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Most smartphones are tilt-sensitive, but what device makes this possible? Additionally, how does it (and the sensors associated with it) work?

Also, since the working of these sensors seem, almost certainly, based on the presence of an external gravitational field (for instance, the earth's), this begs the second question: Do smartphones retain their tilt-sensitivity under zero-gravity (hypothetical) conditions?

(Recently played an aircraft simulator game on my phone...the fact that the plane responded so well to tilting took me aback; hence the urge to ask this question)

Extras:

I put some thought into this myself, so I'll be putting that up here too. For all intents and purposes, my question ended after the second paragraph, but what I've added after this might help tailor an answer that fits my current understanding of physics.

I'm currently in high-school, and if I recall correctly, there are six degrees of freedom for a particle in a 3D Cartesian system. From my experience with the aircraft simulator app, smartphones seem to detect motion in only three degrees of freedom: pitch, roll and yaw

enter image description here

Speaking of tilt-sensitive sensors: The way I assume these sensors/transducers work, is by detecting the minute changes in gravitational potential energy (which may manifest itself as small-scale motion of some tiny components of the sensor) that is associated with the phone's change in spatial orientation.

The way I see it, such a sensor would require moving parts, and cannot simply be another chip on a circuit board.

Under these circumstances, if I were tasked with building a tilt-sensitive device that perceives minute changes in gravitational potential energy, I would probably require at least 3 pairs of sensors (a pair in each of the three coordinate axes). Also, seeing how very sensitive my smartphone appears to be to tilting, I'd have to build a ridiculously large device, with each sensor in a pair placed several meters apart to achieve tilt-sensitivity comparable to that of my phone.

However, smartphones have dimensions smaller than that of a typical sandwich, so having "sensors in a pair placed several meters apart", apart from being impractical, is clearly not the case.

^ I went ranting about this, so that you can get a feel of my genuine perplexity in the sub-question that follows:

How come these sensors are so sensitive, despite their small size?

