Ambient Light Sensor is a component that is commonly used in electronic mobile devices such as mobile phones, tablet computers, and notebook computers. Its function is mainly used to detect the illumination of the environment. When the indoor lights are dimmed, the pupils of the eyes will dilate. At this time, if the brightness of the electronic product screen is too high, the human eye will feel uncomfortable. The ambient light sensor can detect the ambient illuminance, and then adjust the screen brightness of electronic products to the brightness acceptable to human eyes, which greatly improves the user experience. This component not only saves energy, but also extends the battery life of electronic products.
The technological level of modern ambient light sensor components has reached the highest spectral sensitivity of the human eye. Ambient light sensors mainly include four categories: CdS photoresistor, phototransistor, photodiode, and photoelectric IC with integrated amplifier circuit. Its advantages and disadvantages are illustrated as follows:
Figure 1 Classification and advantages/disadvantages of ambient light sensors
As mentioned above, ambient light sensors have been widely used in smartphones, tablets or laptops. When the ambient light sensor is integrated with these electronic products, it is necessary to ensure that the function of the ambient light sensor can operate normally and will not be affected by the electronic product itself including its structural design, for example, the mechanical shading or interference from screen self-illumination. In addition, for the ambient light sensor element itself, there are many important performance parameters that need to be tested:
Generally include: Angular Response, Dynamic Range, Linearity, Field of View (FOV), and the influence of different ambient color temperature.
This refers to the degree of response of the ambient light sensor device to different angles of incident light, the result of which can be presented as an angular curve. Taking A brand’s ambient light sensor as an example, its angular curve is as follows:
Figure 2 The angular curve of A Brand’s Ambient Light Sensor
Simply put, the angular curve is usually used to evaluate the characteristic parameter of the photoelectric sensor; the light distribution curve is used to evaluate the characteristics of the light source. The meaning of the angular curve of the ambient light sensor has already been described in 2.1 above, so it is not repeated here. As for the meaning of the light distribution curve, it refers to the illumination intensity of the light source at different angles. Let’s take C Brand’s light-emitting diode (LED) as an example. Its light distribution curve is presented as follows in Cartesian coordinates:
Figure 3 The light distribution curve of C Brand LED
The human eye will adjust the pupil size according to the bright or dim environment, so that people can ensure clear vision no matter day and night. The ambient light sensor has a similar sensitivity to the human eye to brightness. The performance of ALS’s sensitivity is indicated by the parameter of “dynamic range”.
Under the hot sun, the illuminance is about 100,000 Lux (Lux), and when entering a dark room, the illuminance may not even be less than 1 Lux (Lux). The larger the dynamic range of the ambient light sensor, the wider the applicable ambient brightness range. Take Maxim’s ambient light sensor device, the MAX44009, as an example, which advertises a dynamic range of 0.045 Lux to 188,000 Lux.
With the change of the light and dark of the environment, the ambient light sensor will output the corresponding signal through its photoelectric conversion mechanism. When we use a stable and precise standard light source to illuminate the ambient light sensor, the intensity of the light source is controlled by the program, and its output light intensity changes linearly from 0.1 Lux to 100,000 Lux. At this time, we simultaneously measure the output signal of the ambient light sensor to check whether its signal strength also exhibits the same linear change. This is the linearity characteristic parameter of the ambient light sensor, as shown in Figure 4
Figure 4 The Linearity of T Brand ALS
The value of this characteristic parameter FOV is the angle, which can be expressed in two ways: FFOV (Full Field of View) and HFOV (Half Field of View). According to the definition in Figure 5, FOV can be written as:
FOV = ±θ1/2 or FOV = θ
Figure 5 Field of View (FOV) of Ambient Light Sensor
(Reference: Spec. sheet of T Brand)
When faced with various application scenarios, in order for the ambient light sensor device to function normally, we must first ensure that the ALS can effectively detect light and avoid improper external shading that affects the function. This characteristic parameter of FOV can clearly express the light-receiving path required by the ambient light sensor, so that product designers can follow and produce good design.
