Defining Resolution

by Larry Emmott on November 21, 2016

in Cameras,Radiography

lpmmguageLine Pairs per Millimeter (lp/mm). This is a measure of resolution, which refers to pairs of lines pressed tighter and tighter together until they eventually merge into a single line. The more lines that can be pressed into a millimeter and still be seen as individual lines, the higher the resolution. This type of resolution is needed to see the tip of a #8 endo file, it is not useful in finding incipient decay.

Digital radiography sensors can acquire images ranging from 8 to <20 lp/mm. The device used to measure lp/mm like the one shown here on the right only goes up to 20. If a vendor claims a lp/mm resolution of greater than 20 it is theoretical only and not a measured resolution. The unaided human eye can distinguish about 10 to 14 lp/mm. Is that extra resolution lost? Not really that’s where the software takes over.

Pixel and Megapixel. Pixel is short for picture element. Basically it is a dot on the computer screen. (Actually it is a square) The dots are arranged in rows and columns and are so close together they appear to be connected. The dot can be a shade of gray or a color. The more dots that make up an image the better the resolution. A megapixel is simply one million pixels. A typical 1680 x 1050 image has 1,764,000 pixels. If you display a ten Megapixel image on the 1680 x 1050 monitor almost 90% of the image data is discarded.

8 bit or 12 bit. A bit is short for binary digit. In graphics the term refers to the number of different shades or colors a single pixel can display. An 8 bit image is 2 x 2 x 2 x 2 x 2 x2 x 2 x 2 or 2 to the 8th power. If you do the math that comes to 256. Most monitors display an 8 bit image meaning each pixel could be one of 256 different shades of gray. A twelve bit image is 2 to the 12th power or 4,096. True color is 24 bits which allows for 16 million different shades. At most the unaided human eye can distinguish 100 shades of gray. Is the extra detail useless? It depends on the software.

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