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Wavelength ruler developed by EMRP project for on-site wavelength scale calibration of spectroradiometers in the UV wavelength scale

Spectroradiometers, measuring spectral radiance or irradiance, require calibration of their wavelength scale. This is typically accomplished by using wavelength standards, which are either low-pressure discharge lamps filled with pure gas species (helium, neon, argon) or laser sources. In both cases the lines available can be unequally distributed in the spectral range of interest, they can consist of multiplets or they can be of low intensity. Additionally, not all the wavelength standards can be conveniently used for on-site calibrations.

Within the EMRP project Traceability for atmospheric total column ozone (ENV59 ATMOZ), a wavelength ruler for wavelength scale calibration in the spectral range 290–350 nm has been realised. The main idea of this artificial wavelength standard is based on a single-pass Lyot filter. Such a filter generates a regularly spaced transmission as a function of wavelength, which can serve as a reference for (on site) calibration of the wavelength scale of spectroradiometers, like array spectrometers.

The spectral transmission of the ruler has first been measured at the double-monochromator based permanent facility at VSL, used to perform calibration of transmission of filters. Subsequently, the wavelength ruler has been applied for calibration of two different array spectrometers.