paracetamol
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    What you are talking about are accelerometers see https://www.digikey.com/en/articles/techzone/2011/may/using-an-accelerometer-for-inclination-sensing – JIm Dearden May 11 '17 at 14:06
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    [Accelerometers](https://en.wikipedia.org/wiki/Accelerometer) measure acceleration due to gravity (and other forces). One accelerometer per axis. – brhans May 11 '17 at 14:06
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    Good information on micro electro-mechanical systems (MEMS) here as well. http://www.pcb.com/Resources/Technical-Information/mems-accelerometers – Chris M. May 11 '17 at 14:06
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    Zero-gravity doesn't actually exist, so are you talking about a weightless environment or hypothetical situation? – ks0ze May 11 '17 at 14:07
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    @ks0ze Hypothetical situation, should've mentioned that, thanks! – paracetamol May 11 '17 at 14:08
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    They have accelerometers. To an accelerometer, gravity looks the same as acceleration. This is how they sense orientation. The feature will not function correctly in zero gravity. You are correct that accelerometers have moving parts, however they are very small moving parts. Google accelerometer. Many smartphones also have something similar to a gyro to detect the rate of pitch, roll and yaw. Sometimes these sensors are called "angular rate sensors." The accelerometer is not good at detecting pitch, roll and yaw, but it is good at detecting orientation in a gravitational field. – user57037 May 11 '17 at 14:09
  • @mkeith You're probably at it right now, but... could you post that as an answer? [Since this sounds interesting, you'll get my upvote ;) ] – paracetamol May 11 '17 at 14:12
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    Looks like a couple of other people are going to do a good job answering. But if I feel I still have something to offer after they answer, I will write an answer or comment on their answer. – user57037 May 11 '17 at 14:13
  • @mkeith an accelerometer should not care about gravity. – Trevor_G May 11 '17 at 14:14
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    @Trevor have you heard of "acceleration due to gravity"? – pjc50 May 11 '17 at 14:14
  • @mkeith Great! I'll wait for a day (for the answers to pool in), and then I'll go about "accepting" an answer. I'm looking forward to yours ^_^ – paracetamol May 11 '17 at 14:17
  • @pjc50 ya ya, but that is not relevant in a static situation. An accelerometer should still measure it's own acceleration in a weightless environment. – Trevor_G May 11 '17 at 14:17
  • @Trevor But it won't be able to measure the tilt since there won't be any reference vector. – Eugene Sh. May 11 '17 at 14:18
  • BTW, you won't be able to feel any "tilt" as well in the weightless environment. Because there is no "straight" position defined there. – Eugene Sh. May 11 '17 at 14:20
  • @mkeith yup I know. I'm just being pedantic. It's just that a TRUE accelerometer measures change in speed. Weight on a spring is something different. – Trevor_G May 11 '17 at 14:20
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    The tilt or orientation sensors basically assume that they are in a gravitational field. They measure the direction of acceleration and call that direction "down." If placed in a path or location where "down" does not exist, I expect that the sensor would not function correctly, but I don't know exactly what it would do or whether the firmware covers this operational mode. By now, I imagine someone has taken a smartphone into space. Would be interested to hear about it. – user57037 May 11 '17 at 14:24
  • @mkeith probably it will rotate the screen like crazy. You can simulate this behavior by dropping the smartphone from the Tower of Pisa :) – Eugene Sh. May 11 '17 at 14:27
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    @mkeith If the phones software does have code specifically for a zero gravity situation it will assume the phone has been dropped. Some laptops with mechanical HDDs will park the drive in this situation to avoid data loss, HDDs are better able to withstand high G forces when parked. There isn't much a phone can do to help protect itself when dropped so I doubt they added anything special to cope with the situation. – Andrew May 11 '17 at 14:29
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    @Andrew is right. If you look at some accelerometer chips you will see some pins for the "drop detection" interrupt. – Eugene Sh. May 11 '17 at 14:31
  • I am aware of the drop detection. I don't think it is out of the question that programmers would anticipate operation in a weightless environment, though. In particular, if the orientation is not strongly determined by something close to 1g, the device may simply suppress display orientation changes. – user57037 May 11 '17 at 14:43
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    Yup, what @Andrew is saying is what I was saying too. The fact that your cell phone might not work the same in the international space station, is more a factor of how the software works, not how the accelerometers work. Though the software would probably indicates the phone has been thrown up rather than dropped. – Trevor_G May 11 '17 at 14:44
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    This raises the question: Do laptops on the ISS require special firmware, to prevent the harddrive constantly going into drop-protection mode? – Joren Vaes May 11 '17 at 14:47
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    @JorenVaes I would use SSD instead :) – Eugene Sh. May 11 '17 at 14:53
