How to select an ambient light sensor

Source: Internet
Author: User
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With the development of semiconductor analog sensors and encapsulation in recent years, end users now have a wider range of options for optical sensors. Today, designers use more light sensors than ever before in consumer products, automotive, medical, and industrial applications. There are several main reasons for this, including improved adaptive response, small footprint, low power consumption, high integration and ease of use. You can select a specific type based on the functionality, performance, and environmental functionality required by the designer and application.

There are several key technical factors that can help users and designers decide how to choose one.Ambient Light Sensor. First, the output of the sensor must be linearly related to the light intensity, and the spectral wavelength sensitivity should be very close to that of the human eye. In addition, the output of the device should be directly proportional to the light intensity of the radiation on the integrated photosensitive diode. The peak response of 540nm is close to the peak sensitivity of the human eye. Most optical sensors can sense micron nm ~ 770nm ambient light.

Currently, a design starting point for most applications is to prolong the battery life. New Energy-saving approaches include ensuring optimized brightness management, along with improved photosensitive resistance and/or photosensitive diodes, which have worse linearity and usually require additional amplifiers.

There are now many different types of ambient light sensors that offer these benefits, including analog and digital output sensors, ultra-low-power versions, non-linear versions, and next-generation low-power, ultra-low-brightness versions. Currently, the light sensitivity of the best digital products can be as low as 0.015lux, the dynamic range is as high as 64,000 lux, and the required operating current is less than 65mA. These products also provide ideal spectral response and Low-error light output changes, and simplify the use of mathematical algorithms. Some new digital optical sensors have emerged in the market. From the current perspective, digital optical sensors are a good solution to performance and flexibility problems, especially in automotive applications, because I2C digital output signals used in automobiles provide lower noise, they can networking several sensors on the same bus to control the characteristics of the sensors well and have good overall performance.

As a world leader in technology, intersil is constantly expanding its optical sensor family and providing various types of products. Intersil continuously enhances and improves the isl29000 series products with low power consumption and provides the best performance for all applications. Recently, intersil released the ALS technology using proximity sensing.

 

Light Source

Different Light sources have different spectral characteristics. users need to understand these characteristics when selecting products. For example, the spectral response of sunlight is very wide, and about 50% of the spectrum falls in the infrared range. Filament-based light sources, such as incandescent and halogen lamps, also have high infrared radiation.

For example, spectrum response is used as a criterion for selecting an appropriate optical sensor. Ordinary pin photosensitive diodes (either passive or active) have a wide spectral response interval from UV to IR ). If the goal is to design an ambient light sensor that only detects visible light, these devices are not applicable, not to mention infrared and UV devices. Therefore, only visible light (380nm ~ Nm), and can reduce the redundant infrared and ultraviolet signal light sensor is the best choice, such as isl29020 and isl29023.

Figure 1 below shows how to use isl29023 to measure different light sources. It is worth noting that the error between the measurement curve and the ideal reading line (dotted line) is very small within the wide dynamic range. If we want to use a sensor that is particularly sensitive to infrared rays, the figure above will show that there is a larger deviation between the measurement curve and the ideal reading line. (Insert Graph 1)

The main point is that, even if not all, at least most applications require optical sensors, that is, to accurately measure the visible light that the human eye can see, and to weaken the light with a large amount of infrared and ultraviolet components.

 Optical sensor application-FirstPCMachines, telephones,PDAAnd new cars

The following are the basic information about optical sensors in different markets and applications. From smartphones, PDAs, laptops, and portable music players to similar products, optical sensors are visible in the portable consumer market. Optical sensors are also widely used in the consumer TV market (TFT-LCD, plasma, rear projection and crt TV), as well as medical and industrial applications. Now, manufacturers are developing a new generation of systems for the automotive market and are already in use.

This article gives a panoramic introduction to the design problems and the utility of sensors around the working environment of automobiles. The main applications of optical sensors are as follows:

Backlight Control for information entertainment/navigation/DVD systems to display ideal brightness in various ambient light conditions

Backend entertainment Display Backlight Control

Backlight of a combined dashboard instrument (speedometer, speedometer, etc)

Automatic rearview mirror dimming (usually requires two sensors, one facing forward and the other Facing backward)

Automatic head light and rainfall detection (for specific applications, change as needed)

Rear-view camera control (for specific applications, changes based on user needs)

Because cars need to have perfect backlight lighting in various ambient light conditions, optical sensors have become one of the most effective solutions to achieve more comfortable display quality thanks to sensing functions similar to human eyes, meets the safety and comfort standards of automobiles.

For example, during the day, you need to add the brightness to the maximum to achieve the best visual effect. However, this brightness is too bright in the evening, so an optical sensor with effective spectral response (good IR weakening), dynamic range and overall output signal conditioning can easily adapt to this application. You can set several thresholds, such as low, medium, or bright, or allow the sensor to dynamically change the brightness of the backlight.

The same control method is also used in automotive rearview mirror dimming. When the surrounding environment is dimmed or bright in the rearview mirror, it is no longer appropriate to use intelligent dimming management.

