What is narrowband imaging?
In normal color imaging, three filters (red, green, and blue) are used to separate the primary colors of the visual spectrum. Red, green, and blue (RGB) filters are designed to approximate the color sensitivity of the human eye, so that the resulting image is true color. Each of the RGB filters covers approximately one third of the visual spectrum and the filters overlap slightly so that the whole spectrum is detected by the CCD. (There is sometimes a gap between the green and red filters to block a prominent light pollution emission line, as in the diagram below.) Narrowband filters instead capture only a very small part of the spectrum. They are said to have a narrow bandpass. The bandpass is simply how much of the spectrum the filter allows to pass. This is usually measured in nanometers. The entire visual spectrum runs, approximately, from a wavelength of 400nm (blue) to 700nm (red). Therefore, a typical RGB filter might have a bandpass of 100nm. In contrast, a typical narrowband f