  • @EugeneSh. indeed. Tilt would be referenced to the last known direction. That is, it would know it turned 90 degrees. but whether the displayed image is shown in the right direction would be indeterminate. For an ideal system, it would not matter, since everything would be cumulative from when the thing first came to life, was powered up, and was presumably self-calibrated. In reality, like a gyro, occasional reference point fixing would be required in a weightless scenario. – Trevor_G May 11 '17 at 14:54
  • Not so sure the rotational information in all phones would work in the space-station though. Since, they are using G as the reference signal from each accelerometer, you don't need anything else to detect rotation since the G-Factor will change between the three axis accelerometers. So unless they have a different spin sensor they probably don't. Though I guess, doubling up on accelerometers, and enough math would give you the visibility. – Trevor_G May 11 '17 at 15:02
  • The XBox 360 game console can be operated either lying flat on a table or standing vertically. It would detect this orientation not with an accelerometer but by a very simple two-position tilt switch. The sensor was basically a set of contacts positioned such that a tiny gold-plated metal ball would make or break one of two mutually-exclusive switches. And thus it could figure out which way up it was. It used this information to (among other things) select which of two icons would show up in notification messages. Pretty pointless but it was cheap and easy to implement. –  May 11 '17 at 16:52
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    I'm giving you a +1 on your question simply for correctly spelling "For all intents and purposes" correctly, especially as a high school student! The rest of it is good, but boy did that jump out at me, congrats! – FreeMan May 12 '17 at 12:59
  • @Free Meh... :3 – paracetamol May 12 '17 at 14:44
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    And I'll give myself a comment down-vote for incorrectly and redundantly using "correctly"... meh indeed! – FreeMan May 12 '17 at 14:46
  • that's not what begging the question means, though: http://begthequestion.info/ – njzk2 May 12 '17 at 14:49
  • @njzk2 *Utterly shocked* [I'll be looking for a better reference though, just to confirm that ;) ] – paracetamol May 12 '17 at 14:52
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    @paracetamol there is grammar girl: http://www.quickanddirtytips.com/education/grammar/begs-the-question-update (not sure if you count that as a good reference, though) – njzk2 May 12 '17 at 15:15
  • there are a number of **utterly different** devices at work; that seems to be a basic confusion in this QA. – Fattie May 12 '17 at 19:44
  • "The feature will not function correctly in zero gravity" this is completely wrong. the _usual software app creators write_ will _likely_ not function in zero g. – Fattie May 12 '17 at 19:46
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    Good question! I've linked here in [How do iPads on the ISS know which way is “up” for their users?](https://space.stackexchange.com/q/21493/12102). – uhoh May 13 '17 at 02:51
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    «There isn't much a phone can do to help protect itself when dropped so I doubt they added anything special to cope with the situation.» but I would totally pay for an app that made the phone scream when falling. – JDługosz May 13 '17 at 12:26
  • @Trevor if you can (even theoretically) detect the difference between gravitational acceleration and other acceleration, you need to contact your nearest physics department immediately. That would be a significant development! – Jeanne Pindar May 13 '17 at 15:39
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    What does it mean to even "work" in zero gravity? What's the definition of "down"? "Toward Earth"? – user541686 May 13 '17 at 18:57
  • **"What does it mean to even "work" in zero gravity? What is down?"** it couldn't be more straightforward, Mehrdad. In an ordinary working spaceship, when the crew are sitting around, they refer to "down" as "the floor". No big deal. If (remarkably!) you were making a small game on a phone for space crew, let's say a steering wheel (some sort of car game).. Quite simply, you'd just define "down" as the rest state when the user was sitting there ("at the starting line"), and then detect "left and right turns" (twist on the z axis) from that basis (ie, the gyros would tell you that trivially). – Fattie May 15 '17 at 10:54
  • Many of the comments here are quite incorrect. *"They have accelerometers."( it's a bit like saying "a car has brakes". Overwhelmingly, a car has a motor (yes, it also has brakes). Gyros are far more important in phones. *"[accelerometers] will not function correctly in zero gravity."* Totally incorrect. Obviously from a physics stand point, they of course work in freefall. Practically you can get a hundred apps on the app stores, that are those "action sports" apps (put in your pocket while you do diving, ski jumping, whatever); they perfectly show your action during the freefall phase. – Fattie May 15 '17 at 10:57
  • @Mehrdad the enemies gate is 'down' – Baldrickk May 15 '17 at 13:28
  • @JeannePindar that's not what I was saying, I was intimating that when you use accelerometers, the first thing you do on power on is calibrate them and remove the offset caused by the gravitational effects on the mass involved. After that it's about the delta. This would be the same in any environment. – Trevor_G May 15 '17 at 13:30
  • @Fattie: That doesn't make any sense though. Even on Earth if I lie down on my side on my bed and hold my phone, it will rotate the screen, whereas the same relative position to me will not result in a rotation if I'm standing up. So unless you claim the behavior is already "wrong" on the ground, in which case it's obviously less likely to be "right" elsewhere, clearly "correct" behavior is not relative to the person, it's relative to gravity. So it doesn't make sense to ask what it is in zero gravity... – user541686 May 15 '17 at 20:18

