When the frame rate of a video matches the speed of a particular part, the effect is the same as using a strobe light. In the video, that part freezes.
Since the frame rate of a video can be adjusted, this effect can be used to determine the rotational speed of a part.
The smartphone must support slow-motion video recording (filming at a high frame rate). This is the case for all smartphones starting with iPhone 5S. The range is as follows:
• 6 - 240 Hz (360 - 14400 RPM), for all phones from iPhone 6 onward
To get a clear image, good lighting is important. The clarity of the image is controlled by the exposure of the video (exposure time and light sensitivity (ISO)). If the exposure time is reduced, the image will increase in clarity, but brightness will lower. To increase the brightness, you need to raise the camera's light sensitivity (ISO). You also need to manually adjust the focus of the camera.
The video tachometer measures the rotational speed in exactly the same way a stroboscopic tachometer would.
Measurement of rotational speed by video tachometer
To determine the rotational speed (e.g., to determine the fan rotation speed), a plane perpendicular to the axis of rotation should be visible, and only one mark should be on this plane. For example:
YOU SHOULD BEGIN MEASUREMENTS FROM THE LARGEST FREQUENCY THAT IS POSSIBLE FOR THE TESTED FAN. TO REDUCE THE FRAME FREQUENCY, SIMPLY CLICK AND HOLD THE BUTTON.
The observed pattern will change. For example, if the actual speed of the fan is 2660 RPM, and you began measuring the frame frequency at 11000 RPM, then as you reduce the frame frequency, the following still images will appear:
Four or three marks can be seen at other frame frequency; for example, three fixed marks can be seen at a frame frequency of 3990 rpm (real RPMs multiplied by 1.5), four marks at a frame frequency of 3546 rpm (real RPMs multiplied by 1.333.) and five marks at a frame frequency of 3325 rpm (real RPMs multiplied by 1.25).
The wrong conclusion can be made when defining the RPMs by increasing the occurrence of the frame frequency. For example, at a frame frequency of 887 rpm (real RPMs divided by 3), one mark is visible.
This frequency can be misleading, since after multiplication or division of this reading by 2, one may obtain the same pattern as what is observed at the correct frequency (in this case, at 2660 RPM). THEREFORE, YOU SHOULD NOT START WITH A SMALL FRAME FREQUENCY AND THEN INCREASE IT.
1. To display the strobe effect on the smartphone screen, the application needs access to the camera. Data received from the camera is not transmitted anywhere.