Differential Area Photodiodes for Ambient Light Rejection
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Solution Overview
Problem
Conventional laser bar code readers face challenges in rejecting parasitic ambient light, which degrades the signal-to-noise ratio, especially at increased reading distances, due to the wide frequency range of ambient light and the inability to use additional detectors to remove ambient light without affecting the bar code signal.
Innovation Solution
A bar code reader system utilizing differential area photodiodes, where three photodiodes are used - one main photodiode for the laser signal and two smaller ones for ambient light, with the total active area of the smaller photodiodes equal to the main photodiode, allowing for effective subtraction of ambient light from the bar code signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single photodiode is used to detect the reflected bar code signal, then the device complexity is low, but the ambient light severely degrades the signal to noise ratio
Solution Approach 1:
The photodetector array is segmented into multiple photodiodes with different area configurations. The main photodiode detects the bar code signal while auxiliary photodiodes with specific area ratios (1:2:4 or 1:1:1) detect ambient light components, enabling selective measurement of different signal types from the same optical path.
Solution Approach 2:
Different photodiodes are assigned different active areas tailored to their specific detection functions. The main photodiode has a larger area optimized for capturing the reflected bar code signal, while auxiliary photodiodes have smaller areas optimized for capturing ambient light, creating local optimization of detection quality for each component.
2Object-affected harmful factors
If additional photodiodes are added to remove ambient light, then the ambient light rejection improves, but the device complexity increases
Solution Approach 1:
Multiple photodiodes are combined into a single integrated photodetector array that simultaneously performs both bar code signal detection and ambient light detection. The array processes multiple signals through electronic circuitry that combines the outputs, eliminating the need for separate detection systems while achieving effective ambient light rejection.
Solution Approach 2:
The photodetector array performs multiple functions: the main photodiode detects the reflected bar code signal while auxiliary photodiodes detect ambient light components. This multi-functional design allows a single integrated component to handle both signal types, reducing overall system complexity while achieving effective ambient light rejection.
3Measurement precision
If retro reflection systems are used to extract signal from random noise, then the signal extraction capability improves, but the system becomes large, costly, and has low scanning frequencies
Solution Approach 1:
The invention extracts only the necessary functional elements from complex retro reflection systems. Instead of using complete retro reflection assemblies, the patent selectively uses a simplified photodetector array with specific area ratios that can be directly integrated into handheld devices, removing unnecessary complexity while retaining signal extraction capability.
Solution Approach 2:
The patent creates a simplified copy of the essential function of retro reflection systems using a photodetector array with specific area configurations. Rather than replicating the entire retro reflection mechanism, the invention copies the core function of extracting signals from noise through a more compact and affordable photodetector arrangement suitable for handheld applications.
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
This configuration enhances the signal-to-noise ratio by accurately isolating and removing ambient light, improving the quality of the bar code signal, especially at greater distances, without introducing additional noise or increasing system complexity.
Implementation Method 1
three photodiodes are used - one main photodiode for the laser signal and two smaller ones for ambient light
Data Source
AI summary
A system and method of reading an indicia and rejecting ambient light is disclosed. An imaging signal and an ambient signal are received by the system where the imaging signal corresponds to indicia information and a first portion of the ambient light, and the ambient signal corresponds to a second portion of the ambient light. The imaging signal and the ambient signal are mathematically manipulated to subtract the contribution of the first portion of the ambient light from the imaging signal.