Improved ultraviolet measurement capabilities of NIST’s reference transmittance spectrophotometer

The National Institute of Standards and Technology (NIST) has maintained and operated custom-built reference instruments to establish the national scale for regular spectral transmittance for more than 50 years. In this paper, we report on improvements to the performance of NIST's current iteration of the reference instrument, the reference transmittance spectrophotometer (RTS), in the ultraviolet (UV) and visible spectral range. The improvements include the installation of a new UV source for increased spectral radiance and a monitor line for a reduced noise-to-signal ratio. A new uncertainty budget and validation measurements are presented. A reduction of uncertainties by 70% or more is achieved for UV wavelengths.