposure control menu, wherein said user-initiated control is selected from a daylight exposure control variable setting and an indoors exposure control variable setting.  
     
     
         20 . The optical reader of  claim 11 , wherein said reader in generating said illumination signal generates an illumination signal that varies in accordance with a time averaged value of an image sensor signal output by said image sensor.  
     
     
         21 . A method for controlling an exposure period of an image sensor based optical reader, said method comprising the steps of: 
 (A) generating an illumination signal based on an output of said image sensor;    (B) establishing an initial exposure control variable, said exposure control variable determining a duration of said exposure period;    (C) detecting whether said illumination signal is within a window bounded by minimum and maximum illumination signal values;    (D) increasing said exposure control variable if said illumination signal is below said minimum illumination value; and    (E) decreasing said exposure control variable if said illumination signal is above said maximum value.    
     
     
         22 . The method of  claim 21 , wherein said detecting step includes the step of detecting a plurality of times during a scan period of said reader whether said illumination signal is inside or outside said window.  
     
     
         23 . The method of  claim 21 , wherein said detecting step includes the step of detecting a single time during a scan period of said reader whether said illumination signal is inside or outside said window.  
     
     
         24 . The method of  claim 21 , wherein said increasing step includes the step of increasing said exposure control variable by a predetermined increment.  
     
     
         25 . The method of  claim 21 , wherein said decreasing step includes the step of decreasing said exposure control variable by a predetermined increment.  
     
     
         26 . The method of  claim 21 , wherein said increasing step includes the step of multiplying or dividing said exposure control value by a constant.  
     
     
         27 . The method of  claim 21 , wherein said decreasing step includes the step of multiplying or dividing said exposure control value by a constant.  
     
     
         28 . The method of  claim 21 , wherein said window comprises first and second component maximum illumination values that define separate window exit and re-entry thresholds.  
     
     
         29 . The method of  claim 21 , wherein said establishing step includes the step of establishing said initial exposure control variable in response to a user-initiated control input into said reader is response to a request presented by an exposure control menu, wherein said user-initiated control comprises one of a daylight exposure control variable setting and an indoors exposure control variable setting.  
     
     
         30 . The method of  claim 21 , wherein said generating step includes the step of generating an illumination signal that varies in accordance with a time averaged value of an image sensor signal output of said image sensor.  
     
     
         31 . An optical reader comprising: 
 an image sensor comprising an array of pixels;    an optical assembly for focusing an image onto said image sensor;    a circuit for controlling an exposure period of said optical reader, said circuit being configured to establish an exposure control variable of said reader to one exposure control variable value out of a step pattern of possible exposure control variable values, said circuit being adapted to: 
 (A) generate an illumination signal based on an output of said image sensor;  
 (B) establish an initial exposure control variable, said exposure control variable determining a duration of said exposure period;  
 (C) detect said illumination signal;  
 (D) increase said exposure control variable if said illumination signal is below a value indicative of said exposure control variable being at a desire value; and  
 (E) decrease said exposure control variable if said illumination signal is above a value indicative of said exposure control variable being at a desired value.  
   
     
     
         32 . The optical reader of  claim 31 , wherein said optical reader in increasing said exposure control variable increases said exposure control variable to a next adjacent value of said step pattern of exposure control variable values.  
     
     
         33 . The optical reader of  claim 31 , wherein said optical reader in decreasing said exposure control variable decreases said exposure control variable to a next adjacent step value of said step pattern of exposure control variable values.  
     
     
         34 . The optical reader of  claim 31 , wherein said reader in detecting said illumination signal detects whether said illumination signal is within a window bounded by minimum and maximum illumination signal values.  
     
     
         35 . The optical reader of  claim 34 , wherein said reader in detecting whether said illumination signal is within said window detects a single time during a scan period of said reader whether said illumination signal is inside or outside said window.  
     
     
         36 . The optical reader of  claim 34 , wherein said reader in detecting whether said illumination signal is within said window detects a plurality of times during a scan period of said reader whether said illumination signal is inside or outside said window.  
     
     
         37 . The optical reader of  claim 31 , wherein said reader in increasing said exposure control variable increases said exposure control variable by a predetermined increment.  
     
     
         38 . The optical reader of  claim 31 , wherein said reader in decreasing said exposure control variable decreases said exposure control variable by a predetermined increment.  
     
     
         39 . The optical reader of  claim 31 , wherein said reader in increasing said exposure control variable multiplies or divides said exposure control value by a constant.  
     
     
         40 . The optical reader of  claim 31 , wherein said reader in decreasing said exposure control variable multiplies or divides said exposure control value by a constant.  
     
     
         41 . The optical reader of  claim 34 , wherein said window comprises first and second component maximum illumination values that define separate window exit and re-entry thresholds.  
     
     
         42 . The optical reader of  claim 31 , wherein said reader includes an exposure control menu, and wherein said reader in establishing said initial exposure control variable establishes said initial exposure control variable in response to user-initiated control input into said reader is response to a request presented by said exposure control menu, wherein said user-initiated control comprises one of a daylight exposure control variable setting and an indoors exposure control variable setting.  
     
     
         43 . The optical reader of  claim 31 , wherein said reader in generating said illumination signal generates an illumination signal that varies in accordance with a time average value of an image sensor signal output by said image sensor.