Harold 'Doc' Edgerton, "The Electronic Flash Lamp” - Dept. of Electrical Engineering Lecture

[MUSIC PLAYING]

EDGERTON: Now, gentlemen and ladies, the electronic flash equipment always consists of three parts, an energy source and a device for storing the energy and discharging suddenly into a converter, which we call a flash lamp. The object of this talk this morning is to discuss flash lamps to help prepare you for the experiment that you're going to do on flash lamps. Electrically, a flash lamp is an extremely interesting thing. Because it's completely non-linear in every possible way, shape, and form.

And every lamp is individual. It's different than other lamps. So when you get through experimenting, I hope that you will be convinced that this type of device does have these variations.

Here's a typical electronic flash lamp. This is one of the large real powerful ones that's used for stimulating lasers. It's just, discharges capacitor, a very large capacitor with an inductor and series.

And one of the biggest uses of flash tubes is for making laser lights. If I put a high voltage on this, I can break it down. You can see the discharge in the device.

There's about 40,000 volts coming out of this. That's the kind of electricity that doesn't hurt you. You don't treat other kinds of-- electricity the same way.

I did that for effect. You can try this yourself. Don't jump too high when you try it the first time.

I have a small flash tube on this display equipment. This is one of them that you will be using. There are four equipments like this in the laboratory. So no one can, three of you can work simultaneously.

And I have a flash tube on here. And I'm, you'll notice there's a sound associated with this. When the sound is on, the juice is going in. And there's a variac. And I'm watching the voltage, 2,000 volts, 2,500 volts, as high as I can go on this particular unit.

And of course, nothing happens. Because I am below the self-starting voltage. Every flash lamp has a self-starting voltage. And usually we cannot reach this in the laboratory with without going to a lot of trouble.

The self-starting voltage on that little lamp is probably over 4,000 volts. So it'll be off of this scale. But I can make the lamp go at my command by putting on a stimulator.

By an external electrode, it breaks down. And if you look closely, you can see a discharge that follows the same path as the spark wire. This is a very important aspect of this whole concept, is be able to control the flash to occur at the time as you wish it to occur.

Now I'll put on my little spark coil. And I will turn on a high voltage and bring it up to 2,500 volts. And I'll turn on the spark. And of course, nothing happens because I'm down here at too low voltage. So I'll run the voltage up with this tenuator.

This controls the frequency at which it flashes. So that's 10. That's about 2KV. That starts very good. 2KV down here at about 3,500 volts, so I get a point right there.

Now I want to go down to about 2,000 volts where this tube is normally supposed to be used-- 2,000 volts, maybe a little bit less. Now I have to put more voltage on to make it go, going up, going up. Thanks, ouch, it's still on. There we go.

I've got about 2,000 volts on there now. And the spark voltage is still very reliable. Now this is the point, about this, I want you to understand is that reliability is bad.

So sometimes it goes. Sometimes it doesn't go. So I'm up around 2,000 volts. And I'm putting on, according to this curve, about 3,000 volts, somewhere in here. And I get some uncertainties.

Let's make, if down here we get zeros, up here we get Xs. But we get some zeros in between. If I go to 1KV, let's see what happens then.

You're going to have a lot of fun turning these knobs, trying to figure out what's going on. There we are. That's, the uncertainty point is around 30 on here, which is about 6,000 volts, so up in here somewhere, the uncertainty value for 1KV.

There is a curve, a bottom curve, which delineates the no go area, no go. This is a place to stay away from. There's another area which is maybe. And up here is yes, go.

Well, if you ever design a flash equipment, you want it to be in this area and be up here in good and strong. So it never gets down into this point. There's nothing more discouraging than to work marginally with a flash unit where you haven't got enough spark.

So the rule here is put plenty plenty of zip into this thing so that it will go. Now the object of this first experiment, I haven't read the sheet, is to determine that curve for one of these tubes. Most these tubes, I think we've got enough for everybody.

You'll be able to select out of this pile one of these tubes. And there are many different shapes and sizes and dimensions. And you can have the tube that you test if you want it. You have your opportunity of testing as many of these tubes as you want.

If there's a dud, leave it in the box. Don't take it. But only take a good one. How do you tell a good tube?

First of all, it should be able to start at, have a minimum starting voltage somewhere down in here around 400 volts. If it will not start at 400 volts, throw it out. In other words, and also if you have one that starts with a 3,000 or 4,000 volts on it, throw it out. What you want is to get a tube that will be in the range of which you want to use that particular tube.

The electrical characteristics of the tubes are fascinating to me. Of course, the light output is the most important thing. But as electrical people, we must learn something about how they operate electrically.

You'll notice in this display, there's a very nice little filament of electricity that tends to follow the starting wire. If I put more energy in, this film, it gets bigger. And also gives quite a bit more light, and it fills the tube.

