This document describes measurement services, instrumentation, and techniques for regular spectral transmittance over the spectral range from 250 nm to 2500 nm at the National Institute of Standards and Technology. Section 2 explains the basic theory in deriving the transmittance, while a more thorough derivation is reserved for the interested reader in Appendix A. Section 3 describes the reference instrument hardware, software, validation, procedures, and uncertainties. Section 4 provides this same detailed information for the transfer instrument. Sample calibration reports and sample data are provided in Appendices B and C, respectively, and Appendix D details instructions for requesting Regular Spectral Transmittance Calibrations. The measurement services are described for the benefit of customers and potential customers, and the instruments, measurement procedures, and uncertainties are described in sufficient detail to allow comparisons with measurement methods at other laboratories.