MadSci Network: Physics

Re: more information on stroboscopes?

Date: Sat Nov 4 12:15:06 2000
Posted By: Harry Adam, Research Associate, Research Division, Kodak Limited
Area of science: Physics
ID: 972929587.Ph

Hi, Linda, lucky you - having inherited the stroboscope! You can have a 
lot of fun experimenting with this. Ill try to answer your question, but 
you will have to make some experiments to get the ideal set-up yourself, 
as some things will depend on the type of camera you have, the field of 
view of its lens and things like that.
Anyway, Im going to assume that what you are trying to do is produce a 
single photograph which contains multiple images of your milk drop as it 
falls including just one shot of the drop hitting the plate. That last bit 
is probably the trickiest bit

The first thing you need to work out is roughly how long the sequence will 
take. Newton springs to our help, with his equation: distance = (initial 
velocity x time) + (.5 x acceleration x time squared). In this case, 
initial velocity is 0, acceleration is 32 ft per second per second, so it 
is easy to derive that the time is (roughly) one quarter of the square root of 
height (in feet) through which the drop falls. For example, if the drop 
falls four feet it will take half a second, if through two feet, 0.35 
seconds, and if one foot only a quarter of a second.

Next, you need to decide how many images of the drop, including the one of 
it hitting the plate you want to have showing in your picture. Clearly you 
want them reasonably separated, but if you have too big a gap it is going 
make it more difficult to stop exposing after the drop has hit the plate, 
before the rebound splashes blur the image.

Im going to assume 1 foot as an example, but as I say, it will all depend 
on the camera and the focal length of lens available to you whether you 
will be able to fill your frame with good focus and decent sized images of 
the drops. With a quarter second drop-time, and assuming you might want 10 
images of the drop in the picture, you can see that the frequency of the 
flashes needs to be 10 per quarter second or 40 per second (40 Hz.) 5 
images  20hz will do. In general, the higher the flash frequency the 
closer and more abundant your separate images will be for a given exposure 
time and the faster the subject moves the fewer images you will capture 
between the beginning and end of the action. You could imaging shooting 
only the last foot of descent of the droplet falling from a greater 
height. This would mean a faster speed for a given frequency  another way 
of changing the number of images in the picture. I dont know what range 
of frequencies are available on your stroboscope, so you may need to 
change the drop height or the number of images depending on how it works 

The set-up is important. Because you are exposing the same frame multiple 
times, you dont want fixed objects to be exposed correctly each time  or 
they will end up being very over-exposed. However you do want the drops 
correctly exposed. The fixed objects you must have are the plate and the 
background. If you can make these matt black, they will reflect hardly any 
light and should not show up very much and provide a good contrast for the 
white milk. A largish piece of black velvet would be ideal for the 
background, and you will have to improvise something non-absorbent, but 
matt black to serve as the plate.

To make the exposure, you have some choices  but again it may depend on 
your camera and particularly the type of shutter. SLR shutters have focal 
plane shutters which generally send a slit sort of scanning across the 
film. This makes flash a bit tricky, and the simplest way to overcome this 
is by using the bulb setting which is where the shutter remains open as 
long as you hold the shutter release button down. (For this a cable 
release is best to avoid vibrating the camera.) If on the other had, you 
have a shutter in the lens, you can probably (in this example just set the 
shutter speed at  second. 
You want to do this in the dark or very dim lighting so that the flash 
dominates the exposure  and in fact by varying the level of the 
background lighting you can get a nice effect where the flash causes 
distinct images of the drop in a streak of fainter motion blurred exposure 
caused by the background light.

Now the trick bit  synchronisation. You need to release the drop, fire 
the shutter and flash simultaneously, or if using bulb, close the shutter 
just as the droplet hits the plate. If you can use the camera via a flash 
socket to trigger the flash, that will be easiest, and you can adjust the 
height by trial and error to get the drop just hitting the plate as the 
shutter closes. If you have to use bulb, then open the shutter with a 
cable release in darkness, and arrange to get the drop to fall as you fire 
the flash. (Not easy). You will also want to release the cable, closing 
the shutter as the drop lands  (again, not easy, but not impossible.) 
Providing you are prepared to experiment and burn a bit of film you should 
get some good images. As I have said the hardest thing in all of this is 
synchronisation, as you may be dealing with a pretty short time  a 
fraction of a second, and if you are really clever you might be able to 
design your own system for achieving automatic synchronisation. Theres a 
lot of fun to had trying to do these things  usually more fun than in 
looking at the result!! Good luck!

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