In astronomy, the term ‘narrowband’ refers to the width of the frequency spectrum over which a given observation takes place, and is used to distinguish between spectral line observations and those of continuum (see the entry for broadband for the latter).

As spectral line emission is by definition over a narrow frequency range, observations of emission in a particular line require a filter which closely samples the frequency range of the line and does not pass excessive continuum emission or confusing emission from other lines at nearby frequencies.

In optical astronomy, narrowband filters typically cover less than 5 nm and are used for imaging emission lines such as doubly-ionised [OIII] at 500.7 nm seen in planetary nebulae, and the H transition at 656.3 nm which is bright in HII regions. In radio astronomy, narrowband filters are typically of order 4 MHz.