6 Answers6

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You are right, in a sense. These sensors do need moving components. However, they are a chip on your board.

Tiltsensors (actually, accelerometers), and gyroscopes (and pressuresensors, ...) are part of a family called MEMS: Micro-electromechanical systems.

Using similar techniques as already common in integrated circuit fabrication, we can make amazing little devices. We use the same processes of etching away things, depositing new layers, growing structures, etc.

These are incredibly tiny devices. this is an example of a gyroscope:

enter image description here

enter image description here

link to the original website.

Most of these work by sensing changes in capacitance. A gyro would sense the changes due to rotation (the big thing in the picture would twist around the center axis. This will bring the tiny teeth that are interleaved closer together, and increase capacitance. Accelerometers work under a similar principle. These teeth can be spotted in the rightbottom corner of the second image.

What about zero-gravity?

It would not change much in terms of the functioning of the devices. You see, accelerometers work by sensing acceleration. The key however is that gravity is the same to them - it just feels like you are being accelerated up at 1G, all the time. They use this "constant" to get an idea where "down" is. This also means that while the chips will function just fine in micro gravity, your phone would not - it will be confused as there seems to be no "down".

Quick addition to address a (very good) point that user GreenAsJade brings up: When you look at the common definitions of gyroscopes on sources like wikipedia they are often described as something along the lines of a spinning disk. The pictures above don't seem to have any spinning parts. What's up with that?

The way they solve this is by replacing the rotation with vibration. The disk shaped object in the pictures here are only connected with very thin and flexible structures to the center axis. This disk is then made to vibrate around it's axis at high frequency. When you move the entire structure along an angle, this will cause the disk to try and continuously resist this - similar to a classic gyroscope. This effect is called the Coriolis effect. By sensing the amount of tilt of the disk compared to the surrounding solid material, it can measure how fast it is spinning.

Joren Vaes
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    The mass of such sensor is incredibly low, thus there is no inertia slowing the sensing down. An iPhone is able the determine the RPM of an engine up to 3000 RPM using these sensors. incredible. – Jeroen3 May 11 '17 at 14:16
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    @Jeroen3 I find the fact that they are so sensitive, yet able to withstand the massive G-forces from dropping and being thrown around in peoples pockets mind-blowing. – Joren Vaes May 11 '17 at 14:19
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    Square-cube law helps a lot with durability, as it does for insects. – pjc50 May 11 '17 at 14:23
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    Could you explain how what we are looking at is a gyroscope? I thought a gyroscope has to have something spinning around, but I can't spot what part of that spins... – GreenAsJade May 11 '17 at 14:48
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    @Jeroen, how does one read an iPhone when it is spinning at 3000rpm? – Octopus May 11 '17 at 15:47
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    @Octopus You wouldn't attach the phone to a rotating part and sense rotation with a gyro (obviously a bad idea). You would place the phone against the engine block and use the accelerometer to pick up vibration, analyze it to get the fundamental frequency and print out the equivalent RPM. – jms May 11 '17 at 16:43
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    Gyroscopes are not angular accelerometers- they drive (vibrate) the wheel around the axis and measure the *tilt* of the wheel. – Spehro Pefhany May 11 '17 at 16:43
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    @Octopus You don't. You simply touch the phone chassis to the motor. And run [this app](https://itunes.apple.com/nl/app/vibration-analysis/id817385888). – Jeroen3 May 11 '17 at 17:52
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    A fun thing to do is to download an app onto your phone that plots the accelerometer data live as a function of time. Then drop it onto a pillow and watch all three graphs (x, y, and z) go to zero while it's falling. – Michael Seifert May 11 '17 at 18:30
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    @Jeroen3 What you mean is there is very little inertia. – user253751 May 11 '17 at 21:11
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    @JorenVaes Thanks! So obvious, but I never thought of it that way before! Really amazing stuff, and now the picture makes perfect sense! It leaves me with one question in my mind: is this _really_ a "gyroscope" or something else? – GreenAsJade May 12 '17 at 01:13
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    @Octopus By corotating at 3000 rpm. Well, briefly, at least. – user May 12 '17 at 08:33
  • How does this work to detect motion in 3 axes? Does the phone require 3 of them, each aligned along a different axis, or are there parts built in to the single chip to handle all 3 in one device? (Mind blowing images, too, thanks!) – FreeMan May 12 '17 at 13:08
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    @FreeMan there's actually 6 total sensors, 3 gyros and 3 accelerometers, and there may be a redundant set as well. They are physically oriented along the different axes. This is produced as a single chip. I've used this one before, which includes 3 gyros, 3 accelerometers, and a 3-axis compass (magnetometer): https://www.sparkfun.com/products/11028 – Jasmine May 12 '17 at 17:41
  • All modern devices also have a crappy 20 cent hall effect compass; everyone seemed to forget this! – Fattie May 12 '17 at 19:45
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The sensory device is a weight on a spring. It is indeed "small-scale motion of some tiny components of the sensor", and it's also "another chip on a circuit board".

The key word here is MEMS. It's possible to build small silicon structures and then etch away underneath them, leaving a free-floating piece. If the piece is long and thin, it will deform under gravity (or any acceleration) by an amount proportional to its Young's modulus. The change in position affects the capacitance between the moving part and stationary parts around it, which can be measured electronically.

Generally they have only one three-axis accelerometer. Better precision can be achieved by adding gyros or another accelerometer separated by a distance; Nintendo did this with Wiimote add-ons.