We can fix the ambient light sensor to be measured at the axis, and fix a standard collimated light source on a cantilever that can rotate according to the axis (as shown in Figure 6). The angle perpendicular to the light-receiving surface of the ambient light sensor is defined as 0 degrees, and the angles to the left & right parallel to the light-receiving surface of the ambient light sensor are +90 degrees & -90 degrees respectively. Trigger the cantilever, so that the position of the standard collimated light source starts from -90 degrees, and continuously move to +90 degrees. During the process, the signal of the ambient light sensor is measured in real time. In this way, we can obtain the signal output by the standard light source irradiating the object to be measured at an equal distance of 180 degrees. By marking the signal value on the rectangular coordinate diagram, the angular response data of the ambient light sensor can be obtained, as shown in Figure 2 above.
Figure 6 The internal structure of Enlitech’s ALS angle response measuring instrument
The test method is the same as that in 3.1. The FOV (Field of View) can be obtained by calculating the angle vs. signal strength data through trigonometric functions.
We change the intensity of the standard light source linearly, and measure the signal intensity output by the ambient light sensor in real time under the illumination of the standard light source, so as to know the linearity of the ambient light sensor.
When choosing a measuring instrument, we will first consider whether the specifications of the instrument can meet the measurement needs, including: accuracy, resolution, reproducibility, reproducibility, convenience of operation, etc. In addition, the price and maintenance of equipment, and even the replacement of consumables, will also be the focus of most people’s assessment.
After querying Baidu Encyclopedia, we have sorted out 10 standard color temperatures as follows:
|International Standard Artificial Daylight
|Simulates the average northern sunlight
|Simulate European store lighting
|Simulate American store lighting
|Simulate another American store lighting
|Simulates specified store lighting
|Simulation of warm lighting in a family or hote
|Simulation showroom spotlights
|Simulate horizontal sunlight
The standard spectrum most commonly used for ambient light sensor calibration is shown in the table above. These standard spectrum and color temperature represent the ambient light state of most of human daily life. The ambient light sensor is calibrated according to these standard spectrum and color temperature, so that it can be close to the actual use situation on the application side and provide users with a better actual experience.
In a dark room, the illuminance of ambient light can be less than 1 Lux; when the weather is sunny outdoors, the ambient illuminance under direct sunlight is about 100,000 Lux. We can image such a situation: people use smartphones to look for recently released movies under the bright sun, then walk into the cinema. Before the movie start, people use their mobile phones to browse social messages. This is exactly what happens in our daily life, which also means that the ambient illuminance range from 100,000 Lux to less than 1 Lux should be the proper dynamic range characteristic that an ambient light sensor needs to have.
The disadvantage of collimated light is that it is not easy to design. It is even more difficult to have a light source with high collimation and spatial uniformity at the same time.
However, using collimated light as the light source for an ambient light sensor tester has the following advantages:
Illuminance is the luminous flux received per unit area; it can be understood as how much light is obtained per unit area. The SI unit of illuminance is lux (lx=lux) or radiance (ph=phot), 1 lux = 1 lumen/square meter, 1 ph = 1 lumen/square centimeter, 1 ph = 10,000 lux.
The magnitude of the illuminance depends on the luminous intensity of the light source and the distance between the object and the light source. For the same light source, when the distance of the light source is twice the original, the illuminance is reduced to a quarter of the original, showing an inverse square relationship.
Several common ambient illuminances:
|Baseball field at night
|Under the street light
This is for the low light environment (eg: <5 Lux), we hope that the ambient light sensor can still maintain sufficient sensitivity characteristics, so the test resolution of 0.5 Lux is required.
More about Ambient Light Sensor manufacturers and Ambient Light Sensor industry structure, etc., will be introduced in more detail in the second article of this series, so stay tuned.