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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], = 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

(= 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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