• A stroboscope is used to measure revolutions, velocity and frequency of rotating components or moving parts.

• It is a simple, portable and manually operated device.

• The instrument consists of a source of variable frequency of flashing being set by an operator called as strobotron.

• The working of the instrument is based on the principle that if a strong light is caused to flash on a moving object which occupies a particular position; at the time of flashing that object appears to be stationary

• The speed is measured by adjusting the frequency so that the moving objects are visible only at specific intervals of time.

• A distinctive mark is made on the shaft or a disc is attached to a shaft whose speed is to be measured.

• A strobotron is used as source of light to flash light directly on the mark.

• The frequency of flashing is adjusted until the mark appears stationary.

Consider a case with single mark on the disc. When flash occurs, it appears stationary. This happens when N = f, where N is speed of shaft in rpm, f is flashing frequency in Hz.

•However if N = 2f, 3f, 4f….etc. means if speed of shaft is a whole multiple of flashing frequency, the mark would appear as stationary.

•Hence the mere stationary image does not guarantee that shaft speed is equal to flashing frequency.

•This problem does not arise if N < f.

•The flashing rate is initially kept maximum and reduced gradually. The consecutive flashing frequencies are noted for single stationary images.

•If the single stationary images are obtained at m different flashing rates f1, f2,…fm, the shaft speed is given by, N = (fm.f1(m-1))/((fm-f1))

Advantages This method imposes no load on the shaft. It is a contactless method. This method is very useful when actual physical contact is not possible. It does not require any special attachment with shaft.

Disadvantages Variable frequency of strobotron cannot be stabilized to give a fixed frequency. The method is comparatively less accurate hence requires digital meters. If surrounding light is above some level, this instrument cannot be used.