In smart phones, the power consumption of the LCD panel backlight accounts for about 40% of the total power consumption of the device. Therefore, if the brightness of the backlight changes with the brightness of the ambient light, there will be many benefits. In a relatively dark environment, power can be saved by reducing the brightness of the display, while also reducing the user's visual fatigue and improving the user experience.
In fact, Ambient light sensors (ALS) have been widely used in smartphones to provide information about ambient light levels to support backlit LED power circuits. However, this application is simple to say, but it actually encounters many challenges. It is because on the one hand, the power saving effect is obvious enough, and on the Other hand, the user has to look comfortable.
The ALS must be placed on the back of the monitor screen, which can be said to be inch-inch gold, and the same component must be able to achieve proximity detection (close the display when close to the user's face) and ambient light measurement. These and other conditions severely limit the design engineer from being able to freely optimize the design.
This article describes the main challenges encountered in implementing ambient light sensing in smartphones and how to overcome these challenges to achieve higher response sensitivity of backlights and to accurately adjust backlight brightness based on ambient light.
The first problem with bright visual response is the way photodiodes react to light and are different from the human eye. The human eye is not sensitive to infrared (IR, wavelengths greater than 780 nm) and ultraviolet (UV, wavelengths less than 380 nm). On the other hand, standard silicon photodiodes typically sense light at wavelengths between 300 nm and 1,100 nm.
This means that the designer's first challenge is how to remove the infrared and UV components from the sensor output. The function of the ALS is to obtain the brightness of the light incident on the display of the smartphone (measured in luminosity (lux)). If the brightness measurement contains ultraviolet and infrared light and visible light, it is not for the display backlight controller. What the eye actually sees is that the sensor's response to ambient light is different from the "lightpic" response of the human eye. In short, the sensor "feels" that the ambient light brightness is higher than the brightness perceived by the human eye.
This is because both natural and artificial light contain infrared components. This is the case, for example, for sunlight (Figure 1) and for light from incandescent lamps. An effective way to remove infrared light is to add an optical infrared filter to the sensor. In smartphones, however, the same sensor is typically used for proximity detection (along with infrared LEDs) to turn off the display and touch controller when the phone is close to the user's face.
Figure 1 The spectral power distribution of sunlight, where the intense power infrared component is invisible to the human eye.
Of course, smartphone designers can add a separate IR photodiode for proximity sensing only, but this is a cumbersome solution: in this case, the design must bear the optical filter on the ALS and The cost of both independent infrared photodiodes, the infrared photodiode also takes up extra space, and must be perforated on the surface of the display to allow infrared rays to pass.
Amers Semiconductor has proposed a better solution to this problem: dual-diode modules. One of the photodiodes (channel 0 shown in Figure 2) is used to sense the full spectrum, and the other (Channel 1 in the figure) is primarily used to sense the infrared portion of the spectrum. By subtracting the output of the infrared photodiode from the output of the full spectrum sensor, the measurement of visible light can be obtained.
Figure 2 The spectral sensitivity of the TMD2772 of the Amers Semiconductor's dual diode module series. Other products include the TMD27721 and TMD27723 series.
This sensor is very insensitive to UV light, and in most cases the common source emits very little UV radiation. In most cases, it is sufficient to remove the ultraviolet rays in order to achieve ambient light sensing as long as a packaging material capable of absorbing ultraviolet rays is used.
After removing the infrared elements from the ALS output, smartphone designers now have to solve the second problem: how to limit the viewing angle of the ALS/proximity sensor module without affecting its performance. This is about the balance between the ALS and the proximity sensor.
In terms of ambient light sensing, the ideal viewing angle is 180 degrees (which is virtually impossible) because it is the angle at which the ambient light hits the display, but for proximity sensing it is the opposite: it needs It is a narrow viewing angle to limit the possibility of crosstalk between the infrared LED and the infrared sensor. Ideally, the infrared sensor should only sense the infrared light reflected by the user's face, and the LED should not directly illuminate the sensor, nor should it detect the light reflected from the top and bottom of the touch panel. Therefore, it is necessary to make a trade-off between the demand conflict between the ALS and the infrared sensor.
