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Optical pulse sensors, market situation

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Training devices (whether watches or other types of devices) are increasing their performance with each generation and have been gradually receiving novelties. At first there were only pulsometers (or pulse meters), at first with analog connectivity and later shifted to digital technology solving problems of interference with similar signals that showed incorrect data.

Later came the step meters or footpods, an accelerometer that was placed in the shoe and sent the data to the watch to help you control rhythms and distances. This accelerometer has ended up being integrated into the watches themselves, but its use is much more residual, as it is now used almost exclusively for 0-cadence data for running indoors.

The same path has been followed by GPS. Gone are the years when it was a separate accessory to the watch and was worn on the waist. Now we are all used to wearing a GPS receiver on our wrist.

The optical pulse sensor is the next revolution in training devices. It is not a new technology, in fact it is quite old and has been used in hospitals for years (these are the pulse sensors that are placed on the tip of the finger). What is new is to be able to use it in situations of high activity. This type of sensor has been around for a while, first as an external accessory to the watch with wireless connectivity, but now more and more watches are starting to integrate it into the device itself.

Operation of the optical sensors

There are different manufacturers offering their solutions, with different results. But they all base their operation on the same type of measurement, the photoplethysmography.

This technique requires the use of a sensor capable of interpreting changes in light absorption. It needs the support of a powerful light source, but with one condition, it must be green.

The reason for the choice of this colour is very simple: it is not because the inventor is Irish or because he is sponsored by Heineken, it is because the red colour absorbs the green and does not reflect it. Therefore the sensor (there are two types, infrared or electro-optical light) what it does is measure the changes in the reflected light.

Cuando hay un latido, fluye más sangre en las venas y capilares, por lo que disminuye la cantidad de luz reflejada y permite al sensor determinar tu pulso en ese preciso instante. Por tanto, la dificultad no radica en «ver el pulso», eso está más que solucionado y cualquier fabricante puede hacerlo. La complejidad se les presenta a los fabricantes a la hora de poder interpretar los datos recibidos. Es aquí donde el algoritmo usado cobra tanta importancia y donde los desarrolladores consiguen un producto que, simplemente, funciona. Por tanto, el secreto no está en el sensor, reside en el software que interpreta los datos que éste recibe.

Para que te hagas a la idea de cómo es su funcionamiento, este vídeo del programa de Antena 3 «El Hormiguero» sirve perfectamente.

This is how the sensor sees our veins and capillaries. From there it must proceed with the data recording.

Current limitations

A día de hoy los sensores de pulso presentan ciertas limitaciones. En su funcionamiento básico no existe problemática alguna, el pulso instantáneo en la inmensa mayoría de los casos se va a leer sin problema. Pero hay casos en los que no funcionarán de forma correcta. Por ejemplo estos últimos días Apple ha estado en la picota precisamente por este detalle, que en las redes sociales se ha visto reflejado con el «hashtag» #tattoogate.

Algunos usuarios de Apple Watch, aquellos con la piel muy oscura o tatuada, se han quejado de que su sensor de pulso no les estaba funcionando de forma correcta. No es un fallo de Apple, es simplemente que una piel que no permita que la luz penetre en los tejidos o interponer «una capa» de tinta es algo que no ayuda a la hora de que el sensor pueda ver la luz reflejada. El problema es más notorio con tatuajes con tinta oscura, ya que no permite que la luz «ilumine» las venas y capilares, con lo que el sensor no tiene nada que poder leer.

This only affects a small number of users, but the main shortcoming of optical sensors today is the inability to accurately measure heart rate variability (perhaps you know this best from HRV, from Heart Rate Variability), and this affects all users as it is a technical limitation. To do this you need to know the R-R intervals, which is the time between two consecutive heartbeats. It may seem that heart rate and R-R intervals are similar, but they are not.

It should be noted that R-R intervals by themselves do not provide any useful information, but used in conjunction with a mathematical algorithm is what facilitates heart rate variability.

These values are used to obtain data such as status and recovery times, training level or VO2Max estimation. But without the R-R interval values all this additional information will be impossible to know exactly. To see it graphically, you can check it in the image below, from the Marco Altini.

Comparison of RR values
Image: Marco Altini

In the first of the three graphs you can see the data obtained by an optical sensor (a Mio Alpha) together with the data from the sensor used for reference. These differences cannot be seen in either of the other two cases where the reference sensor is compared to a traditional chest mounted pulse sensor.

For the moment, this is the limitation of all pulse sensors, but it does not mean that the measurement system cannot advance further and its evolution stops here. In fact Valencell (one of the main developers) announced at CES in Las Vegas at the beginning of the year that they are working on a sensor capable of reading R-R intervals. In fact they have already developed the algorithm.

Sensors on the market

Pero antes de continuar hablando de lo que llegará en un futuro próximo lo mejor es centrarse en el «ahora». ¿Qué ofrece el mercado, a día de hoy, con medición de pulso óptica? ¿Quiénes son las empresas que están trabajando en este tipo de tecnología?

Mio (Phillips)

Mio is a major player in optical pulse measurement, in fact their entire product range is focused on the use of optical pulse sensors and they were the first to apply this technology to the world of sport.

