| Summary
The physiology of vision determines how we perceive images,
regardless of whether they are analog or digital. Similar to most
digital image sensors, the human eye processes images by dividing
light into its three primary colors, red, green and blue. The
human eye is limited to perceiving approx. 40 shades of grey,
200 colors, and a resolution of 10-14 line pairs per mm (at a
viewing distance of 25 cm or 10 in.). Digital images are primarily
characterized by resolution and color depth. Resolution means
clarity and sharpness of an image, commonly expressed in dots
per inch. Color (or bit) depth (typically expressed in bits) indicates
how many colors an image can contain (e.g. 8 bits = 256 colors).
Various file formats are used to store images on a computer, such
as jpg, gif, tiff and png. File formats differ in their purpose,
compression type, bit depth and primary application. Compression
can be either lossy or lossless. For images in dentistry, limited
lossy compression is acceptable since its results cannot be perceived
by the human eye. The most common file format for clinical images
and xrays is jpg.
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Objectives
After studying this chapter, you should be able to:
- explain how the physiology of human vision relates to the
characteristics of digital images and discuss implications for
the space required to store images
- define common terms and acronyms related to images, such
as pixel, megapixel, color (or bit) depth, dpi, lpi, jpg, gif
and tiff
- contrast the resolution of various output media, such as
paper, computer screens and computer projectors to each other
- understand attributes of different image file formats
- discuss the impact and relevance of lossy and lossless compression
in digital imaging
- describe the clinical relevance of various attributes of
digital images
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Benefits
Understanding the material in this chapter will increase
your ability to:
- understand technical specifications of digital imaging devices,
such as intraoral and digital cameras
- choose equipment that produces images suitable for diagnosis,
treatment and patient education
- differentiate between various image attributes and how they
relate to clinical practice
- determine capacities for hard drives and other storage media
for digital imaging
- choose appropriate file formats for image storage
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Key Concepts
- human vision
- primary colors
- rod and cone cells
- color and grey shade perception
- acuity
- resolution
- pixel/megapixel
- color (or bit) depth
- output device characteristics
- image file formats (tiff, bmp, pict, gif, jpg and png)
- compression (lossy and lossless)
- compression artifacts
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