Ambient Light Sensor Behind OLED Display for Brightness Control
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Solution Overview
Problem
Ambient light detection through OLEDs is challenging due to the opaque nature of the display, which requires sensitive sensors and results in undesirable fluctuations in display brightness as the sensor detects both ambient and display-generated light.
Innovation Solution
An ambient light sensor system where the integration time is unsynchronized with the display screen's frame rate, using a PWM blanking signal with a duty cycle to control brightness, and estimating the ambient light signal by identifying highest and lowest sample values to separate ambient light from display light, allowing for accurate brightness adjustment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If the ambient light sensor is disposed behind the OLED display screen, then the screen area can be maximized and bezel width reduced, but the display becomes opaque requiring very sensitive sensors and causing brightness fluctuations
Solution Approach 1:
The patent applies periodic action by using PWM (Pulse Width Modulation) blanking signals to periodically turn the display off during sensor integration periods. The display is pulsed on and off at specific frequencies, allowing the ambient light sensor to collect clean measurements during OFF periods while maintaining normal display operation during ON periods. This periodic switching resolves the contradiction by enabling accurate ambient light detection through the opaque OLED display.
2Measurement precision
If the ambient light sensor detects light through the display, then ambient light can be sensed, but the sensor also detects display-generated light causing brightness fluctuations
Solution Approach 1:
The patent applies segmentation by dividing the sensor's integration period into distinct segments: one segment integrates during display OFF periods (capturing only ambient light), and another segment integrates during display ON periods (capturing both ambient and display light). By separately measuring and then subtracting the display light component from the total measurement, the system isolates the ambient light signal and eliminates brightness fluctuations.
Solution Approach 2:
The patent uses feedback by continuously monitoring the display brightness level and using this information to compensate for display light interference in the sensor measurements. The system measures the display's light output, feeds this information back into the signal processing algorithm, and subtracts the displayed light component from the total sensor reading to obtain accurate ambient light measurements.
3Measurement precision
If a very sensitive sensor is used to detect ambient light through the opaque OLED, then ambient light detection becomes possible, but the sensor becomes susceptible to display light interference
Solution Approach 1:
The patent converts the harmful effect of display light interference into a beneficial measurement strategy. By intentionally having the sensor detect both ambient and display light during ON periods, then separately measuring ambient light during OFF periods, the system uses the display light signal itself as a reference to subtract from the total measurement. This transforms the interference problem into a solvable equation where the display light becomes known information used to isolate the ambient light component.
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 accurate ambient light detection and display brightness control, ensuring the display appears sharp and readable while reducing energy consumption by decoupling ambient light from display brightness fluctuations.
Implementation Method 1
An ALS sensor will detect not only ambient light (e.g., background light, sunlight, etc.) passing through the display, but will also detect the light generated by the display itself
Implementation Method 2
OLEDs are generally opaque primarily as a result of a protective film on their backside
Data Source
AI summary
An apparatus includes a display screen, an ambient light sensor disposed behind the display screen, and an electronic control unit. An integration time of the ambient light sensor is unsynchronized to a frame rate of the display screen. The electronic control unit is operable to control a brightness of the display screen based on a duty cycle of a PWM blanking signal, wherein at least one OFF time of the PWM blanking signal occurs fully within a first integration period of the ambient light sensor, and wherein at least one other integration period ON time of the PWM blanking signal occurs fully during an ON time of the PWM blanking signal. The electronic control unit is further operable to acquire samples of an output of the ambient light sensor, to identify a highest value and a lowest value from among a consecutive group of the samples, and to estimate a magnitude of an ambient light signal based at least in part on the highest value and the lowest value.