Ambient light sensors are now widely used in many LCD display applications, from consumer electronics to automotive applications. By automatically adjusting the display brightness, they help save device battery power. Furthermore, these sensors work well under various light sources, including natural sunlight, fluorescent light, and incandescent light. Many such products have recently been released, and they all share a common characteristic: these newly developed ambient light sensors are designed to match the requirements of the human eye, which is crucial for reducing eye strain.
An ambient light sensor detects surrounding light conditions and instructs the processing chip to automatically adjust the display's backlight brightness, reducing power consumption. For example, in mobile applications such as smartphones, laptops, and tablets, the display consumes up to 30% of the total battery power; using an ambient light sensor can maximize battery life. Furthermore, ambient light sensors help the display provide a softer image. When the ambient light is high, an LCD display using an ambient light sensor automatically adjusts to high brightness. When the ambient light is low, the display adjusts to low brightness.
Ambient light sensor technology principle
An ambient light sensing system requires three main components: a light sensor to monitor ambient light intensity, a data processing device (usually a microcontroller), and an actuator to control the backlight input current.
Figure 1 is a block diagram illustrating a system for implementing backlight control. In this assembly, the light sensor is a critical component, as it provides ambient light intensity information to the other modules of the system. The light sensor must include a signal converter (such as a photodiode or CdS photoresistor) to convert the light signal into an electrical signal, a signal amplification and/or conditioning device, and an analog-to-digital converter (ADC).
Figure 1. System block diagram for implementing backlight control
Figure 2 shows a discrete photodiode circuit. As can be seen from the figure, this circuit requires one or more operational amplifiers: one for current-to-voltage conversion, and possibly another stage for amplification to provide additional gain. It also includes several branch circuits for power supply and to ensure a highly reliable signal chain. In space-constrained applications, an excessive number of required components can lead to space-limited problems.
Figure 2. Discrete design of photodiode circuit
There is a more subtle problem here. Specifically, ideally, the measurement of ambient light should simulate the human eye's response to light. This is typically achieved using the visual brightness curve provided by the CIE (Figure 3). However, photodiodes rarely perfectly simulate this response mechanism because they generally have very high infrared (IR) sensitivity. Under illumination conditions with high IR intensity (such as incandescent lamps or sunlight), this infrared sensitivity can lead to misjudgments of light intensity.
Figure 3. CIE curve and typical photodiode
One way to solve this problem is to use two photodiodes: one that is sensitive to both visible and infrared light, and the other that is sensitive only to infrared light. Finally, by subtracting the response value of the latter from the response value of the former, infrared interference is minimized, resulting in an accurate visible light response.
Electrical characteristics of ambient light sensors
1. Low dark current, low illumination response, high sensitivity, and current changes linearly with increasing illuminance;
2. Built-in dual sensors automatically attenuate near-infrared light, and the spectral response is close to the function curve of the human eye;
3. Built-in micro-signal CMOS amplifier, high-precision voltage source and correction circuit, with large output current, wide operating voltage range and good temperature stability;
4. Optional optical nanomaterial encapsulation allows visible light to pass through while blocking ultraviolet light and relatively attenuating near-infrared light, thus enhancing the optical filtering effect;
5. Complies with EU RoHS directive, lead-free and cadmium-free.
Factors to consider when selecting an ambient light sensor
There are several methods for detecting light, such as using phototransistors, photoresistors, or photodiodes, but for the overall light sensing requirements of today's applications, IC-based monolithic photodiodes are one of the best choices.
A photodiode is a semiconductor used to detect light and generate current. It is constructed from a single-crystal silicon wafer, similar to the crystalline silicon wafers used to manufacture integrated circuits. A typical sensor application framework includes a photodiode, a current amplifier, and a passive low-pass filter to detect and process the output voltage signal caused by the light input. The ability to integrate all these devices into a small package is highly beneficial for end users, and this is precisely what the current market demands.
Another important aspect of selecting the appropriate optical sensor for an application is understanding which specifications are most critical and which require the most attention for the application.
Generally, the following factors should be considered when selecting a light sensor:
1. Spectral response/IR suppression: The ambient light sensor should only be sensitive to the range of 400nm to 700nm.
2. Maximum Lux range: Direct sunlight can reach up to 130,000 Lux, but most applications require a maximum range of only 10,000 Lux.
3. Low Lux photosensitivity: Depending on the type of lens at the top of the light sensor, light attenuation can range from 25% to 50%. If low photosensitivity is critical (<5 Lux), it is essential to select a light sensor that can operate within this range.
Integrated signal conditioning (i.e., amplifier and ADC): Some sensors may be available in very small packages, but still require an external amplifier or passive components to obtain the desired output signal. More integrated optical sensors eliminate the need for external components (ADC, amplifier, resistors, capacitors, etc.).
