* Astronomy

The sun has two prominent element spectral lines: Hydrogen Alpha and the Calcium K-line (at 3933 Angstroms). A hydrogen-alpha filter, or H-Alpha filter, blocks all parts of the solar spectrum except the alpha spectral line of Hydrogen at 656.281nm (6562.8 Angstroms), and allows you to see fine detail on the solar surface or prominences.

A hydrogen-alpha filter is an optical filter designed to transmit a narrow bandwidth of light generally centred on the H-alpha wavelength. They are characterised by a bandpass width that measures the width of the wavelength band that is transmitted. These filters are manufactured by multiple (~50) layers of vacuum-deposited layers. These layers are selected to produce interference effects that filter out any wavelengths except at the requisite band
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Credit rockinho131

H-Alpha filters are sophisticated and thus expensive.  

2 Angstrom (±0.5 Angstrom) H-alpha-Filter 1'/4"

£662.40

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H-Alpha filters are sophisticated and thus expensive. The quality of the filter is denoted by the bandwidth of the H-Alpha part of the spectrum which the filter passes - a narrower bandpass means less visual "noise" and more detail. The bandpass is measured in Angstroms (one ten-millionth of a millimetre) and traditionally the less expensive H-Alpha filters have been 1.5 Angstrom filters while the most expensive have been 0 .8 or 0.7 Angstrom filters. However, there is a difference in what you can best see, between these two types.
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Venus Transit 2004

Credit BaaderPlanetarium

Note: an emission line H-Alpha filter used for viewing or imaging nebulae or deep sky objects should NOT be used for solar observation. These filters have a too wide bandwidth (15-7nm, and therefore not contrasty enough) compared with a 'proper' solar H-Alpha filter (with a narrow ~0.7nm bandwith. 

An Angstrom is 1/10th of a nanometer; and a nanometer is 1 billionth of a metre.