Moreover, its business is not limited to selling products, but also to licensing technology to third parties. But it should be noted that Mio Global did not actually create the technology, but is licensed by Philips, but with full rights (which is why they can, in turn, license it to a third party).

MIO Link 8

We can find your technology in many products. Of course in the whole range Mio Link/Fuse/Velo/Alpha, but also in watches like Adidas Smart Run and other Fit Smart devices of the brand, TomTom Runner Cardio and Multisport Cardio, and the one that promises to be the spearhead for optical sensor watches, the Garmin Forerunner 225.

Garmin Forerunner 225

The inclusion of an optical pulse sensor watch in Garmin's range can be a major turning point. They are not the first to offer the technology, but within the three main manufacturers (Garmin, Polar and Suunto) they have been the first to venture into it. Other companies such as TomTom or Adidas have launched very valid products using this technology, but the market volume they represent is insignificant compared to what any of these three may represent.

Valencell

Valencell is the other big leader in terms of sports measurement accuracy. Unlike Mio, this company does not manufacture devices, only licenses its technology to other companies so that they can create them.

On Valencell's website you can find a list of companies for which they have licensed the technologyFor example, Scosche and its RYTHM+, a sensor with a concept very similar to Mio Link.

Scosche RYTHM+

We can also see the technology in headphones, such as the Jabra Sport Pulse or iRiver On.

jabra

Basis

Basis, a company that is now in the hands of Intel for a year, also has a few devices equipped with optical pulse sensors. Currently, the Basis Peak is the model that is on the market. It is a clock with integrated activity monitor.

intel_basis_titanium_back_sensor

It follows in the footsteps of the Basis B1, a device that came on the market in 2013 and was the first to offer constant heart rate monitoring (as well as Fitbit Charge HR and Fitbit Surge).

Probablemente la compra de Basis por parte de Intel se deba a su creciente interés por el mercado de los «wearables», y querrán hacer uso en la tecnología de medición de pulso óptica más que en el desarrollo de la marca en si. En estos momentos adquirir patentes es crucial para cualquier empresa que piense en desarrollos a largo plazo.

Epson

Epson is best known for making printers. What few of you know is that Epson began its journey in Japan as a watch parts manufacturer for Seiko in 1942. So it should come as no surprise to see watches bearing her brand name. One of the ones in their catalogue, the Epson Runsense SF-810The optical pulse sensor is developed by them.

Epson-Runsense

In this case the development is theirs, and at the moment they don't offer the technology to any other manufacturer. I haven't had the chance to test it, but from the data collected its operation is more than correct.

Other OEM manufacturers

There are many other manufacturers offering optical pulse sensors for third parties, for example Texas InstrumentsBasically, they offer economical solutions to add one more feature to the specification sheet. Their performance is usually quite poor, with data that has no rigor beyond a mere indication that you may have been active or at rest, but without any clinical validity.

This type of sensors are the ones we can find in more generic products and not so sport-oriented as Samsung, Motorola or Microsoft.

apple-watch-sensors-580-90

 

For example the Motorola Moto 360 has this type of sensor, and as you can see in the relevant testThe information provided is not at all rigorous.

Its operation can be on a variety of scales, from disastrous, such as the devices shown by Samsung, to the Motorola Moto 360; hasta «bastante decente», como los sensores que hemos visto en Apple Watch o en los últimos productos de Fitbit, el Charge HR or arises..

Fitbit Surge

In short, companies buy the sensor, of greater or lesser quality, and are themselves responsible for creating the algorithm that processes the data obtained. They acquire the hardware itself, not a complete technology, so the final quality is determined by the development of software and the momentum that puts each of them in the product.

What the future holds

There is still a lot of room for improvement, but the way ahead is quite clear. The performance of the optical sensors will continue to increase over time, but don't expect to see this in the new ranges of all watches. Implementation will be gradual.

Garmin was the first to launch the Forerunner 225They have opted for a solution without high costs, as it is basically the same as the one used in the previous years. Forerunner 220 to which the sensor and two software functions (activity monitor and heart zone data display) are added. In other words, there have been no high engineering costs in designing a new device or creating the sensor from scratch.

But this product will allow them to carry out extensive market research at no cost to them, and they will be able to reliably determine what their customers' real interest is in this type of product. Based on these results, they will be able to analyse whether they should continue along the same path or even design their own sensor and save the fee they have to pay to Mio.

But you shouldn't expect the market to be filled with watches with optical sensors. The implementation will be gradual and will start with the simplest and most affordable models. The reason is very simple, and is that all high-end devices have advanced features such as VO2Max calculation or recovery states, for which it is essential to have reliable R-R interval values. In addition, a company with the sales volume of Suunto, Garmin or Polar cannot risk launching a product to the market that will not work properly with the 100% of their buyers. It would be fatal for their brand image.

Valencell has already announced that they are working on a solution to this problem, but they estimate that they will start licensing the sensor in 12 to 18 months, so there are still many months of testing ahead. It is likely that Mio or Philips are also working on a similar solution.

When this happens it will be time for us to start looking at the optical pulse sensors as a basic feature in any GPS watch. We are talking about a period of approximately 24 months, more than enough time for the technology to reach the point of maturity needed to become a reference and we can finally leave behind the sensors placed on the chest.

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