Typical applications of RGB sensors

Since the physical structure design is very important for many of these applications, the RGB ambient light sensor needs to be small and accurate. If the device side length is less than 2 mm, it can be easily packaged near the image sensor or mounted in an inconspicuous position in the display glass panel frame without reducing the effective display area.

The sensor also needs to be able to be flexibly mounted in many different situations, such as behind a specialized dark glass used in infrared (IR) remote control ports on televisions or proximity sensors on mobile devices that sense infrared signals from a user's touch function. Unlike standard glass, this material effectively transmits infrared signals at greater distances than most other forms of glass. IR filtering or compensation can prevent strong IR signals from affecting visible light measurements.

One approach is to put an IR filter on the RGB sensor itself, but this adds cost and complexity. Another approach integrates not only the visible light photodiode but also the IR sensor on the device, allowing the processing electronics to perform digital compensation. A major advantage of this approach over IR filtering is that it adds a level of intelligence to the light detection. Different types of visible light sources produce very different amounts of IR light. For example, incandescent light sources generate a lot of IR light, while fluorescent light produces very little IR light and is generally lower than the IR light found in natural sunlight. By interpreting this data, software can compensate for the image based on different lighting conditions.

Because infrared glass filters out most visible light, a highly sensitive RGB photodiode array is required to ensure accurate color response for sensors used in these conditions. The sensor needs to be able to measure color in dark environments with illumination as low as 0.005 lux (such as when working under infrared glass), but it also needs to support applications with very high glass transmittance and illumination up to 10,000 lux. For ease of use, the sensor should be directly connected to the core processor of the device and continuously adjust the brightness based on the current lighting conditions to maintain consistent color and ideal brightness of the display.

Accuracy and repeatability are critical to ensure a consistent user experience without time-consuming calibration during production. Deviations of less than 10% are sufficient to ensure effective color matching of the environment. Because many RGB sensors are used in mobile devices, low power consumption is essential. Not only is it necessary to reduce the operating current of the analog front end, but it is also important that the sensor electronics can enter a low-power standby mode when not in use. Since lighting conditions are unlikely to change rapidly, taking several readings in a short period of time is an effective strategy for using such devices, such as when the LED photodiode of a proximity sensor flickers. Once the required readings are taken, the sensor can be powered down and wait until the next cycle in a few seconds. This strategy can reduce the energy consumption of the sensor subsystem by several orders of magnitude.

Being able to validate sensor performance using software tools is also important. By showing how closely ambient light readings match the industry standard CIE 1931 chromaticity diagram, developers can more easily assess the effect of a particular piece of glass on the readings and adjust the color compensation needed for the final product. The software can also show how close the IC sensor's accuracy can be to that of a dedicated handheld colorimeter.

The Intersil ISL29125 is an RGB sensor that meets the needs of this rapidly growing market. This sensor can operate at low power consumption in power-down mode. It uses a standard I2C interface to transmit data to the host. The ISL29125 also provides a wide dynamic range of 0.005lux to 10,000lux, allowing designers to apply it to a wide range of working environments and applications. In power-down mode, the Intersil RGB sensor consumes less than 0.5μA; in operating mode, its current consumption is about 85μA.

In addition to high accuracy and excellent performance, the ISL29125 sensor is the smallest 6-pin RGB sensor in the industry. Only a very small aperture is required in the device design to obtain enough ambient light to accurately measure color temperature, which greatly improves industrial design. The evaluation board supports software to decode the display readings according to the CIE chromaticity diagram to assist in development.

Through integration and efficient IC design, the new generation of RGB sensors can significantly improve the user interface reading experience in consumer, automotive and industrial applications.