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Understanding WWII Aviation Microfilm

What we can learn from this now outdated media

When microfilm became popular it was the cat’s pajamas, the bees knees, the best thing since sliced bread. Museums, libraries, and yes, even the Air Force, used the technology to preserve their paper records and make documents easier to find. Unfortunately, the practice of converting records to microfilm often meant that the original was destroyed – but thankfully not always! While microfilm made the distribution of large quantities of information substantially easier, there were drawbacks, some of which we are still dealing with today. This blog will briefly discuss the history of microfilm, and more specifically delve into its close ties to the technical information surrounding WWII aircraft.

 

History

“Microform” is the umbrella term used to describe any scaled down reproductions of media on film. The history of scaling down images started earlier than you might imagine, with original roots in the mid 1800’s. There are several types of film that fit into the microform category: microfilm, microfiche, aperture cards, and microcards, but I am only focusing on microfilm in this blog.

If you’ve never seen a roll of microfilm in the flesh, it looks like a larger version of the negative reel in a film camera (if you’re old enough to remember using one of those!), without the perforations on the edges. There are two standard sizes of microfilm reels, 16 and 35 millimeter. 16mm film has a thinner ribbon and contains a larger number of images per roll than 35mm film. On a daily basis I typically deal with 35mm film, which usually contains an average of about 650 slides of engineering drawings.

Microfilm During The War

During WWII the need to quickly deliver information was key to victory, and microfilm became the way to accomplish this. Hundreds of thousands (if not millions) of engineering drawings were photographed, and the images were placed on microfilm rolls to assist field office mechanics, technicians, and factory workers do their jobs more efficiently. If you are already familiar with aviation microfilm from WWII you can attest to the insane number of images that were photographed during this time. Every individual part in an aircraft had a drawing that told the fabricators how to manufacture it. On top of the simple single part drawings, there were more complex drawings that illustrated how to assemble these smaller individual parts into larger assemblies.

Drawings for specific aircraft were photographed and put on multiple rolls of microfilm, what I call “sets” of film. The number of microfilm rolls needed for each aircraft varied depending on its size and complexity. For example, the set of microfilm for the Boeing Stearman, a popular trainer airplane used at the beginning of WWII, contained 3 rolls of 35mm film with around 2,500 total slides. In contrast, the set of film for the Douglas DC-6 contained 47 rolls of 35mm film, with over 27,000 slides!

Drawing Sizes

As you might imagine, the drawings for different aircraft parts were different sizes. For example the drawing for a small bracket or spring could easily be done on an 8 ½” x 11” (or “A” size) piece of paper, while a larger assembly drawing would need much more space to incorporate the adequate details. When the drawings were photographed, the size of the original drawing that could be captured was limited by the camera frame. Smaller drawings were easily photographed, but when the drawings got too large, they had to be photographed in multiple frames.

As I mentioned earlier, 35mm microfilm rolls are made up of approximately 650 “slides”. Each of these slides contains a single image. Smaller drawings are found on a single slide, but if a drawing was very long and had to be photographed in multiple frames, each of those frames ended up as its own slide. Essentially, this means that large drawings had more than one slide associated with a single drawing or part number. Most of the time drawings on a single roll of microfilm were grouped together by size, so smaller drawings were on rolls together and larger, multi-slide drawings would be found on separate rolls. 

Numbers along the bottom margin of drawings helped photographers know that they were overlapping frames appropriately and capturing all the necessary information. These numbers also help to locate information on drawings, similar to the letters and numbers found on the borders of a map. Interesting fact – the numbers on drawings done by North American Aviation are approximately 12 inches apart on the original drawings, meaning that each number you see corresponds to one foot!

A roll of 35mm microfilm and its original air force box from WWII
Each slide on a roll of film is separated by a black border that keeps the images from running together
Three different sets of microfilm for the P-51 Mustang. Each roll is stored in its own box.
This "A" size drawing for the B-25 Mitchell fuel vent line was drawn on the equivalent of our normal 8 1/2" x 11" piece of paper, and would have fit on a single microfilm slide with room to spare. The drawing below for the B-25 cowling installation was too large to fit on a single slide, so it had to be taken in four different photos.

Slide Numbers

If you have utilized microfilm before you will be familiar with “slide numbers”. These are usually a combination of a letter and number, and can be found at the bottom of each slide. Each roll of film in a given set was identified by a letter, so in a set of microfilm that had 5 rolls of film, the rolls would have been identified with letters “A” through “E”. From there, each slide on a roll was given a number. Example: you see a slide that has the slide number “C 438” at the bottom. From this information you can determine that this particular drawing originally came from Roll C, and it was the 438th slide on that roll. The photographers used a small black plate and removable letters and numbers to update the slide number for each drawing they were photographing.

Slide numbers were key to helping technicians find the drawings they were looking for. Every set of film had a “Microfilm Index”. These indexes listed the part numbers of every drawing needed to build an individual aircraft, and their corresponding slide number. If someone needed to look at the drawing for a specific part number, they would find their part number in the microfilm index, and from there its corresponding slide number. Then they would go to the cabinet and pull the specific roll of film they needed. The film would then be placed on a microfilm reader, and the technician would scroll through to the correct slide on the roll to view the drawing. Not exactly quick and easy!

