The Making of Modern Michigan -"Digitizing Michigan's Hidden Past"

For 1 bit/bitonal images: Scan at 600 dpi. Save in TIF format if available; otherwise save in GIF format

For 8 bit/grayscale images: Scan at 400 dpi. Save in TIF format.

For 24 bit/full color images: Scan at 400 dpi. Save in TIF format.

Create JPG reductions. Size may vary, but 750 pixels wide, with length in proportion to width, is generally a good choice. Always save the TIF version as the master image.

Explanation of Terms:

Bit Depth (or Color Depth):

BIT DEPTH is determined by the number of bits used to define each pixel. The greater the bit depth, the greater the number of tones (grayscale or color) that can be represented. Digital images may be produced in black and white (bitonal), grayscale, or color.

1 bit (bitonal): Each pixel can be either black or white

8 bit (grayscale): Each pixel can be one of 256 shades of gray

24 bit (full color): Each pixel can be one of 16.8 million colors

Bit Depth Examples

Resolution (Pixels Per Inch/Dots Per Inch):

RESOLUTION is the ability to distinguish fine spatial detail. The spatial frequency at which a digital image is sampled (the sampling frequency) is often a good indicator of resolution. This is why dots-per-inch (dpi) or pixels-per-inch (ppi) are common and synonymous terms used to express resolution for digital images. Generally, but within limits, increasing the sampling frequency also helps to increase resolution.

Pixels per inch (PPI) reflect the sharpness of the image

Dots per inch (DPI) is used in printing as measure of the quality of the printed image on the paper. With computers, dpi is a measure of sharpness on a computer monitor.

Image Formats:

FILE FORMATS consist of both the bits that comprise the image and embedded information which tells software applications how to read and interpret the file. File formats vary in terms of resolution, bit-depth, color capabilities, and support for compression and metadata.

TIF or TIFF – Tagged Image File
JPEG – Joint Photographic Experts Group
GIF – Graphic Image File Format

For more detailed information on image file formats, see:
http://www.library.cornell.edu/preservation/tutorial/presentation/table7-1.html

Pixel Dimensions:

PIXEL DIMENSIONS are the horizontal and vertical measurements of an image expressed in pixels. The pixel dimensions may be determined by multiplying both the width and the height by the dpi.

A digital camera will also have pixel dimensions, expressed as the number of pixels horizontally and vertically that define its resolution (e.g., 2,048 by 3,072). Calculate the dpi achieved by dividing a document's dimension into the corresponding pixel dimension against which it is aligned.

For example, an 8" x 10" document that is scanned at 300 dpi has the
pixel dimensions of 2,400 pixels (8" x 300 dpi) by 3,000 pixels (10" x 300 dpi).