Bezeless designs in mobile devices provide a higher display aspect ratio while retaining features such as the receiver, front camera, light sensor, and distance sensor.
Mobile designers have been working hard to reduce the size of these functional modules, but a sticking point is that the receiver and camera performance rely heavily on size, so not much can be done about them. As a result, figuring out how to reduce the size of light and distance sensor modules has become a critical task for engineers.
Part I: Distance Sensor
Reducing the size of a phone screen bezel from 20mm in 2017 to 0.8mm in 2019 can be achieved with:
- Very narrow modules or devices
- Integrated infrared laser transmitters instead of normal LEDs (to reach achieve ultra-small sizes and transmission angles)
- More rigorous optical design and high levels of relevant experience
These requirements present significant challenges for the design engineer. The figure below illustrates an ultra-narrow border design featuring a notch. While at a glance there are few differences from conventional designs, the distance from the chip to the touch screen (air gap) is much larger.
A big air gap is problematic because when the distance between light reflection and refraction is too large, the gap causes more of the emitted light to reflect back to the touchscreen glass. This raises the noise floor, inundating the effective signal and potentially causing the distance sensor to fail.
Ultra-narrow border designs with a notch feature have a much larger air gap between chip and touch screen, compromising the effective signal.
Addressing this problem requires several considerations:
Background noise: In this case we are specifically referring to signals the sensor receives when there is no occlusion. The figure below illustrates the main reflected path of infrared light emitted by the light source. Path 1 and Path 2 have the greatest impact on background noise, followed by Path 3. The focus on background noise reduction is dependent upon barrier design to control light paths. Using a VCSEL laser as a light source can centralize the beam and make it easier to control, reducing reflection.
Tolerance control: Tolerance control in manufacturing is critical for achieving quality performance for a large air gap distance sensor. Tolerance control reduces background noise and minimizes the hindrance of infrared light from emission to reception, providing the strongest possible signal. For example, for a 1mm window design with a tolerance of +/- 0.25mm, the window may be open to 0.75 mm which will block some light. Designers must carefully consider the width, height, and barrier locations for difference tolerance levels to control the end-product within an acceptable quality range.
Keypoint setting: A keypoint is a receiving point at the edge of the emission and reception angle after refraction from the touchscreen glass. The blue rays and its intersection are shown in the figure below. The keypoint is determined by the device angle barrier location and is chosen primarily to balance background noise and blind spots.
If the keypoint is placed too low, too much light will be reflected from the upper surface of the touchscreen, resulting in a high noise floor; if the key point is placed too high, the blind spot will be too large, reducing the amount of light reflected by objects close to the touchscreen. When the reflectance of the measured object is small (such as with black hair), there is a possibility of not enough reflection energy, causing the sensor to "fail to see" the measured object.
The main reflected path of infrared light emitted by the light source.
Part II: Light Sensor
In this instance, the light sensor refers to the ambient light sensor, which allows the phone to check the surrounding ambient light brightness so it may adjust screen brightness. Ambient light sensors are relatively simple because they do not emit, only receive.
Designers aim to spread the light in order to ensure that a portion of light enters the sensor, thereby expanding the viewing angle.
When the air gap is too large and the window is too small, the sensor's viewing angle is severely affected, as shown in the red angle in the figure above. When the viewing angle is too small, the measured brightness is inaccurate because the sensor does not see light outside the viewing angle.
To expand the viewing angle, designers typically find ways to spread the light so a portion of the light slanting into the window also can enter the sensor, as shown in the yellow line.
There are two methods effective at spreading light: The use of white ink, whose particles diffusely reflect light, with some light slanting into the sensor; and diffusion film, which is similar in principle.
Diffusion film is much more effective than white ink, but more expensive and inconvenient to assemble.
A negative effect of diffusion is the reduction of transmittance as some light is reflected or scattered elsewhere. Lower transmittance means less light arrives at the sensor, lower test accuracy, and higher device sensitivity requirements. As such, the right balance between transmittance and diffusivity must be achieved. In general, ams recommends the lowest total transmittance be no less than 1%. This includes the transmittance of components that light passes, such as the touchscreen, ink, and more.
Selecting more sensitive sensors also provides additional redundancy in optical design. ams' approach to ultra-narrow borders is based on appropriate module design, optical design experience, and a range of test data. It is designed to help engineers accelerate their design cycles and increase their success rates.