Osram introduces new fitness photodiode
Osram Opto Semiconductors has introduced the Topled D5140 photodiode for monitoring fitness levels, for example in fitness armbands. The device is also suitable as an ambient light sensor.
An updated version of the SFH 2440 photodiode, the D5140 has a spectral sensitivity optimised for visible light and allows precise heart rate measurements using red or green light. The device allows customers to design more compact sensors, preserving the high signal quality offered by the SFH 2440, according to Osram
A pre-molded package soldered to the bottom of the package forms the basis of the new component. Previously offered packages had to be soldered to side legs. Because these legs are no longer there in the new design, the Topled D5140 is around 1.4 mm shorter, now measuring 5.1 millimeters (mm).
The electrical and optical properties of the new photodiode are identical to those of its counterpart, the SFH 2440. Along with the short switching time of typically 90 nanoseconds, the spectral sensitivity is optimised for visible light, with a maximum sensitivity of 620 nanometers. However, in the infrared spectral range the sensitivity of the two photodiodes is greatly suppressed.
The Topled D5140 is said to offer a high signal-to-noise-ratio because it registers the reflected light particularly well while at the same time suppressing the infrared light. Thanks to its short switching time, the light signal modulated with the heart rate can be time-resolved well.
"It works by shining visible light on the surface of the skin, some of which is absorbed or reflected to the detector. As arterial blood absorbs more light than the surrounding tissue, the strength of the detector signal changes with the volume of blood through which the light passes. The periodicity of the signal indicates the heart rate, " explained Chris Goeltner, expert for infrared products at Osram Opto Semiconductors.
"However, infrared light which shines onto the measuring point from the surroundings and disperses in the body also reaches the photodiode. In practice, the sensor consisting of juxtaposed light source and detector is located directly on the skin, usually on the wrist or fingers. Due to the location the measurement is made at different wavelengths - green light has established itself as the best option for the wrist, red and infrared light for the finger."
The global wearables market is estimated to be worth $30bn in 2016, and expected to grow to $100bn by 2023, according to analysts IDtechEx. Opto semiconductor technology is core to many applications within this sector, from infrared light for precise heart rate measurements to iris scans, Biofy sensors, finger print recognition and also the visible light e.g. display lighting.
