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NIST SPECIAL PUBLICATION 260-1 23

U. S. DEPARTMENT OF COMMERCE/Technology Administration

National Institute of Standards and Technology

Standard Reference Materials ®

Infrared Transmittance Standards —SRMs 2053, 2054, 2055, and 2056

S. G. Kaplan and L. M. Hanssen

QC

100

1.U57

NO. 260-123 2001

ional Institute of Standards and Technology

nology Administration, U.S. Department of Commerce NIST CENTENNIAL
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NIST Special Publication 260-123

Standard Reference Materials®

Infrared Transmittance Standards —SRMs 2053, 2054, 2055, and 2056

S. G. Kaplan

L. M. Hanssen

Optical Technology Division

Physics Laboratory

Gaithersburg, MD 20899-8442

U.S. DEPARTMENT OF COMMERCE, Donald L Evans, Secretary

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Karen H. Brown, Acting Director

Issued May 2001
Certain commercial equipment, instruments, or materials are identified in this paper in order to specify the experimental procedure adequately. Such identification is not intended

to imply recommendation or endorsement by the National Institute of Standards and

Technology, nor is it intended to imply that the materials or equipment identified are

necessarily the best available for the purpose.

National Institute of Standards and Technology Special Publication 260-123

Natl. Inst. Stand. Technol. Spec. Publ. 260-123, 23 pages (May 2001)

CODEN: NSPUE2

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NIST Special Publication 260-123

Standard Reference Materials:

Infrared Transmittance Standards—SRMs 2053, 2054, 2055, and 2056

Table of Contents

Abstract 1

1. Introduction 2

2. Material Preparation and Instructions for Use 3

3. Theory of Transmittance for Thin Film Filters 5

4. Measurement Techniques and Instrumentation 8

5. Effects of Temperature and Spatial Non-Uniformity 10

6. Determination of Uncertainty in Transmittance 13

7. Summary 19

8. Acknowledgements 20

Certificate, SRM 2053, Infrared Transmittance Standard 23

Certificate, SRM 2054, Infrared Transmittance Standard 29

Certificate, SRM 2055, Infrared Transmittance Standard 35

Certificate, SRM 2056, Infrared Transmittance Standard 41

iii

NIST SPECIAL PUBLICATION 260-123

Standard Reference Materials:

Infrared Transmittance Standards - SRMs 2053, 2054, 2055, and 2056

S. G. Kaplan and L. M. Hanssen

Optical Technology Division

Physics Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899

ABSTRACT

Standard Reference Materials 2053, 2054, 2055, and 2056 are transmission

filters designed to have nominally neutral attenuation over the 2 um to 25 |um

wavelength region, with optical densities near 1, 2, 3, and 4, respectively. The

filters are made of polished single-crystal Si substrates, 25 mm in diameter and

0.25 mm in thickness, with thin Ni:Cr or Cu:Ni coatings appropriate for the given

attenuation level. They are intended to be used for checking the accuracy of the

ordinate scale of infrared spectrophotometers. The regular spectral

transmittance of each filter has been measured with 8 cnv1 resolution using an

FT-IR spectrophotometer. The uncertainty in transmittance of each filter has

been determined, including both type-A (statistical) and type-B (systematic)

components.

Keywords: attenuation; FT-IR spectrophotometer; infrared filter; optical density;

transmittance; uncertainty analysis

1
1. Introduction

Historically, NIST has developed extensive high-accuracy

spectrophotometry instrumentation and provided U.S. industry with a variety of

Standard Reference Materials (SRMs) and Calibration Services covering

ultraviolet, visible, and near-infrared wavelengths [1-4]. These include both

regular transmittance and reflectance of specular materials, and measurements of

diffuse materials in particular geometries [5]. Equivalent standards and services

covering the thermal infrared spectral region have been much sparser, in part

because of the traditionally smaller demand for them, as well as the technical

difficulties in establishing reliable scales at these longer wavelengths. However,

one standard which has been quite popular is SRM 1921, a polystyrene

transmission wavelength standard covering the 2 \xm to 18 urn wavelength

region [6]. The wide use of infrared spectrophotometry in chemical

identification and analysis has led to the need for wavelength calibration

standards.

With the increasing use of Fourier-Transform infrared (FT-IR)

spectrophotometers for quantitative as well as qualitative analysis of a wide

variety of materials and optical components, the need has grown for standard

materials to test or verify the ordinate scale accuracy of the instruments. Though

FT-IR spectrophotometers provide good spectral resolution, signal-to-noise ratio,

and measurement repeatability, they are subject to more complex sources of

error than are traditional dispersive-based systems [7]. While different types of

measurements and samples will emphasize different sources of error, an

important check of the baseline ordinate accuracy can be obtained by measuring

the transmittance of a neutral filter with an attenuation level similar to the

sample of interest.

A new set of Standard Reference Materials, SRMs 2053, 2054, 2055, and

2056, have been produced. They are intended for use in checking the ordinate

scale accuracy of infrared spectrophotometers for regular transmittance

measurements. Each SRM is a transmission filter consisting of a thin Ni:Cr or Cu:Ni coating on a 25 mm diameter, 0.25 mm thick polished single-crystal Si

substrate. The coatings are designed to produce neutral attenuation over the

2 um to 25 um (2053, 2054, and 2055) or 2 jam to 20 um (2056) wavelength region

for optical densities (OD = -logio(T), where T is the transmittance) near 1, 2, 3, or

4. This special publication discusses in detail the measurement theory and

instrumentation, material preparation, and uncertainty determination for the

certification of these standard reference materials.

2
2. Material Preparation and Instructions for Use

These transmission filters consist of thin metallic alloy coatings on singlecrystal double-side polished <100> Si substrates. The metallic coatings were

deposited by Luxel Corporation* of Friday Harbor, WA, while the Si substrates

were supplied by Virginia Semiconductor, Inc., of Fredericksburg, VA. The

250 um thick, 25 mm diameter substrates were specified to be parallel to within

1 um, and have a resistivity greater than 20 Q/cm. The metallic layers are overcoated with a thin (= 20 nm) layer of SiO to protect them from oxidation.

Table I lists the nominal optical density levels and coating thicknesses for the

different SRMs, while Figure 1 shows a plot of the transmittances versus wavelength for a representative filter from each set. The Ni:Cr films were grown

from a Ni:Cr evaporation source, while the Cu:Ni films were grown by co- depositing Cu and Ni from separate sources. In both cases an ion-beam was

used to improve the density and structure of the deposited films. More details of

the coating design have been given previously [8, 9].

Each SRM 2053 through 2056 is stored in a special container to minimize

degradation of the surfaces of the filter. The plastic wafer tray holds the sample

securely, while the outer anodized aluminum box protects the sample holder and

clearly identifies each filter. When not in use, the filter should be kept in its protective box, preferably in a dessicator cabinet if available. The SRMs are quite

fragile and should be handled only by the edge, with clean plastic or thin rubber

gloves. The surfaces should never be touched. Dust can be removed by gently

blowing the surface with clean, dry air.

SRM NOMINAL OD FILM ACTUAL OD

NUMBER LEVEL COMPOSITION RANGE FROM 2 um

AND THICKNESS TO 25 um

2053 1 20 nm Ni:Cr 0.95-1.05

2054 2 90 nm Ni:Cr 1.8-2.2

2055 3 77 nm Cu:Ni 2.95-3.2

2056 4 97 nm Cu:Ni 4.0-4.5

Table I. Nominal OD levels, film composition and thicknesses, and actual spectral OD ranges

from 2 um to 25 um wavelength for the four SRM filter sets described in this document.

* The use of a trade name or company name is for identification only and does not imply endorsement by

the National Institute of Standards and Technology, nor does it imply that the material or product identified are necessarily the best available for the purpose.

