Slit Scan and Strip Photography Overview

Andrew Davidhazy
Imaging and Photographic Technology
School of Photo Arts and Sciences
Rochester Institute of Technology

The  photographs below were made in my special effects class using a traveling slit scan box made out of some wood, a pair of rollers, a high torque variable speed DC gear head motor, and some ancillary parts as described in another article at: http://people.rit.edu/andpph/text-slit-scan.html

They were made  with a Canon Digital Rebel attached to the back of the box and the shutter held open for about 20 seconds or so giving time for the slit up front to move from top to bottom of the front of the box.

      

OK - now where do we go from here? If you do a recording of the video out from a video or still camera that provides a video out all you will see is a traveling slot moving across a dark field.

Now if you could accumulate the image of the moving slot one line at a time and display the resulting summation as the slot moves then you'd be able to see the resulting image "develop" on the screen ... filling up more and more of the frame.

There is a video converter or control box made by Colorado Video that does a similar function. It records any change that rises above a set value - let's say black and displays the result continuously on the video out.

Another method is to obtain a camera that operates on the principle of a "rolling shutter" - the digital equivalent of a focal plane shutter. Most CMOS phone cameras for example have this kind of shutter built-in - but they move the area of sensitivity or "line" very quickly so they are not well suited to make distortions.

Of course, animating still frames is also an option. It is an option that some (many these days) people have "exploited". I believe one of the people who is a promoter of the method is the one who runs the following website and whose name is Bryan Mumford. and his web address is: http://www.bmumford.com/photo/camctlr.html

Golan Levin put together an extensive list of people who photograph or make images based on either continuous recording or extraction of several lines fro each still frame of a video record and ultimate assembly into a final presentation. The list does not seem to discriminate between systems where the slit moves or where the recording surface moves. These are really different systems. One is best described as a "streak" system, another as a slit-scan system and the third as a strip system. In any case his list can be found here: http://www.flong.com/texts/lists/slit_scan/

To illustrate the difference between the three generally referred to slit imaging systems below is a record of the order of finish of racers at end of a race with what is referred to as a strip camera. It was made by looking, fixing the recording field of the camera, to a narrow vertical slit and moving photosensitive material behind it as the images of the racers traveled over the slit. An approximate match between image speed and recording material speed is desirable but not essential. The counterpart of these film cameras in the digital realm are called linear array scanning cameras and they have now pretty much replaced film-type photofinish cameras at most racetracks. Their recording rate is adjusted so that the resulting image of the racers appears more or less appropriate in terms of the aspect ration to the original. But the principle of operation is the same.


Of course one could put the camera in motion relative to the subject. For example by placing it in a moving vehicle and causing the passing landscape or cityscape or row of parked cars to move over the slit in the camera or the linear array in a digital version. As shown below:



The same camera can be used to photograph any object that moves across the slit or linear array orientation. In this mode they are called "strip" cameras and they are used to photograph objects that put the subject features in motion by rotating the subject so it present (for a 360 degree rotation) a different location on its surface to the recording system. These are called peripheral or rollout cameras. They can be sued to photograph archeological specimens or humans as shown in the examples below:





A logical variation is to put the camera in rotational mode and in this case the camera will acquire a 360 degree panoramic record of the surrounding scene.





The alternative is to keep the slit fixed to some location and still move the recording surface or clock a digital linear array at some rate and then have the subject, remaining essentially stationary over the slit or moving along it leave a trace of its image position or size along the slit versus time. These systems are most often referred to as :"velocity recorders" or "time recorders". They are the equivalent of strip chart records but use light instead of a pen to write on a moving surface as lie detectors or seismographs or cardiac monitors work. These cameras or imaging systems can be used to time the duration of an event, velocity of the subject or whether events are simultaneous or not and if not the provide timing error or discrepancy information. These cameras are still widely applied in fusion research to determine the timing and duration of laser pulses to a target area. In the illustration below such a camera is used to determine the velocity of 2 .22 caliber rifle bullets.


To demonstrate the power of these "streak" cameras used to determine simultaneity discrepancies in events that are supposed to be (or that are desirable) to be simultaneous consider the example below. Here a flash is slaved to another one and the difference in the time that the 2nd flash fires is off from the time that the first, or trigger, flash fires is shown to be about 25 microseconds or so.


On the "visual" and possibly creative side the following illustrations also rely on looking at one line in space and having the subject, in this case some colored objects, rotation on a turntable so that their images would traverse the width of the slit on a regular basis.


Finally both in the film realm and the digital one further variations can be achieved by moving the image of the subjects, the location of the slit within the camera and also the orientation of the camera. I don't have many examples of these variations but the one below should serve as an example. It demonstrates that during the time the model turned 180 degrees in front of the camera the camera itself panned across the field of view from one side of her head to the other.



Now for a variation not often seen it is also possible to move the film in rotary fashion instead of linear fashion as in every example included above. This can also be accomplished with digital systems but it calls for a varying rate of image acquisition along the linear array. This results in the information from the linear array to be displayed as a part of a circle. The digital versions are beyond me but the film version is shown below as applied to the accurate peripheral record of a conical teapot and a panoramic record made with a strip panoramic camera where the axis of the camera was vertical but the camera's field of view was elevated. This is unlike linear strip panoramic records where the camera is adjusted to view at right angles to the rotation axis of the camera as shown above.
          

I am positive that there are many other variations possible on this theme. It is all about images being presented to the slit in a film type camera or to a linear array in a digital version of these cameras. The resulting images are all  records of time as opposed to some physical dimension like width or length. These cameras include the time dimension as one of their dimensions and give up the 2nd dimension that is captured by most other standard types of cameras.

These cameras are mostly about time and the relationship of of images that happen to pass by the slit of linear array over time. They are truly records of time itself. Amazing instruments don't you agree?
If you have any coments or observations to share you can reach me at:
Andrew Davidhazy, andpph@rit.edu