OLED Post Display Technology: the New Ambient Light Sensor Technology Greatly Improves the Managemen

Mathew Hubbard, Ames semiconductorAmbient light sensor (ALS) or color sensor has become a necessary feature option for mobile phone design. ALS can measure the incident ambient light intensity of the display screen and let the processor adjust the display screen backlight according to the measured ambient lighting conditions. This helps improve the viewing experience and reduce power consumption.ALS or color sensor for display management is usually installed on the top frame of mobile phone client, which can receive ambient light without shielding. However, the new design trend of high-end smart phones is to minimize or even eliminate the border to maximize the display size, and OLED display is widely used. Therefore, mobile phone manufacturers hope to obtain an ambient light sensing system that can be installed under the display screen.

There is no doubt that it is very difficult to perceive the incident light from behind the OLED light-emitting layer. To this end, optical sensing technology suppliers have proposed various solutions. At present, Ames semiconductor has developed a solution that not only greatly exceeds the requirements of mobile phone manufacturers for measurement accuracy, but also takes into account the consideration of design flexibility - als can be installed anywhere behind the display screen, and supports the common sensor hub architecture adopted by traditional border ALS devices.This paper introduces the motivation of moving ALS to OLED display screen, and summarizes the innovative technologies that enable ALS devices to operate under light-emitting screen.Display size and performance: key differentiators

Facing the lucrative high-end smartphone market, manufacturers have launched fierce market competition, and the display size and performance are the two strongest weapons possessed by mobile phone OEMs. For a long time, users have attached great importance to the media player function of mobile phones. Users pay more and more attention to the ability of mobile phones to render virtual reality (VR) and augmented reality (AR) content and play traditional videos and display images.This demand is driving the transformation from traditional liquid crystal display (LCD) technology to organic light emitting diode (OLED) display technology: OLED technology has higher contrast, more vivid and accurate color reproduction, and lower power consumption. OLED technology can also realize more flexible structures, such as curved display or folded screen.But this will affect another aspect of display performance: the response to ambient light. Today, when using a mobile phone in a dark environment, the display will automatically darken to avoid glare and save electricity. When using the mobile phone in bright sunlight, the brightness of the display will rise, so that the perceived contrast can be maintained and users can view the content comfortably.

If consumers value display quality, they will also consider display size: the larger the better. Of course, the size of the display screen is limited: the mobile phone must be portable and easy to hold. Therefore, in order to maximize the size of the display within a given size range, mobile phone manufacturers want to eliminate the border and extend the visual display area to the top and bottom of the mobile phone case.If the phone is equipped with a color sensor instead of ALS, it can not only manage the brightness of the display, but also manage the color reproduction to adapt the display to different viewing environments, such as fluorescent lighting, LED lighting and natural sunlight.However, the disappearance of the frame and the development of OLED display technology make OEM manufacturers face great challenges: they can't continue to install ALS devices or color sensors on the frame, because the frame position can receive ambient light unimpeded. However, ALS or color sensors must be installed somewhere, because they are the key factor to achieve efficient display management.

This "somewhere" refers to somewhere under the OLED display. But how to perceive the incident ambient light from under the screen that emits high-intensity light?Ultra high sensitivity alsIn fact, OLED displays are translucent to some extent: complex structures can scatter incident light without completely blocking it. Most of the area under the OLED display of the mobile phone is an opaque reflective thin backplane, because it is necessary to provide a uniform optical background for the light emitted by the display. However, if a small hole (1mm in diameter) is opened in the reflective film on the back of the OLED, part of the ambient light can pass through the OLED to the ALS sensor behind it.

The difficulty of realizing ambient light sensing under OLED screen is that the light emitted by the display screen will also reach ALS, thus interfering with the measurement of ambient light. Therefore, the challenge is how to subtract the light emitted by the OLED display from the ambient light sensed by the photosensitive sensor of ALS device, so as to obtain the real ambient light intensity.One method is to use the duty cycle of OLED display. At low display brightness, the brightness of OLED display screen is controlled by PWM modulation, and the duty cycle of PWM is less than 100%. When the screen becomes dark, OLED display screen provides a short "interval time" and the screen goes out. The ALS operation can be triggered only at intervals to obtain pure ambient light measurement without display light interference.This method is feasible, but it only solves some problems. Because when the display brightness is high, the duty cycle is usually 100%. This means that ALS must have the ability to deal with the luminous interference of the display screen.