Many phones also contain a magnetometer, which tells you vaguely where magnetic north is relative to the phone, although the calibration tends to be bad on these.

Addressing specific parts of the question:

  • What makes smartphones tilt-sensitive?

MEMS accelerometers. Few mm square chip package, $0.50 or less in quantity.

  • Will they retain this ability in zero-gravity conditions?

Not exactly. They no longer have a convenient reference vector. However, they can still detect acceleration, so if you have one of those "lightsaber" apps and wave it around it will still work on the ISS. But neither you nor the phone have a clear idea of "up".

(The Raspberry Pi kit sent up there has an accelerometer and a bunch of programs written by schoolchildren, so there's almost certainly a video demonstrating this somewhere)

The raw output of a 3-axis accelerometer is a vector of 3 values measured in m/s^2. The magnitude of this vector will usually be about 1g, but the direction varies. For a stationary phone it will point downwards. If you move it then the acceleration vector will change direction. If you drop the phone, i.e. it goes into freefall the same as a phone on an orbiting craft would be, then the magnitude goes to near-zero. This makes the direction of the vector swing wildly and turn to noise.

The use of accelerometers as drop detectors for hard disk safety was popularised about a decade ago by Macbooks. People found other uses for them.

  • how does it work?

Answered in more detail by other answers.

pjc50
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  • Perhaps worth mentioning that magnetometers are also very easily disturbed by metal structures, magnetic fields, ... – Joren Vaes May 11 '17 at 14:17
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    Small point, but the ISS is not in zero-gravity - it's in orbit. – Christian Palmer May 11 '17 at 15:28
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    Mac nerd here, but I think the IBM/Lenovo ThinkPads had accelerometers a little earlier. At least I remember them talking up the feature. The MacBook ones might have been more easily accessible in software though, and hence more hacking friendly. Digital cameras also had auto screen rotation a little while before smartphones made it commonplace. – Flambino May 11 '17 at 23:32
  • @ChristianPalmer, it's in free fall. There is no such thing as zero gravity anywhere. – Wildcard May 12 '17 at 03:25
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Theoretically, yes, a phone or tablet could work just as well in say the International Space Station (ISS) as it does here on the ground.

Let's break this down a bit.

There are two types of motion a device needs to detect.

Linear Motion

Self-contained accelerometers use the deviation of a spring-coupled mass from a normal rest point as a measure of acceleration force in that axis. Obviously you need three of these to detect motion in any axis.

Knowing, and tracking those forces you can "dead-reckon" the speed and direction of travel of the device from its original "power-on" location. Factor in an accurate clock, and you can also figure the current position.

That sounds simple, but the math is actually quite complex and errors in the system cause a drift over time.

Rotation

Rotation is obviously spin about any axis.

Spin Sensors

Rotation can be measured using a gyroscope or a spin sensor. These devices again have a loosely coupled mass that is free to rotate, or is driven, in a particular axis. When the body of your device rotates, the difference between rotations tells you how much the device is rotating.

Spin sensors and gyroscopes don't care about gravity, other that perhaps some frictional differences.

Gravity Referenced Accelerometer Rotation

Since accelerometers measure the force acting on a loosely suspended mass, when that sensor is vertical relative to the earth, there will of course be a deflection in the spring due to the weight of the mass due to gravity. This offset is mathematically removed by the software in order to extract the acceleration part.

However, since the three axis accelerometers will produce different offsets depending on their orientation, it is possible to mathematically detect spin from the difference in the offsets.

However, though this method works, it is subject to variances in G. It would not work in space. It would also be significantly less functional in a maneuvering aircraft. Even a car going round a tight bend at speed could be problematic.

Accelerometer Spin Detection

It is possible, with two sets of sufficiently sensitive accelerometers, to detect spin from the difference in acceleration between accelerometers.

Since each accelerometer has to move relative to the other, there will be a difference in acceleration in that axis between each. Those values can again be used mathematically to predict the spin.

Simply put, if you can tell from the accelerometers centered at one end of the phone that the centre point has moved to \$X_1,Y_1, Z_1\$, and the other end is now at \$X_2,Y_2, Z_2\$, calculating the three angles is trivial.

This method is NOT affected by gravity.

Will YOUR Phone or Tablet Work On The ISS

As you can see from the above it really depends on which methods your device uses.

