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References:

Adobe Photoshop CS, Studio Techniques; Ben Wilmore, 2004, Peachpit Press
ISBN: 0-321-21352-1

Adobe Photoshop 7.0, Studio Techniques; Ben Wilmore, 2002, Peachpit Press
ISBN: 0-321-11563-5

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Scanning

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Glossary

Dithering:

Mixing tiles of nearby colours or shades of grey. Dithering looks at the colours or greys in one row of tiles and colours or greys in an adjacent row and then averages the two rows to create a third row between them. The goal is to smooth the transition between contracting elements.

Re-screening:

When you scan photos from an offset printed piece, the halftone screen used in printing will cause image distortions in your scan. Avoid re-scanning printed material. If you must scan it, you may try placing the printed material diagonally on the bed of the scanner. There are also tools in Photoshop which can help reduce such distortions.

Scanning:

When scanning on a flatbed scanner, light is reflected from an original image to an electronic sensing area that records the value for each point on the image. The number of points is your resolution. The amount of grey or colour information sensed for each point is your bit depth.

A Good Scan:

To scan properly, you must obtain good resolution, colour balance, tonal range and sharpness. To scan for print, the resolution must be 1.5 to 2 times the line screen.

Ppi (Pixels per inch):

A measurement of the amount of information scanned from an image.

Dpi (Dots per inch):

A measurement of the output resolution of a printer, copier, imagesetter, or monitor.

Lpi (Lines per inch):

A measurement of the frequency of the halftone screen used to print an image. Lpi is also referred to as "line screen".

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Scanning Line art

Line art would seem to be the simplest to scan, being only black and white pixels. The problem is that it often results in either black splotches or too little detail.

To effictively scan line art images, you need to avoid the “Line Art” setting on your scanner and scan in greyscale.

You need to match the output device’s resolution. Make the resolution the same as the smallest dot your printer can print. Too high a resolution adds no quality to the image, only excessive size.

If your image is in Bitmap mode, you can use the “Image >Image Size” dialog to increase the file’s resolution. Simply check the “Resample Image ” box, select “Bicubic”, enter the resolution of your printer in the “Resolution” field and click “OK”. (The resolution of the printers in the classrooms is 600dpi)

You can allow your self to use this technique on a Bitmap image (line art) because it cannot become blurry, being only black and white pixels. If you did this to colour images, they would become blurry.

See my Unsharp Mask Filter instuctions towards the bottom of this page to sharpen your greyscale image.

Threshold Adjustment Layer

Your image should be in Greyscale mode at this point. To see what your image could look like in true bitmap mode, go to the bottom of the layers palette and add a “Threshold” Adjustment Layer. Move the slider to a position which includes enough detail in your image.

Fine Tuning

You can use the following tools to refine the image quality. Use these tools on the Threshold Adjustment Layer.

• Sharpen Tool
• Burn Tool
• Dodge Tool
• Despeckle or Median Filters

If you simply need an image which looks like it’s in Bitmap mode, you can leave it as it is. If your really need it to be in true Bitmap mode, go “Image >Mode >Bitmap”. This is where you input the resolution of your output device, if you didn’t do it at the start. Click “OK”.

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Scanning Colour Images

When you’re scanning an image or having it scanned at a service bureau, you should consider the maximum size and/or resolution the image may be used.

If you are scanning an image for use in a printed publication, you must use a “pixels per inch” (ppi) number at least twice the line screen (lpi) of the publication.

Common LPI Settings

Publication Type	lpi	ppi
Newspaper advertisements	85 lpi	170 ppi
Newspaper editorial section	100 lpi	200 ppi
Magazines & Brochures	133 lpi	266 ppi
High quality Magazines and brochures	150 lpi	300 ppi
Annual reports & high-end brochures	175 lpi	350 ppi
300dpi laser printers	53 lpi	106 ppi
600dpi laser printers	106 lpi	212 ppi
1,200dpi laser printers	212 lpi	424 ppi