1.
Introduction
Holmium oxide glass is commonly used as a wave-
length standard for the calibration of spectrophoto-
meters. It has a number of desirable features that have
made it a commonly used wavelength standard. It does
not induce a slit positioning error as atomic emission
lamps may. It is also compact, easy to use, and most
importantly, stable over long periods of time.
NIST, formerly known as the National Bureau of
Standards (NBS), began distributing holmium oxide
glass filters in 1961 [1]. The glass was initially devel-
oped by Corning Glass, NY and sold as glass No 3130
[2]. The thickness of the glass selected was typically less
than 3 mm, which provides 1 % transmittance or greater
at the absorption peaks. The absorption bands were
determined by consensus of several laboratories using
different instruments. Eleven bands were selected based
on their sharp and symmetric absorption peaks [1].
NIST has been involved in the dissemination of sever-
al different types of holmium oxide standards. These
include 52 mm square glass, glass within a cuvette
mount, and a liquid solution in cuvette cells. The glass
samples were once offered for direct sale (calibration
service 38050C and 38051C) and the solution was pro-
vided as a Standard Reference Material (SRM 2034)
[3]. NIST no longer provides these samples, as they are
readily available through commercial suppliers, and
traceability for such measurements can be achieved as
described below.
Although both the holmium oxide glass and the solu-
tion are based on the same rare earth oxide, they exhibit
slightly different wavelength bands. Additionally, there
are some near infrared bands in the glass that are not
present in the solution due to absorption by the water.
The bands discussed in this paper should only be used in
conjunction with the glass standard. For the purpose of
this paper, reference to holmium oxide filters refers to
the glass variety and not the liquid solution.
One well recognized difference observed between dif-
ferent batches of holmium oxide filters is the presence or
absence of the 241 nm band, labeled 1 in Fig. 1. Some
filters have either a less distinct absorption peak or no
peak at all. This is likely due to the variation in the base
Volume 112, Number 6, November-December 2007
Journal of Research of the National Institute of Standards and Technology
303
[J. Res. Natl. Inst. Stand. Technol. 112, 303-306 (2007)]
Holmium Oxide Glass Wavelength
Standards
Volume 112
Number 6
November-December 2007
David W. Allen
National Institute of Standards
and Technology,
Gaithersburg, MD 20899-8442
david.allen@nist.gov
Holmium oxide glass has been used as a
wavelength standard for over four decades.
These standards have shown insignificant
spectral variation from batch to batch and
from one manufacturer to another. The
National Institute of Standards and
Technology (NIST) has certified and recer-
tified 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-
more, traceability may be established either
through the supplier or by the end user
without the need for NIST involvement.
Key words: calibration; holmium oxide;
spectrophotometer; wavelength scale.
Accepted: November 30, 2007
Available online: http://www.nist.gov/jres
glass composition causing reduced UV transmittance.
Typically, the remaining bands are sufficient to serve
the needs of the user.
It is important to note that while these standards are
inherently stable with respect to the wavelength scale
(abscissa), the transmittance scale (ordinate), also his-
torically referred to as the photometric scale, can be
subject to change due to temperature, surface contami-
nants and other environmental sources. Therefore, it is
important that these holmium oxide standards not be
used in the calibration of the transmittance scale.
Typically, the transmittance scale is determined using a
set of neutral density filters. For a complete set of pro-
cedures for measuring the performance of spectro-
photometers see reference [4].
The following figures show the typical signature for
holmium oxide glass. Although the band depth may
appear different from those shown here, the band posi-
tion is the key factor. The certified bands are listed in
Table 1.
304
Volume 112, Number 6, November-December 2007
Journal of Research of the National Institute of Standards and Technology
Fig. 1. Spectral transmittance of holmium oxide glass showing 11 NIST certified bands. A line has been inserted for band 8 for
clarification. The position of the wavelength minima is the critical parameter, while the level of transmittance can be considered
irrelevant.
Table 1. Certified wavelengths of minimum transmittance of the
holmium oxide glass filter for a spectral bandwidth of 1 nm. Band 1
may be absent from some glasses due to UV absorbance.
Band
Certified wavelengths [nm], k = 2
1
241.5 + / – 0.2
2
279.3 + / – 0.2
3
287.6 + / – 0.2
4
333.8 + / – 0.2
5
360.8 + / – 0.2
6
385.8 + / – 0.2
7
418.5 + / – 0.2
8
453.4 + / – 0.2
9
459.9 + / – 0.2
10
536.4 + / – 0.2
1 1
637.5 + / – 0.2
2.
Traceability
The transition of the holmium oxide glass standard to
one that does not need direct involvement of NIST
through a calibration service parallels the transition
which was described by Travis, et al. for the holmium
oxide solution [5]. That paper discusses the bands as
intrinsic values with the establishment of traceability
through atomic absorption lines of the Hg lamps
that were used to establish the certified bands. Those
atomic lines are directly traceable to the values of NIST
standards for the SI unit, the meter.