Converting to Microfilm

In the factory, technicians would place original drawings on a light table so that details would be clearly visible, and then photograph them. Occasionally, you will even see a hand or finger in some slides if the operator didn’t move out of the way quickly enough before the photo was taken!

Distribution

Sets of microfilm were distributed to field offices and other locations where aircraft maintenance and manufacturing were taking place. Today there are many sets of microfilm floating around for WWII aircraft, and places like the Smithsonian Air & Space Museum are continually making additional copies from their master sets for people who purchase them. Many sets of WWII era microfilm are original to the 1940s, and sometimes can even be found in their original Air Force boxes!

Quality issues

The age of the microfilm does not always correspond to the quality of the images found on the rolls. Storage conditions, and the amount of use the rolls have seen, are much more likely to affect the images. Microfilm is not the most stable type of media, and tends to do best in conditions that are continuously dark, cool, and under 50% humidity. Microfilm is also prone to what is called “Vinegar Syndrome” which causes the film to become brittle and the images to darken or become cloudy. Once started, there is no way to reverse Vinegar Syndrome, so it’s best to think about how microfilm is being stored to stave off this deterioration.

Rolls of film that were used regularly are often prone to scratches and other general wear and tear. Similar to Vinegar Syndrome, scratches and wear are irreversible, and can cause areas of the film and images to become hard to read or simply illegible.

What You See Is What You Get

Because every image on a roll of microfilm was taken by a person, there is always some level of human error. If the photo was taken with the incorrect exposure, the resulting image could have ended up either darkened in areas, or very light. Occasionally you will even come across images that are blurry. The images on microfilm are not “born digital” like the photographs we see today that were taken using a digital camera. While the resolution of the camera used during WWII is all we have to work with, sometimes you can view more details by adjusting the contrast or brightness of a digitized microfilm image. However, there is no way to “improve” the resolution of an image on microfilm – what you see is what you get.

The horizontal lines on the lower half of this PBY drawing indicate areas where the film has been scratched. This can happen from overuse, or from particles that become stuck in the glass plate that holds the film in place while it passes through the viewer.

Digitizing and Modern Practices

A large part of what we do at AirCorps Library is convert microfilm rolls to digital images, in order to make the information more accessible to mechanics, restoration shops, and enthusiasts. There are a handful of different scanners that perform the task of turning each slide on a roll of microfilm into a separate digital file. These machines range from small desktop versions that work well for scanning specific images when needed, to commercial grade machines that can digitize an entire roll of film in under 10 minutes!

Commercial grade machines are not only fast, but they scan at the highest resolution available for microfilm digitization. The files that are produced by these commercial machines are a resolution that allows us to always be able to print the drawing at its original size, and even much larger if we need it! These high resolution files (we output to JPEG files because they are easier to store and open) are what you see when you view a drawing on the AirCorps Library website. These high resolution files are also what you get when you purchase a download of a drawing from the website.

Why Go Digital?

Once our microfilm has been digitized, we use a specific software program that we developed to rename each individual JPEG file to reflect the part number on the drawing. This practice is what separates AirCorps Library from other sites, and gives our members the ability to search for a part number without using a microfilm index or a microfilm reader. I often explain the benefit as: imagine finding the drawing you need in 5 seconds rather than 20 minutes.

Converting microfilm to digital files can save an incredible amount of time over the course of any project, whether it be a complete aircraft restoration, or just research. But perhaps the most important benefit is that the process allows younger generations to access this information in a way they are familiar with. Without the interest and excitement of young people, the warbird industry will quickly lose its relevance, and I know we can all agree that we can’t let that happen!

Numbers in boxes along the bottom margin equal 12 inches on NAA drawings. In this case the drawing above for the B-25 cowling installation would have actually been 10 feet long!
Here, the slide number H 25 (boxed in red) indicates that this drawing for the DC-6 parking brake pulley could be found on the 25th slide of Roll H. This drawing is also a good example of how much extra space surrounds a small drawing on a slide.
This frame assembly for fuselage station 104 drawing for the P-51 Mustang clearly shows the technician caught in the act of unrolling the drawing. The version that we have on the AirCorps Library website is the retake : )
An example of mild darkening on a BT-13 Valiant drawing
More severe darkening on an A-26 Invader drawing
Lightening of an image can either happen with age, or is the result of a poor photograph. Here, the details of this P-38 Lightning drawing are barely visible. The contrast of drawings can be adjusted to bring minor details info view, but for the most part the majority of information on drawings like this is lost.
Blurry images such as this one for the Beechcraft C-45 are simply due to operator error. This is understandable when you think about how many drawings these technicians were likely photographing on a daily basis!
This scanner is a common desktop option that works well for digitizing microfilm on an as-needed basis.
Larger units like this commercial microfilm scanner are incredibly fast, not to mention accurate, when scanning large batches of film.
This zoomed in area from the F8F Bearcat drawing above demonstrates the quality of scans that are possible. Digitizing at this resolution means that the smallest details are captured - making the drawing usable for any purpose!
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