4. Power Consumption: For optical sensors that need to withstand high Lux levels (> 10,000 Lux), it is best to use a non-linear light-to-analog output optical sensor or a light-to-digital output optical sensor. This will be explained in more detail later.
5. Package Size: For most applications, smaller packages are better. Currently available packages are 2.0 × 2.1 mm optical DFNs, while 1.3 × 1.5 mm 4-lead packages are the next generation. Once these important specifications are determined, the next question to consider is which type of output signal is most beneficial to the target application. For most optical sensors, the most common output is a linear output current. While this works for some applications, there are now more options, including linear voltage output, digital output (via I2C interface), or non-linear current or voltage output. Each has its advantages, as listed below.
6. Linear Analog Output—Current or Voltage Output: More common sensor output, with a fast response time (digital outputs are limited by integration time). An ADC converter is integrated into the controller. Voltage output eliminates the need for an external resistor (to convert current to voltage) and provides a low-impedance output. Current output requires adding passive components to convert current to voltage, setting the sensor's gain range, and adding low-pass or high-pass filters as needed.
7. Nonlinear Analog Output – Current or Voltage Output: Allows for extremely low light sensitivity and maximum dynamic range (up to 100,000 Lux), sensing light in a way more similar to how humans perceive light (nonlinear and linear), with the option of voltage or current nonlinear output, with low impedance for voltage output and high impedance for current output.
8. Digital output: The output can be directly connected to the controller (without ADC). Digital output is more noise-immune than analog output, allowing the sensor to have more digital functions (i.e., a more intelligent light sensor), easier to work on networks on the general I2C bus, easier to allow multiple light sensors to be placed on the same I2C bus (address selection pin), and constant power consumption (analog output circuit losses are proportional to incident light density).
Application of ambient light sensing technology
With advancements in semiconductor analog sensors and packaging in recent years, end users now have a wider range of choices when it comes to optical sensors. Today, designers are using more optical sensors than ever before in consumer products, automotive, medical, and industrial applications. This is primarily due to several reasons, including improved light response, small footprint, low power consumption, high integration, and ease of use. Specific selections are typically made based on the functionality, performance, and environmental requirements of the designer and application.
Several key technical factors can help users and designers decide how to choose an ambient light sensor. First, the sensor's output must be linearly related to light intensity, and its spectral wavelength sensitivity should be very close to that of the human eye. Additionally, the device's output should be directly proportional to the light intensity illuminating the integrated photodiode, with a peak response of 540nm close to the peak sensitivity of the human eye. Most light sensors are capable of sensing ambient light in the 380nm–770nm range.
Main applications of ambient light sensors in automobiles
A. Backlight control of infotainment/navigation/DVD systems to ensure ideal brightness under various ambient light conditions;
B. Backlight control of the rear seat entertainment display screen
C. Backlight of the instrument panel combination
D. Automatic rearview mirror dimming
E. Automatic headlights and rain detection
F. Rearview Camera Control
Since automobiles need perfect backlighting under various ambient light conditions, light sensors, with their human-eye-like sensing capabilities, have become one of the most effective solutions for achieving more comfortable display quality, meeting automotive safety and comfort standards.
Mainstream ambient light sensors and their characteristics
The AMIS-74980x ambient light sensor utilizes the company's proprietary CMOS image sensing technology, offering greater design flexibility and cost-effectiveness for devices with luminous displays. It is an affordable and ideal solution for mobile phones, PDAs, handheld displays, LCD monitors, portable DVD players, laptops, large LCD displays, and MP3 players. It is also well-suited for automotive applications such as in-vehicle infotainment systems, GPS displays, headlights, rearview mirrors, and dashboards.
The AMIS-74980x is an analog/digital output device that offers excellent performance over a supply voltage range of 3.0V to 3.6V , an operating frequency of 10–100kHz, and a temperature range of 0°C to 70°C. They operate well under a variety of light sources, including natural light, fluorescent light, conventional incandescent lamps, and halogen lamps. Furthermore, a key feature is their very low dark current, which enables display controllers to adjust display brightness in low-light environments. This series of sensors also offers a variety of output options, programmability, and low power consumption.
Similarly, OSRAM Opto Semiconductors Inc. has introduced what is claimed to be the first ambient light sensor for mobile products that precisely matches or simulates the sensitivity curve of the human eye, allowing for more accurate adjustments to the display and brightness levels of mobile products. The company developed "human eye" characteristics for this sensor, model ALSSFH5711, by using a new material system instead of traditional silicon. A logarithmic amplifier integrated circuit (IC) enables it to detect a wider range of illuminance (from 3 lux to over 30,000 lux) with greater accuracy, and eliminates the need for various series resistors, allowing for smaller and more accurate product designs.
