The ambient light sensor (ALS) or color sensor has become a common feature of mobile phone designs. An ALS measures the intensity of environmental light incident on the display screen, providing information that lets a processor adjust the brightness of the display backlight in response to ambient lighting conditions.
When equipped with a color sensor in place of an ALS, the mobile phone can also adjust the display’s rendition of color in response to the difference in color temperature between, for instance, indoor LED lighting and sunlight. Either sensor type provides for a better viewing experience and lower power consumption.
An ALS or color sensor for display management has traditionally been located in the bezel at the top of the user side of the phone, where it has an unimpeded view of 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.
The difficulties of sensing incident light from behind an emissive layer are, not surprisingly, considerable. 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 behind an organic light-emitting diode (OLED) display.
OLED display technology is rapidly replacing liquid crystal displays because it offers higher contrast, more vivid and accurate color reproduction, and lower power consumption. But how is it possible to perform sensing of ambient light incident on a surface that itself emits light at an intensity high enough to provide a vivid and engaging viewing experience?
Ultra-high sensitivity ALS
An OLED display is somewhat transparent: its complex structure diffuses incident light, but does not block it. An OLED display in a phone has an opaque, reflective thin backing which provides a uniform optical background for the display’s emissions. A small (approximately 1mm in diameter) aperture cut into this backing will allow ambient light to pass through an OLED display to an ALS sensor mounted on the PCB beneath it.
The difficulty in implementing ambient light-sensing behind-OLED is that light emitted by the display also reaches the ALS, interfering with the ambient light measurement. The challenge is therefore to subtract the light emitted by the OLED display from the ambient light sensed by the ALS device’s photosensors, to give a true measure of ambient light intensity.
The ALS performance requirement for effective display management is lux measurement accuracy of at least ±10%. (Lux is the unit of luminance, or light intensity incident on a surface.) When calibrated in the handset, ams ALS devices mounted in the bezel typically achieve lux measurement accuracy of better than ±5%.
Achieving the required accuracy when mounted behind an OLED screen is much more difficult than when bezel-mounted. This is why ams has developed the TCS3701 ALS chip which offers very high photo-sensitivity – some ten times higher than that of ALS chips used in conventional bezel-mount designs.
This high sensitivity provides a very broad full-scale lux measurement range: the benefit of this is that small lux values attributable to ambient light may be detected when a much larger lux value attributable to the display is subtracted from the total measured value.
The operation of this solution for ALS Behind-OLED also depends on the ability to cancel out the effect of the light emitted by the display: this is achieved with complex algorithms developed by ams.
The ams Behind-OLED sensor: proven performance
It is this combination of high-sensitivity hardware and sophisticated software which enables the effective operation of the ams Behind-OLED solution. It has achieved calibrated lux measurement accuracy of better than ±10%, exceeding the performance target set by phone manufacturers.
The effective operation of the ALS device is supported by the optically diffusive structure of the OLED display, which means that the sensor works at most incident angles without requiring a wide field of view.
The ams Behind-OLED solution enables phone manufacturers to retain the valuable display brightness adjustment function as they make the transition to OLED display technology in the new sleek, bezel-less display designs favored by today’s users.
For more information about the ams portfolio of Behind-OLED solutions visit https://ams.com/display-management.