Methods and Setup

Methods and Setup

Equipment Used:

Equipment setup:

The pipe was suspended above the tank by ring stands and clamps
A hose attached to the sink was attached to the left end of the pipe
The pellet catch was placed behind the suspended pipe to stop the speeding pellet
The flash unit with wide-angle fresnel lens was placed to the right and slightly behind the pipe and a light baffle was put in front of it. This was done in order to backlight the subject and obtain an evenly light image. Also, a variable resistor was used to obtain the shortest flash duration possible.
The gun was placed in the rifle rest directly in front of and center to the pipe. (the gun was moved occasionally to accommodate the changes in water flow. Delay also had to be adjusted for these changes.)
The sound trigger was placed ~.5m away from the gun on the right.
The sound trigger was connected to the interface box and computer which in turn triggered the flash. This was done to allow for precise delay adjustments.
The camera and tripod were setup at a variable angle to the pipe in order to capture several different views of this high speed event.

Diagrams of Equipment Setup:

Top View

Side View

Experimental Method:

In the process of taking a single picture, experimental method went as follows:

All systems turned on: a piezoelectric sound trigger, computer, flash unit, camera, and water flow from faucet.
Intervalometer program on computer (Apple 2+) set to receive the signal from the sound trigger, initiate a specified delay, and set off the flash unit (Vivitar 283).
Camera (Nikon Coolpix 990 or Nikon D1, depending on photo shoot session) set to bulb, F9-F11, manually focused, completely zoomed in, thereby made ready to take a picture.
Protective eyewear readied, then rifle aimed, pumped, and loaded.
Lights turned off. Both lab partners assume ready positions at gun and camera.
Countdown from three, shutter opened on two, gun fired on one. The sound from the gun firing sets off the sound trigger, which signals the computer. The computer initializes a specified delay, then sets off the flash unit.
Lights turned on, water flow shut off.

Producing and Lighting the "water wall":

To produce the water wall we took an aluminum pipe, formerly part of a tent pole, and took a very thin saw and sawed a single slit in it that was about 1-2mm wide and 26 cm long. We attached this pipe to the sink through a tube and fed water directly into it. The slot was pointed down and this produced, depending upon water pressure, a pretty nice water wall. To light this we decided a diffused backlight was appropriate to reduce glare and fully light the entire phenomenon. We knew this would work well because of a previous experiment involving splashes which used backlighting³, which can be found through the NCSSM websites, called Dual Liquids and Splashes on a Coin². We placed a flash unit facing straight at the camera behind the pipe with a piece of frosted plastic in front of the flash unit to act as a diffuser. This seemed to work quite well, as almost all of our pictures had good lighting and contrast with this setup.

Overview of Computer-Interfaced System¹:

We used a computer-interfaced system to control the timing. This system consisted of a Apple II+ computer (w/ Intervalometer software), sound trigger, interface box and finally the flash unit. The sound trigger picks up the sound from the event and sends an electric signal to the interface box. The interface box then interprets this signal and passes it on to the computer. The intervalometer software delays any further signaling for the desired amount. Since this is high speed imaging, these delays are usually in milliseconds. At the right moment a signal is sent back to the interface box and then a signal is sent to discharge the flash.

Imaging, Triggering, and Timing Methods:

To capture the water wall being shot, we synchronized opening the shutter with shooting the gun. We used two different cameras: the Nikon Coolpix 990 and Nikon D1. Both cameras were set on bulb through the entire experiment, so the flash alone provided the light for the image. We chose bulb shutter speed because it allowed completely manual control of whether the shutter was opened or closed. By using the camera on bulb shutter speed we could be positive that the shutter was open to capture the image.
Our sound trigger was placed a set distance from the gun for the entirety of the experiment. When the gun shot, the vibrations in the air were picked up by the microphone, which transferred the signal to our interface box. From the interface box the signal entered the Apple II+. The computer program we used, Intervalometer, then initiated a time delay averaging at four to six milliseconds. After the time delay was complete, the computer output a signal to the interface box which was sent to the flash unit, a Vivitar 283, causing the flash unit to discharge. For all our photographs, the flash unit had a variable resistor inserted in place of its auto-thyristor and dialed to the least resistance possible so that the flash length was always the same and always as short as possible. We chose these settings because the shooting of the water wall was a very high speed phenomenon that would be blurred with a longer flash duration, and we wanted sharp, precise photos.