And that, of course, is what we want. When we use a lamp, we want it to be efficient. And we want to get lots and lots of light out of it.

Now, the electrical characteristics of these tubes are shown in a slide that will come on the screen. This is a slide of time versus the voltage across. The tube is the upper curve. You'll notice it's marked wrong.

The current rises to a peak and then decays to zero. Usually the voltage comes down and leaves a residual voltage on the capacitor. Now every flash tube it should take will have a different characteristic of its type.

Some of these have these little ripples in them. And they're always different. And this is one thing that makes this type of experimenting interesting is because you never know for sure what is going to come out.

As electrical engineers, we're in such a habit of depending on Ohm's law and the other electrical waves which calculates circuits that we try to do everything we can by calculation. But when you work with flash lamps, you have to back off every once in a while and go to an analog solution of the equations. It's determined by what the tube itself does.

The laws of the tube have not been completely studied yet. So we have to do as best we can by calculating from examples and then rushing to the laboratory. And in a few microseconds, out comes the answer. And it's usually different than what you had. But that is the real answer for that particular tube.

So this is why people that work with electronic flash do lots and lots of experimenting. Because you have to be in the laboratory. And you have to do a lot of these things on these nonlinear devices by using the thing itself in order to find out what is going on.

The second slide shows the same data plotted on a volt ampere curve. On the bottom is current. On the top is volts. At the upper left hand corner, it starts with the maximum voltage you have a cross a tube, such as 2,000 volts.

Then when the current starts, this current rises very rapidly to thousands of amperes. And then it decays slowly as the capacitor discharges. And it comes down to a residual voltage, a small voltage is left on the capacitor.

Now, if the tube did follow Ohm's law, then you could draw a resistance line. Let's go back to the slide once more. There is a straight line in that. Can you back up to that resist-- back up to the other slide. There is a resistance there.

You'll notice that straight line is a resistance line. And everybody that works with flash units gets into a habit very quickly of calculating an approximate resistance. You know that the answers that you're going to get with resistance are going to be wrong.

But they're better than nothing. And so you use them. And one way in which to calculate this resistance is to use the maximum voltage you have on the capacitor, divide it by the maximum current that you get during a discharge.

And you'll notice that that curve, that straight line intersects the real volt ampere curve in several places. And it gives you a sort of an average. And this is a useful concept. Because if you don't know the voltage, you do know the resistance, the tube is defined that way, you can immediately calculate the maximum current, as long as the current is limited entirely by the tube.

Now the people that stimulate lasers also put an inductor in there. So this whole thing collapses. Because the inductor is what determines the maximum peak current that flows. But the resistance concept is still useful because it enables you to estimate for a capacitor discharge the time in which the thing collapses.

The next characteristic I want to show you is the one that you've already experimented with in the laboratory. And you know this from old times, is the voltage from a photo cell that tells you the number of [INAUDIBLE], usually in the millions of [INAUDIBLE] as a function of time. These curves have several bumps and ripples on them.

They rise to a peak value in a certain time and it takes to discharge to rise. And then it decays exponentially, in an exponential form to zero. Well those of us that work in this field define flash duration as the time between third peak, which is shown very clearly in this slide that shows the time duration of a flash.

Now how can all this information be presented into a form where you can use it for designing other tubes or for understanding some kind of a tube? This has all been, all information on a standard tube has been assembled on this sheet that I'll hand you out at the end of the class. It shows the dimensions of the flash tube.

And we're going to look at these curves on the view graph in a minute and try to explain to you what they mean so that you'll be able to use this information. The lamp on which this experimental work was done was a six inch lamp, about that long, of this four millimeter inside diameter, filled with xenon gas at about 20 centimeters. That's about a fifth of an atmosphere.

It's excited externally. And the variables that you have are the voltage across the tube, that capacitance you have connected across the tube. And you want to know a lot of things about this when you want to apply it to some particular problem.

So I'll go over the view graph and show you these curves. Well, these are things that we've picked out. Maybe they're the right things. Maybe they're not.

But the first one that we're interested in is, how much peak current do you get? If you're designing a circuit, how many thousands of amperes do you get? Well, here is a curve of the peak current.

This goes up to about 3,000 amperes. And it's done for capacities that go from one microfarad to 200 microfarad and voltages that go from 500 volts to 3,500 volts. So there's a tremendous range of things that are covered in this experiment.

And I think you realize that the data for this was obtained by going into the laboratory and measuring the peak light as a function of these things and mounting them on this log plot so that you could get the whole data on that particular chart. The full rated value of that particular tube is 100 microfarad at 2,000 volts.

And so here, 2,000 volts, you read 1,200 amperes. And if you want to know the lamp resistance, you divide 2,000 by 1,200. And it's equal to approximately 2 ohms, 2 ohms, close enough. So you say that is a 2 ohm lamp. And from then on, you can deal with it as a 2 ohm lamp.

