Scanning Photographic Processes
 
Andrew Davidhazy
Imaging and Photographic Technology
Rochester Institute of Technology
70 Lomb Memorial Drive
Rochester, NY 14623 USA
andpph@rit.edu

 
Key words: Scanning, imaging, peripheral, panoramic, photofinish  and strip photography, cartography, cartographic projection.
 
Abstract
 
Photographs imply that they are representations of a particular scene in terms height, width and an instant in time. There are cameras that display time itself as a dimension of the final record. These are sometimes called "streak" or “strip” cameras. These cameras can be thought of as strip chart  recorders  where the subject information is gathered optically. This makes streak cameras powerful tools for non-contact measurement of subject changes over time. But they can also be used for other than purely technical applications.
 
In this presentation several improvised cameras of this type based on film and CCD or solid-state technology are presented and illustrated with applications. based the application of a linear CCD removed from an inexpensive hand-scanner and installed in the back of a 35mm camera body. I’ve used them to demonstrate a variety of applications where quantitative data about subject performance is desired and have also applied the camera for more aesthetically oriented purposes such as peripheral and panoramic photography. The cameras and their applications will be described in this presentation.
 
1.  Scanning approach to photography
 
The process of scanning is becoming almost a household word in the field of imaging and indeed photography in general. This is due to the introduction of electronic scanning devices that approach imaging in a fundamentally different mode than the one normally associated with that of our own visual system.
 
We look at the world around us in an  instantaneous fashion and perceive our surroundings all at the same time although we may concentrate on some aspect of a scene more closely than others. Standard photographic methods essentially duplicate our personal experience with remembering a slice of time, an instantaneous reality that presented itself to our eyes at some specific time.
 
Sometimes referred to or classified as 2-dimensional imaging, photography has assigned to it  the characteristic of being a “witness of reality” because it captures an instant in time and makes a faithful record of whatever appeared in front of the camera at the selected time. We assume that photographs are to a large extent a reflection of reality, However, there are a host of film-based as well as digital image acquisition processes that operate in a fundamentally different manner than standard camera and when the images produced by these systems are seen under conventional conditions our perception and understanding of these images becomes confused and our imaginations often stretch to the limit.

At the heart of this alternative approach to image making is the focal plane shutter.This consists of a narrow slit or slot that travels across the film plane of a camera during the course of making a record of the scene it is recording on film (or a digital array). When the rate at which this slot travels across the image plane is short in comparison o the rate that images might move at during the exposure process the final photograph looks very much like the original scene. However, if the images move significantly during the time the slot travels across the image then various types of distortion are introduced in the final photograph. This effect is appreciated by photographers as “focal plane shutter distortion”.

It is a type of distortion that is actually seldom seen. However, as shown in Figure 1., it can be made visible by altering the mechanical characteristics of a shutter or by chance as Henri Lartigue’s famous photograph of a race car appearing to lean forward in its direction of motion while the background shows bystanders and telephone poles leaning in the opposite direction demonstrates.

Generally we assume that there is no such thing as focal-plane shutter distortion. We assume that photographs are made instantaneously all over the surface of the film or of a solid state sensor in a digital camera. However, such  instantaneity of recording , quite possible with standard cameras, is very difficult to achieve with electronic cameras when a truly high quality image are required. Instead, simultaneity of capture is sacrificed and a one dimensional array of CCDs (some call them pixels) is used and it is sequentially exposed to an image.  Because the sensor is a 1-dimensional, not a 2-dimensional, sensor it is much cheaper to manufacture.  In order to capture the full detail in a 3-dimensional scene and reproduce it as a 2-dimensional reproduction (as regular photographs do) the a single row of sensors is made to scan the image plane by mechanically moving it across the image. This means that large amounts of image bearing data can be extracted and stored. This leads to extremely high quality digital images.
 
This process of scanning a scene or the image of a subject has been known in earlier times and has been exploited by various specialized photographic technologies but has never gained the widespread acceptance that the electronic derivatives of these techniques will probably achieve.
 
The images recorded by slits or linear arrays can themselves be stationary in which case the slit or array is moved or the slit or array may remain stationary and the image moved past it. In this manner current electronic imaging systems are related to traditional scanning techniques associated with strip cameras and to traditional cameras equipped with focal plane shutters.
 
2. Streak photographs made with stationary slits or CCD arrays
 
If we turn the “slit-scan” or focal plane shutter method of scanning images around so that instead of moving the slit or the array, instead we move the film or simply “dump” data from a stationary CCD linear array into computer memory, we have an imaging system often described as a “streak” camera system. These cameras, while not often used to capture real looking reproductions of subjects, provide time or timing information that is often crucial for a quantitative analysis of a particular subject.

2.3 Panoramic photography. The last common example of the application of these cameras is in the use of the scanning principle for the making of ultra-wide angle panoramic photographs. Scanning panoramic cameras able to cover angles of view extending to a full 360 degree record. This is accomplished by rotating the strip camera. The film moving beneath the slit (or across the CCD array) then records various locations around the camera at different times and eventually a complete photograph of the scene surrounding the camera is secured.