Through experiments, smartphone designers have found that 90-110 degrees of viewing angles provide high-performance proximity detection while also allowing ambient light sensing systems to perform well; narrowing the angle to below 90 degrees can significantly impair ALS performance. In addition, if the system works at a 90 degree angle of view, the air gap between the bottom of the touch screen and the top of the sensing module must be very small.
The viewing angle is not the only mechanical design issue that affects ALS performance. In order for the light to pass through the screen to reach the sensor module, the designer must open a hole in the display. OEMs hope that the smaller the aperture, the better, to avoid damaging the rounded, smooth shape of the touch screen. They also add ink to the underside of the screen glass to mask the opening, which darkens it and blends its color with the color of the phone case. Both ink and openings reduce the amount of light incident on the sensor module.
In addition, OEMs must strictly control the change in transmission of inks in production online. For example, if a 17% transmittance ink is used, only a ±1% change in ink transmission will result in an additional error of 5.9% for the ALS output.
The third major challenge in implementing ambient light sensing in smartphones is the need to handle very high dynamic range light output. Smartphone manufacturers want to make the brightness of the display backlights set appropriately, whether the phone is in an environment that is almost completely dark (lower than 0.1 lux) or direct sunlight (brightness up to 220 klux).
This requires the sensor to have high sensitivity over an extremely wide dynamic range while still maintaining a very low noise floor. In addition, the gain of the device should also be controlled to accommodate changes in ambient light levels.
The implementation of fine-tuning has demonstrated the compromise between ambient light sensing and the benefits of dual-photocell solutions in smartphones, as well as the ALS module features that OEMs need to focus on. However, the appearance, mechanical design and ink of each device Different, this requires various characterization features to develop a tailored brightness equation.
This equation is used to accurately remove the infrared component of ambient light and to compensate for limited viewing angles.
In order to achieve such characterization characteristics, smartphones must be exposed to a variety of different light sources that emit different proportions of infrared and ultraviolet light. Then, under the same lighting conditions, the illumination of the environment is measured by a high-precision lux meter, and the illuminance of the environment is measured with the ALS module, and then the output of the ALS is corrected based on the output of the lux meter. The metering surface of the lux meter should be covered with a hood to simulate the limited viewing angle of the light sensor.
To characterize sensor modules, such as TMD27721 or TMD27723 from Austrian Microelectronics, the following equations can be used:
CPL=(ATIME_ms×AGAINx)/20 Lux1=(C0DATA-a0×C1DATA)/CPL Lux2=(b0 × C0DATA-b1×C1DATA)/CPL Lux=MAX(Lux1, Lux2, 0)<,b>
In the above equation, CPL, a0, b0, b1 are parameters that characterize the characteristics.
CPL: Counts per Lux. C0DATA: Read data from Channel 0. C1DATA: Read data from Channel 1. C0DATA-a0x C1DATA: A light source with a high ratio of weighted count infrared. b0xC0DATA-b1x C1DATA: A light source with a low weighted infrared ratio. MAX: The maximum value of Lux1, Lux2 and 0.
In general, the more data sets collected under more light sources, the more accurate the characterization characteristics will be.
By defining an appropriate mechanical design, tightly controlling ink transmission during production, and carefully characterizing, the systematic error of ambient light sensing can be limited to no more than ±15%. In some cases, the error can be as small as ±10%. This is good enough for the purpose of adjusting backlight brightness to reduce power consumption and improve the user experience.
Of course, OEMs may be based on functions other than display backlight control, and require higher accuracy ALS, which requires the use of highly sensitive ambient light sensors (eg, stand-alone devices that do not have proximity detection). For such applications, Austin Microelectronics' TSL25911 is ideal.
Summary ALS has been widely used in smartphones to provide information about ambient light levels to support backlit LED power circuits. However, this application is simple to say, but it is not easy to do it – it is because on the one hand, the power-saving effect is obvious enough, on the other hand, it has to make the user look comfortable.
The ALS must be placed on the back of the monitor screen, which can be said to be the size of the gold - the component must be able to achieve proximity detection (close the display when the user's face is closed) and the main ambient light measurement function. These and other conditions severely limit the design engineer from being free to optimize for the design.
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