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There are apps like Instant Heart Rate and Cardiograph which claim to be able to measure heart rate by reading color changes on arterial pressure change. They generally require you to place your finger over the flash led and camera.

From my tests the values don't quite match rates measured traditionally (by feeling arterial pressure changes, counting them over 10 seconds and multiplying by 6) but I'm not a physician so I might be measuring incorrectly.

So, are these apps more precise than I can be or is it the other way around?

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Hi! I changed my answer a bit to reflect the test I made this weekend with a friend's BPM watch. – Coyote Mar 26 '12 at 12:02

5 Answers

up vote 2 down vote accepted

The operative term here is photoplethysmograph. It's the term for what your phone is acting as.

Basically, your blood vessels rhythmically dilate and contract with every heart beat. This is measurable as either a change in reflectance or transmittance (depending on how the photoplethysmograph is configured. In the case of your phone, it's measuring reflectance, as the light is adjacent to the camera.

In medical instruments, it's generally done with infrared light, because blood absorbs shortwave infrared more then red or white light, and your tissue is more transparent to infrared. However, there is no reason it wouldn't work with a phone camera.

If done properly, it should as or more accurate then manual measurement. However, a phone is not a proper photoplethysmograph, so the realistic achievable accuracy is likely dependent on a number of factors, including camera performance, proximity of the illuminating LED to the camera lens, and the algorithm used to extract the pulse from the video return.

However, you need to be aware that all photoplethysmographs are very sensitive to mechanical movement. It is essential that you hold your finger as still as possible, and avoid moving the phone relative to the finger.

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up vote 5 down vote

Very interesting question! I wasn't even aware that these kind of apps exist. In order to answer your question, I decided to put one of them to the test against a chest strap heart rate monitor. (I was using a Garmin brand chest monitor similar to this one.) I was using both heart rate monitors at the same time when I performed my tests and compared their values.

I tried out Instant Heart Rate Monitor, and it turns out that it is remarkably accurate... when it worked.

The main problem was that it only worked about 1 in 30 times. Every other time it would say it could either not detect a finger, or it started detecting the pulse, but then never finished the process.

When it did work, though, the results were surprisingly similar to my other heart rate monitor. After performing 5 (non-scientific) tests, it was off by an average of only 1.26%. Had it worked more consistently, I would have tried it out more than 5 times.

A few notes:

  • While it is beeping and gathering info, the heart rate it shows is usually way off from your actual heart rate. Sometimes it's almost exactly half your real heart rate.
  • The final heart rate it outputs is usually very close to your actual heart rate.

Therefore, you should only trust the last heart rate value it outputs (i.e. the average).

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Much better experiment! At least you have a point of comparison. Now we just have to assume that the chest strap is itself accurate ;) Incidentally, I have tested this and have pretty much found the same thing as you. I used a Timex T5G941 (which was calibrated to ~6 cardio machines, such as treadmills, stairclimbers, etc.). Like you, I found it was accurate but that wasn't really the issue, it was the reliability. I had a better rate of detection than you did, but it wasn't great that's for sure. The spotty detection really makes it more for entertainment purposes. – cksum Mar 26 '12 at 11:04
@cksum - I get a very good rate of detection after some practice (>95%). It's a very close tie between this answer and Coyote's, I'll leave the question open a couple more days before accepting. – Vlad Mar 26 '12 at 13:56
@cksum: That's true, but since the main purpose of this Garmin chest strap is to monitor heart rate, I would hope it's accurate! From what I've read, at least, chest strap monitors tend to be more accurate than the wrist watch variety. – Senseful Mar 26 '12 at 22:23
up vote 3 down vote

I suspected that assuming you are used to checking your BPM regularly apps which rely on the flashlight and camera weren't more precise than a manual check. Event if they rely on a technique used by medical apparatus.

But then I did some testing.

Testing

vs myself

Comparing with my own readings the results seem to be consistent with a ~5 BPM margin based on the approximation due to the multiplication. I actually am less precise than these apps.

