Working Principle Of Stroboscope 3a4c3w

A stroboscope, also known as strobe, is an instrument used to make a cyclically moving object appear to be slow moving or stationary. The principle is used for the study of rotating, reciprocating, oscillating or vibrating objects. Machine parts and vibrating strings are common examples. Stroboscopic instrument is a monitoring and measuring device that makes use of stroboscopic effects and is employed in the observation of rapid periodic motions. Such instruments are used, for example, to measure the oscillation frequencies of mechanical and electronic systems, to measure resonance frequencies, to study the vibrations of various bodies, and to visually monitor rapidly moving parts of machines. The principle of operation of stroboscopic instruments is as follows: the object performing periodic motion is illuminated and made visible in separate time intervals that are very small by comparison with the period of the object’s motion. If the frequency fstr of the light pulses is the same as the frequency fobj of the period of the object’s motion, then the object appears stationary. When these two frequencies are somewhat different, the object appears to be executing a motion that is slower than the actual motion. The frequency F of the slowed motion is the difference between the two frequencies—that is, F = fobj – fstr.

Fig. Schematic diagram of stroboscope

The stroboscope is in some respects like a movie camera. The camera shutter, operating at very high speed, chops up the action into a series of very small elements in which movement is not apparent in any one element. The film can then be projected at any desired speed, recreating the

original motion at a rate faster than, slower than, or equal to the original motion. The series of projected frames takes on the appearance of continuous, rather than interrupted, motion because of what is known as persistence of vision —the ability of the human eye to hold each image for a fraction of a second, thus filling in the gaps between frames. The enjoyment of movies and the optical illusions of stroboscopy depend to a great extent on persistence of vision.

A movie-camera shutter can produce a stroboscopic effect if it is at or near synchronism with some cyclic motion. If, because of the chopping action of a camera shutter, we are allowed momentary glimpses of a spinning wagon wheel exactly 200 times a minute, and if that wheel is spinning at the rate of 200 revolutions a minute, then at each glimpse of the wagon wheel, it will be in the same position. Since the camera shutter never allows us to see the wheel in any other position, it appears to stand still. If the camera speed is advanced so that we are now given, say, 205 frames a minute, then each frame occurs 1/205 of a minute later than the previous frame. However, the wheel needs more time (1/200 of a minute) to return to a given position so that each successive picture catches the wheel at a slightly earlier part of its cycle. The effect, not uncommon in motion pictures, is that the wheel appears to rotate very slowly backwards. If we slow down the camera speed to 190 frames a minute, each frame captures the wheel at a slightly later part of its cycle, and the wheel appears to rotate slowly forward. Thus, by controlling the rate at which we interrupt vision, we can produce a replica of the high-speed motion at almost any slow speed we desire, forward or backward. Intermittency of observation can be provided by mechanical interruption of the line of sight (as with the motion picture camera) or by intermittent illumination of the object being viewed.

The modern industrial stroboscope is basically a lamp plus the electronic circuits necessary to turn it on an off very rapidly - at rates, in fact, as high as 150,000 flashes per minute. Electronic control of flashing lamp permits accurate setting and knowledge of flashing rate, and this capability leads to the widespread use of stroboscopes as tachometers. If one can make a moving device appear stationary by illuminating it with a light flashing at a rate equal to the device speed, one can also adjust the flashing rate until the device appears stationary and then determine the device speed from knowledge of flashing rate.

Figure. How the stroboscope produces a slow-motion image. The stroboscope above is flashing once every 11/8 revolution of the disk. In A, a single flash catches the disk in its 0° position. In B, while the stroboscope is not flashing, the disk, rotating clockwise, makes better than a full revolution. In C, the next flash catches the disk at its 45° position. The next flash, in E, occurs after the stroboscope has made another 11/8 revolution. The eye, retaining each image it receives for a split second, weaves A, C, and E into an image of apparent slow forward motion.

Types of stroboscope Mechanical In its simplest mechanical form, a rotating cylinder (or bowl with a raised edge) with evenlyspaced holes or slots placed in the line of sight between the observer and the moving object. The observer looks through the holes/slots on the near and far side at the same time, with the slots/holes moving in opposite directions. When the holes/slots are aligned on opposite sides, the object is visible to the observer. Alternately, a single moving hole or slot can be used with a fixed/stationary hole or slot. The stationary hole or slot limits the light to a single viewing path and reduces glare from light ing through other parts of the moving hole/slot. Viewing through a single line of holes/slots does not work, since the holes/slots appear to just sweep across the object without a strobe effect. The rotational speed is adjusted so that it becomes synchronised with the movement of the observed system, which seems to slow and stop. The illusion is caused by temporal aliasing, commonly known as the stroboscopic effect.

Fig: early disk type stroboscope Electronic In electronic versions, the perforated disc is replaced by a lamp capable of emitting brief and rapid flashes of light. Typically a gas-discharge or solid-state lamp is used, because they are capable of emitting light nearly instantly when power is applied, and extinguishing just as fast when the power is removed. By comparison, incandescent lamps have a brief warm-up when energized, followed by a cooldown period when power is removed. These delays result in smearing and blurring of detail of objects partially illuminated during the warm-up and cool-down periods. The frequency of the flash is adjusted so that it is an equal to, or a unit fraction of the object's cyclic speed, at which point the object is seen to be either stationary or moving slowly backward or forward, depending on the flash frequency. Neon lamps or light emitting diodes are commonly used for low-intensity strobe applications, Neon lamps were more common before the development of solid-state electronics, but are being replaced by LEDs in most low-intensity strobe applications. Xenon flash lamps are used for medium- and high-intensity strobe applications. Sufficiently rapid or bright flashing may require active cooling such as forced-air or water cooling to prevent the xenon flash lamp from melting.

Fig: electronic stroboscope