A PRIMER OF STROBOSCOPY
WHAT IS A STROBOSCOPE?
The electronic stroboscope is a light that can be flashed on and off very rapidly - many hundreds of times per second, in fact, with each flash lasting only about a millionth of a second.
Now WHY, you may well ask, should anyone of sound mind want to flash a light on and off hundreds of times a second? Because it's there? Because it's psychedelic? No.
People all over the world are flashing strobe lights on and off because a light flashing that fast can do some pretty amazing things. Like making a machine that is operating at high speed look as if it's standing still or moving very slowly.
Like making it possible for you to take pictures of almost anything, no matter how fast it's moving, with a simple camera.
Some of the things you can do with a stroboscope are almost unbelievable. Just about anything driven by a motor is fair game - mixers, fans, drills, saws, shavers, etc. You can make them appear stopped. You can make them appear to move slowly forward. You can make them appear to move slowly backward. You can take pictures of 1ightning-fast events, like bottles breaking or bubbles bursting or your nine-year old son washing his face. And, one of the beauties of the stroboscopic technique is that no physical contact is needed with the subject under observation.
HOW DOES IT WORK?
Suppose, for the purposes of explanation, that we are in a dark room with an electronic stroboscope and a three—bladed fan. Let us call the three blades of the fan Blades 1, 2, and 3, respectively. After some groping, we switch the fan on, and it rotates clockwise at, say, l800 revolutions per minute. We also switch the stroboscope on, we set its flashing—rate control so that it is flashing l800 times per minute, and we aim it at the fan, an incredible sight greets our eyes: The fan appears to be standing absolutely still.
What is happening, of course, is that each flash of light occurs with the blades in the same position. If Blade l is at the 12 o'clock position at the time of one flash, it will again be at 12 o'clock at the next flash, and we will not have seen the blade at all during its travel between flashes.
Now suppose that we advance the flashing-rate control so that our l800 rpm fan is illuminated not by l800 flashes per minute but by 1810. If one flash catches Blade 1 at l2 o'clock, the next flash will catch it a bit before it can get all the way to l2 o'clock again. And the next flash will catch it even earlier, and so forth. The fan appears to be turning backwards. (The same principle is behind all those backward- spinning wagon wheels you see in Western movies.)
If we slow down the flashing rate, say to 1790 flashes per minute, Blade l will make a little more than one revolution between one flash and the next, and the fan will seem to be rotating very slowly forward.
When a light is flashing that fast, of course, it seems continuous, the eye having a "memory" that carries one image until the next arrives. The faster the flashes and the motion being observed, the better the illusion. The stroboscope thrives on superspeed machines like high- speed dentists‘ drills.
We set our opening scene in a darkened room for purposes of illustra- tion, but the surroundings really don't have to be dark at all. The stroboscope's flashes are usually much brighter than the surrounding light, and that's all that matters. Of course, you can't expect the stroboscope to compete with direct sunlight; we'll have to consider stroboscopy basically an indoor sport.
The flash of an electronic stroboscope lasts only a millionth of a second or so, and any light that's that short is of interest to photographers. The traditional way to take a picture is to open and close the camera shutter, exposing the film to the subject for a split second. If the subject is a small child moving about, you set the shutter speed faster than usual - to open and close in, say, l/l00th of a second. If you are a sports photographer shooting a key play in a baseball game, you might set your camera so that shutter opens and closes in only l/l000th of a second.
When you want to photograph a rifle bullet in flight, you start looking for something much faster than the fastest camera shutter. A little simple arithmetic tells you that a rifle bullet traveling at a speed of 2700 feet per second moves only about l/30th of an inch in a millionth of a second. That means that it's practically standing still during one flash from a stroboscope. So if you use a dark room, leave the camera shutter open, and arrange for one flash to occur while the bullet is in camera range, you should get your picture.
Of such plotting have many spectacular pictures been born.
HOW DO WE MEASURE SPEED WITH THE STROBOSCOPE?
The point has been made that when a rotating device is illuminated by a stroboscope, and when the stroboscope is flashing at the same rate as the device is rotating, the device will seem to be standing still. From that nugget of truth it is only a hop, skip, and syllogism to another; If a moving device seems to be standing still under stroboscopic light, and we know how fast the stroboscope is flashing, we can easily figure out how fast the device is going.
