Inspections, Compliance, Enforcement, and Criminal Investigations


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Date: 8/3/73 Number: 12
Related Program Areas:
Food, Drug, Device, Manufacture and Packaging


The stroboscope is an intense, high speed light source used for visual analysis of objects in periodic motion and for high speed photography. Objects in rapid periodic motion can be studied by using the stroboscope to produce an optical illusion of stopped or slowed motion. When the flash repetition rate of the stroboscope is exactly the same as the object movement frequency, or an integral submultiple thereof, the moving object will appear to be stationary. When the strobe flash rate is near these speeds, a slow motion replica of the actual object motion can be seen. Using this stroboscopic effect, the stroboscope can be used for a wide range of applications, including quality control and inspection operations. Examinations and speed measurements of rotors, gears, cams, shafts, spindles, liquid spray patterns, package filling and sealing, etc., can be realized. The stroboscope has been used to monitor operation of high speed bottle filling and capping machines in breweries. Proper fill can be approximately determined by monitoring the amount of head or foam that spills over the top of the bottles. With the aid of a specially marked disk and a steel rod on which the disk can rotate freely, linear speeds of conveyor belts can be measured. When the disk is held against the moving belt and made to rotate by the belt, a stroboscopic measurement of disk speed can be easily converted to belt feet per minute. The stroboscope is also used to photograph objects at high speed by providing exposure times as small as a fraction of a microsecond.

The General Radio Type 1538-A Strobotac {{Registered Trademark}} is a good example of a versatile commercially available stroboscope. The type 1538-A has two basic operating modes; Internal flash Control and External Flash Control. In the Internal Flash mode, the unit's flashing rate is controlled by an internal oscillator with flash rate adjustable from 110 to 150,000 flashes per minute. This overall range is divided into four direct reading ranges displayed on a large illuminated range control knob. A larger control concentric with the range knob provides precise setting of the flashing rate.

When the Strobotac {{Registered Trademark}} range control is adjusted for a single stationary image, the operator must be careful to avoid being confused by spurious images. There are several ways the operator can identify spurious images. When a single stationary image is located, he can decrease the flashing rate until another single image appears. If this occurs at half the first range control reading, the first reading was the actual speed of the object. If the image appears at some other reading, the first value was a submultiple. The operator can also double the flashing rate and check for a double image. If there is a double image, the first speed was the fundamental. The simplest method is to flip the range switch to the next higher range. Because of the type 1538-A six to one relationship between ranges, a six to one image pattern will appear if the first speed is the fundamental speed. When symmetrical objects (such as a four bladed fan) are viewed, some part of the moving object should be marked to avoid being deceived by harmonic images.

When observed motion is not periodic, an external means of triggering, by the object, is necessary to synchronize the Strobotac {{Registered Trademark}} flashes with the observed object motion. In the External Flash mode, the type 1538-A Strobotac {{Registered Trademark}} can be externally triggered by closure of a mechanical contact or an electrical trigger. To trigger by mechanical closure, the mechanical contact is attached to the observed object in such a manner that the contact closes once during each cycle of the object motion. Mechanical contacts are usually used at low speeds and in situations where loading of the observed object is not critical. The photoelectric pickoff is the most common electrical trigger source used to trigger the strobe. This device contains a photosensitive element that creates an electrical pulse whenever a change in light level occurs, such as reflection from a spot on an object in periodic motion or reception of light through a hole in an object with periodic motion. Various means can be devised to obtain a change in light level.

When using the stroboscope, the user is cautioned not to become careless and touch the object under study or allow clothing to become entangled. Precaution should also be taken when operating the stroboscope in the presence of persons subject to flicker-vertigo. The periodic flash of the stroboscope could induce an epileptic state in such persons, even though previously undiagnosed as epileptic prone.

The General Radio Type 1538-A may be operated from either a 100 to 125 volt, 50 to 60H Z power source or a rechargeable nickel-cadium battery pack. The basic unit consists of a carrying case (containing the electronics) and an attached, hinged, lamp/reflector that swivels 360 degrees. The carrying case cover doubles as an adjustable stand. The 1538-A is 10.63 x 6.63 x 13 inches, weighs 7.5 lbs., and is presently listed on the Federal Supply Schedule at approximately $575. The battery pack, in leather carrying case, with charger, is listed at approximately $125. One General Radio Type 1538-A Strobotac {{Registered Trademark}} with battery pack and one each type 1536-A and type 1537-A Photoelectric Pickoff and type 1531-P2 Flash Delay are now available on loan basis for District use from DEIO/Investigations Branch (HFC-132). The type 1531-P2 Delay provides amplification and power to the type 1536-A Photoelectric Pick-off and is also used to delay the flash so that any element of the observed object cycle may be viewed. Any Districts interested in experimenting with the Strobotac {{Registered Trademark}} or that require further information concerning the Strobotac {{Registered Trademark}} should contact the DEIO/Investigations Branch on (301) 443-3340.


  1. Handbook of Stroboscopy, 1966, General Radio Co., West Concord, Mass.
  2. Handbook of High-Speed Photography, Second Edition, 1967, General Radio Co., West Concord, Mass.

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