Building a Book Scanner (Part II: Tray)

Last week, I showed how I built the lectern for my book scanner prototype. This week, let’s have a look at the tray holding the glass needed to keep the book evenly opened.


After completing the lectern, I needed to build a tray (for lack of a better term) that holds the glass that keeps the books evenly opened, and as flat as possible. The complete setup looks like this:


The tray is build like a drawer, with glue and nails rather than finger joints or dovetails (I’m not equipped to make either of those).


The glass itself isn’t held by a groove, just blocks. If I need to change them later, I won’t have to break everything.


The light sources are positioned so that they light evenly, but do not reflect in the glass (from the point of view of the camera).


* *

It’s a rather crude setup, but let’s see it if works. First, let’s pick a book (the book is Vrai zen pour chats). The first raw image looks like this:


It’s white balanced, but misaligned (something that will need to be fixed in the future). Cropped and rotated, the image is satisfactory:


The second image…


… exemplifies one of the problems: removing the glass, turning the page, and replacing the glass may change alignment if you’re not careful! It still gives a satisfactory page:


* *

For now, pretty much everything requires manual intervention. Given a better background—neutral gray or movie-matte green?—the book’s outline, and therefore alignment, could be automagically determined. Automatic cropping could be done as well.

While movie-matte green seems like a good choice, a neutral gray flannel might be better for color balance. Unless you don’t mind taking an image with the Kodak color patches every time you scan a book.

Depth of field might be a problem with a single-lens system. If you decrease the opening (high F-stop) you increase focal depth but also exposure time and you may be liable to vibration effects. To flatten the image, a greater distance and greater focal length may be desirable. Long focal lengths “flatten” the scene and might help compensate for the angle of the lectern, and therefore of the book. Further experiments are clearly needed.

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