The ALSSFH5711 is also an enhanced ChipLED version of Osram's popular SFH3411. It utilizes a very small and robust surface mount technology (SMT), saving costs for consumer applications while providing excellent optical performance. The ALSSFH5711 has a spectral sensitivity of 560 nanometers (nm), simulating the region most sensitive to the human eye. The ALS's V (Lambda) characteristic represents the wavelength spectrum of colors seen by the human eye. By more quickly and automatically simulating human eye characteristics, ALS is suitable for adjusting the brightness of displays and components to create optimal reading conditions in frequently changing lighting environments.
Employing an ultra-low voltage of 2.3 ~5V, this sensor offers power efficiency and is suitable for natural light sensor applications, including mobile phones, digital cameras, laptops, PDAs, and handheld GPS devices. They can also be used in automotive applications to control headlight switching based on ambient light.
Avago Technologies recently announced two new ambient light sensor products using chip LED packages. The APDS-9005 (6-pin) and APDS-9006 (4-pin reverse mounting) are analog current output ambient light sensors (ALPS) in miniaturized, environmentally friendly, lead-free chip LED packages. With a peak spectral response of 500nm, the visual intelligence provided by ALPS effectively simulates the response range of the human eye by excluding ultraviolet and infrared light. The APDS-9005 supports a power supply from 1.8V to 5.5V , while the APDS-9006 supports 2.4V to 5.5V. Both sensors can operate within a temperature range of -40°C to +85°C. Furthermore, the more compact arrangement of the devices results in greater reliability and higher performance.
The APDS-9005 and APDS-9006 ambient light sensors are designed to closely mimic the human eye's response to spectral changes, significantly reducing power consumption and extending battery life. These new sensors are suitable for backlight control applications in mobile products and commercial LCD displays, such as portable devices, laptops, lighting management systems, and automotive interiors—applications that consume large amounts of current. They can also be used to turn indoor and outdoor lighting, streetlights, electronic signs, and traffic lights on or off. These sensors perform equally well under various light sources, including natural sunlight, fluorescence, and traditional incandescent and halogen lamps.
Capella Microsystems states that its Filtron series of ambient light sensors effectively eliminates excessive backlight in portable devices such as laptops, mobile phones, DVD players, PDAs, MP3 players, iPods, and GPS devices, thereby extending the battery life of these mobile products. These chips can detect ambient light levels and adjust LCD brightness to meet the requirements of the human eye.
The CM3200HS (high sensitivity), CM3200, and CM3000 photoluminescence sensors in this series consist of an optical filter, a photodiode, a digital filter, and a 9-bit D/A converter (capable of producing an analog output that varies depending on ambient light intensity). The CM3200HS operates in an ambient light range of 0.5 Lux to 350 Lux, while the CM3200 operates in an ambient light range of 10 Lux to 10,000 Lux. The CM3000 uses an external resistor to set the threshold of an external comparator, which generates an on/off output signal for the desired ambient light range. This series of photoluminescence sensors typically consumes 80 microamps, less than 1 nA in the off state, and operates on a voltage range of 2.7–5.5 V.
Lingyao Technology states that backlighting typically consumes more battery power than any other function. The newly launched CM3200HS, CM3200, and CM3000 can increase laptop battery life by up to 41% and mobile phone battery life by up to 100%. In addition to portable electronic products, the Filtron series can also be used to control the brightness of plasma and LCD TV screens and automotive dashboards.
Because these chips closely mimic the brightness requirements of the human eye, they also reduce eye strain, and the built-in digital filters eliminate the flicker of fluorescent lights in the environment. The company claims that, thanks to its proprietary Filtron light filter technology, these chips are the industry's best at simulating human eye conditions. Filtron detects wavelengths only in the 450-650nm range. Furthermore, this light filtering technology is reportedly the first to be manufactured using semiconductor processes, eliminating the additional step of rotating plastic filters onto photodiodes.
These chips are manufactured using mixed-signal technology and contain approximately 7,000 transistors. The CM3200HS and CM3200 are available in CMP packages (3×4mm) or OPLGA packages (1.8 × 2.35 ×1mm). For CMP packages, both types of devices are priced at $ 0.60 per 1,000 units. The CM3000 is also available in both packages; for 1,000 units, the CMP package price is $ 0.55 .
In addition, Vishay Intertechnology's ambient light sensors, TEMT6200F, TEPT5700, and TEPT4400, are designed for advanced automotive functions. These devices enable automatic brightness control for liquid crystal displays (LCDs) and support features that enhance driver comfort and safety. Vishay offers designers three package options for these devices: a surface-mount 0805 (TEMT6200F), a 5mm flat-top package with leads (TEPT5700), or a 3mm flat-top package (TEPT4400). In automotive applications, these sensors will enable automatic headlight control and allow tunnel sensors and displays to automatically adjust to changes in ambient light. These sensors offer simple two-pin connections and will be used to reduce power consumption in consumer electronics applications by controlling keyboard backlighting and adjusting LCD brightness. Furthermore, these sensors are environmentally friendly alternatives to cadmium-based photoresistors.
