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New Optical Sensor from ams Targets Enhanced White Balance for Mobile Device Cameras

February 26, 2019 by Robert Keim

The AS7341 can be used for optical-spectrum analysis and color matching in mobile phones and various other electronic devices.

The AS7341 can be used for optical-spectrum analysis and color matching in mobile phones and various other electronic devices.

ams describes the AS7341 as a “complete spectral sensing system” and I have to say that this description seems justified.

 

The circuit symbol for a photodiode is shown on the left, and the diagram on the right depicts the semiconductor structure.

 

Featured Resources

If you don’t have much experience or familiarity with optical sensors, AAC has quite a few resources that can help you get up to speed. If you’re interested more in the practical side of the issue, you may want to start with this project built around a color sensor IC made by ROHM.

If you’re inclined more to theoretical or introductory topics, you could read about photodiodes in the AAC textbook, laser diodes, or D-star, which is a parameter that allows us to compare different optical sensors with respect to their sensitivity.

 

Color, NIR, and Flicker

The diagram below gives you a general idea of the AS7341’s functionality and interface.

 

Diagram taken from the AS7341 product page.

 

As you know, it is possible to generate (or detect) a large number of colors using only red, green, and blue color channels. However, the folks at ams apparently wanted to surpass the performance offered by the RGB approach. The AS7341 uses 8 channels to cover the visible spectrum, as indicated by the 8 colored squares in the upper portion of the diagram. Sensitivity to a specific color is achieved by means of an optical filter.

The ninth square in the diagram is a photodiode without a filter. This channel responds to overall light intensity rather than to the intensity of light that falls within a limited range of wavelengths.

 

This gives you an idea of relative sensitivities for a sensor that incorporates multiple color channels and a clear (i.e., non-filtered) channel. Sensitivity plots for the AS7341 were not available on the part’s product page.

 

The AS7341 takes its optical detection a step further by incorporating a sensor that responds to electromagnetic radiation in the NIR (near-infrared) band. This is an interesting feature—why would an optical sensor be designed to detect wavelengths that human beings cannot see? We’ll discuss this issue in the next section.

As mentioned in the title of this article, the AS7341 is an 11-channel device. So far we have eight color detectors, one unfiltered detector, and an NIR detector. That makes 10. Number 11 is a sensor that is dedicated to flicker detection. It seems to be intended primarily for detecting the flicker associated with 50 Hz or 60 Hz AC voltage, but it is compatible with flicker frequencies up to 1 kHz.

Automatic White Balance

If you’re a photographer, you know that white balance is a fundamental aspect of producing accurate and visually satisfying images. The various types of light that illuminate our indoor and outdoor spaces have significant variations in color content.

The human eye adapts readily to these different color conditions, but image sensors do not. The term “white balance” refers to the technique of applying corrections to sensor data in such a way that the colors in the final image are more consistent with how a person would perceive the scene.

The AS7341 can be used as a component in an automatic white balance system. The idea is that the optical sensor determines the color content of the ambient light, and this information can be delivered to an algorithm that applies the appropriate corrections to the image data.

I would expect that the AS7341’s eight-channel coverage of the visible spectrum would provide excellent information on color content, but this is also where the NIR channel comes into play. One way of enhancing a white balance algorithm is to provide specific information about the source of the ambient light, and the additional data provided by an NIR sensor can facilitate light-source detection.


 


 

The AS7341 seems like a fairly sophisticated device that integrates a lot of optical and signal-processing functionality into a small package. If you decide to use this device in one of your upcoming prototypes, please leave a comment and let us know.

Featured image courtesy ams.

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