3
10

l

10"

0

oc -2 M

« 10

E

CO

co 10

10

10"

-4

Infrared Transmittance

Standard Reference Materials

c—i

1 1 1 1 1 1—i

1 1 1 1 : ! 1

1

2053 "

; : ; 2054

:

"

: ! 2055

2056

'

10 15 20

Wavelength (|im)

25

Figure 1. Transmittance versus wavelength of representative samples from the four SRM filters described in this publication. The 2053 and 2054 filters are Ni:Cr coatings on Si, while the 2055

and 2056 filters are Cu:Ni coatings on Si. The films provide fairly neutral attenuation over the

2 um to 25 urn wavelength region; absorption features at 9 um and 16 um are due to the 0.25 mm

thick Si substrates.

Only the central 10 mm diameter portion of the filter should be used in the

transmittance measurements. If possible the spectrophotometer beam should be

centered on the sample. The certified transmittance values and associated

uncertainties are intended to be correct for normally incident light. In practice,

the values will be valid for an unpolarized beam with f/3 or slower geometry,

where the central ray is within 2° of the sample normal. For larger angles of

incidence, or polarized beams, the transmittance values will change.

If possible the spectral resolution of the instrument should be set to 8 cm4

, which was that used in the calibration measurements. At higher resolution

(smaller value in cm-1

) Fabry-Perot fringes will appear in the spectrum because of

interference in the substrate. At lower resolution, some of the spectral features

(notably the prominent Si substrate absorption peaks at 9 \xm and 16 jam) will

broaden. The filter temperature should be between 20 °C and 30 °C during the

measurements.

4
3. Theory of Transmittance for Thin Film Filters

The transmittance of a thin metallic layer on an optically thick substrate for TE

polarization, Tte, can be calculated from the following formulas [10, 11]:

lfTE"

flTE'

f2TE'

2fTE'

2n | cos

n ^ -cos -h n

f cos

n

y -cos ( 9 i

) - n f cos f n ycos [ b j tn f-cos (0

f 2n f-cos id f

n 2' cos ( ° 2/ "l" n

f' cosf

n o-cos 9 o - n f cos

f.

n 2'cos ^0 2) + n fcos (9 f

r f2TE ;= " r 2fTE

2-71 •n ^-cos io

f

T 12TE :_Re

n 2'Cos

n i cos

1 lfTE t t2TE exP< 1 'P)

1-

r

flTE r f2TE exp(2 i -(3)

(1)

(2)

(3)

(4)

(5)

(6)

(7)

R 21TE'

r 2fTE+r flTE'exP^ 2 i

l_ rf2TE. rflTE exp(2-i -p)

(8)

T 23TE :_Re

n g-cos (9 ^ n 2

cos

2 n 2

C0S 1 0 2

n 2' cos 1 9 2) n 3' cos " 3

(9)

R 32TE'

4-71

n 2

C0S - n 3 COS

n 2C0S -h n

3

-cos

a := -Im^r^-cos^ 2

A.

^ PTE ^ 23TE exP(~ 0' ^ 2 TE

1 - R 21TE'R 32TEexP(" 2 a d

2

(10)

(11)

(12)

5
For TM polarization, the following equations are used:

2-n ycos 1 0

j lfTM

r flTM :=

n f-cos b

i

) + n

i' cos [o f

n f-cos (0 A - n j-cos (0

f

n ^-cos (0 j) +- n j-cos 0

^

2-n f-cos id

f f2TM

n f-cos it) 2j + n 2' cos y f

n f-cos ( b i ] — n o-cos 2fTM2'COS id

^

n f-cos I b 2] + n 2' cos ( 0

f

r f2TM ;- " r 2fTM

T 12TM :_Re

' n 2'Cos (t

n j cos

1 lfTM' 1 f2TM exP(' -P)

* - r flTM' r f2TM' exP^ ' -P)

(13)

(14)

(15)

(16)

(17)

(18)

R 21TM :_

r 2fTM")" r flTM'exP<-" ' -P)

l - r f2TM r flTM exP( 2i -P>

(19)

T 23TM :_Re

n 3-cos

n 2'cos

2-n 2'Cos id 2

n 3'cos ( b 2

J + n 2'cos lb 3

(20)

R 32TM :_ n 3 COS - n 2'cos

n 3-cos I b 2

j + n 2'Cos I b 3

(21)

T 12TM T 23TM'exP _a ' d

2 TM

1 _ R 21TM R 32TM exP

" 2 a d

2

(22)

Here ni, nf, r\2, and n3 are the complex refractive indices of the incident medium,

film, substrate, and exit medium, respectively. (We take ni=ri3=l for air.) The

incident angle at the first surface is 61, and the remaining angles 0f, 02, and 03 are given by the complex form of Snell's Law, nx sin(0i) - nisin(Oi). These equations

treat the wave in the metal film coherently, but the wave in the substrate

incoherently, an appropriate approximation for the low-resolution spectral data,

free from Fabry-Perot fringes, supplied with these SRMs.

6
The complex index of refraction of the film, n/co), along with the film

thickness df, determines the transmittance level for the filters. The index of

refraction is in turn a function of the complex ac conductivity cr/co) of the film

material:

Here £ is the high-frequency dielectric screening constant contribution from

electronic transitions in the metallic ions.

The functional form of the ac conductivity of the metallic alloys is highly

dependent on the composition and structure of the films. In general, the

behavior has been found to be non-Drude-like at high frequencies [12]. The alloy

compositions and film thicknesses for the four different optical density levels

were largely determined by trial and error, using Eqs. (1-22) as a guide in

modeling the transmittances. The resulting spectra all have less than 0.8 OD

variation over the 2 um to 25 [im wavelength range. The filters attenuate mostly

by reflection, especially for the higher OD levels.

Eqs. (1-22) can also be used to model the incident beam geometry

dependence of the filter transmittances. The sensitivity to incident angle is quite

small, especially for unpolarized light, or light with a conical focussing

geometry. Figure 2 shows the predicted TE and TM polarized transmittances

from 0° to 10° for an OD 1 filter at a wavelength of 10 \im, relative to the normal

incidence transmittance. The approximately ± 2 % change in transmittance at 10°

is fairly insensitive to wavelength and film thickness and depends mainly on the

Si substrate. For an f/3 focussed geometry at near-normal incidence

(corresponding to the geometry in our FT-IR instrument) the two polarization

modes are effectively averaged, leading to a small residual effect of 0.1 % to 0.2 %

due to a slight (5 %) degree of polarization of the beam. The effects of path

length change in the Si substrate at wavelengths where the Si is absorbing, lead

to a maximum change in transmittance for the f / 3 geometry versus normal plane

wave illumination on the order of 0.05 %.

(23)

7
1.02 1—I—I—I—I—I—I-—I—I—I—I—I—I—I—I—I—I—I—I TM /"

--

s /

s

s

s s

s s

s

- -

avg

f

>» v - -

>. X X

X

X

X

N X X X

X

s

\

TE \-

1—I—I—-J—I—I—I-—I—I—I—I—I—I—I—I——I—I—I 0.98

8 10

e (deg)

Figure 2. Simulated change in transmittance of an OD1 filter on Si versus incident angle for TEand TM-polarized light (dashed lines) and average (solid line) for a wavelength of 10 um. The

changes in transmittance for the two polarization modes average out to a very small residual

effect for unpolarized radiation.

4. Measurement Techniques and Instrumentation

The measurement setup is shown in Figure 3. A Bio-Rad FTS-60A FT-IR

spectrophotometer configured with a ceramic-coated globar source, a KBr:Ge

beamsplitter, and a room-temperature pyroelectric detector was used for the

2053, 2054, and 2055 (OD 1, 2, and 3) filter measurements, covering the 2 um to

25 um spectral range. A Bomem DA3 FT-IR spectrophotometer with a 77 K

mercury-cadmium telluride photoconductive detector was used for the 2056 (OD

4) measurements, covering the 2 um to 20 um spectral range. Both systems were

modified by adding half beam-blocks before and after the sample position to

eliminate inter-reflections involving the interferometer, sample, and detector, as

8
has been described previously [13]. Also, a field stop was placed in the sample

compartment before the sample position in order to reduce the effects of thermal

emission from the source aperture of the instrument. The temperature in the BioRad sample compartment was 29 °C, and 24 °C in the Bomem sample

compartment. Both instruments were purged with dry, CC^-free air.

