For some background on this page, please refer to this post.

A Microtek S400 scanner has sufficient resolution and color detail to resolve the watermarks printed on color lasjerjet printers. In Photoshop, you can play around with the color balance and thresholding to see the faint yellow dots, but you can do better! In order to enhance the watermarks I modified my S400 scanner to emit blue LED light instead of the normal white light from the internal CCFL tube.

First, I built a bar of LEDs using surface-mount LEDs that I ordered from Digi-Key. A pair of 22-gauge solid copper wires, stripped back to bare copper, served as the power distribution busses for the LEDs. I soldered the LEDs between the wires, about one every centimeter or so.

Installation of the LED bar turned out to be relatively easy. It just so turns out that the S400 has a slot for a second light source. I’m guessing they designed the scan head to be extendable to some future version with alternate lighting sources. Great news for us! I hot-glued the LED array into the slot and put some strain reliefs on the wires so that I don’t interfere with the scanner mechanics too much during operation.

On the left, the LED array. On the right, the bright white reflective strip is the CCFL light normally used by the scanner. You can see a bit of the blue and green wires that I use to provide power to the LED array to the right.

Here is the LED array in operation.

Now, the tricky part is that the scanner actually calibrates the sensor array prior to scanning. It does a light-dark calibration using a dummy white strip located off the scan area. If you just leave the blue LEDs on, the scanner will actually calibrate out the blue light and scan a relatively normal looking image (when the CCFL is enabled) or just scan darkness (when the CCFL is unplugged). Kudos to Microtek for such a robust calibration sequence!

In order to bypass this system, I had to unplug the CCFL and the LED array during the calibration sequence (it seems to happen at the start of every page scan) and then turn the LED array on once the scan head moves into the page area. Bypassing the calibration yields a very contrasty but somewhat noisy image, but it accomplishes the goal of making the watermark easily visible to the naked eye.

The results can be seen below!

Unretouched scanned image using the modified scanner; approx resolution is 1200 dpi (click to see full-size version).