Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Multiflash photography

Multiflash photography creates successive images at regular time intervals on a single frame.

Method 1: Using a digital camera in multiflash mode 
You can transfer the image produced direct to a computer. 
 
Method 2: Using a video camera 
Play back the video frame by frame and place a transparent acetate sheet over the TV screen to record object positions. 
 
Method 3: Using a camera and motor-driven disc stroboscope 
You need a camera that will focus on images for objects as near as 1 metre away. The camera will need a B setting, which holds the shutter open, for continuous exposure. Use a large aperture setting, such as f3.5. Digital cameras provide an immediate image for analysis. With some cameras it may be necessary to cover the photocell to keep the shutter open. 
 
Set up the stroboscope in front of the camera so that slits in the disc allow light from the object to reach the camera lens at regular intervals as the disc rotates. 
 
Lens to disc distance could be as little as 1 cm. The slotted disc should be motor-driven, using a synchronous motor, so that the time intervals between exposures are constant. 
 
You can vary the frequency of ‘exposure’ by covering unwanted slits with black tape. Do this symmetrically. For example, a disc with 2 slits open running at 300 rpm gives 10 exposures per second. 
 
The narrower the slit, the sharper but dimmer the image. Strongly illuminating the objects, or using a light source as the moving object, allows a narrower slit to be used. 
 
Illuminate the object as brightly as possible, but the matt black background as little as possible. A slide projector is a good light source for this purpose. 
 
Method 4: Using a xenon stroboscope 
This provides sharper pictures than with a disc stroboscope, provided that you have a good blackout. General guidance is as for Method 3. Direct the light from the stroboscope along the pathway of the object. 
 
In multiflash photography, avoid flash frequencies in the range 15-20 Hz, and avoid red flickering light. Some people can feel unwell as a result of the flicker. Rarely, some people have photosensitive epilepsy. 
 
General hints for success 
You need to arrange partial blackout. See guidance note Classroom management in semi-darkness
 
Use a white or silver object, such as a large, highly polished steel ball or a golf ball, against a dark background. Alternatively, use a moving source of light such as a lamp fixed to a cell, with suitable electrical connections. In this case, place cushioning on the floor to prevent breakage. 
 
Use the viewfinder to check that the object is in focus throughout its motion, and that a sufficient range of its motion is within the camera’s field of view. 
 
Place a measured grid in the background to allow measurement. A black card with strips of white insulating tape at, say, 10 cm spacing provides strong contrast and allows the illuminated moving object to stand out. 
 
As an alternative to the grid, you can use a metre rule. Its scale will not usually be visible on the final image, but you can project a photograph onto a screen. Move the projector until the metre rule in the image is the same size as a metre rule held alongside the screen. You can then make measurements directly from the screen. 
 
Use a tripod and/or a system of clamps and stands to hold the equipment. Make sure that any system is as rigid and stable as possible. 
 
Teamwork matters, especially in Method 3. One person could control the camera, another the stroboscope system as necessary, and a third the object to be photographed.  

  • Switch on lamp and darken room. 
  • Check camera focus, f 3.5, B setting. 
  • Check field of view to ensure that whole experiment will be recorded. 
  • Line up stroboscope. 
  • Count down 3-2-1-0. Open shutter just before experiment starts and close it as experiment ends.