Non-Contact tachometers:

 Non-contact tachometers use a light source, generally a laser or infrared light, to measure rotational speed. This requires no physical contact between the tachometer and the target. The tachometer is aimed at the spinning target to which a piece reflective tape has been affixed. As the light source hits the target it is reflected off the tape and back to the tachometer. The tachometer measures the rate at which light pulses are reflected back to the tachometer which is proportional to the rotating speed of the shaft or disc.

Like many instruments, tachometers are becoming increasingly digital, which requires the application of membrane swicth or graphic overlay as the control panel. Though it’s still possible to find analog tachometers, especially in cars, digital models are much more prevalent. Digital tachometers offer a number of advantages over their analog cousins, primarily in the advanced functions available with each type. Analog tachometers have no provision for features such as memory, or statistical functions such as averaging or changing measurement units. These functions are common on most digital instruments.