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Osram modifies its ambient light sensor to achieve outstanding signal quality

The new SFH 2440 photodiode from Osram Opto Semiconductors combines short switching times with a spectral sensitivity optimized for visible light. The component enables high-quality heart rate measurements to be made with the aid of green LEDs. Typical applications include wearables such as fitness armbands and smartwatches. With its greatly reduced sensitivity to infrared light, SFH 2440 is also ideal as an ambient light sensor.

SFH 2440 is a large-format photodiode which is particularly good at detecting visible light while filtering out virtually all infrared light. This makes it an ideal detector for optical pulse meters because these measure the absorption of visible light in blood. It works by shining light from LEDs on the surface of the skin. Different amounts of this light are absorbed by blood and the surrounding tissue. The light not absorbed is reflected to the detector. The amount of light at the receiver depends on the volume of blood through which the light passes. The changes in the signal over time indicate the heart rate. However, ambient light that penetrates the skin and is diffused by the tissue also hits the detector. This effect is particularly strong with infrared wavelengths. Thanks to its spectral sensitivity, SFH 2440 is highly effective in suppressing this interference and therefore offers an exceptionally good signal-to-noise ratio.

A modified ambient light sensor offers optimum spectral sensitivity To achieve this property, Osram’s developers modified the SFH 2430 ambient light sensor, a large-format photodiode. Ambient light sensors are photodetectors with a spectral sensitivity that corresponds to that of the human eye. In other words, it mainly registers visible light. For measuring pulse rates the switching time of the component had to be drastically reduced so that the light signal modulated with the heart rate could be time-resolved. SFH 2440 has a capacitance of only 135 picofarads and therefore achieves rise and fall times of 1.3 microseconds. Osram also improved the filter for infrared wavelengths, with the result that the relative sensitivity in this spectral range could be reduced to five percent. In addition, the developers doubled the sensitivity to green light, resulting in much higher signals for SFH 2440 at the same operating current for the LEDs. Alternatively, designers can use the new photodiode to create sensors that consume less power for the same signal quality. “SFH 2440 enables optical sensors to be produced for measuring heart rates with optimum signal quality and excellent precision”, said Chris Goeltner, Senior Manager Product Marketing at Osram Opto Semiconductors. “The component is a high-quality product that fully meets customer requirements even in demanding applications.”

Excellent infrared suppression for optimum brightness measurements

SFH 2440 is also ideal for measuring ambient brightness, for example for adjusting display backlighting. Designs in which the ambient light sensor is mounted under dark housing covers benefit in particular from their excellent spectral sensitivity. The dark covers block visible light more than infrared wavelengths and therefore worsen the ratio between the useful signal and the interference signal. SFH 2440 can greatly offset this effect because not only does it filter out a large proportion of the infrared light but also offers extremely high sensitivity for green wavelengths. The geometry of the package and the chip remains unchanged from the predecessor version, the SFH 2430. With a photo-sensitive area of just over seven square millimeters, SFH 2440 offers a sensitivity of 0.37 A/W for green light (550 nm). The SFH 2440 is already available on request for first customer projects. Volume start-up is planned for mid of 2016.

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