For portable applications, a typical display consumes the same amount of energy until the user changes the system settings, which is usually brightness control. In very bright places such as outdoors, the user will increase the brightness of the display, which will increase the power consumption of the system. When the environment changes, most users do not change the settings if they enter the building, resulting in the system still in high power consumption. With optical sensors, the system can automatically detect changes in environmental conditions, adjust the display to the optimal brightness, and reduce the overall energy consumption. In general consumer applications, the use of ambient light sensor feedback for automatic brightness control can greatly prolong the battery life of smart phones, laptops, PDAs and digital cameras. This kind of control is useful because recent research shows that in a laptop, the battery power consumed by display backlights accounts for 33%.

Sensing ambient light is not a new concept, but unlike photosensitive diodes, sensor sensing of ambient light can reduce excess infrared light and ultraviolet light while sensing ambient light, moreover, these features are provided in a very small-size package while supporting stringent requirements for automotive standard AEC (Q-100), ensuring that the device is at-40 ℃ ~ + The temperature range of 105 ℃ can work reliably and meet the requirements of other standards.

To meet the temperature requirements of AEC (Q-100) Grade 2, there are still some optical problems to solve. Any light sensor, led transmitter, or receiver that changes encapsulation color at a continuous high temperature (> 85 ℃ ). This problem is not common in the system of Standard IC Encapsulation. Therefore, if a black mold compound with an extended temperature of + 125 ℃ is used to make the system more stable.

So far, all environmental light sensor applications have been applied in Vehicle Driving warehouses and have not yet been applied in engine warehouses or out-of-vehicle environments. Even with such applications, optical encapsulation is not designed for such harsh environments (+ 125 ℃ or + 150 ℃). Therefore, in the current optical encapsulation technology, these optical sensors may not be able to withstand such an environment.

When selecting a device, it depends on what features, performance, and environment functions the designer needs. With the digital output light sensor, the setting process becomes quite simple and clear.

The inherent advantages of digital output optical sensors include high noise resistance, data access through I2C bus, programming on sensors, and many other features, making these multi-functional products a choice for many engineers. The functional diagram of isl29011 (as shown in figure 2 below) shows that it is easy to design the device into any system, moreover, the system can also benefit from precise and highly programmable ambient light sensors.

 


 

In the isl29000 series, isl29011 adopts the latest and most advanced technologies. The device is an integrated environment and infrared to digital converter with a built-in infrared LED driver and I2C interface (compatible with SMBus ). The device provides ambient light sensing to achieve stable backlight/display brightness control. Infrared sensing with interrupt function can achieve close estimation. (Proximity sensing is an important feature. More details will be discussed below)

For ambient light sensing, the ADC inside the sensor is designed using the charge balance A/D conversion technology. The nominal conversion time of the ADC is 90 ms. Based on the oscillator frequency and ADC resolution ~ Adjust within 90 ms. The ADC can suppress the flickering noise of 50Hz and 60Hz produced by the artificial light source. The illumination range selection function allows you to program the illumination range to optimize the Count value per lux.

For proximity sensing, when the internal infrared LED driver is closed and enabled at the user-selected modulation frequency according to the preset time period, when the external infrared LED is driven, the ADC digitizes the output signals from the photosensitive diode array. As the proximity sensor uses a noise elimination mechanism, redundant infrared noise can be greatly reduced, and the digital output of proximity sensing decreases with the increase of distance. The output current of the driver is optional and can reach up to 100mA. It can drive different types of infrared emission LEDs.

The best ambient light sensor with proximity sensing provides six different working modes, which can be selected through the I2C interface. These modes include: One programmable als and one programmable infrared sensing for automatic power outages, one programmable proximity sensing, programmable continuous als sensing, programmable continuous infrared sensing, and programmable continuous close sensing. The programmable one-operation mode greatly reduces power consumption, because the subsequent automatic shutdown will reduce the overall power supply current to less than 0.5 μA.

The latest generation of important devices can be found in V ~ 3.63v works at the power supply voltage. Hardware and Software interruptions are maintained until the master controller clears these interruptions through the I2C interface and starts ambient light sensing and proximity detection.

Basic optical technology for ambient light sensing

Most light sources emit light that contains visible light and infrared band radiation. Based on the Lumen, different light sources may have similar visible light intensity, but the response in the infrared band is very different. The spectral characteristics of light and the spectral sensitivity of light sensors must be taken into account when measuring light intensity. A cmos optical sensor can detect most infrared radiation (peak sensitivity at 880nm), leading to false alarms in real (visible) environments.

For light sources such as bulbs, the sensor signal is much higher than the number seen by the human eye. Responses to lighting schemes controlled by these sensors may be inconsistent with environmental spectra, limiting the longest distance close to sensing. As part of the close-to-sensing system solution, to establish a more appropriate dimming or lighting control, the basic requirement is to have a sensor that can imitate the human eye, in addition, the maximum infrared signal is used. Figure 3 shows the spectral response of an optical sensor, which is ideal for ambient light sensing. Figure 3 also shows the infrared wavelength spectrum used in proximity sensing.

