Single Optical Sensor Colour Feedback for LED Backlight
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Solution Overview
Problem
Conventional LED-based backlights face challenges in maintaining consistent intensity and color due to temperature and age-related variations, leading to slow response times and high costs associated with high dimming ratios and expensive A/D converters, while existing solutions result in color break-up and require high refresh rates.
Innovation Solution
A method and system that utilize a single optical sensor coupled with a sample and hold circuit to determine and adjust drive settings for colored light sources, ensuring their ON times do not coincide, allowing for peak luminance measurement and temperature compensation to maintain a predetermined color point, using pulse width modulation and current control, and switching to temperature control algorithms when PWM pulses are too short.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple optical sensors are used to measure different colour channels simultaneously, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent segments the measurement process in time by sequentially activating different colour channels (RED, GREEN, BLUE) at different time intervals. The single optical sensor measures each colour channel separately during its designated time slot, eliminating the need for multiple simultaneous sensors while maintaining measurement precision through temporal separation.
Solution Approach 2:
The system implements periodic activation of different colour channels with specific duty cycles. Each colour channel is activated in periodic intervals, allowing the single optical sensor to capture measurements at appropriate moments. This periodic action enables one sensor to perform the work of multiple sensors through time-multiplexed operation.
2Measurement precision
If colour filters with overlapping spectral response are used in optical sensors, then sensor sensitivity is improved, but colour interference occurs during measurement
Solution Approach 1:
The patent uses periodic activation of colour channels with non-overlapping time windows. When RED channel is active for measurement, GREEN and BLUE channels are inactive, and vice versa. This temporal separation eliminates colour interference while allowing the optical sensor to use broadband filters for high sensitivity in each measurement window.
Solution Approach 2:
The patent extracts the colour separation function from the optical domain (filters) and moves it to the time domain (sequential activation). Instead of relying on spectral filtering to separate colours, the system separates colours by activating them at different times, removing the harmful overlap effect while preserving sensor sensitivity.
3Measurement precision
If sensor values are integrated to DC for measurements, then measurement accuracy is improved, but response time becomes very slow
Solution Approach 1:
The system uses periodic PWM-driven colour channel activation with duty cycles that create distinct ON and OFF periods. Measurements are taken during the ON periods when each colour channel is actively emitting light, allowing rapid AC-coupled measurement without requiring long DC integration times. This enables fast response while maintaining accuracy through synchronous detection.
4Measurement precision
If colour sequencing is used to avoid interference in optical sensors, then colour measurement accuracy is improved, but colour break-up occurs and dimming ratios are reduced
Solution Approach 1:
The patent implements a feedback control system where the controller measures the actual luminance output of each colour channel using the optical sensor, compares it with the desired target values, and adjusts the PWM drive settings accordingly. This closed-loop feedback ensures colour uniformity across the display while maintaining high dimming ratios, preventing colour break-up by continuously compensating for variations.
Solution Approach 2:
The system dynamically changes PWM duty cycle parameters for each colour channel based on measured luminance values and temperature compensation data. By adjusting the ON time duration and intensity of each colour channel in real-time, the system maintains colour balance and prevents break-up effects while achieving high dimming ratios without the limitations of fixed colour sequencing.
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 approach enables stable and efficient control of brightness and color over a high dimming range without recalibration, using cheaper sensors and preventing color break-up, thus improving the reliability and efficiency of LED-based backlight systems.
Implementation Method 1
measuring a first peak luminance value of the one or more first light sources wherein the step of measuring comprises the step of sampling and holding the peak luminance value
Implementation Method 2
an optical sensor coupled to a sample and hold circuit
Data Source
AI summary
A method for controlling an illumination system comprises determining first drive settings for each of a plurality of coloured light sources, the first drive settings generating an ON time and an OFF time of the light sources; for each of the light sources of a first colour and the light sources of a second colour changing the first drive settings so that the ON time of the light sources of a selected one of the first and second colour does not coincide with the ON time of the light sources of the other colours for at least a period of time, and during that period of time, measuring the peak luminance of the light sources of the selected colour; and for each of the light sources of the first colour and the second colour recalculating the drive settings into second drive settings, based on the measured peak luminance for the light sources of that colour, so as to maintain a pre-determined colour point.