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An overview of photometry applications, trends

13 May 2013  | Robert Burnham, Antoine Lourdes Praveen Aroul, Lijoy Philipose

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Placement of the LED light source and the photo-diode, along with separation of the light and detector, differentiate these two device types. The transmissive method projects a light through the tissue and a photodiode measures the light, which makes it through to the opposite side. In the reflective method, the light source and photodiode are the same surface. The signal levels for reflective are much smaller, so the user needs to be mindful of where it's placed and how the circuit is designed.

Pulse-oximeter trends
There are several new market trends driving growth in the pulse-oximeter market. These include: home monitoring / telehealth, emerging markets, and sports and fitness. Additionally, technological advances in electronic hardware and software are fueling market growth.

Home monitoring / telehealth: Home monitoring enables the elderly to stay in their own homes rather than moving to some form of assisted living. Telehealth enables continuous monitoring and aids physicians to determine treatment and drug effectiveness. In the case of the elderly, who typically have more than half a dozen different drugs, oximeters could be used to monitor drug interactions and whether the user has taken the prescribed medications.

Emerging markets: To date, many developing countries in Asia, Africa and other regions have minimal availability of medical diagnostic equipment such as electrocardiogram (ECG), blood pressure meter, pulse oximeter, and many others. Historically this has been due to cost. However, advances in electronic technology have changed this by creating significant improvements in integration and reduction in costs. The increase in integrated analogue front-end devices has reduced time-to-market and electrical know-how needed to build some equipment. The circuits often are designed to interface directly to the sensor with minimum external components. What used to take more than a dozen chips and over six months is now done with one chip and as little as one month.

Sports and fitness: Information on the number of steps, distance, speed, power, cadence, skin conductivity and heart rate, are all parameters that are useful in the calculation of activity, calories burned, efficiency, and so on. Until recently, the only way to get continuous heart rate was with the use of a bio-potential chest strap. Heart rate monitor watches have been available for several years and provides periodic measurement. But in order for the user to be able to read their heart rate, they need to wear the heart monitor on one wrist, then either press a button or touch an electrode on the watch face using the other hand.

Monitoring the heart rate continuously can provide significantly more valuable information, such as active, resting and sleeping rates. Over time, it is possible to quantify improvements in fitness levels. Watches that use an optical method of measuring heart rate have the distinct advantage of being continuous and passive. The biggest hurdles to overcome are power, motion cancellation, and ambient light cancellation.

Motion: When a user is active, motion artifacts are introduced into the signal. This motion creates an error in the heart rate measurement. Advanced algorithms are needed to process the data prior to calculation of the heart rate. It is commonplace to use some form of multi-sensor motion chip to identify motion and compensate the heart rate measurement.

Ambient light cancellation: Normally in a clinical environment, the ambient light levels are at a more constant level. For sports and fitness, ambient light interferers are more varied. In many cases, the user can be running through woods on days with very bright sunlight. This gives a wide range of ambient light levels ranging from the shadows of the trees to direct sunlight.

Power: As with all portable battery-powered devices, battery life is very important for product success. A device that requires frequent recharging is likely to be less successful than a comparable device with a long battery life. In order to significantly reduce the power of an optical-based heart rate monitor, there are several parameters that need to be optimised. LED's are the dominant power hog for a photo-based continuous heart rate monitor. Duty cycle of the LED on time, intensity and frequency of measurements significantly impact the average LED power.

Figure 2: A block diagram of an integrated analogue front-end.

Engineering advances
Software: Along with the availability of lower-power microcontrollers, significant advancements have been made in the development of physiological algorithms that can be used to calculate physical parameters. Example: using the ECG and pulse-oximeter information, along with other sensor information, it is possible determine the user's energy level and warn when he/she is approaching exhaustion.

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