The use of slit scan photography is actually quite old. It is often called line-scan, photo finish, or streak photography. Slit scan photography has a rich and colorful history rooted in chemical analog photography. This technique is often used to visualize high-speed events, such as missiles and bullets, although it is probably best known as photo finish photography that is used to determine the outcome of races.
In the past, slit scan photographic systems used a sheet of film that was moved past a slit. These cameras were most commonly used as photo finish cameras at races and, for example, could very precisely measure the time one horse might have won the race by. There were a number of designs of these types of systems. One of the most interesting slit scan cameras had the camera and film moving at the same time to create a panoramic picture. The last camera on the market to use this technique was the Spinner Dolphin 360 made by Lomography.
Modern times have replaced film with digital sensors, making slit scan photography more popular than ever. Slit scan imaging is still used to solve a number of problems found in industrial applications. Quality control inspections found in a high-speed assembly lines can look for product defects in real time. This is the most common uses of the technology today. Unfortunatly, most people have never seen the results of split scan imaging. Admittedly, it’s not the most popular form of photography, but the resulting images are often a surprise to the photographer and can be quite stunning and beautiful in their own right.
To make a slit scan image, a large number of images are collected in video format. To create a slit scan image, one row of pixels is extracted from each frame of the video and placed adjacent to the same row from the next frame. Each row of pixels represents a duration of time. If the video was recorded at a rate of 30 frames a second, then the row of pixels will represent 1/30th of a second. The resulting image is really a representation of motion and time. To figure out how much video to collect, count each frame of the video as one pixel in the width of the image to be part of the final image. For example, when I collected 4 x 60 x 30 or a grand total of 7,200 images, my final image was 1920 high and 7200 pixels wide. The 1920 comes from the video frame placed vertical to get a larger pixel count in that direction.
To extract the slit scan image out of the finished MOV file, I use a useful program written by Martin Dixon called Slit Scan (it’s a free download). The program is available for both Mac and Windows and requires the programming environment called Processing to running. This is currently by far the easiest way to make slit scan images.
To measure the results of a toy car race, students in my class recorded two Matchbox cars going down a track. The video was recorded at 1000FPS with an Edgertronic camera.
Above is a typical photo finish image of a Hot Wheels car race. The red car crossed the finish 114 pixels in front of the blue car, and since each pixel in the horizontal direction represents 1/1000th of a second, the red car won the race by .114 seconds. The slit scan image is a very powerful tool and useful to make image measurements. If the length of the car is known, then the pixels can be measured in the horizontal direction to determine the amount of time for the car to pass a fixed point. From this information, the velocity of the car can be calculated.
Here’s a slit scan image of ocean waves that were recorded when moving. Since each pixel in the horizontal direction represents 1/30th of a second, the wave motion can be measured.
If the camera is rotated, the extracted slit scan becomes a panoramic image. Here, a junction in the college’s hallways is imaged by Nate Dileas, Scott Semler, and Makayla Roof from my high-speed imaging class. Since the camera was moving very slowly, Nate ducked around the camera and was recorded twice. This is a classic strategy for this technology and dates way back. If you look at old panoramic images, you will almost always see at least one individual that is included twice in the photograph.
In this picture, my colleague Dan Hughes volunteered to sit patiently on a rotating chair. The resulting image is a peripheral slit scan image that’s sometimes called a peripheral portrait and it reveals the full circumference of his head. This technique has been used to record Roman vases, and tread wear patterns on tires.
Instead of a human subject, this peripheral slit scan image was collected from a rotating Dahlia flower. The slit of pixels extracted from the video is parallel to the axis of rotation. If the extracted row of pixels is not parallel or taken at an angle, the resulting images can be quite weird and surprising.
I used the same dahlia flower as above, but here the row of extracted pixels is at a 90-degree angle to the axis of rotation. I personally find these images new and exciting to make. Even after making these images for 20 years, I still never quite know what to expect.
Above is an off axis image of another Dahlia flower. This image was extracted from a full resolution .mov file taken with a Canon 5DMkIII.
Here’s the same Dahlia flower movie file used to make a peripheral slit scan image of the flower.
A bouquet of spring tulips that’s imaged off axis makes a unique twisty picture. The flowers were placed on a turn table that took three minutes to make one complete revolution. This image was collected in camera by using a Better Light scan back camera which had the scanner parked in a fixed position. The camera has a feature that lets you take very high resolution panoramic images. The full image requires approximately five minutes to collect, because the camera’s operation is slow.
Here a Dahlia flower is imaged with the axis of the rotation and the camera off set by 15 degrees. Strange effects are quite common with this technique.
This photo of downtown Richmond, Virginia, was imaged by collecting a high-speed video from an iPhone 6 pointed out the window of a moving car. The handheld image shows the aspect ratio is not correct — the car should have been moving slower. Slit scan imaging is commonly used to map the ground from high flying aircraft. Surprisingly, aerial slit scan photography still uses large and long rolls of film even in 2017.
Here I used slit scan to record the patterns on a cone seashell. Another example of peripheral streak imaging.
Here I used the technique to capture the arrangement of corn kernels in Flint corn. Flint corn is also known as Indian corn or calico corn and is a common decorative corn seen here in the United States throughout the Thanksgiving holiday. This image shows the variation of the placement and color of the kernels of corn around the full corncob in one image.
Many images just look strange, like this slit scan image of a historic flashbulb firing. The slit image is used to determine the timing of the flash; the M3 flash bulb is brightest for about 20 milliseconds. As in all of the slit images, time goes from left to right here.
This is an abalone paua shell from New Zealand in an off axis slit scan image. The constant pattern at the bottom is due to the rotation stage being turned off.
Have you ever wondered what beach glass placed on a rotation stage that’s placed on a second rotation stage imaged in off axis silt scan photography would look like? I have, and pictured above is the result. Oddly enough, this image will be familiar to readers that are used to looking at rotation charts for the moons of Jupiter, which are displayed in a similar pattern.
For a number of years, I have been intrigued by the unique patterns displayed by objects in a variety of ways through the use of slit scan imagery. I hope the interested reader will give this technique a try.