FEATURE OPTOELECTRONICS AMBIENT LIGHT SENSORS IN THE
WORLD OF WEARABLES Dr. Christian Merfort business development of infrared & optocouplers at Everlight Electronics Europe explores how ambient light sensors are being used to convert light into energy for wearable devices
T
he development of ambient light sensors in the area of wearables has
created new opportunities for health analysis with regard to rapid detection and mobile data tracking. Latest developments, especially in the area of optical detectors, and constant improvements to coatings that carry out the filtering of the ambient light equivalent to the luminous efficiency curve of the human eye, have had an enormous influence on health analysis methods. Ongoing photodiode optimisation in
terms of chip size and spectral sensitivity is leading to a vast quantity of new applications including IoT, which includes self-sufficient solar based power supplies and, due to the simplification and miniaturisation of measuring systems, the independence of specialised lab environments. Ambient light sensors (ALS) are
optoelectronic devices, allowing the direct physical conversion of light into electrical energy, the photo current. Common methods of health analysis like heart beat detection, pulse or blood pressure measurement are based on the principle of the photoelectric effect – the electrical behaviour is determined by the energy provided by the irradiated photons. ALS are based on a photodiode (PD),
which consist of different layers of doped silicon. A filter coating at the top layer limits the spectral sensitivity to the human eye’s response. The coating design reduces the interference from UV light below 400nm and infrared light above 650nm to ensure accurate measurements. The PD’ operating point is carried out
under reverse voltage conditions, meaning the generated charge carriers can move through the layers of the PD only once. The benefit is a very low dark current, leading to a higher dynamic bandwidth in terms of bright/dark sensitivity. Combined with a large sensing area, at low light conditions the sensor’s signal calculation failure rate is reduced, the current efficiency increases.
DIFFERENT TYPES OF AMBIENT LIGHT SENSORS – AN OVERVIEW Everlight’s ALS portfolio can be subdivided into three classes. Photodiodes (PD), Photodiodes with integrated amplifiers (PDIC) and Phototransistors (PT).
22 NOVEMBER 2016 | ELECTRONICS
light bulb, to white fluorescent light at a colour temperature of 6500K. Compared to a PD, the light current at EV=100Lux is 36 times higher. In addition to the integrated amplifier, this higher light current is also due to the shift of maximum peak sensitivity to shorter wavelengths. Another member of the PDIC group is
A representative of the first class, the
surface ALS-PD70-01C, is moulded in a water clear miniature SMD package with a flat top. The light current at EV=100Lux is IL=1.1uA, the peak sensitivity wavelength λp = 630nm. A preferred application of PDs with a wide operating temperature performance ranging from -40°C to 85°C is power saving of display backlighting of mobile appliances. The second, photodiodes with integrated
amplifiers, are the company’s largest group. More than ten devices with analogue output of the light current or advanced I²C digital output are
available.The ALS-PDIC17- 77C/TR8 features output linearity across a wide range of spectral power density and low sensitivity for various light sources. This ensures a high degree of independence from light sources starting from CIE standard illumination type A (2856K), well known as a
Figure 1:
Everlight’s ALS-PD50- 42C series is the ideal component for heart rate measurements in the wearable technology fitness market
the ALS-DPIC17-78C. The main difference is the advanced digital output type with I²C protocol interface compatible to the SMBus with 20bits effective resolution. This sensor converts light intensity to digital data with a high resolution of 0.006Lux/count, allowing a high dynamic sensing range from 0 Lux up to 1,000,000Lux. This is realised by a second, light shielded photodiode. Its dark noise is subtracted from the signal of the opened PD. A 50Hz/60Hz flicker rejection supplements the increase in dynamic range. The third group, based on a
phototransistor, is represented by the ALS-PT17-51C. This low cost ALS provides a good linearity across a wide illumination range. Due to the peak wavelength of λp = 630nm this component shows a strong dependence between light current and illuminance with different light sources like fluorescent light or incandescent lamp. Due to an increasing degree of health-
Figure 2: Overview of Everlight’s ALS portfolio
awareness, more and more people manage their wellbeing and progress through their own daily activities. Advanced technology plays an important role in the growing fitness market. Advanced sensors are among the most innovative and convenient ways to measure one’s heart rate. Based on the principle of PPG (photoplethysmography), the heart rate signal is calculated according to the photo current changes in transmission and reflection between the green light LED and the ALS to detect the systolic and diastolic blood vessels. Close to the human skin, the green light
penetrates the epidermis and is reflected by the blood vessels. Due to the effect that their filling rate is not constant, the strength of reflective light differs, causing specific photo current values. Recurring maximum peaks of the generated photo current indicate the heartbeat.
Everlight Electronics Europe
www.everlight.com e:
christian.merfort@
everlight-eu.de
/ ELECTRONICS
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