Field block

Stop

Figure 3. Schematic optical layout for the transmittance measurements of the neutral density

filters. The FT-IR spectrometer is modified with the addition of half-beam blocks, a field stop near the sample position, and additional filtering as described in the text, in order to eliminate

major sources of radiometric error.

The transmittance measurements of the 2053 filters were performed using

an empty beam path as the reference. The incident flux was reduced by placing

an ~ OD 1 filter on the field stop, in order to linearize the detector response. The

source aperture of the instrument was set to 2 mm, with an = f / 1 collection

geometry, which is transformed in the sample compartment to a = 6 mm

diameter spot at the sample location, with an f/3 focussing geometry. The filter

transmittance was calculated as

T= J{filter)_

(24)

I (empty)

where I(filter) and I(empty) are the spectra obtained with the sample and blank in

place, respectively. The 2054 and 2055 filters (OD 2 and OD 3) were measured

9
using a 2053 filter as a reference, with the additional filter on the field stop

removed, and the transmittance was calculated as

1 {filter) n-. 7= * I (reference) \^)

I (reference)

where I(reference) is the spectrum recorded with the 2053 filter in place. The 2056

filters were measured using a 2054 filter (OD 2) as a reference, again using Eq. 25

to calculate the filter transmittances. The spot size on the 2056 filters was 5 mm

in diameter, with an f/4 focussing geometry.

The sample and reference spectra were recorded with 8 cm-1 resolution,

using either Hamming or Happ-Ganzel apodization. Each measurement

consisted of an average of from 64 to 4096 scans, giving a total measurement time

ranging from several minutes to approximately 1 hour. The measurements were

repeated several times in succession, alternately moving the sample or reference

into the beam path. Measurements on a subset of the filters were then repeated

approximately 6 months later, after removing and remounting the filters in the

spectrophotometer. These sets of measurements were analyzed to estimate

uncertainty components for repeatability and reproducibility, as discussed in

section 6. The average of all the measurements on a given filter was used for the

calibrated transmittance value for that filter.

5. Effects of Temperature and Spatial Non-Uniformity

In addition to uncertainties in transmittance due to the measurement process

itself, the effects of temperature and non-uniformity in the filter coatings needs to

be investigated in order to certify the transmittance of the filters. These effects

were investigated for a subset of filters from each SRM set in order to estimate

uncertainty components due to temperature dependence and spatial nonuniformity.

(1) Temperature Dependence

The transmittance of the filters is expected to be only weakly temperature

dependent near room temperature. The ac conductivity of the films and the

absorption coefficient of the Si substrate both change with temperature. The

near-ambient temperature dependence was tested by mounting one filter from

each set in an optical access cryostat and measuring the transmittances at 298 K

and 308 K. These data are shown in Figure 4. Each measurement was performed

twice, and the drift between successive measurements was on the order of 0.2 %.

Within this uncertainty due to non-repeatability of the measurements, there is no

observed temperature dependence. Any temperature dependence is expected to

be linear near room temperature. In order to certify the transmittances of these

10
filters over the 20 °C to 30 °C temperature range, we take 0.2% as the uncertainty

component due to temperature dependence.

4 8 12 16 20 4 8 12 16 20

Wavelength (urn)

Figure 4. Measured near-ambient (25 °C to 35 °C) temperature dependence of the transmittance

of a representative filter from (a) SRM 2053, (b) SRM 2054, (c) SRM 2055, (d) SRM 2056. Within

the uncertainty due to drift in the spectrometer between measurements, we did not observe any

change in transmittance over this temperature range.

(2) Spatial Non-Uniformity

The certified transmittance values were obtained using a 5 mm or 6 mm

diameter spot located at the center of each sample. The transmittance of each

filter is in fact non-uniform because of gradients in the thickness of the metallic

films. These gradients are a result of the vacuum deposition process, and are

n
typically less than 1 % of the thickness across the face of the sample. However,

for the higher OD levels, this can lead to a large variation in transmittance. In

order to assess the transmittance uncertainty component due to spatial variation,

we selected two filters at random from each set of 10 and measured the

transmittance with a 3 mm spot at five different locations on the sample, as shown in Figure 5.

Figure 5. Diagram of the five spots on each sample measured to estimate the spatial uniformity

of the transmittance. The beam diameter was nominally 3 mm, and was moved ± 3.5 mm from

the center of the sample in two perpendicular directions.

Figure 6 shows the ratio of the transmittance measured at points 2 through 5

on four samples to the transmittance measured at the center (point #1). As can be seen from this figure, the non-uniformity is on the order of 0.5 % to 0.7 % for

the OD 1 and OD 2 filters, but 3 % to 4 % for the OD 3 and OD 4 filters. The

thickness gradient appears to be fairly uniform over each sample, so a standard

uncertainty component was derived by taking the maximum observed deviation

from the center transmittance and dividing by V3 . Since all 10 filters in each set

were produced in the same deposition run, and mounted on rotating platters

during the deposition, it is expected that they will have similar thickness and

composition gradients.

12
2 4 6 8 10 12 2 4 6 8 10 12 Wavelength (p)

Figure 6. Measured spatial uniformity of a representative filter from (a) SRM 2053, (b) SRM 2054,

(c) SRM 2055, and (d) SRM 2056. The ratio of the transmittance at a given position to that at the center is plotted for the five positions indicated in Figure 5. The non-uniformity over the central 10 mm diameter is on the order of 0.5 % for 2053 and 2054, and 3 % to 4 % for 2055 and 2056.

6. Determination of Uncertainty in Transmittance

The combined uncertainty in transmittance for each filter consists of a number of systematic components, evaluated by Type-B methods, as well as

statistical components, evaluated by Type-A methods [14]. In addition to the

spatial non-uniformity and temperature dependence components described

above, there are a number of other potential sources of error in the measurement

process. Most of these effects have been discussed before [13] in the context of

transmittance measurements on ultra-thin (100 nm thickness) neutral density

filters. Here we briefly discuss the major radiometric error sources for the filter

transmittance measurements and give estimates for the standard uncertainty

components associated with each one.

13
1. Interreflections

Interreflections involving the interferometer, sample, and detector are potentially

a large source of radiometric error. The half beam blocks mentioned in section 4

eliminate most of the interreflected flux components, but we estimate a 0.2 % to

0.3 % residual effect may remain, depending on the reflectance of the filter.

2. Detector nonlinearity

Care was taken to ensure that the detectors used for the transmittance

measurements were operating in a linear regime, by successively attenuating the

beam and looking for changes in apparent transmittance. Also, the spurious

signal at wavenumbers lower than the detector or beamsplitter cutoff frequency

was observed and found to be on the order of 10"4 to 10~3 of the peak single-beam

signal. While no effects of nonlinearity were directly observed for the neutraldensity filters, some false harmonic signals were observed for narrow-band

filters under similar measurement conditions. We take 0.05 % to 0.1 % as standard uncertainty components due to detector nonlinearity.

3. Detector nonequivalence

This effect was clearly observable using the pyroelectric detector with the

unattenuated (= 50 mW) beam from the interferometer, which yielded as much

as a 3 % error in transmittance due to the change in detector temperature

between the sample and reference measurements. However, by limiting the

change in flux on the detector, this effect was reduced, leaving an estimated

uncertainty component of 0.1 % to 0.2 %.

4. Non-source emission

Emitted flux from the source aperture can overfill the sample and cause

errors in the transmittance measurement. This effect was reduced by placing the

field stop in front of the sample, as shown in Fig. 3. Also, emission from the

sample can be modulated by the interferometer and collected by the detector,

yielding an error in measured transmittance. This effect was unobservable with

the pyroelectric (room temperature) detector, and less than 0.1 % with the 77 K

HgCdTe detector used for the OD4 measurements. The estimated uncertainty

component for non-source emission is 0.15 %.

