Behind-Display Ambient Light Sensing With Refresh-Synced Sampling
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Solution Overview
Problem
Ambient light sensors (ALS) mounted behind a display face challenges in accurately measuring ambient light levels due to design constraints and the need to distinguish between ambient light and display-emitted light, leading to inaccurate results, especially with modulated light sources like 50 Hz or 60 Hz PWM controlled displays.
Innovation Solution
A method where the display is periodically switched on and off to generate three signal counts: one during the on-state, one during the off-state, and one over the total integration time, allowing the light sensor to differentiate between ambient light and display light, enabling accurate ambient light level determination without controlling the display.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the display is repeatedly switched off to allow ALS measurement, then the light sensor can measure ambient light without display interference, but the measurement sampling rate becomes too slow to satisfy the Nyquist criterion for 50 Hz/60 Hz modulated light
Solution Approach 1:
The patent applies periodic action by utilizing the display's existing periodic refresh cycles (at 50 Hz or 60 Hz) as the measurement window. Instead of introducing a separate slow measurement cycle, the system performs ALS measurements during each display refresh period, specifically during the off-subinterval of the PWM-controlled display. This allows the measurement sampling rate to naturally synchronize with and match the display refresh rate, satisfying the Nyquist criterion for detecting modulated light while maintaining accurate ambient light sensing.
2Measurement precision
If the display off time is extended to cover the full modulation period, then complete ambient light measurement is possible, but the display refresh rate drops to unacceptable levels
Solution Approach 1:
The patent applies segmentation by dividing the display refresh period into distinct subintervals: an on-subinterval where the display emits light and an off-subinterval where the display is off and ALS measurements are performed. This segmentation allows both display operation and ambient light sensing to occur within the same refresh cycle without requiring the display to remain off for extended periods. The measurement is completed during the off-subinterval while the overall refresh rate is maintained by the rapid switching between on and off states.
3Device complexity
If the light sensor is mounted behind the display to save space, then the sensor is hidden from sight and space is optimized, but the sensor cannot distinguish between ambient light and display-emitted light
Solution Approach 1:
The patent applies periodic action by exploiting the periodic on/off switching of the display during each refresh cycle. During the off-subinterval, the display emits no light, so any light detected by the sensor must be ambient light. During the on-subinterval, the display emits light that mixes with ambient light. By comparing measurements from these two periodic states, the system can mathematically separate and identify the ambient light component from the total light signal, enabling accurate ambient light sensing despite the sensor's position behind the display.
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 allows for accurate measurement of ambient light levels at high picture refresh rates, avoiding under-sampling effects and providing reliable data for display brightness adjustment, thus improving user experience and power efficiency.
Implementation Method 1
Light which is incident on the display may penetrate through the display and can be received by the light sensor arrangement
Data Source
AI summary
A method is suggested for sensing light being incident on an electronic device. The electronic device comprises a display and a light sensor arrangement mounted behind the display such as to receive incident light through the display. The method comprises periodically switching the display on and off depending on a control signal, wherein a period is defined by a succession of an on-state and an off-state of the display. A sensor signal is generated by integrating the incident light by means of the light sensor arrangement for a total integration time comprising a number of periods. A first signal count is determined from the sensor signal being indicative of an amount of integrated incident light during an on-state. A second signal count is determined from the sensor signal being indicative of an amount of integrated incident light during an off-state. A third signal count is determined from the sensor signal being indicative of an amount of integrated incident light during the total integration time. Finally, an ambient light level is determined depending on the first, second and third signal counts.