Calibrate Your Flat Bed Scanner

The first step is to make a step wedge in Photoshop. The next step is to print out the gray scale step wedge. If you flat bed scan a lot of color photographs, you probably want your photo lab to print out the step wedge. Bring your digital file to the lab for an output. The last step is to flat bed scan the output and make a "levels" adjustment that corrects the step wedge. The "levels" adjustment is then saved or played back on any new flat bed scans.

Here are the steps:
Open Photoshop

File: new
Set up the file: 5x7 inches at 300dpi with a white background. You can set the file up at any size, but 5x7 inches is an inexpensive output from the lab.

Press the letter "D" on the keyboard, and then the letter"X" on the keyboard, this will set the "foreground" color on the toolbar to white. The background color on the toolbar will now be black.

Choose the "linear gradient tool". Click and drag the gradient tool across the white canvas from left to right. At this stage your canvas should look like this:

Now choose:
image / adjust / posterize. Set the levels to 11. Your canvas should now look like this:

Activate the info palette and place the mouse on the white strip. It will read 255,255,255. The black strip will read 0,0,0. The middle gray will read 127,127,127. You now have created a totally neutral step wedge. Bring the file to the lab for an output.

You are now ready to calibrate your scanner. Scan the output on your flatbed scanner with no scanner adjustments. Your adjustments will be made with a "levels" adjustment in Photoshop, and saved to be used on future scans.

Double click the "eyedropper" tool on the tool bar and set the point sample to 3 by 3. For Photoshop 6.0, set the point sample on the top option bar for the eyedropper tool.

The first correction will use the middle eyedropper of "levels" to color correct the scan. Click once on the middle gray eyedropper in "levels".

Run the mouse across the step wedge to the middle gray. Counting from the white, middle gray is six steps over. Look in the info palette and note the RGB readouts. They will probably read different values. This means the scan has a color cast. Click once with the mouse and read the RGB values again. They should now read the same or within 1 of each other. They probably won't read 127,127,127 at this stage, but that's OK. The next step is to use the middle slider in "levels" to adjust the scan so that the info readings will read 127,127,127. Adjust until the mid-tone reads 127,127,127, or within one of each other. Your "levels" adjustment has now corrected the scanner.

The next step is to save this "levels" adjustment so you can play it back on future scans.

The next time you scan an image, open up "levels" and click on the "load" button.

The last step is to highlight the adjustment and click on the "load" button.

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