Lasers target classical guitar for rolling shutter effect (string oscillation) - Is it safe?

When I gave physics lessons to high school students during my Bachelor’s degree, I thought that presenting both theory and simple visualisations of physical phenomena was key to enjoy learning physics. 

I also started to play the guitar quite intensively at that time. Solving standing wave equations to relate the tension, thickness and material type of the strings with the length of the guitar neck in order to calculate string vibration frequencies was one thing. Learning those classic rock songs and keeping the guitar in tune was another!

That was a good mix between learning the theory behind the sound and understanding why a A string provides a satisfying 110 Hz oscillation when tuned properly along with the other strings!

Our ears are sensitive to the variations of air pressure caused by the propagation of acoustic waves in the air. Our brain interprets it as sounds. Waves are made of oscillations that propagate in a medium. Acoustic waves in the air are generated by the energy that is transferred from the mechanical waves that propagate along a guitar string to the air molecules in motion around it.

When we look at guitar strings with the naked eye, we only see the envelope of positions allowed by the string knowing its boundaries conditions: the string is fixed at the nut and the bridge of the guitar and the human eye cannot resolve the position of the string in time.

Would there be a way to actually see the mechanical waves propagating along the string like a lasso in slow motion would do? Laser light shows enthusiasts and finger picking guitarists, this way please. Here comes the rolling shutter effect!

Stroboscope-like effect

Have you already watched these videos where helicopter propellers appear completely still while the chopper is actually moving in the air? This very counter-intuitive visualization is made possible when the camera shutter speed is matched precisely with the rotation frequency of the blades.

 

 

What happens when the blades move faster than the shutter speed such as with propeller planes? Have you ever seen the blades getting slow and twisted when shooting a video of the engines from a plane window?

That’s because the position of the fast-rotating blades changes between the beginning and the end of the pixel readout process of each images typically produced by a CMOS-based camera.

This illusion is then caused by what is called the rolling shutter effect: the camera electronic shutter opens and closes as a rolling window would do, but slower than the frequency of what is filmed.

Imagine you could speed up the frame rate of your camera to match with the oscillation frequency of guitar strings like a stroboscope would do, and set your exposure time short enough to fix the motion of the string and get no blurring.

In theory you should be able to slow down the apparent motion of the string to the point it would eventually stop in your video. You would need a lot of light because short exposure times mean less light entering your sensor per frame. 

A lot of light?! That sounds like laser! Let’s shoot laser beams on guitar strings and harness that rolling shutter effect by using proper camera settings and create the illusion of slow string oscillation we’re after. 

Show time!

Laser beams do provide a lot of light that is very focused, working well if you need to build a high-contrast video setup for maximum visual effect. I used a green laser pointer and a red circular laser (usually used for alignment purposes) attached to my guitar. That allowed to illuminate a good portion of the strings area.

I found that using laser power less than 50 mW with a camera able to push its ISO level enough without increasing the readout noise too much, allowed to provide the illusion of mechanical waves propagating along the classical guitar strings. Have a look at this video! 

The camera used here is a Canon 5D Mark III. Settings during guitar playing are 1920x1080, 24 fps, 1/1000 shutter speed, f/2.8 (maximum allowed by the lens I had) and ISO 4000.

I was actually more impressed by the circular pattern of the green laser beams reflected by the surface of the three nylon strings than by the oscillations of the strings themselves.

When strumming hard, one can see these green circles of light getting cut at regular intervals (due to the moving strings going back and forth in front of the laser beam) and showing an unexpected wave-like scanning behavior going from the bottom to the top of the frame, a bit like what happens when filming a TV or computer screen that has a different native scanning frequency than the frame rate of the camera.

That was more than I expected!

How bright?

I already explained the importance of protecting our eyes during laser light shows and the importance of working with a laser safety officer to ensure maximum safety.

Laser power needs to be measured in order to be sure that we are using the lasers safely in the field. I used a PRONTO-Si laser power meter in my rolling shutter effect video.

I measured a total of 45 mW of power from the 360-degree red laser (635 nm, 2π.200 mm x 2 mm beam surface area). Since only a 7 mm x 2 mm red laser beam would reach my retina (7 mm being the average diameter of a fully dilated pupil), it yields a diluted power in this area of 0.25 mW, which is rather safe.

On the other hand, I measured a total of 31 mW of power from the green laser (532 nm wavelength, 2 mm beam diameter). Working with such a focused Class 3B laser beam requires protection. I measured that my sunglasses attenuate by a factor of 10 at 635 nm and 7 at 532 nm, making it safe with now less than 5 mW of remaining power that could reach my eyes, assuming that the gap between my skin and the glasses was small enough to not end up with a direct beam in the eye.

Conclusion

Acoustic, electromagnetic, mechanical, gravitational... the physical world around us is made of waves (and particles and energy but that would be for another article!). Visual representations of waves and oscillations have a strong impact on us. It provides a feeling of action and liveliness that relates to our very desire to be in motion and to accomplish things. It also tricks our minds!

Here we succeeded to create the illusion of guitar string oscillations in the range of 1 Hz to 10 Hz (therefore visible to the eye) for what is typically happening in the range of 70 Hz to 300 Hz in the physical world (the guitar strings oscillation frequencies) which cannot be resolved temporally by the human eye. That was a fun demonstration!

Always use safety eyewear adapted to your lasers and use a laser power detector, laser energy detector and beam profiling camera adapted to your laser specifications in order to build the exact setup you need for your application. Contact us to find the right measurement solution!


Félicien Legrand
Technical Sales Physicist
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