  

 

 

Figure 3 environmental light sensor and proximity sensor spectral response

  Importance of proximity sensing: a typical system

The basic principle of proximity sensing is: infrared LED occurs in infrared light, and infrared light will be emitted from the irradiation object. The reflected infrared light is detected by an infrared sensor, and the closeness of the object is proportional to the magnitude of the detected infrared light. Applications include proximity detectors, reflected object sensing, ambient light detection, backlight control, and lighting control.

Proximity sensing is achieved by collecting infrared signals and mathematical processing. It usually requires two components to form an optical front-end: an infrared LED and an optical sensor. An infrared LED emits an infrared signal to the sensing object. Some of the signals are reflected back and detected by an infrared CMOS optical sensor. Digital infrared signals can be sent to the microprocessor for post-processing through signal conditioning and modulus conversion on the chip for various close-to-sensing purposes.

A typical infrared proximity sensing system is composed of an optical front-end, analog hybrid signal processing circuit, and a certain mechanical structure. This paper briefly introduces the basic Optical Principle, circuit function module, mechanical design, proximity sensing algorithm and software. The design of mechanical structures is usually related to the design compromise between different application platforms, such as mobile phones, PDAs, laptops and various consumer electronic products.

Design compromise includes device selection, placement size, Lens Properties and optical design, as well as application algorithms and software implementation.

Integrated ambient light sensing and proximity sensing systems measure the ambient light environment and detect the proximity or exit of an object. In this way, the microprocessor or MCU can implement very complex control, or adjust settings to further improve the system efficiency measured by power consumption, just as in many other applications. For example, when a user moves his or her phone near his or her ears and receives a call, a mobile phone equipped with a sensor can close the screen to save battery energy during a user's call.

  Basic Optical Principle of proximity sensors

Ambient light sensors use the visible frequency of light, while proximity sensors use the UV band. Figure 4 shows that when no sensing object is located near the probe path, no reflected infrared signal is captured by the sensor. The close reading is sent back to the default baseline counter. When the sensor object appears within the measurable distance between the center of the infrared LED and the infrared sensor, the sensor is close to the sensor and captures the reflected infrared signal, as shown in Figure 5. The close-to-read relationship is linearly proportional to the signal strength of the captured infrared light, and is inversely proportional to the square of the distance.

  

 

 

Figure 4. No sensing objects in the proximity detection Area

  

 

 

Figure 5: an object with a sense of test in the proximity detection Area

The design of the optical sensor IC has evolved, and now integrated digital ambient light and proximity sensors have emerged. This new-generation advanced device provides many excellent design features, such as ambient Infrared Suppression during close-to-sensory testing, which allows the sensor to work normally in direct sunlight. Another feature is the provision of four different ambient light sensitivity ranges, which can sense both 0.015lux light intensity and 64,000 lux light intensity. The interrupt function is used to generate an alarm or monitoring function to determine whether the ambient light level or the level close to the probe level has exceeded the upper or lower limit. It also allows users to configure the interrupt duration through the digital interface.

  Typical function diagram of digital proximity sensor

Figure 6 shows the circuit function diagram of a typical digital ambient light and proximity sensor. The photosensitive diode array is a part of the optical front end for signal conditioning and acquisition. The integrated ADC converts the captured optical signals into digital data streams, and the microcontroller processes the data for different application objectives. Through the I2C interface, you can write different configuration commands and use the same digital interface to read the ambient light and close-to-distance data streams. The interrupt function is directly sent to the MCU, which controls the infrared LED driver and outputs the required forward current, so that discrete or integrated Infrared LEDs can emit infrared signals.

  

 

 

Figure 6 typical digital proximity sensor function Diagram

Another important component of the optical front-end is the infrared LED. Different Infrared LEDs have different peak wavelength, luminous intensity, and angle of view. The typical peak wavelength is nm ~ The Infrared LEDs with a high intensity of NM match the spectrum of the proximity sensor isl29011. Narrow viewing angle and higher luminous intensity can expand the distance close to the probe. When selecting an infrared LED, it is important to strike a compromise and balance between the angle of view, mechanical placeholder, luminous intensity, and power consumption.

Size and position of the glass window

For a flat surface lens, the angle of view is a function of the refractive index of plastic or glass materials. If the density of the material is higher (the refractive index is higher), the effective angle of view is smaller, so the low density material has a larger angle of view. Windows lenses have clear limitations on the perspective of optical sensors. The window lens should be placed directly on the top of the sensor, and the lens thickness should be as thin as possible to reduce the loss of light intensity.

System Algorithm for proximity sensing

After device selection and design, a stable and durable proximity sensing system also requires optical sensors for dynamic self-calibration of different sensing objects under various ambient light conditions. An excellent proximity sensing algorithm is essential to help close sensing hardware cleverly bypass the barriers from different mechanical design limitations and harsh peripheral environments, to continuously and stably detect the distance.

After a variety of design and implementation techniques are used in the proximity sensing system, we can get a good proximity sensing test system, eight. The ambient light and proximity sensing system is optimized according to the requirements of specific user applications. After balancing the above design compromise, consumers can accurately adjust all aspects of the system to meet application requirements.

  

 

 

Figure 8. Close sensing distance and LED current driving strength

 

How to select an ambient light sensor

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