2 different iPhones models with flash

The results of some testing using 2 different iPhone models, the iPhone 4 and the iPhone 4S shows that on 6 readings (3 per device), I had a difference of 2 to 0 bpm with less than ideal conditions.

vs a Gamin Forerunner

Testing of one of the apps agains a GPS sports watch with BPM Showed very close results on a dozen readings. A 3 BPM difference was be the least precise it would get in when testing in good conditions, most of the time they are off by 1 or 2.

Conclusion

Convenient and precise

If you don't press hard on the camera (to avoid blocking blood flow), remain calm, not moving and there are no strong variations in lighting around you you should get a decent reading without having to concentrate on feeling the blood flow and counting the heart beats.

history

Apart from the convenience, I believe the real advantage of these apps is the tracking and history of your bpm.

If you want to use these apps before and after exercising they will be enough. Based on my tests, the accuracy seems very good (better than I believed) if you make sure to place your finger properly, covering the camera without pressing hard as not to stop the blood flow (again this is important).

I think I will trust the iPhone from now on ;)

As a side note. If you want a really good way of keeping track of your bpm and blood pressure this product works great on the iPhone/iPad.

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Good roundup of usage advice to get a good reading. – Vlad Mar 22 '12 at 21:26
2  
Your experiment is flawed and the only thing you've shown is reliability (that the app works across a number of devices). But you haven't shown actual validity (that the app is actually getting an accurate reading of your heart rate). – cksum Mar 22 '12 at 22:50
1  
@cksum comparing the reading of the iPhone to my own reading done "by hand" gives me good results. I got 69 and 70 BPM on the iPhone vs approximatively 72 BPM by hand. it seems all right, and seeing that the readings are consistent across devices I feel confident they are not off by too much. – Coyote Mar 22 '12 at 23:19
@cksum: I tried a test similar to the one you describe. See my answer here. – Senseful Mar 26 '12 at 7:35
up vote 0 down vote

It would be much better if they had you tap out the pulses with one finger and using your other hand to palpate an easy to find artery like your carotid artery. Also note, for exercise, new low energy bluetooth 4.0 accessories are now shipping so if you have a new iPad (3rd generation) or iPhone 4S you can expect much better apps that have the correct devices for measuring heart rate as well as blood pressure.

The motion of your finger (side to side as well as pressing harder or less hard) will be hard to distinguish by the camera when all you are looking to do is tell when fresh blood is washing into your fingertips. By the time the blood gets to the extremity - the pulse is less strong so this is one of the worst areas to try to detect pulses (but it's very convenient and the skin is fairly translucent to the LED flash and oxygenated blood does reflect the light so it does work fairly well in some body types).

I like the wahoo fitness blue heart rate belt, but there should be many more on the market soon.

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1  
I only use them for informative purposes and I like the convenience of not having to carry specific equipment or do the reading manually. It's also a great demonstration of technology progress in areas you would least expect. Provided they are accurate enough. – Vlad Mar 22 '12 at 21:26
They are amazing "flying car" things when they work, and in my experience, once it works for a certain person, they work for that person reliably. It is truly cool that they can use the camera in that manner. – bmike♦ Mar 22 '12 at 23:45
up vote -1 down vote

Unless for some reason you are not able to count the pulse beats, these apps are never more precise than standard manual check.

The way they work is when you put you finger on the led light and the camera they monitor colour changes of your skin. Your skin colour changes rhythmically as the blood is pumped through veins. This could be a fairly accurate method, but requires very strict behaviour of the person who is being checked. Any move of an arm, hand, or a finger can, and most likely will skew the result.

I believe both apps say, the results provided are not definitive and cannot be used as a source for medical diagnosis.

So, to sum up - they're almost never as accurate as the standard manual check.

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The apps employ some sort of autocalibration and have a settle time before they give out a reading. Sometimes they report they were unable to get a consistent reading, if you don't use them correctly. – Vlad Mar 22 '12 at 21:24

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