LED Driver PWM Modulation for Smartphone Camera Stripe Reduction
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Solution Overview
Problem
Smartphone cameras capture bright and dark stripes when taking pictures or videos in areas illuminated by PWM-controlled LED lighting, causing undesirable effects in both the illuminated surfaces and the lighting itself.
Innovation Solution
A method and driver unit that modulate the current through LED lighting using PWM, adjusting the on-time or off-time to maintain a constant long-time average current, with a sigma-delta modulator, and varying the duty cycle within a threshold of 25% to 75%, ensuring a frequency above 60 Hz to reduce stripe effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If PWM modulation is used to control LED lighting, then energy efficiency and dimming control are improved, but bright and dark stripes appear in photos and videos taken by smartphone cameras
Solution Approach 1:
The patent applies periodic action by using PWM modulation to switch the LED lighting on and off periodically. This periodic switching allows the lighting to achieve high energy efficiency while the modulation frequency is specifically designed to avoid interference with camera exposure timing, thereby preventing the bright and dark stripe artifacts in captured images
Solution Approach 2:
The patent employs parameter changes by dynamically adjusting the PWM duty cycle and modulation frequency based on operating conditions. By changing these parameters, the system maintains energy efficiency while avoiding specific frequency ranges that cause camera interference, thus eliminating the stripe effect in photos
2Measurement precision
If PWM duty cycle is varied to control lighting intensity, then dimming control precision is improved, but stripe effects become more pronounced at certain duty cycle values
Solution Approach 1:
The patent implements feedback mechanisms that monitor the PWM duty cycle and modulation frequency, and automatically adjust them to avoid ranges that produce stripe effects. This feedback control maintains precise dimming capability while preventing the occurrence of bright and dark stripes in captured images
Solution Approach 2:
The system uses dynamic adjustment of PWM parameters based on real-time operating conditions. By making the duty cycle and frequency adaptive rather than fixed, the system maintains precise dimming control while dynamically avoiding parameter combinations that cause stripe artifacts
3Stability of the object's composition
If high frequency PWM modulation is used to reduce visible flicker, then human eye perception stability is improved, but interference with camera sensors increases
Solution Approach 1:
The patent applies local quality by treating the requirements for human eye perception and camera sensor interference differently. It uses frequency ranges that are imperceptible to humans (satisfying perception stability) while specifically avoiding frequencies that interfere with camera sensors, thus addressing each requirement with appropriate local optimization
Data Source
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AI summary
The invention relates to a method (100) for operating a LED based lighting means (202), wherein the method (100) comprises the steps of pulse width modulating (101), PWM, a current through the LED lighting means (202) by a driver unit (201) such that a long-time average value of the current through the LED based lighting means (202) corresponds to a value set by a feed-forward or a feedback control performed by the driver unit (201), and modulating (102), by the driver unit (201), this set long time average value by varying an on-time of the PWM modulation while maintaining a constant off-time of the PWM modulation, in case the PWM duty cycle is more than 50%, or an off-time of the PWM modulation while maintaining a constant on-time of the PWM modulation, in case the PWM duty cycle is less than 50%.