Ambient light sensors for wearables
Latest developments, especially in the area of optical detectors, and constant coating improvements to filter ambient light in accordance to the luminous efficiency curve of the human eye, have had an enormous influence on health analysis methods. Ongoing photodiode optimization in terms of chip size and spectral sensitivity is leading to numerous new applications.
Technical background
Ambient light sensors are optoelectronic devices. These devices allow the direct physical conversion of light into electrical energy, the photo current. Some of the most 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 behavior of ambient light sensors is determined by the energy provided by the irradiated photons E = h * ν. Ambient light sensors are based on a photodiode consisting of different layers of doped silicon. A filter coating at the top layer limits the spectral sensitivity to the eye’s range of responsivity. The coating design effectively reduces the interference from UV light below 400nm and infrared light above 650nm to ensure accurate measurements.
for heart rate measurements in the wearable
technology fitness market.
Source: Everlight Electronics.
The photodiodes’ operating point is carried out under reverse voltage conditions. That means the generated charge carriers can move through the layers of the photodiode only once. The benefit is a very low dark current, ID = 2 nA at VR = 5V for example, leading to a higher dynamic bandwidth in terms of bright/dark sensitivity. Combined with a large sensing area, the sensor’s signal calculation failure rate is reduced at low light conditions, the current efficiency increases.
Different types of ambient light sensors – an overview
Everlight’s portfolio of ambient light sensors (ALS) can be subdivided into three classes. Photodiodes (PD), Photodiodes with integrated amplifiers (PDIC) and Phototransistors (PT). A representative of the class based on photodiodes is the surface mounted ALS-PD70-01C, which is molded in 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 PD application is power saving of display backlighting of mobile appliances such as mobile phones and PDAs, with a wide operating temperature performance ranging from -40°C to +85°C.
The group of photodiodes with integrated amplifiers is Everlight’s largest group, with more than ten devices available with analog output of the light current or advanced I²C digital output.
A representative of this class is the ALS-PDIC17-77C/TR8 with a good output linearity across a wide range of spectral power density and low sensitivity across 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 light bulb, to white fluorescent light at a color temperature of 6500 Kelvin. Compared to a photodiode, 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 this group of photodiodes with integrated current amplifier is the ALS-PDIC17-81B. 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 0.04 Lux/count. This allows a high dynamic sensing range from 0 Lux up to 10,500 Klux. This is realized by a second, light shielded photodiode. Its dark noise is substracted from the signal of the opened photodiode. The increase in dynamic range is supplemented by a 50Hz/60Hz flicker rejection.
The third group, based on a phototransistor, is represented by the ALS-PT17-51C/L177/TR8. This low cost ambient light sensor 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.
Application
With the increasing demand for health-awareness, more and more people manage their health and progress throughout their own daily activities. Advanced technology plays an important role in the growing market for fitness and wellbeing. Currently, these advanced sensors are the most innovative and a convenient way to measure one’s heart rates. 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 sensor to detect the systolic and diastolic blood vessels.
To carry out the measurement, the setup based on a green led (peak wavelength 530nm) and an ambient light sensor has to be close to the human skin. The green light penetrates the epidermis and is reflected by the blood vessels. Due to the effect that the filling rate of the vessels with blood is not constant, the strength of reflective light is different, causing specific photo current values. As the blood flow is controlled by the heart which pumps blood through the vessels at every beat, heartbeat causes alternating filling rates of the vessels.
Recurring maximum peaks of the generated photocurrent indicate the heartbeat. The systolic values are given by the rising edge, when the heart muscle contracts and the vessel got filled up with blood. The diastolic value is given by the photo current which is measured between two heart beats, when the blood level in the vessels goes down.
Thanks to a large sensing area of up to 8.1mm2, the signal quality of Everlight’s advanced ambient light sensors proves to be good and reliable. A bigger than usual sensing area also improves accuracy in instances when there are signal interferences caused by people’s skin color, tattoos and hair on the skin. This feature allows for exact and reliable detection capabilities of wearable heart rate technology, especially a benefit for runners who need to rely on precise wearable equipment.
About the author:
Dr. Christian Merfort is in charge of Business Development, Infrared & Optocouplers at Everlight Electronics Europe GmbH – www.everlight-eu.de