Other things that are of interest is the peak light. If you're doing experiment in the laboratory and you want to know how many millions of candlepower are coming out, you can measure it. And you go from one million, 10 million, 100 million, doesn't go up that high, with same way capacitors. Go from one microfarad to 200 microfarad. And the voltage covers the same range.

This again covers a tremendous range of light over this range. Now, this standard tube at full load rating again reads eight million, 500, 8.5 times 10 to the sixth candlepower light. Quite often, when we wish to test the photoelectric cell in a laboratory or some device, we drag out this six inch tube.

We put a capacitor of 100 microfarad across it at 2,000 volts. We flash it. And out will come eight and a half million candlepower, which makes it easy for us to do our experiments.

This curve is very important, one flash duration. Because the first thing that you wish to know when you do any experiments on a subject, to stop its motion, you need to know what the flash duration. The flash duration is too long, you'll get a blur.

So here is flash duration running from 10 microseconds up to 500 microseconds for capacities going from 1 to 100. For the different voltages, you'll notice that there's a tendency for it to be flat. It would be flat if this resistance were constant.

But it's not constant. The resistance goes up as you go to lower voltages. And that means these curves go up. And the discharge lasts longer. In other words, if you have a small capacitor on a flash lamp like I showed you, you will have a nice little, skinny, dim filament, which doesn't go anywhere near as high and light level and it lasts a much longer time. So this curve is a very useful one.

Now, the final curve, and I think one of the most useful ones, is the one of efficiency. Efficiency is given on the axis in candlepower per watt. If you could double the efficiency of a flash tube, it would be a great thing. So how do you get the efficiency higher than 5? I just don't know.

But anyway, these are the highs as far as we've been able to get it. But you'll notice that they come up. Again, these are a function of capacity and voltage.

And you would think that you would be able to design all the flash units to be efficiency of 5. Well, that's a good idea. We should keep working on it and try to get those up.

So I think these four curves, you'll have copies of these to take home with you. And you can check these in the lab or as you wish. And you can help to use these for designing other flash tubes.

All the information on designing other flash tubes is given in this book that you've probably heard about. And the rules are quite simple on how to do this. Suppose you want to make your tube twice as long.

The physics of the tube require that you have exactly the same voltage per unit length. So if you have a 6 inch tube with 2,000 volts on it and you make a 12 inch tube, how many volts do you need on it?

STUDENT: 4,000.

INSTRUCTOR: Hooray, 4,000. If you put 100 watt seconds into a 6 inch tube, how many watt seconds do you have to put in the others? The other requirement, you have to put that same number of watt seconds per cubic centimeter in these two tubes. So you'd have to put 200 in the second tube.

And the reasons for this are that the molecules of xenon that are in here are swimming around, all they know is that they have so many joules or watt seconds per molecule and that the electrical gradient in here, which is causing them to ionize, is exactly the same. And if you could get out one xenon molecule from this thing and look at it with an eyeglass, it would react exactly the same in these two tubes. And it would not know whether it was in a 6 inch tube or a 12 inch tube or 1,000 inch tube.

So those rules are simple and straightforward. And it's all documented in the technique. And all I can guarantee to you when you use this theory is that you'll get the wrong answer. Because this theory does not include the electrode effects and the wall effects.

So what you do is use the theory. You can do it quick. And you grind out what the new tube will probably do. And do I need to tell you? You rush into the laboratory and clip it onto one of these various machines that we have there, put the telescope up, and bang, out comes a number, which will tell you exactly how far off your theory is.

We have used this technique a great deal in our various, development of very large strobe equipment. And it's very useful there, like the one up on top of the prudential building is a tube. It's about that long. And it's a tube about that big. And there's about 5,000 watt seconds into it.

And just by taking these dimensional effects, we can hook it up. We can find out what it will do as an optical device. We can get the flash duration. We can get the efficiency. And all these other factors that we need to know can immediately come out via a preliminary try at it.

Now, the next thing I have to show you as is discussed as a problem that I do not know the answer to very well. And it still is one of the most important things in all flash equipment. And that is the life of the tube.

When most people ask me, what is the life of the flash lamp? I say, well, it's usually too long. Of course, that's the view of the man in the factory that's making them. He wants it to go on a lot of production.

But you can make a flash lamp that will run for years and years and years. In fact, the one on the Museum of Science, I think, has been up there about eight or 10 years. It quit about five years ago.

And I rushed over. And I said, glory be. The thing is finally broke. But somebody pulled the plug out of the wall. And it is still going.

And whenever you see the Museum of Science, that little blinker up there, it is, let's see that curve of efficiency. Curve of efficiency, this is a log plot starting at the bottom with one. You don't have to go below a one flash.

