What is the characteristic function of a longpass filter?

A longpass filter is designed to allow light transmission above a specified wavelength while blocking or attenuating light below that wavelength. In this case, the characteristic function of the longpass filter is to transmit wavelengths that are above 650 nm. This means that any light with a wavelength over 650 nm passes through the filter, enabling the observation or measurement of that portion of the spectrum.

Understanding the distinction in filter types is crucial. Unlike shortpass filters, which only allow shorter wavelengths to pass, longpass filters specifically serve the opposite purpose. They essentially "cut off" or block wavelengths shorter than the defined threshold—in this scenario, 650 nm—while permitting longer wavelengths to flow through unimpeded. This characteristic makes longpass filters valuable in various applications, including fluorescence microscopy, where detecting longer wavelength emissions is essential.