Holmium oxide glass has been used as a wavelength standard for over four decades. These standards have shown very little variation from batch to batch and from one manufacturer to another. NIST has certified and recertified holmium oxide glass samples for over four decades. Over this period of time there has been no recorded instance of a spectral shift of the certified bands for any of the samples measured. Moreover, these samples are known to be robust and relatively insensitive to a normal range of temperature and humidity. Based on the extensive experience that NIST has with this material and its long term stability, NIST will no longer recommend the recertification of these standards. Further, traceability may be established either through the supplier or by the end user without the need for NIST involvement.
Holmium Oxide Glass Wavelengths Standards, Journal of Research (NIST JRES), National Institute of Standards and Technology, Gaithersburg, MD, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=841100
(Accessed April 23, 2021)