To meet this requirement, researchers propose another method: display synchronization. If ALS knows that the above pixel display is black, the degree of optical interference from the display is zero or close to zero.Again, this is only a theoretically feasible solution, but in practice, it requires close coordination between two completely discrete functions - als and display driver IC (DDIC). The current mobile display architecture is difficult to achieve this coordination. ALS has a single interface to the sensor hub IC to run drivers for each sensor supported. The output of ALS is transmitted to the mobile application processor through the hub, and the display management software is also running. In addition, connecting ALS to DDIC will add considerable additional complexity to circuit board layout and system software.Therefore, Ames semiconductor has developed an OLED off screen ambient light sensing solution without display synchronization (although complex display management systems based on color sensors rather than ALS may still need display synchronization). The new ALS solution of Ames semiconductor integrates the photosensitive sensor with greatly improved performance and the advanced software developed by Ames semiconductor.

Maintain the measurement accuracy behind the OLED screenEfficient display management requires lux measurement accuracy in ALS performance to be at least 10% (Lux is the brightness unit, that is, the intensity of incident light on the surface). After calibration, the lux measurement accuracy of Ames semiconductor ALS device usually installed in the frame of mobile phone is improved to 5%It is much more difficult to make the device under the OLED screen achieve the required accuracy than when the device is located on the frame. Therefore, Ames semiconductor has developed a new ALS chip with high photosensitivity, which is about 10 times higher than the ALS chip with traditional frame design.

The detection range and resolution of lux are improved: the advantage of this method is that even when the ambient light is weak relative to the light leakage of the display screen, the influence of the display screen can be reduced from the detected light intensity signal to get the emphasis of the actual ambient light.The operation of this OLED off screen ALS solution also depends on the ability to offset the display light: This is achieved through a complex algorithm developed by Ames semiconductor.It is this combination of highly sensitive hardware and complex software that enables Ames semiconductor ALS solutions to run efficiently. The calibrated lux measurement accuracy of the OLED off screen ALS system integrating the latest generation of Ames semiconductor devices and algorithms is better than 10%, far exceeding the performance indicators required by mobile phone manufacturers.

The optical diffusion structure of OLED display supports the efficient operation of ALS devices, which means that the sensor works at most incident angles without special wide field of view design.Inverse integration trendHowever, this diffuse structure also means that the infrared transmitter for proximity sensing will encounter major challenges when operating under the OLED display. Therefore, the ambient light and proximity sensing modules of the original Amis semiconductor OLED under screen ALS solution were not fully integrated. However, with the introduction of infrared user face recognition technology into high-end smart phones, OEM will have other proximity sensing options.

In order to cope with this reverse integration trend, Ames semiconductor OLED under screen ALS solution also integrates led hardware driver and infrared receiver, which can cooperate with external IR led to realize proximity detection function. If the infrared LED is installed on the outermost edge of the display screen or on the parting line, the user's face can be clearly viewed, and the reflected infrared light will be detected by the integrated receiver under the OLED screen.ALS under OLED screen: reliable performanceTherefore, Ames Semiconductor provides an ALS solution to support mobile phone manufacturers to eliminate the frame of the new OLED technology display screen to maximize the display size.

With the high sensitivity semiconductor technology developed by Ames semiconductor and complex interference compensation algorithm, ALS function can operate efficiently under OLED screen. The algorithm can run outside or inside the AMS chip.This enables mobile phone manufacturers to retain the practical display brightness adjustment function, so that they can adopt the fashionable borderless display design favored by today's users in the process of transition to OLED display technology.[End]

SummarysummaryThe ambient light sensor ALS or color sensor has become a common feature of mobile phone designs. An ALS measures the intensity of ambient light incident on the display screen providing information that lets a processor adjust the brightness of the display backlight in response to the ambient lighTIng condiTIons. This provides for a better viewing experience and lower power consumpTIon.

Ambient light sensor (ALS) or color sensor has become a common function option in mobile phone design. ALS can measure the incident ambient light intensity of the display screen and let the processor adjust the display screen backlight according to the measured ambient lighting conditions. This helps improve the viewing experience and reduce power consumption.An ALS or color sensor for display management is typically located in the bezel at the top of the user side of the phone where it has an unimpeded view of the ambient light. New designs for high-end smartphones however have eliminated the bezel to maximize the size of the display. Mobile phone makers are therefore looking to implement an ambient light-sensing system which can be mounted behind the display. In the new bezel-less phone designs this display will generally use OLED-display technology.The ALS or color sensor for display management is usually installed on the top frame on the user side of the mobile phone, which can receive ambient light unhindered. However, the new design of high-end smartphones eliminates the border to maximize the display size. Therefore, mobile phone manufacturers hope to obtain an ambient light sensing system that can be installed under the display screen. In the new borderless mobile phone design, this display will generally use OLED display technology.

Various approaches to the problem have been suggested by providers of light-sensing technology. Now ams has developed a soluTIon which exceeds the mobile phone manufacturers requirement for measurement accuracy while providing the design flexibility to locate the ALS anywhere on the PCB behind the display and supporting the familiar sensor hub architecture used