Bar code reader for reading high to low contrast bar code symbols

An indicia reader for digitizing bar code symbols that include black and white code bars comprising:

• an image sensor for scanning said bar code bars, and for generating therefrom voltages that vary in accordance with the blackness and whiteness of said code bars;
• a waveform generating block in electrical communication with said image sensor for generating a signal varying in accordance with the blackness and whiteness of said code bars;
• a black and white peak detecting circuit responsive to said signal for generating black and white peak tracking signals which approximately track the peak black and peak white values of said signal;
• a threshold generating block for generating a threshold signal that varies in accordance with changes in said black and white peak tracking signals;
• a comparator responsive to said signal and to said threshold signal for generating a digital signal for use in decoding said bar code symbols;
• threshold switching means for switching said threshold signal between a first state, wherein said threshold signal is a dynamically variable signal bearing a first predetermined relationship to said peak values, and a second state wherein said threshold signal is a dynamically variable signal bearing a second predetermined relationship to said peak values;
• and
• a controller in communication with said threshold switching means and said image sensor, for causing said threshold switching means to switch said threshold signal between said first state and said second state during alternate scans or sets of scans by said image sensor.

In an indicia reader for digitizing code symbols that include black and white code elements in combination:

• image sensing means for scanning said bar code elements, and for generating therefrom voltages that vary in accordance with the blackness and whiteness of said code elements;
• waveform generating means in electrical communication with said image sensing means for generating a signal varying in accordance with the blackness and whiteness of said code elements;
• black and white peak detecting means responsive to said signal for generating black and white peak tracking signals which approximately track the peak black and peak white values of said signal;
• threshold generating means for generating a threshold signal that varies in accordance with changes in said black and white peak tracking signals;
• comparing means responsive to said signal and to said threshold signal for generating a digital signal for use in decoding said symbols;
• and
• manual adjusting means for manually adjusting said threshold signal to a state wherein said threshold signal bears a substantially constant relationship to said peak values.