This is a Tachometer- which works on the principle of Stroboscope. It is pocket size, 2 Nos. AA size battery operated, optical, non-contact type, which can be used for remotely viewing of blades of a RUNNING turbine of an aircraft while on the ground in an airfield.

This Tachometer works on the principle of Stroboscopic Effect.
This means that at certain speeds of rotation, a rotating object, seen in steady light, will appear to be stationary to a human observer.

A variation of Stroboscopic Effect is that if a rotating object-in steady light- is observed through a hole in another rotating plate , the observed object would appear to be stationary, when rotating plate is rotating at the same speed as observed object (irrespective of the speed). This variation has been used in design of this instrument.
This Tachometer, operates on 4 to 6 volts (2 to 4 AA size dry electric cells, making it truly portable for field applications), can be used for any visible object even if it is in another galaxy - ( so there is no limitation on distance), and does not involve any electronic components.

Another variation of Stroboscopic Effect is that, if a rotating object is seen in flickering light, when frequency of flicker is same as frequency of rotation of the object - the object would appear to be stationary ( irrespective of the speed ). A tachometer based on that variation is existing, but it has the DISADVANTAGE of running on main supply ( which limits its field use), observed object has to be close enough to reflect the light( which limits the distance of observation) and involves fragile electronic components.

This proposed instrument is described as follows:-
a. Instrument works on AA size batteries so it is portable.
b. Distance of the object is no hindrance, as long as it is visible, even if it is in a distant galaxy or nebula. There is no need for any physical contact.
c. Instrument creates no disturbance (optical or otherwise); therefore it can be used undetected also.
d. Instrument does not involve any electronic circuit, therefore it is more robust, and reliable. Electronic circuit may be further incorporated as an optional accessory, but is not a requirement.
e. Cost of production could be less than US$ 5.
f.Market potential is good, especially in the industry where rotary parts like aeroplane turbines need to be inspected in running condition.
g.Manufacturing is simple. In fact, the prototype has been made making small changes in hand held battery operated pocket size portable fan.