### Re: definition about line pairs/mm and effective area

Date: Sun Nov 9 17:22:56 2003
Posted By: Harry Adam, Research Associate
Area of science: Microbiology
ID: 1067330559.Mi
Message:

Hello Hensy - line pairs (or strictly lines) per mm is a measure of limiting resolution of an optical system. Line pairs (rather than lines) is a term used in photography – where the lines in question are usually on a test card and are black on a white background. The lines are of equal width to the spaces between them, hence the spaces are in fact also lines – but white instead of black. Whether referred to as pairs or simply lines the measure is exactly the same and is of the number of black lines which can be resolved (counted) in the image system of concern. The result will depend on a number of factors governing the system in question – and often there is one factor which limits the resolution capability – a bit like the weakest link in a chain. This is not always the case however, and several factors will contribute significantly to the final result. See the following previous answer to a question on this: 981137951.Ph

Your situation refers to a digital camera and a microscope. Either the resolution of the camera – its lens or its CCD sensor may be limiting – or it could be the microscope optics. I’m not sure what you are asking with respect to effective area, but usually resolution will be equal in both dimension of the image. To determine whether your system is up to your needs, you need first to understand what resolution you need and then basically your system needs to have a resolution twice that to avoid aliasing. This is a well know theorem – The Nyquist Theorem - (H. Nyquist), which states that an analogue signal may be uniquely reconstructed, without error, from samples taken at equal intervals. (Which is what your CCD will do). The sampling frequency must be equal to, or greater than, twice the highest frequency in the signal. (This is also sometimes referred to as the Sampling Theorem.)

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