The user of a holmium oxide filter can assess the
presence of the certified bands using a reference such
as another certified holmium oxide filter or, alternative-
ly the user can overlap the spectra for the sample in
question with a reference spectrum (digital copy pro-
vided by NIST). There may be some differences
between the measured sample and the reference, but the
presence of a contaminant would lead to a significantly
different feature not present in the reference spectrum.
Once the user has demonstrated that the same absorp-
tion features are present as the ones shown in Figs. 1
and 2, the user can self-declare traceability to the SI
(International System of Units). Likewise, the same
procedure may be followed by a supplier, which in turn
may provide SI traceability for the holmium oxide
filters sold.
Once a sample has been verified to be holmium
oxide glass, based on the known absorption bands,
there is no need to recertify the sample. Since holmium
oxide is inherently more stable than most instruments,
it is not meaningful to measure the sample with the
intention of calibrating the sample itself. The purpose
of the measurement is instead to identify the standard
as a holmium oxide wavelength standard. The holmium
oxide standard in essence may be considered an intrin-
sic standard.
The certified wavelengths of minimum transmittance
are considered to be valid for spectral bandwidths
not exceeding 2 nm. Spectral bandwidths greater than
2 nm can lead to errors of minimum transmittance due
to asymmetry of the band. The expanded uncertainty
(k = 2) [6] of the certified values is 0.2 nm.
Volume 112, Number 6, November-December 2007
Journal of Research of the National Institute of Standards and Technology
305
Fig. 2. Absorbance peaks of the 11 NIST certified bands are shown for convenience. A line has been inserted for band 8 for clarifi-
cation. The absorbance is a calculated quantity derived from the measured spectral transmittance.
3.
Summary
Based on the extensive experience that NIST has
with holmium oxide glass wavelength standards, NIST
no longer recommends the recertification of the wave-
length standards. Matching to the absorption bands list-
ed in this paper provides traceability to SI through self
declaration.
4.
Disclaimer
Certain commercial equipment, instruments, or
materials are identified in this paper to foster under-
standing. Such identification does not imply recom-
mendation or endorsement by the National Institute of
Standards and Technology, nor does it imply that the
materials or equipment identified are necessarily the
best available for the purpose.
Acknowledgment
The author would like to acknowledge the valuable
input provided by Herbert Hoover (ret.) of Corning
Glass and John Travis (ret.) of NIST.
5.
References
[1] H. J. Keegan, J. C. Schleter, and V. R. Weidner, Ultraviolet
Wavelength Standard for Spectrophotometry, J. Opt. Soc. Am.
51, 1470 (1961).
[2] J. McNeary and Walter Slavin, A Wavelength Standard for
Ultraviolet-Visible-Near Infrared, J. Opt. Soc. Am. 1, 365 (1961).
[3] V. R. Weidner, R. Mavrodineanu, K. D. Mielenz, R. A.
Velapoldi, K. L. Eckerle, and B. Adams, Spectral Transmittance
Characteristics of Holmium Oxide in Perchloric Acid, J. Res.
Natl. Bur. Stds. 90, No. 2, pp. 115-125 (1985).
[4] Standard Practice for Describing and Measuring Performance
of Ultraviolet, Visible, and Near-Infrared Spectrophotometers;
American Society for Testing and Materials; ASTM designa-
tion E 925-02, Philadelphia, PA, 2004.
[5] J. C. Travis, J. C. Acosta, G. Andor, J. Bastie, P. Blatner, C.
Chunnilall, S. C. Crosson, D. L. Duewer, E. A. Early, F.
Hengstberger, C. S. Kim, L. Liedquist, F. Manoocheri, F.
Mercader, A. Mito, L. A. G. Monard, S. Nevas, M. Nilsson, M.
Noel, A. C. Rodriquez, A. Ruiz, A. Schirmacher, M. V. Smith,
G. Valencia, N. van Tonder, and J. Zwinkels, Intrinsic
Wavelength Standard Absorption Bands in Holmium Oxide
Solution for UV/visible Molecular Absorption Spectrophoto-
metry; J. Phys. Chem. Ref. Data Vol. 34, No. 1, 2005.
[6] B. N. Taylor and C. E. Kuyatt, Guidelines for Evaluating and
Expressing the Uncertainty of NIST Measurement Results,
NIST Technical Note 1297 (1994).
About the author: David W. Allen is a research
chemist in the Optical Technology Division of the NIST
Physics Laboratory. The National Institute of
Standards and Technology is an agency of the U.S.
Department of Commerce.
Volume 112, Number 6, November-December 2007
Journal of Research of the National Institute of Standards and Technology
306
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