I hope none of you will have that experience, although it's a lot of fun. One flash, our lab is full of pieces. Then it goes to 10, 100. Most of the tubes, flash tubes are made have a life of 10 flashes.

And tubes of this type, I think that's a 38. No, that's a different number. That's the tube they're talking about on that curve. It's connected in a series with an inductor.

And that curve, let's look at the curve again. We run that curve up as a function of voltage. And you'll notice, it goes to 1,000 volts and then 2,000 volts. And then at 1, there's a kind of a star.

That's where we increase the xenon concentration in the strobe alley. That xenon comes out there and gives you a increase of, also tube flies around the room. And it's not too healthy a place.

If you're going to do this experiment, get something between you and the lamp. Because the glass doesn't go very fast. But it does get in your hair and causes all kinds of trouble.

Now, the 10, how do you rate a 10 lamp flash? It doesn't blow up after 10 flashes. What happens is that the electrodes will melt away. The tube will get dark and black. And so its efficiency will reduce.

So this type of a lamp, which is used for stimulating these very powerful lasers, they give us a tremendous whack. Its life is only 10 flashes. Then you throw it out and put in four or five new ones.

On the other end of the scale, at the top end of that curve, it goes up to 1,000 and 10,000 and 100,000. Most of the electronic flash units that are used in small portable equipments run around, oh, 100,000. They guarantee 10,000. But you'll have a hard time counting after 10,000. Most of them will run over 10,000.

But the ones that you want to put in beacons run one million and 10 million. And I think the one over there on the Museum of Science has run actually about somewhere around 100 million. And it's still going strong.

Of course, this is done by reducing the energy in the tube and tending to your business. So this curve is a very interesting one to follow. And it's going to become more important as time goes on.

Because one of the advantages of using electronic flash lamp as a beacon in a lighthouse or something is a flash, is a point that it will flash for a long, long time without attention. When I was in high school, I lived in a prairie town out in Nebraska that had a courthouse. It went up about a thousand miles. Snow's only 500, 600 feet.

But one of my jobs as a high school student was to go up there, open the window, and reach out and unscrew a 200 watt bulb when it burned out. And I can still remember how, it looked like it was way up. So now that courthouse has four 10 watt second xenon flash tubes in it. And I service it from Boston about once every six months.

They keep right on going. So I'm still doing the same job I did in 1920 when I was a high school student. I still climb up that darn tower and open the window. But I don't have to reach out so far.

And if any of you go out through the prairie in Nebraska, there will be one unique courthouse. There's one every 25 miles. And that's in rural Nebraska. That's the only one out there with four strobes on it. So it's quite distinctive.

In fact, people driving on the road come trudging in and say, what are those blinking lights doing on that courthouse? I'm going to be out there next week. I'm going to take a few spare tubes. And I'll dust them off, clean them off, put them in.

But those tubes will last a long, long time. This is very important for lighthouses and for beacons. You don't want to change them often. The tungsten lamp, if it's used efficiently, will never run more than a month. If it's used inefficiently, it'll run longer.

Xenon flash tubes, of course, will run, nobody really has the patience to find out how long they actually will go. Now I have just a few slides at the end to kind of wind us up. And I thought I'd show you a few pictures, a few applications, this one.

Next slide will show, I'm very much interested in underwater. Here, the strobe is not used because of its short speed. It's not used because if its, any purpose, but the ability to get out a small battery, enough light to take hundreds and hundreds of pictures.

And it's been my pleasure to work in cameras. We've had them down, oh, we've had them down five miles deep. And they're taking pictures of the bottom of the ocean. There's an awful lot of it to be photographed. And we're still working on this, the angle of it.

The next slide shows a color picture of a balloon with a bullet going through it. In case any of you haven't done this, when you shoot through a balloon, there's several possibilities. If the balloon is only partially deflated, then the bullet will go in and out.

And it will not tear. The air will just squirt out through this hole. And you'll notice that there's a ring, a wave where, it's coming out from where the bullet went in.

The other thing is to blow the balloon up very tight. And when you shoot the bullet through it, it will tear. If you put the light around back, you'll see a little line going through where this tear occurs.

The next slide shows a golfer with mother flash. You've had a chance to do some of this in the laboratory. And I hope you'll keep this in mind. Because this is one of the most interesting types of photography there is, where you put a time sequence of a whole lot of pictures onto a stationary film so that you've got a time record of what goes on.

And I don't need to tell you that you have to move your lights around. So that you do not light the background, you use a black background. You use of white subject and a lot of contrast and so forth and so on.

And the last slide, there's one more in there that shows MIT from, with a 80,000 watt second strobe. There's one of these hanging in Strobe Alley, this one made in 1944 with 80,000 watt seconds and two giant tubes in a big airplane flying up in the sky. And someday I hope to build a real powerful strobe, not one of these types. So this concludes this lecture. And I hope that all of you will have some good experiences in the laboratory.