5. Beam nonuniformity

The interaction between the beam and sample nonuniformity leads to an

averaging of the sample transmittance with unknown spatial weighting and thus

contributes to uncertainty in the actual transmittance level. This effect was tested

by flipping the sample over without repositioning it in the beam, and is estimated to contribute a standard uncertainty component of 0.2 %.

14
6. Beam displacement, deviation, andfocus shift

The SRM filters are only 0.25 mm thick, with the maximum angle of incidence

in the f/3 beam about 9.5°. Effects of the sample on the beam geometry were

tested by rotating or tilting the sample, as well as looking at effects of optically

thicker samples. The uncertainty component due to these effects is estimated at

0.1 %.

7. Beam geometry and polarization

As discussed in section 3, the effects of the difference of the measurement

geometry from ideal normal-incidence plane wave irradiation are small due to

the small angles (less than 9.5°) and small degree of polarization of the beam (less

than 5 %). Also, the effects of path length change in the Si substrate due to offaxis rays is small because of the small absorption level (less than 20 %) and large

index of refraction (= 3.4) of Si. The uncertainty component due to these

geometric considerations is estimated at 0.2 %.

8. Sample vignetting

This was tested by placing a blank aperture of the same size as the sample in

the beam path and measuring its transmittance. Some residual effects from

source aperture radiation or scattered light from other parts of the instrument

not rejected by the field stop yield an uncertainty estimate of 0.03 % for sample

vignetting.

9. Sample scattering

The total integrated scatter from a sample was measured using an infrared

integrating sphere. No signal was observed above the 0.02 % threshold of the

measurement, and we take this as a conservative uncertainty component

estimate due to sample scattering.

10. Phase errors

Errors in the calculated phase spectrum can produce large errors in the

transmittance spectrum. In general, the phase spectra of the sample and

reference are individually computed. However, for the higher OD samples this

procedure can cause errors in the lowest signal portions of the spectrum. For the

OD 4 measurements, the phase spectra were computed using the OD 2 reference

spectra. Drift in the phase can produce a false slope in the transmittance curves,

which is folded into the repeatability estimates given below. We estimate the

contributions of phase errors to the uncertainty in transmittance at 0.1 % for

these measurements.

Table II lists the various type-B relative standard uncertainty component

estimates, in percent of the measured value. They are added in quadrature to produce a standard uncertainty component due to type-B effects, <jb. For the

higher OD filters, the dominant source of uncertainty is the spatial nonuniformity of the metallic coatings.

15
UNCERTAINTY SOURCE OD1 OD2 OD3 OD4

Type B

Inter-reflections 0.20 0.30 0.30 0.30

Detector nonlinearity 0.10 0.10 0.05 0.05

Detector nonequivalence 0.20 0.20 0.10 0.10

Non-source emission 0.15 0.15 0.15 0.15

Beam nonuniformity 0.20 0.20 0.20 0.20

Beam displacement, deviation, focus shift 0.10 0.10 0.10 0.10

Beam geometry, polarization 0.20 0.20 0.20 0.20

Sample vignetting 0.03 0.03 0.03 0.03

Sample scattering 0.02 0.02 0.02 0.02

Phase errors 0.10 0.10 0.10 0.10

Sample nonuniformity 0.40 0.40 2.89 3.46

Sample temperature 0.20 0.20 0.20 0.20

Quadrature sum 0.65 0.68 2.93 3.50

Table II. Type-B relative standard uncertainty components for the four different sets of neutral

density filters. Values are given in percent of measured transmittance.

Each filter was measured several times to reduce the effects of drift in the

spectrometer. The measurements were evaluated for repeatability variance, ar2

. A subset of filters from each set of 10 were measured again after a period of

approximately 6 months, in order to test the reproducibility of the values. The

number of measurements for each set of filters is listed in Table III. SRM MEASUREMENT FILTERS REPETITIONS PER

NUMBER DAYS MEASURED DAY

2053 3 10 - day 1 6 - day 1 10 - day 2 2- day 2

4 - day 3 2 - day 3

2054 2 10 - day 1 6 - day 1 3 - day 2 2 - day 2

2055 2 10 - day 1 3 - day 1 10 - day 2 2 - day 2

2056 2 10 - day 1 3 - day 1 4 - day 2 2 - day 2

Table HI. Number of measurements for each filter set on different days, used to establish

repeatability and reproducibility uncertainty components as described in the text. A reproducibility variance, <jr2

, was assigned based on the variation in the mean

value from one set of measurements to the next. A comparison of the two

16
variances was made for each set of filters to determine if the day-to-day variation

in measurements was significant, according to the number of degrees of freedom

in the data:

1 2 7 2 -a. + —at

3 days 54

r (26)

a

2 - " 2 " days

1 ^ " r

1 2 ^ = ^

adays +

5_ 24

(27)

(28)

Eq. (26) applies to the 4 OD 1 filters measured on 3 separate days, Eq. (27) to the

remaining OD 1 and OD 2 filters, and Eq. (28) to the OD 3 and OD 4 filters.

The filter in each set showing maximum daily variance, <7days 2

, was used to

estimate values for this variance component for each set of filters, which were

averaged over several wavelength bands. Values of Gdays are shown in Table IV

as percent of the average measured transmittance for each filter and wavelength

band.

SRM

NUMBER

2 um to 4 um 4 jam to 12 um 12 um to 20 |im 20 m to 25 Jim

2053 0.5 0.5 0.5 0.7

2054 0.5 0.3 0.4 0.8

2055 2.7 2.2 3.0 3.5

2056 2.0 4.0 5.0

Table IV. Relative uncertainty component Odays averaged over four different wavelength bands

for the four sets of filters, in units of percent of measured transmittance value. These were

derived from the maximum variation in daily average observed for filters in a given set.

The repeatability variance, <7r2

, was found to be quite similar for each filter

in a given set, and the average of these variances was used for each filter in the

set. The final type-A variance components, <ta2, for each filter's average

transmittance values were computed for each wavelength position in the

spectrum according to the following formulas:

1 2 1 2 = — a, +-a:

2 ^6

(29)

o\ , 1 2 = <7 , + — cr r days g

' (30)

1 ^2 5 2 — — (J, H £Tr 2

days 24

(31)

„2 , 1 2 = a , + — cx r days 2 (32)

17
where Eqs. (29), (30), (31), and (32) are used for the OD 1, OD 2, OD 3, and OD 4

filters, respectively. Then the type-B uncertainty component for each set of filters

was added in quadrature to the type-A component from the above equations,

ameas = ffA

+crB (33)

For the OD 2 and OD 3 filters, which were measured relative to an OD 1 reference, and the OD 4 filters, which were measured relative to an OD 2

reference, the uncertainty of the reference transmittances were added in

quadrature as well:

2 n

2 _ 2 ,°meas , reference 2 ( ta\ °filter =( Y

2~)* Tfilter (34)

Tfilter ^"reference The final expanded uncertainties were calculated with a coverage factor of 2,

(Tfinai = 2(jfiiter. Thus one set of expanded uncertainty values was generated for

each set of filters of similar OD level. The average transmittance value of each

individual filter over the different measurement days was used as the calibrated

transmittance value for each filter. Table V shows relative expanded

uncertainties at four different wavelengths for the four SRM filter types. Figure 7

shows the transmittance and associated expanded uncertainties for a representative filter from each OD level.

SRM

NUMBER

2 um 4 jim 12 um 20 um

2053 1.5 1.5 1.5 1.7 2054 2.3 2.1 2.2 2.7

2055 7.2 6.8 7.5 7.9

2056 10 11 14 27

Table V. Relative expanded uncertainties at four different wavelengths for the four sets of filters, in units of percent of measured transmittance value.

IX
Wavelength (nm)

Figure 7. Transmittance versus wavelength for representative filters from each SRM set, showing

expanded uncertainties in transmittance. The filters are individually calibrated, but the

uncertainties in transmittance apply to every filter in a given set.

