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Aperture Card Scanning and Image Quality

Aperture Cards and the Importance of Image Quality

Aperture cards (or AP Cards) are a type of punched card with a cut-out window into which is typically mounted a 35mm frame of microfilm. Predominantly the information stored on an aperture card is usually of some type of reference document, such as an engineering drawing or plan. Due to the amount of detail these images hold it is essential to scan at an optimal image quality to ensure an accurate digital copy. It’s all about resolution!

Image Resolution

Resolution is a measurement of how many pixels a scanner can sample in a given image and is measured in a grid. For example, a chessboard with eight squares along each side has a resolution of 8 x 8. If the chessboard had 300 squares along each side, its resolution would be 300 x 300. If scanning an image at a resolution of 300 – that scanner samples a grid of 300 x 300 for every square inch of the image, a single square on the grid is known as a pixel. A one inch grid with 300 x 300 resolution contains 90,000 individual pixels. With a higher resolution scan you get more pixels, with a lower resolution scan you get less.

True Optical Resolution Scanning

A scanner’s optical resolution is determined by how many pixels it can actually see. For example, a scanner using a scanning head with 300 sensors per inch, will sample 300 dots per inch (dpi) along the width of the image. To scan along the length, it will move the scanning head down the image, pausing 300 times per inch, to scan 300 dpi along the length. Each pixel captured is a true digital copy of what the scanner head saw on the original aperture card image.

Interpolated Resolution

Unlike Optical Resolution scanning an interpolated resolution measures how many pixels the scanner can guess at. Through a process called interpolation, the scanner turns a 300 x 300 dpi scan into a 600 x 600 dpi scan by inserting new pixels in between the old ones, and guessing at what light reading it would have sampled in that spot had it been there. The Interpolation scanning process almost always diminishes the quality of the scan, and should therefore be avoided.

A Professional Resolution to Digitising Aperture Cards

At SunRise we have been scanning the world’s microfilm for over two decades. If you’re considering a move away from aperture cards and in to digital we can walk you through all the technical aspects and help you understand the next steps in your digital journey. Should you require assistance we’ll be happy to help.

Please feel free to contact SunRise on 020 8255 2011 or via email at

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