Triggered Time Lapse

There are many unique photographic possibilities with time lapse sequences taken in response to sensor triggered events as opposed to simple fixed intervals of time. This example uses the impulse mechanism of a precision English pendulum clock made by the Synchronome company. The clock has a "count wheel" that causes a weighted arm to fall and give a push to the pendulum on every every 30th swing of the pendulum. The adjustments that cause the arm to fall are critical, but the action happens so quickly it's nearly impossible to see it clearly. The Time Machine can easily take a sequence of pictures of this action that are synchronized to the motion of the pendulum, each frame of which is taken at a slightly longer delay into the action. The Time Machine can skip 29 swings of the pendulum so that the exposures are taken only during the impulse events and are synchronized to the mechanism.

This movie is remarkable not because it shows the action in slow motion, but because it was created with a digital still camera rather than an expensive high speed movie camera. It was created using the Time Machine "Setups" mode. An optical detector was placed near the bottom of the pendulum so that it would trigger the Time Machine on every swing. The Time Machine was configured to trip the shutter of the camera with an increasing delay of a few milliseconds after the trigger event on each exposure. Finally, the Time Machine was configured to "Timeout" for 29 swings of the pendulum. This means that, after the first picture was taken, the camera waited for 29 uneventful swings of the pendulum, and then triggered the next exposure during the next impulse event of the clock mechanism. Becuase the Time Machine can "Timeout" by a number of sensor events it is able to stay synchronized to the action of mechanism.

You will need a QuickTime player to see the movie below. If you don't have the plug-in for your browser, you can get it as a free download from Apple Computer. Click Here to download QuickTime. To play the movie, click on the right arrow button below the image. You can also use the mouse to drag the "play button" manually through the sequence to examine the motion in even greater detail. 

CLICK HERE to download a larger version (667K) of this movie.

 

An explanation of the clock mechanism:
The moving arm at the bottom of the frame is attached to the pendulum rod, which is out of view to the right. As the pendulum swings to the right, a vertical steel support piece near the left side of the image is pulled out from under a horizontal strip that supports a larger brass arm that runs horizontally through the center of the image. This horizontal arm is called the "gravity arm". When the gravity arm is no longer supported, it begins to fall. Mounted to this arm, near the middle of the image, is a roller. This roller runs down the "ski slope" edge of the moving arm, and pushes it to the right. This pushes the pendulum to the right and is what keeps the pendulum swinging. When the roller has come to about halfway down the "ski slope", a switch is closed and an electromagnet pulls the gravity arm back to its horizontal position, and the catch latches to hold it there until the next impulse. This clock mechanism was invented around the turn of the last century and was used in precision pendulum clocks until the 1970s. For more information, search the Internet for "Synchronome".

Here's a similarly made movie of a more conventional clock mechanism. This time the process results in an odd appearance. The escapement seems to be slowed down and you can observe the action in detail, but the gear train appears to be speeded up.

 

The following triggered time lapse movie was also made with the Time Machine "Setups Mode". An optical sensor was arranged near the spout of a container suspended over an inverted champagne glass. The container was filled with milk and a valve was opened just enough to let a drop fall every two or three seconds. The Time Machine was programmed to wait 150 milliseconds after first detecting the drop, then fire two Vivitar model 285 flashes that were aimed at the base of the glass. After each exposure, the Time Machine waited 15 seconds to let the camera store the image, and then opened the shutter of the camera. When the next milk drop fell, an increment of .0005 seconds was automatically added to the previous delay setting and the milk drop was captured a little later. While these pictures are being taken you can stand back and watch, leaving the Time Machine to calculate every delay and take every picture. Or you might even leave the room and do something else. The Time Machine can be configured to take a predefined total number of pictures.

With the lights in the studio turned off, the Time Machine caused the camera to take about 100 automatic exposures in this way. The images were then taken to a personal computer and combined into a QuickTime move. It's interesting to drag the slider in the QuickTime toolbar back through the movie. You can see that, with a frame interval of .0005 seconds, you capture two frames as the drop enters the surface of the pool.