7. Summary

We have produced four sets of neutral-density transmitting infrared filters

designed to be used over the 2 jum to 25 um wavelength region for evaluating the

accuracy of infrared spectrophotometers. The transmittances have been

measured with relative expanded (k=2) uncertainties ranging from 1.5 % for the

OD 1 filters (SRM 2053) to 12 % for the OD 4 filters (SRM 2056). The dominant

source of uncertainty in transmittance for the OD 3 and OD 4 filters is the spatial

non-uniformity of the metallic coatings. The near-ambient temperature

dependence of the filter transmittances has been investigated and found to be

negligible. In addition, the transmittances have been measured over a period of

more than 1 year and found to be stable in time.

19
8. Acknowledgements

The authors gratefully acknowledge the contributions of M. Carroll Croarkin for

consultation in the statistical analysis, and Joylene Thomas for support and

coordination leading to the certification of SRMs 2053 through 2056.

20
9. References

[I] R. Mavrodineanu and J. R. Baldwin, Glass Filters as a SRM for

Spectrophotometry - Selection, Preparation, Certification, and Use - SRM 930,

NBS Special Publication 260-51, U. S. Government Printing Office,

Washington, D. C. (1975).

[2] K. L. Eckerle, J. J. Hsia, K. D. Mielenz, and V. R. Weidner, Regular Spectral

Transmittance, NBS Special Publication 250-6, U. S. Government Printing

Office, Washington, D. C. (1975).

[3] K. D. Mielenz and K. L. Eckerle, Design, Construction, and Testing of a New

High Accuracy Spectrophotometer, NBS Technical Note 729, U. S. Government Printing Office, Washington, D. C. (1972).

[4] K. L. Eckerle, V. R. Weidner, J. J. Hsia, and Z. W. Chao, Extension of a Reference Spectrophotometer into the Near Infrared, NBS Technical Note 1175,

U. S. Government Printing Office, Washington, D. C. (1983).

[5] P. Y. Barnes, E. A. Early, and A. C. Parr, Spectral Reflectance, NIST Special

Publication 250-48, U. S. Government Printing Office, Washington, D. C.

(1998).

[6] D. Gupta, L. Wang, L. M. Hanssen, J. J. Hsia, and R. U. Datla, Standard

Reference Materials: Polystyrene Films for Calibrating the Wavelength Scale of

Infrared Spectrophotometers - SRM 1921, NIST Special Publication 260-122,

U. S. Government Printing Office, Washington, D. C. (1995).

[7] J. R. Birch and F. J. J. Clarke, "Fifty Categories of Ordinate Error in Fourier

Transform Spectroscopy," Spectros. Europe 7, 16-22 (1995).

[8] A. Frenkel and Z. M. Zhang, "Broadband High Optical Density Filters in

the Infrared," Opt. Lett. 19, 1495-1497 (1994).

[9] G. Lefever-Button, J. F. Lindblom, and F. R. Powell, "An Ultrathin Neutral

Density Filter Set for the Infrared," Proc. of SPIE 3425, 64-72 (1998).

[10] O. S. Heavens, Optical Properties of Thin Solid Films, Dover, New York

(1965).

[II] C. J. Gabriel and A Nedoluha, Optica Acta 18, 415 (1971).

[12] Z. M. Zhang, R. U. Datla, and L. M. Hanssen, "Development of NeutralDensity Infrared Filters Using Metallic Thin Films," Proc. of MRS (1994).

[13] S. G. Kaplan, L. M. Hanssen, and R. U. Datla, "Testing the Radiometric

Accuracy of Fourier Transform Infrared Transmittance Measurements,"

Appl. Opt. 36, 8896 (1997).

[14] B. N. Taylor and C. E. Kuyatt, Guidelines for Evaluating and Expressing the

Uncertainty of NIST Measurement Results, NIST Technical Note 1297, U. S. Government Printing Office, Washington, D. C. (1994).

21

Certificate

Standard Reference Material" 2053

Infrared Transmittance Standard

SenalNo.: 1-1

This Standard Reference Material (SRM) is intended for use in checking the accuracy of the transmittance (ordinate)

scale of spectrophotometers in the infrared (IR) spectral region from 2um to 25 um (400 cm"

1 to 5000cm" 1 ). SRM 2053 is a neutral-density transmitting filter with an optical density near 1.0 (OD = -logio(T), where T is the

spectral transmittance). It consists of an approximately 20 nm thick nickel-chromium film on a 250 um thick, 25 mm diameter, high-resistivity, single-crystal <100> silicon substrate. The metal film is overcoated with a thin (30 nm) layer of silicon oxide to protect it against oxidation from the atmosphere. SRM 2053 also includes a floppy

disk that contains the certified transmittance values and associated expanded uncertainties.

Certified Transmittance Values: The transmittance of this filter was measured at equally spaced wavenumber

values from 400 cm"

1 to 5000 cm"1 (2 um to 25 um) with an apodized resolution of 8 cm"1 and a data spacing interval of approximately 4cm" 1

. The transmittance was measured at the center of the filter using a 6 mm diameter spot size with the filter at the focus of an f/3 optical beam in the spectrophotometer. The certified transmittance values and

associated expanded uncertainties at each wavelength are listed in a text file on the accompanying floppy disk and

plotted in Figure 1 . These values and their associated uncertainties are valid for normal incidence transmittance

within a 5 mm radius of the center of the filter and at temperatures from 20 °C to 30 °C.

Expiration of Certification: The certification of this SRM is deemed to be valid until 31 December 2001,

provided the SRM is stored and handled in accordance with the Storage and Handling section of this certificate. Certification will be nullified if the SRM is damaged, contaminated, or exposed to excessive temperatures or humidity.

Maintenance of SRM Certification: The neutral-density filter has been measured over a period of approximately one year, and no significant change in transmittance has been noted. A reference filter is kept at NIST and will be monitored over time. Users will be notified if a significant change occurs before the stated expiration date.

The technical measurements leading to certification were performed by S.G. Kaplan and L.M. Hanssen of the NIST

Optical Technology Division. The overall direction and coordination of the technical measurements were performed

by R.U. Datla of the NIST Optical Technology Division.

Statistical consultation was provided by M.C. Croarkin of the NIST Statistical Engineering Division.

The support aspects involved in the preparation, certification, and issuance of this SRM were coordinated through

the NIST Standard Reference Materials Program by J.W.L. Thomas.

Albert C. Parr, Chief

Optical Technology Division

Gaithersburg, MD 20899

Certificate Issue Date: 09 August 2000

Nancy M. Trahey, Chief

Standard Reference Materials Program

SRM 2053 Page 1 of 5 23
Source of Material: The silicon substrates used in making the filters were obtained from Virginia Semiconductor

Corporation 1 of Fredericksburg, VA. The metal films were deposited on the substrates by Luxel Corporation 1 of

Friday Harbor, WA.

CERTIFICATION ANALYSIS

Measurement Conditions: The certified transmittance values were measured using a Bio-Rad FTS-60A 1 Fourier transform infrared (FT-IR) spectrophotometer. The temperature in the filter compartment of the instrument was

approximately 29 °C, and the spectrophotometer was purged with dry, carbon dioxide free air. Details of the measurements and analysis can be found in Reference [1].

Determination of Uncertainty in Transmittance: The combined uncertainty includes "Type A" uncertainties,

which are evaluated by statistical methods, and "Type B" uncertainties, which are determined by other means. Type A

uncertainties include the reproducibility, listed in Table 1, and the repeatability, listed on the accompanying floppy disk. Each filter was measured six times in one day, then removed from the spectrophotometer and measured twice,

approximately six weeks later. A subset of the 10 nominally identical filters in this batch were again measured

twice, approximately six months after the first set of measurements. Each set of measurements was evaluated for repeatability variance, s r2

. The day-to-day variance was also computed for each filter and no evidence of long-term

drift in the measurements was found. The maximum daily variance of all the filter measurements was used as the value of reproducibility variance, Sdays , which was averaged over several different wavelength bands. Values for a number of systematic (Type B) sources of uncertainty were also estimated (Table 1) and added in quadrature to produce a variance component, Sb

2

- The combined uncertainty for the filter was calculated as

2 -

1 2

_I_ 1 2

, 2 (I)

s filter _ "7 S r + — s days + S B

v ' 6 2

The expanded uncertainty (coverage factor of 2) was calculated as 2s finer [2]. Representative curves for the certified transmittance values and associated expanded uncertainties in transmittance are shown in Figures 1 (a) and (b). The

certified values and uncertainties are listed in the data file on the accompanying floppy disk.