The modern stroboscope is designed so that its flashing rate is both controllable and accurately known. Therefore, measuring the speed of an object should be a simple matter of turning the flashing-rate control until the object appears motionless, then reading the speed right off the stroboscope dial. But nothing in life is that simple anymore.
Things are complicated slightly by the fact that you can get a stopped image at flashing rates other than the rate at which the object is moving. Returning to our l800-rpm fan, we find that we can make it "stop" with a flashing rate of 900 flashes per minute, . where there is one flash for every two revolutions of the fan. If the fan were four-bladed, we could also product a stopped image by flashing the stroboscope 3600 times a minute. The flashes would then be coming at each half revolution of the fan, but a four—bladed fan looks just the same before and after a half revolution, unless you can distinguish one blade from the other somehow.
But that's not all. The four—bladed fan looks just the same after 1/4 revolution or after 3/4 revolution, or 5/4. The three bladed fan will look the same after l/3 revolution, 2/3 revolution, etc. This means that there will always be many flashing rates at which a moving object will appear to stand still.
It probably has already occurred to you that you could put an end to most of this nonsense by painting one of the fan blades red. Then any red showing in more than one blade position would be a dead giveaway that the stroboscope was flashing at some speed other than the fan speed.
Marking a device so that it no longer looks symmetrical is always a .good idea. Of course, the marking needn't be as drastic as a coating of red paint. It could be yellow. A simple scratch or piece of tape stuck to the device will also do. ‘
Even with markings, doubts remain. The l800 rpm fan will look stationary under strobe light at 900 flashes per minute just as it does at 1800, no matter what the decorations. And it will look the same at 600 flashes per minute (one flash for every three revolutions), although there will be some noticeable flicker as the flashes occur less frequently.
There are two ways of stamping out this last shred of uncertainty. One is a common-sense appraisal of the situation to find out if there really is a reasonable doubt. Usually you know something about the speed of the object before you start out. If it's supposed to be traveling at about 1800 rpm, you probably wouldn't let a stroboscope fool you into thinking it was running only half that fast.
The second approach, for those who want to be sure, is to set the flashing—rate control to the highest setting that produces a stopped single image (you have already marked the device so you know a double image when you see one). If you have the right setting — and you certainly should —— you won't be able to achieve another single image at any flashing rate between that setting and one half that setting. One final wrinkle develops if the machine you're watching goes faster than the stroboscope can flash. This is not likely to happen, because stroboscopes are almost always much faster than machines. But even if you do manage to find a 30,000 rpm machine and your stroboscope has a 25,000 rpm ceiling, the speed measurement is pretty easy. You set the stroboscope to its maximum flashing rate and adjust downward until you have a stopped single image. You note the flashing rate and continue downward until you have another stopped single image, and you note that flashing rate as well. Then you divide the product of these two numbers by the difference, and you have the actual device speed. Suppose, for instance, that the highest flashing rate at which you can obtain a stopped single image is 22,000 rpm and that the next such image appears at 16,500 rpm. Then
22,000 x 16,500
---------------------- = 66,000 RPM
22,000 - 16,500
A neater way to cope with speeds like this is to use a stroboscope designed for them. You can buy a stroboscope that will flash as fast as 150,000 times a minutes, and it is a very rare machine indeed that can go faster than that. With this high—speed stroboscope and a bit of computation, you can measure speeds as high as l,000,000 rpm!
What about linear motion? At this point, you may have the idea that the stroboscope can 5e used only for measuring speeds of rotating devices. Not true. By using a simple device called a linear speed wheel that is mounted on a shaft, you can determine the linear speed Of something like a drive belt. You simply hold the wheel against the belt, adjust the strobe flashing rate until the wheel appears stopped (there's a mark on the wheel), and apply some simple arithmetic to get your answer. Since you know the circumference of the wheel and the flashing rate, just multiply the two to get inches/minute.
STROBOSCOPES AND THEIR ACCESSORIES
Candor compels us to say right off that the highest—quality, highest- performance electronic stroboscopes are those made by General Radio Company.
