Ambient Light Interference Removal in Laser Barcode Readers
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Solution Overview
Problem
Ambient light, especially from fluorescent lamps and LEDs operated at kilohertz frequencies, interferes with the information signal in moving laser beam readers, causing decoding errors and degrading reader performance due to close frequency components.
Innovation Solution
A dual filtering system and photodetector setup that equalizes ambient light signals and subtracts them from the information signal, using first and second optical filters and photodetector systems to isolate and attenuate the laser frequency while passing ambient light frequencies, ensuring the information signal is not impeded by interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If filters are used to suppress ambient light signal, then interference from ambient light is reduced, but decoding accuracy deteriorates when ambient light frequency is close to information signal frequency
Solution Approach 1:
The detector is divided into two separate detector systems: a first detector system that receives both return laser light and ambient light, and a second detector system that receives only ambient light through a second optical filter. This segmentation allows independent measurement and subtraction of ambient light interference from the information signal.
Solution Approach 2:
A second optical filter is introduced as an intermediary component that selectively passes ambient light frequencies while blocking laser frequency. This intermediary filter enables the second detector system to measure only ambient light interference, which is then subtracted from the first detector system's output to recover the clean information signal.
2Object-affected harmful factors
If a single photodetector system is used, then device complexity is reduced, but ability to remove close-frequency ambient light interference is insufficient
Solution Approach 1:
The single photodetector system is segmented into two separate detector systems with distinct functions: the first detector system captures the combined signal of return laser light and ambient light, while the second detector system captures only ambient light through a selective optical filter. This segmentation enables precise interference removal despite increased complexity.
Solution Approach 2:
Different optical filters are applied to different detector systems: the first optical filter passes both laser and ambient light frequencies to the first detector, while the second optical filter selectively passes only ambient light frequencies to the second detector. This local differentiation in filter properties enables targeted interference measurement and subtraction.
3Reliability
If ambient light signal magnitude is large compared to information signal, then signal-to-noise ratio deteriorates, but filtering out DC component only partially solves the problem
Solution Approach 1:
The ambient light interference signal is extracted from the combined signal received by the first detector system through the second detector system, which measures only ambient light through the second optical filter. This extracted ambient light signal is then subtracted from the first detector system's output, effectively removing the harmful ambient light component while preserving the information signal.
Solution Approach 2:
The system uses feedback by continuously measuring the ambient light signal through the second detector system and using this measurement to compensate for ambient light interference in the first detector system's output. This real-time feedback mechanism dynamically removes ambient light interference, improving signal-to-noise ratio while preserving information integrity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Effectively removes interference from ambient light, enhancing the decoding accuracy and performance of moving laser beam readers by isolating the information signal from ambient light signals, even when frequencies are close, thereby ensuring successful reading of bar code symbols.
Implementation Method 1
a first filtering system for passing the return laser light from the target and the ambient light, and a first photodetector system for receiving the return laser light and the ambient light passed by the first filtering system
Implementation Method 2
a first photodetector system for receiving the return laser light and the ambient light passed by the first filtering system, and for generating a first analog electrical output signal constituted of an information signal bearing information related to the target from the return laser light
Data Source
AI summary
A reader for electro-optically reading a target in the presence of ambient light, includes a scan component for scanning a laser beam across the target, a first filter for passing return laser light from the target and the ambient light to a first photodetector system for generating a first output signal constituted of an information signal and a first ambient light signal, a second filter for passing the ambient light to a second photodetector system for generating a second output signal constituted of a second ambient light signal, and signal processing circuitry for measuring a difference between the first and second ambient light signals, for equalizing the first and second ambient light signals when the difference exceeds a threshold, and for subtracting the second output signal from the first output signal to obtain a receiver output signal substantially constituted only of the information signal.