INSTRUCTIONS FOR USE

Storage and Handling: SRM 2053 is quite fragile and should be handled with care when mounting and

dismounting it from the measurement apparatus. The filter may be handled by wearing clean plastic or thin rubber

gloves and holding it gently by the edge. The surfaces should never be touched. Dust can be removed by gently

blowing the surface with clean, dry air. When not in use, the SRM should always be kept in its accompanying

protective box, which has the SRM serial number engraved on the bottom. It is recommended that the SRM be

stored in a dessicator cabinet, if available.

Test Measurements: The spectrophotometer system should be set up under the following conditions. The spectral

resolution should ideally be 8cm" 1 in order to match that used in the SRM calibration measurements. Lower

(i.e., larger cm"

1 value) resolution may be used and the certified values averaged to match the resolution of the measurement. At resolutions higher than 8 cm"

1

, Fabry-Perot fringes will appear in the spectrum due to interference

in the silicon substrate. In addition, the depth and width of the prominent silicon absorption features near 9um and

16 urn wavelengths, and weaker structures at intervening wavelengths, will be affected by choice of resolution. It is recommended that transmittance levels away from these features be used to compare the user's data with the SRM

calibration data if the user's resolution is different from 8 cm"

1

.

The IR beam parallelism may range from collimated up to f/3 focusing geometry. The beam should be centered on

the filter with a spot size less than 10 mm in diameter. For most FT-IR spectrophotometers, a spot size in the 1 mm

to 5 mm diameter range will be most convenient. The average angle of incidence at the filter surface should be less than 6°. The filter should be reproducibly moved in and out of the beam path and the spectra, with and without the

'Certain commercial equipment, instruments, or materials are identified in this certificate in order to adequately specify the experimental procedure. Such identification does not imply recommendation 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 this purpose.

SRM 2053

24

Page 2 of 5
filter in place, ratioed to produce a transmittance spectrum. The measurements should be repeated several times in order to assess their reproducibility.

Steps should be taken to reduce the effects of detector nonlinearity, ambient thermal emission, and inter-reflections involving the filter, spectrophotometer, and detector. The magnitude of each of these effects will depend on the user's system and may be reduced by using additional filters and/or stops in the beam path, tilting the filter slightly or blocking half of the beam, and performing additional measurements to test for the effects of ambient thermal emission not coming from the source. A measurement should be made with the beam path at the filter position

blocked with an opaque object in order to test for stray light or electronic offset which could produce a false transmittance signal. If a significant signal is found, it should be subtracted from both the filter and reference measurements before ratioing them to obtain the filter transmittance.

Comparison with Certified Values: The primary purpose of this SRM is to check the accuracy of the transmittance (ordinate) scale of the user's spectrophotometer system. In conjunction with SRMs 2054, 2055, and

2056, transmittance measurements of this filter can be used to assess the linearity of the instrument response scale. The user's transmittance spectrum should be compared with the certified values at the wavelength(s) of interest. If the user's values differ from the certified values by less than the quadrature sum of the certified expanded

uncertainties and the expanded uncertainty of the mean of the user's values, then the user's values are accurate to this level and no correction should be attempted on the basis of comparison with this SRM.

If the user's measured values differ significantly from the certified values, then additional steps along the lines suggested in the previous section may be taken in order to attempt to improve the transmittance (ordinate) scale accuracy of the instrument. Because of the wide variety of possible spectrophotometer systems and the number and

complexity of the various sources of error that may be present in different systems, it is not possible to give a general correction algorithm that will work for all systems. The user of FT-IR systems is referred to the literature on

the accuracy of measurements with these instruments [3]. In particular, it should be noted that different types of

filters will introduce very different sources of error into the measurements. This SRM is highly reflective and has a transmittance significantly less than 1, so it will tend to reveal errors due to detector nonlinearity and

inter-reflections. However, it is optically quite thin and will not yield much difference in the beam

geometry, with and without the filter in place, which can be a significant source of error for thicker filters.

REFERENCES

[1] Kaplan, S.G. and Hanssen, L.M., "Standard Reference Materials: Infrared Transmittance Standards - SRMs

2053, 2054, 2055, and 2056," NIST Special Publication 260-123, In Preparation (2000).

[2] Guide to the Expression of Uncertainty in Measurement, ISBN 92-67-10188-9, 1st Ed. ISO, Geneva,

Switzerland, (1993); see also Taylor, B.N. and Kuyatt, C.E., "Guidelines for Evaluating and Expressing

Uncertainty of NIST Measurement Results," NIST Technical Note 1297, U.S. Government Printing Office,

Washington DC, (1994); available at http://phvsics.nist.gov/Pubs/ .

[3] Kaplan, S.G., Hanssen, L.M., and Datla, R.U., "Testing the radiometric accuracy of Fourier transform infrared transmittance measurements," Appl. Opt. 36, 8896 (1997), and references therein.

Users of this SRM should ensure that the certificate in their possession is current. This can be accomplished by

contacting the SRM Program at: telephone (301) 975-6776; fax (301) 926-4751; e-mail srminfo@nist.gov; or via

the Internet http;//www. nist. sov/srm.

SRM 2053

25

Page 3 of 5
Table 1 . Standard uncertainty components, in percent of measured transmittance value,

for systematic uncertainty sources as well as reproducibility in the measured values.

Uncertainty source Value (%)

TypeB

Inter-reflections 0.20 Detector nonlinearity 0.10 Detector nonequivalence 0.20 Non-source emission 0.15 Beam nonuniformity 0.20 Beam displacement, deviation, focus shift 0.10 Beam geometry, polarization 0.20

Filter vignetting 0.03

Filter scattering 0.02 Phase errors 0.10

Filter nonuniformity 0.40

Filter temperature 0.20

Quadrature sum, Sb 0.65

Type A (Reproducibility, Sdays ) Daily variation 2 urn to 4 urn 0.50 Daily variation 4 |im to 12 um 0.50 Daily variation 12 um to 20 um 0.50 Daily variation 20 um to 25 um 0.70

SRM 2053

26

Page 4 of 5
0.12— (a)

0.115 +

0.11

0.105

0.1

0.095 +

0.09

:

-i—

h

SRM 2053

Serial No. 1-1

-i—i—i—i—|—i—i—i—i—|—i—i—i—i- -i—i—i—i0.003

| 0.0025 c

CO

& 0.002 nt

CDo

cD

TJUD

C

toQ.X

UJ

H—I—|—I 1—I

1—| 1 1—I—I—|1—I—I1—|1 1 1—

h

5 10 15 20 25

Wavelength (fim)

h—i—|—i—i—i—i—|—i—i—i—i—| i i i

y

0.0015- 5

0.001 -t—i—|—i—i—i—i—|—i—i—i—i—|—i—i—i—i—|—i—i—i—i- 5 10 15 20 25

Wavelength (iim)

Figure 1. (a) Certified transmittance versus wavelength, and (b) expanded uncertainty in transmittance of the neutral-density filter. The graphs should not be scaled to produce data for the filter. Data should be taken from the

file on the accompanying floppy disk.

SRM 2053

27

Page 5 of 5

• ffiutiaxml institute oi J^tanfrarfrs 8c ^lecl}naiam

states <* (Certificate

Standard Reference Material 2054

Infrared Transmittance Standard

Serial No.: 1-1

This Standard Reference Material (SRM) is intended for use in checking the accuracy of the transmittance (ordinate)

scale of spectrophotometers in the infrared (IR) spectral region from 2um to 25 um (400 cm"' to 5000cm" 1 ). SRM 2054 is a neutral-density transmitting filter with an optical density near 2.0 (OD = -logio(T), where T is the

spectral transmittance). It consists of an approximately 90 nm thick nickel-chromium film on a 250 um thick, 25 mm diameter, high-resistivity, single-crystal <100> silicon substrate. The metal film is overcoated with a thin (30nm) layer of silicon oxide to protect it against oxidation from the atmosphere. SRM 2054 also includes a floppy

disk that contains the certified transmittance values and associated expanded uncertainties.