Of these, there are several types. Two of these answer to the trade name "Strobotac ". The Type l53l Strobotac® electronic stroboscope has a flashing—rate range up to 25,000 per minute; the Type 1538 will flash as fast as 150,000 times a minute. The l538 has a few other things going for it, such as battery as well as ac operation.
Two other General Radio stroboscopic instruments are the Strobolume and the Stroboslave light source. The Strobolume is a high—intensity light for covering large areas; the Stroboslave is a compact, inexpensive unit with a smaller lamp at the end of a five-foot cord -— a perfect arrangement for poking into hard-to-reach places.
A fundamental difference distinguishes the Strobotac from the Stroboslave. The former has a flashing—rate control and can therefore be used to measure speed as well as to create the illusion of 'stopped motion. The Stroboslave, on the other hand, has no flashing- rate control, must be externally triggered, and thus cannot be used as a tachometer, The new 1540 Strobolume, soon to be announced, will be available either with or without a flashing—rate control.
Stroboscopes can also be flashed by external means. Such devices - which can be used to trigger the Strobotac as well as the Strobolume and Stroboslave - may take the form of a mechanical contactor, something like the distributor in your car, or a photoelectric pickoff, which senses light from the object being viewed.
Mechanical contactors work well as long as the speeds involved are relatively low - under 1,000 rpm. The photoelectric pickoff, on the other hand can handle almost any speed. What's more, it doesn't have to be attached to the object being viewed, and this is often a point of some consequence. The pickoff is typically a Small cylinder enclosing a photocell and a lamp. The cylinder is mounted so that the lamp can be aimed at the shaft or wheel or whatever it is you want to look at. If a single small piece of reflective tape is attached to the object you're looking at, the photocell can be triggered by light reflected from this tape, and the stroboscope can thus be synchronized with the cyclic motion being observed.
One problem is that the flash, though synchronized with the action of the wheel or shaft, or whatever, may be occurring at the wrong part of the cycle. This is easily solved by means of an important accessory called a flash delay, which "holds" for a time (selectable) the electrical impulse that passes from a contactor or pickoff to the stroboscope. Thus, to observe the relationship between a cam and its follower at different points in the cycle of the cam, you simply increase the time by advancing the flash-delay control progressively. Insertion of delay doesn't upset synchronization, so the image remains stationary at each control setting.
THE STROBOSCOPE AND THE CAMERA
The flash of a stroboscope takes only about a millionth of a second. Let's pause a moment to consider the significance of this fact to a photographer.
The significance, as you've probably guessed, is that by using the duration of the flash rather than the opening and closing of the camera shutter to control the exposure time the photographer can take high-speed photographs of events that occur in unbelievably brief intervals. Consider: a rifle bullet moves less than a thirtieth of an inch during the flash of a stroboscope. The trick in photographing a rifle bullet, therefore, is in timing the flash to occur as the bullet is passing in front of the open-shutter camera. This is really not as hard as it sounds, though you'll probably shoot a few holes in the wall before you get everything properly adjusted. Details are given in our Handbook of Stroboscopy and Handbook of High- Speed Photography.
The wonderments of high-speed photography with the stroboscope are well beyond the capacity of this little booklet, and we refer the reader interested in this subject to our larger handbooks. One fact deserves emphasis here, however: To take absolutely incredible stop-action pictures you do not need a super Haggendorfer twin—reflex Zoomomatic camera with a roomful of lenses. A simple camera, preferably one with "X" synchronization, will do very nicely.
THE ENDLESS VERSATILITY OF THE STROBOSCOPE
There's no limit to the number of uses for a stroboscope. You can look at, measure the speed of, or photograph machines of all kinds. You can put on dramatic classroom demonstrations, monitor production equipment, measure projectile velocity, observe air currents, measure torque and horsepower, measure the slip between two shafts, calibrate tachometers, create stage effects, study fuel-nozzle spray patterns, and so on into the farthest recesses of your imagination. It has been said that there's a place for a stroboscope wherever things are moving. If this is an exaggeration, it's only a small one. Certainly no modern manufacturing facility lacks the kind of problems that are most easily solved under the strobe's flashing light.
Reprinted from A Primer of Stroboscopy
published by General Radio Corporation.