Choose the six indicators to consider when choosing a light sensor

Light sensors are ubiquitous. From the portable consumer market (smartphones, PDAs, desktop PCs, portable music players, etc.) to the consumer TV market (including LCD, plasma, rear-projection, and CRT TVs, etc.), to the medical, industrial, and automotive markets, light sensors are everywhere. Some applications, such as barcode readers, laser printers, and autofocus microscopes, use optical detection of reflected light to sense position; others, such as portable electronics such as digital cameras, mobile phones, and laptops, use light sensors to measure the amount of ambient light. System products that use ambient light sensors can provide more comfortable display quality. This is very important for some applications. For example, a car dashboard requires a clear display under all ambient light conditions. During the day, users need maximum brightness for optimal visibility, but this brightness is too harsh at night. Light sensors bring many benefits to portable applications. Systems with light sensors can automatically detect changing conditions and adjust the display settings to ensure that the display is at the optimal brightness, thereby reducing overall power consumption. For example, mobile phones, laptops, and digital cameras can automatically control the brightness of the display by using ambient light sensors, thereby extending the battery life. Photodiodes are the best implementation method . Light sensors include photoresistors, phototransistors, or photodiodes. Among them, the simplest light sensor is a photoresistor. Low-end photoresistors are made of CdS (cadmium sulfide) materials, while more expensive photoresistors are made of GaAs materials. GaAs has a small band gap. It can absorb low-energy photons in infrared light and cause electrons to jump to the conduction band. Its illumination range is from 1 lux to 100 lux. Photodiodes are more complex. Photons bombard the semiconductor junction, generating current. A reverse bias is applied to the photodiode. A larger reverse bias can increase the sensing speed and linearity of the sensor, but it also increases the dark current and shot noise of the sensor. Photons bombard the semiconductor junction, which will generate a forward current and reduce the reverse bias current. In the design, an external circuit can be added to the photodiode to linearize the IV curve. The general characteristics of a phototransistor are the same as those of a photodiode, but with the addition of an amplification function. It requires a larger bias current, but the noise associated with the current forces the sensitivity of the sensor to a higher lux range, from 1000 lux to 100,000 lux. The detection response time of a phototransistor is similar to that of a photodiode and can be adjusted using the bias current. The bias current can also vary with the detected signal level. Phototransistors can roughly determine ambient light levels, such as indoor/outdoor, day/night, and bright light/shadow, so external circuitry is required to calibrate the output signal. Currently, IC-based monolithic photodiodes are one of the best implementations of light sensors. Photodiodes are made of single crystal silicon. A typical sensor application component includes a photodiode, a current amplifier, and a passive low-pass filter. For end users, it is important to be able to integrate all of these devices into a small package. Six indicators of light sensors The most important point when choosing a light sensor is to understand which specification parameters are the most critical. Generally speaking, when choosing a light sensor, the factors that need to be considered include spectral response/IR suppression, maximum lux number, light sensitivity, integrated signal conditioning function, power consumption and package size. The specific descriptions of these six specifications are as follows: 1. Spectral response/IR suppression: The ambient light sensor should only be sensitive to the spectral range of 400nm to 700nm. 2. Maximum lux number: Most applications are 10,000 lux. 3. Light sensitivity: Depending on the type of lens of the light sensor, the light attenuation after the light passes through the lens can be between 25%-50%. Low light sensitivity is very critical (<5 lux), and a light sensor that can work in this range must be selected. 4. Integrated signal conditioning function (i.e. amplifier and ADC): Some sensors may provide very small packages, but require an external amplifier or passive components to obtain the required output signal. Light sensors with higher integration eliminate external components (ADC, amplifier, resistor, capacitor, etc.) and have more advantages. 5. Power consumption: For optical sensors that are subject to high lux (>10,000 lux), nonlinear analog output or digital output is preferred. 6. Package size: For most applications, the smaller the package, the better. The smallest package available now is approximately 2.0mm×2.1mm. The 4-pin package with a size of 1.3mm×1.5mm is the next generation package. Once the above important specifications are determined, the next question to consider is what type of output signal will help achieve the target application. For most optical sensors, the most common output is a linear analog output. Although this output is very suitable for some applications, products now offer more output options, including linear voltage output, digital output (via I2C interface), or nonlinear current or voltage output. Each output has certain advantages.