Certified Transmittance Values: The transmittance of this filter was measured at equally spaced wavenumber

values from 400 cm' to 5000 cm"' (2 um to 25 um) with an apodized resolution of 8cm" and a data spacing interval of approximately 4cm"'. The transmittance was measured at the center of the filter using a 6 mm diameter spot size with the filter at the focus of an f/3 optical beam in the spectrophotometer. The certified transmittance values and

associated expanded uncertainties at each wavelength are listed in a text file on the accompanying floppy disk and

plotted in Figure 1 . These values and their associated uncertainties are valid for normal incidence transmittance

within a 5 mm radius of the center of the filter and at temperatures from 20 °C to 30 °C.

Expiration of Certification: The certification of this SRM is deemed to be valid until 31 December 2001,

provided the SRM is stored and handled in accordance with the Storage and Handling section of this certificate. Certification will be nullified if the SRM is damaged, contaminated, or exposed to excessive temperatures or humidity.

Maintenance of SRM Certification: The neutral-density filter has been measured over a period of approximately one year, and no significant change in transmittance has been noted. A reference filter is kept at NIST and will be

monitored over time. Users will be notified if a significant change occurs before the stated expiration date.

The technical measurements leading to certification were performed by S.G. Kaplan and L.M. Hanssen of the NIST

Optical Technology Division. The overall direction and coordination of the technical measurements were performed

by R.U. Datla of the NIST Optical Technology Division.

Statistical consultation was provided by M.C. Croarkin of the NIST Statistical Engineering Division.

The support aspects involved in the preparation, certification, and issuance of this SRM were coordinated through

the NIST Standard Reference Materials Program by J.W.L. Thomas.

Albert C. Parr, Chief

Optical Technology Division

Gaithersburg, MD 20899

Certificate Issue Date: 09 August 2000

Nancy M. Trahey, Chief

Standard Reference Materials Program

SRM 2054 29

Page 1 of 5
Source of Material: The silicon substrates used in making the filters were obtained from Virginia Semiconductor

Corporation 1 of Fredericksburg, VA. The metal films were deposited on the substrates by Luxel Corporation 1 of

Friday Harbor, WA.

CERTIFICATION ANALYSIS

Measurement Conditions: The calibration measurements were made using a Bio-Rad FTS-60A Fourier transform

infrared (FT-IR) spectrophotometer. The temperature in the filter compartment of the instrument was approximately

29 °C, and the spectrophotometer was purged with dry, carbon dioxide free air. A neutral-density filter with optical

density near 1 (SRM 2053) was used as the reference in measuring the transmittance of this SRM. The

transmittance of the filter under test was calculated from

ffilter — Tre i ative X Tre ference ( 1

)

Details of the measurement and analysis can be found in Reference [1].

Determination of Uncertainty in Transmittance: The combined uncertainty includes "Type A" uncertainties,

which are evaluated by statistical methods, and "Type B" uncertainties, which are determined by other means. Type A

uncertainties include the reproducibility, listed in Table 1, and the repeatability, listed on the accompanying floppy disk. Each filter was measured six times in one day, then removed from the spectrophotometer. A subset of the 10 nominally identical filters in this batch were again measured twice approximately six months after the first set of measurements. Each set of measurements was evaluated for repeatability variance, Sr2

. The day-to-day variance was also computed for each filter, and no evidence of long-term drift in the measurements was found. The

maximum daily variance of all the filter measurements was used as the value of reproducibility variance, Sdays2

, which was averaged over several different wavelength bands. Values for a number of systematic (Type B) sources of uncertainty were also estimated (Table 1) and added in quadrature to produce a variance component Sb

2

. The

variance in transmittance of the OD 1 reference is sre ference

2

- The combined uncertainty for the filter was calculated from

s

2 =-s 2 +s2 +s2 3 meas ^ r daV s B

Q

2 2 (2)

s filter - V 2 2 ' fllter ^filter Tre ferenCe

The expanded uncertainty (coverage factor of 2) is 2sf,| ter- Representative curves for the certified transmittance values and associated expanded uncertainties in transmittance are shown in Figures 1 (a) and (b). The certified values and uncertainties are listed in the data file on the accompanying floppy disk. INSTRUCTIONS FOR USE

Storage and Handling: The SRM is quite fragile and should be handled with care when mounting and dismounting

it from the measurement apparatus. The filter may be handled by wearing clean plastic or thin rubber gloves and

holding it gently by the edge. The surfaces should never be touched. Dust can be removed by gently blowing the surface with clean, dry air. When not in use, SRM 2054 should always be kept in its accompanying protective box,

which has the SRM serial number engraved on the bottom. It is recommended that the SRM be stored in a dessicator cabinet, if available.

Test Measurements: The spectrophotometer system should be set up under the following conditions. The spectral

resolution should ideally be 8cm" 1 in order to match that used in the SRM calibration measurements. Lower

(i.e., larger cm"

1 value) resolution may be used and the certified values averaged to match the resolution of the measurement. At resolutions higher than 8 cm"

1

, Fabry-Perot fringes will appear in the spectrum due to interference

in the silicon substrate. In addition, the depth and width of the prominent silicon absorption features near 9 urn and

Certain commercial equipment, instruments, or materials are identified in this certificate in order to adequately specify the experimental procedure. Such identification does not imply recommendation 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 this purpose.

SRM 2054 Page 2 of 5
16 um wavelengths, and weaker structures at intervening wavelengths, will be affected by choice of resolution. It is recommended that transmittance levels away from these features be used to compare the user's data with the SRM

calibration data if the user's resolution is different from 8 cm"

1

.

The IR beam parallelism may range from collimated up to f/3 focusing geometry. The beam should be centered on

the filter with a spot size less than 10 mm in diameter. For most FTIR spectrophotometers, a spot size in the 1 mm

to 5 mm diameter range will be most convenient. The average angle of incidence at the filter surface should be less than 6°. The filter should be reproducibly moved in and out of the beam path and the spectra, with and without the

filter in place, ratioed to produce a transmittance spectrum. The measurements should be repeated several times in order to assess their reproducibility.

Steps should be taken to reduce the effects of detector nonlinearity, ambient thermal emission, and inter-reflections involving the filter, spectrophotometer, and detector. The magnitude of each of these effects will depend on the user's system and may be reduced by using additional filters and/or stops in the beam path, tilting the filter slightly or blocking half of the beam, and performing additional measurements to test for the effects of ambient thermal emission not coming from the source. A measurement should be made with the beam path at the filter position

blocked with an opaque object in order to test for stray light or electronic offset which could produce a false transmittance signal. If a significant signal is found, it should be subtracted from both the filter and reference measurements before ratioing them to obtain the filter transmittance.

Comparison with Certified Values: The primary purpose of this SRM is to check the accuracy of the transmittance (ordinate) scale of the user's spectrophotometer system. In conjunction with SRMs 2053, 2055, and

2056, transmittance measurements of this filter can be used to assess the linearity of the instrument response scale. The user's transmittance spectrum should be compared with the certified values at the wavelength(s) of interest. If the user's values differ from the certified values by less than the quadrature sum of the certified expanded

uncertainties and the expanded uncertainty of the mean of the user's values, then the user's values are accurate to

this level and no correction should be attempted on the basis of comparison with this SRM.