Technically it could be built, and programmed, to do so. You may need to shut it down and power it up again to recalibrate it, but with the right systems in place it should work fine. At least for playing that "aircraft simulation game".

Drift may be a larger issue on the ISS though. Since phones in normal G have the ability to known which way "down" is at that particular moment, they can re-adjust over time. A space-based unit would need an occasional manual reset to indicate the "normal" direction.

Peter Mortensen
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Trevor_G
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All the comments and answers are great to help you understand how it is possible. But, here's something that will help you understand how it's actualized in real products.

Orientation(image source)

This is a tiny IC (3x3x1 mm) by InvenSense. It has a three-axis accelerometer (for lateral movement), a three-axis gyroscope (for rotation), and a three-axis magnetometer (like a compass needle). It has internal code which will do all the complicated math. It consumes very little power.

This is only an example. There are several companies making similar products. Some are more accurate than others, some are cheaper, most don't have the magnetometer, etc...

bitsmack
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This is a rare case on the Electronics site where, none of the answers clearly and crisply answered the question!

Do cellphones retain the ability to detect tilt in zero-gravity conditions?

The answer is:

They retain (at a hardware level) the ability to detecting tilting, but they can no longer detect tilt.

Further,

At the level of app software, in fact, almost all (very likely "all") app-software-writers would not allow for the corner case of zero-gravity, so very likely the gyro-accel functions would act whackily overall, in most/all actual apps.

Regarding how gyros/accels work in phones, you can easily google the APIs for these on the two platforms (example).

Note however that all OSs as of writing, in practice wrap the lower-level gyro/accel functions in some sort of convenient higher-level motion manager:

Accels/gyros, are in fact wrapped together at the OS level

So in fact...

in practice, for any fairly newly-written app (remembering that, let's say, about 25% of apps in the store are decayed / not updated regularly), it would come down to how the team at Apple which wrote (in their case) "Coremotion" handled (if at all!) the zero gravity environment case. (There's a similar situation for Android).

And further, for games as such...

Today almost any game you pick up and play on a phone was created in Unity3D, rather than as a native app. (And as a rule, if you look at the set of "apps which use the accel/gyros", 90% (more?) of them are just games.) So in fact (on all platforms) the software-writers are actually using's Unity's level of software wrappers.

Hence, the actual behavior in the extreme corner case of earth orbit, would depend on what those folks did when writing that.

One confusing point...

that hasn't been clarified. When you're writing software for phones, it's totally commonplace to have to deal with "zero gravity" ... for short periods of time: that is, when the phone is in free-fall. So if you're making one of the (100s of) apps for skateboarders, skiers or the like which measures hang-time and so on, you deal with this as a matter of course.

Gyros were introduced to phones about 2010; accels were in them from the start.

A French/Italian company called STMicroelectronics pretty much makes most of the gyros for both apple and samsung.

Regarding accelerometers, most phones now have a couple of them since it works better that way. I have heard that there is more variety of suppliers of accelerometers (Bosch, etc).

You can literally buy MEMS gyros or accels, if for example you are making an electronic toy that includes such a feature.

Just to repeat, the fundamental quick answer to the question posed is

In "zero g", they retain (at a hardware level) the ability to detecting tilting, but they can no longer detect tilt.

In terms of the software,

  1. it would, almost certainly, "totally fail!" in the whacky "you're in orbit" case. Since no gane or app engineer (I know) would be so OCD as to cover that case, but don't forget...

  2. it's totally commonplace to have "zero gravity" .. during short periods of freefall (this applies as a commonplace matter if you're making one of those "action sports apps").

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Fattie
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  • Good answer; +1. When you say "Most phones now (2017) have a six axis gyro.", don't you mean a 6-axis sensor (or "IMU", Inertial Measurement Unit), consisting of a 3-axis accelerometer and a 3-axis gyro? – bitsmack May 14 '17 at 23:35
  • Bits, thanks - you're quite correct that was poorly phrases; edited. Cheers! – Fattie May 15 '17 at 00:21
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I think they might use a sagnac interferometer in smartphones. A Sagnac Interferometer is a device which produces a constant interference pattern while at rest and its pattern varies when the setup is rotated.

So when 3 such interferometers are placed we can measure the rotation about all 3 axis.

Sagnac interferometers come in very small sizes and it comprises of optical fibers to channel light , a source of light(coherant), and an detector.

Of course it should be calibrated before use.

user_sp
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