Welcome to TI Precision Labs series on light sensing. My name is Rahland Gordon. And I am an applications engineer for Texas Instruments Optical Sensors. This video will discuss the use of light sensors in applications that need to determine whether it is day or night.

A frequent use case for applications is to be able to tell when a light threshold is crossed. This is often used to determine whether it is day time with a high lux value of light in the environment or nighttime, where there is a minimum amount of light in the environment. Applications with this use case change operation based on the amount of light in the environment. With an ambient light sensor, you are able to sense the amount of light in the environment and alert the application when it is time to change operation.

With many applications, such as video doorbells and surveillance cameras, when the surrounding environment is dark, there is a need to turn on IR LEDs so that the camera can still record the environment. The IR LEDs, while invisible to the human eye, illuminate the environment for night mode sensing. Other applications, such as street lamps and landscape lighting, turn on visible light sources when the surrounding environment is dark, most often for safety or aesthetic appeal. In all these cases, light sensors are typically used in order to determine the amount of light in the environment to alert the application to when a change in operation is needed.

Unlike brightness control, where the sensor reading is considered over a range of light intensities, in day versus night detection, a threshold light level is set. Above the threshold is considered day, and below the threshold is considered night. Based on this determination, an action is taken, such as turning a headlight or an IR illuminator on or off.

Similar to brightness control, the lux accuracy of the sensor is important in making an accurate determination. Therefore, human eye matching is an important sensor specification. This allows similar performance under different light source types. For example, taking the correct action in both clear and cloudy days.

As mentioned in the next chapter of this series, many cameras, such as security cameras and video doorbells, use IR LEDs to illuminate the scene under dark conditions. And a light sensor is used to turn the LEDs on and off. As covered previously, the human eye response drops to zero after 700 nanometers, where visible light ends and near IR begins. As a result, sensors with good human eye matching will reject most IR light.

However, when IR illuminators are used in the product, the intensity of IR light can be orders of magnitude higher than the visible light. Therefore, the sensor IR rejection at the wavelength of the IR illuminator should be considered. Common wavelengths for IR LEDs are 850 and 940 nanometers. The figures of merit video in the next chapter of this series covers the IR rejection specifications in more detail.

IR rejection across angles can also be important. For example, in IP network cameras, the IR LEDs are commonly placed in a ring around the camera as shown. While commonly placed behind a glass, the IR light may reflect off the glass and enter the light sensor at all angles. This means that the sensor should reject IR light coming from all angles, and not just from zero degrees, as typically spec. TI's OPT3004 and OPT3005 spec IR rejection over a range of angles of incidence for this reason. OPT3005 is specifically designed for this type of application.

In some applications, such as video doorbells, the product is battery powered and expected to last for long periods of time. In these cases, using a light sensor with very low power draw, such as TI's OPT3005, will minimize the effect of the sensor on the overall system power draw.

Another important consideration in low power applications is how often the microprocessor needs to read from the sensor. In light sensors without an interrupt functionality, the microprocessor must continually read from the sensor and compare it to the threshold. Some light sensors, such as TI's light sensor portfolio, support integrated comparisons with programmable thresholds, different types of interrupt modes, and support an interrupt PIN. These features allow the microprocessor to sleep and only wake up to action when the light levels crosses the threshold between day and night, greatly reducing the MCU active time.

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Please try the following quiz. Question one, which of the following help conserve power for video surveillance applications? Select all that apply.

A low active current light sensor allows for operation with minimal current draw, thus conserving power. An interrupt capability allows the microprocessor to sleep and only wake up to action when the light level crosses the threshold between day and night, greatly reducing the MCU active time, thus conserving power. And controlling the IR illuminators with a light sensor allows the light sensor to turn on the IR illuminators only when the environment is dark, thus conserving power.

Question two, what would happen if the light sensor has poor IR rejection for a video surveillance application?

If the light sensor has poor IR rejection, when the environment gets dark, the light sensor will indicate to turn on the IR LEDs. With the IR LEDs illuminated and a light sensor with poor IR rejection, the sensor will read a high value of lux, thus indicating to turn on the IR LEDs, starting a repeating loop of switching between on and off during the night.