If the user's measured values differ significantly from the certified values, then additional steps along the lines suggested in the previous section may be taken in order to attempt to improve the transmittance (ordinate) scale accuracy of the instrument. Because of the wide variety of possible spectrophotometer systems and the number and

complexity of the various sources of error that may be present in different systems, it is not possible to give a general correction algorithm that will work for all systems. The user of FT-IR systems is referred to the literature on

the accuracy of measurements with these instruments [3]. In particular, it should be noted that different types of

filters will introduce very different sources of error into the measurements. This SRM is highly reflective and has a transmittance significantly less than 1, so it will tend to reveal errors due to detector nonlinearity and inter- reflections. However, it is optically quite thin and will not yield much difference in the beam geometry, with and

without the filter in place, which can be a significant source of error for thicker filters.

REFERENCES

[1] Kaplan, S.G. and Hanssen, L.M., "Standard Reference Materials: Infrared Transmittance Standards - SRMs

2053, 2054, 2055, and 2056," NIST Special Publication 260-123, In Preparation (2000).

[2] Guide to the Expression of Uncertainty in Measurement, ISBN 92-67-10188-9, 1st Ed. ISO, Geneva,

Switzerland, (1993); see also Taylor, B.N. and Kuyatt, C.E., "Guidelines for Evaluating and Expressing

Uncertainty of NIST Measurement Results," NIST Technical Note 1297, U.S. Government Printing Office,

Washington DC, (1994); available at http://physics.nist.gov/Pubs/ .

[3] Kaplan, S.G., Hanssen, L.M., and Datla, R.U., "Testing the radiometric accuracy of Fourier transform infrared transmittance measurements," Appl. Opt. 36, 8896 (1997), and references therein.

Users of this SRM should ensure that the certificate in their possession is current. This can be accomplished by

contacting the SRM Program at: telephone (301) 975-6776; fax (301) 926-4751; e-mail srminfo@nist.gov; or via the Internet http://www.nist. mv/srm.

SRM 2054

31 Pase 3 of 5
Table 1 . Standard uncertainty components, in percent of measured transmittance value,

for systematic uncertainty sources as well as reproducibility in the measured values.

Uncertainty source Value (%)

Type B

Inter-reflections 0.30 Detector nonlinearity 0.10 Detector nonequivalence 0.20 Non-source emission 0.15 Beam nonuniformity 0.20 Beam displacement, deviation, focus shift 0.10 Beam geometry, polarization 0.20

Filter vignetting 0.03

Filter scattering 0.02 Phase errors 0.10

Filter nonuniformity 0.40

Filter temperature 0.20

Quadrature sum, Sb 0.68

Type A (Reproducibility, Sdays ) Daily variation 2 urn to 4 um 0.50 Daily variation 4 um to 1 2 um 0.30 Daily variation 12 um to 20 um 0.40 Daily variation 20 um to 25 (am 0.80

SRM 2054

32

Page 4 of 5
&

u

:S3

SRM 2054

Serial No. 1-1

0.016 --

0.014 --

5 0.012

0.01

0.008

0.006

0.0006

H—I—|

1—I—

h

H H H 1—

h

H—

h

10 15

Wavelength (|im)

20 25

5 0.0005

c

>»c

're

a)o

cD

D

(V "D

§, 0.0002 x

LU

Wavelength (|im)

Figure 1 . (a) Certified transmittance versus wavelength, and (b) expanded uncertainty in transmittance of the neutral-density filter. The graphs should not be scaled to produce data for the filter. Data should be taken from the

file on the accompanying floppy disk.

SRM 2054

33

Page 5 of 5

(Eerttftcate

Standard Reference Material® 2055

Infrared Transmittance Standard

Serial No.: 1-1

This Standard Reference Material (SRM) is intended for use in checking the accuracy of the transmittance (ordinate) scale of spectrophotometers in the infrared (IR) spectral region from 2um to 25 um (400 cm"

1 to 5000cm" 1 ). SRM 2055 is a neutral-density transmitting filter with an optical density near 3.0 (OD = -logio(T), where T is the spectral transmittance). It consists of an approximately 77 nm thick copper-nickel film on a 250 (im thick, 25 mm

diameter, high-resistivity, single-crystal <100> silicon substrate. The metal film is overcoated with a thin (30nm)

layer of silicon oxide to protect it against oxidation from the atmosphere. SRM 2055 also includes a floppy disk that contains the certified transmittance values and associated expanded uncertainties.

Certified Transmittance Values: The transmittance of this filter was measured at equally spaced wavenumber

values from 400 cm"

1 to 5000 cm"1 (2 um to 25 um) with an apodized resolution of 8 cm" 1 and a data spacing interval of approximately 4cm" 1

. The transmittance was measured at the center of the filter using a 6 mm diameter spot size with the filter at the focus of an f/3 optical beam in the spectrophotometer. The certified transmittance values and

associated expanded uncertainties at each wavelength are listed in a text file on the accompanying floppy disk and

plotted in Figure 1 . These values and their associated uncertainties are valid for normal incidence transmittance within a 5 mm radius of the center of the filter and at temperatures from 20 °C to 30 °C.

Expiration of Certification: The certification of this SRM is deemed to be valid until 31 December 2001,

provided the SRM is stored and handled in accordance with the Storage and Handling section of this certificate. Certification will be nullified if the SRM is damaged, contaminated, or exposed to excessive temperatures or humidity.

Maintenance of SRM Certification: The neutral-density filter has been measured over a period of approximately one year, and no significant change in transmittance has been noted. A reference filter is kept at NIST and will be

monitored over time. Users will be notified if a significant change occurs before the stated expiration date.

The technical measurements leading to certification were performed by S.G. Kaplan and L.M. Hanssen of the NIST

Optical Technology Division. The overall direction and coordination of the technical measurements were performed

by R.U. Datla of the NIST Optical Technology Division.

Statistical consultation was provided by M.C. Croarkin of the NIST Statistical Engineering Division.

The support aspects involved in the preparation, certification, and issuance of this SRM were coordinated through

the NIST Standard Reference Materials Program by J.W.L. Thomas.

Albert C. Parr, Chief

Optical Technology Division

Gaithersburg, MD 20899 Nancy M. Trahey, Chief

Certificate Issue Date: 09 August 2000 Standard Reference Materials Program

SRM 2055

35

Page 1 of 5
Source of Material: The silicon substrates used in making the filters were obtained from Virginia Semiconductor

Corporation 1 of Fredericksburg, VA. The metal films were deposited on the substrates by Luxel Corporation 1 of

Friday Harbor, WA.

CERTIFICATION ANALYSIS

Measurement Conditions: The calibration measurements were made using a Bio-Rad FTS-60A 1 Fourier transform

infrared (FT-IR) spectrophotometer. The temperature in the filter compartment of the instrument was approximately

29 °C, and the spectrophotometer was purged with dry, carbon dioxide free air. A neutral-density filter with optical

density near 1 (SRM 2053) was used as the reference in measuring the transmittance of this SRM. The

transmittance of the filter under test was calculated from

Tfilter — Tre iatj ve X Tre ference (1

)

Details of the measurement and analysis can be found in Reference [1].

Determination of Uncertainty in Transmittance: The combined uncertainty includes "Type A" uncertainties,

which are evaluated by statistical methods, and "Type B" uncertainties, which are determined by other means. Type A

uncertainties include the reproducibility, listed in Table 1, and the repeatability, listed on the accompanying floppy disk. Each filter was measured three times in one day, then removed from the spectrophotometer. Each of the 10 nominally identical filters in this batch was again measured twice approximately six months after the first set of measurements. Each set of measurements was evaluated for repeatability variance, s^'. The day-to-day variance was also computed for each filter, and no evidence of long-term drift in the measurements was found. The

maximum daily variance of all the filter measurements was used as the value of reproducibility variance, Sdays , which was averaged over several different wavelength bands. Values for a number of systematic (Type B) sources of uncertainty were also estimated (Table 1) and added in quadrature to produce a variance component Sb". The

variance in transmittance of the OD 1 reference is sreference~- The combined uncertainty for the filter was calculated from

s

2 -i s2 +s 2 +s 2 a

meas ^

r days B

2 2

2 _ , S meas , ^ reference » t 2 S filter - V 2 + —j x i

filter Tfilter Treference

(2)

The expanded uncertainty (coverage factor of 2) is 2sf,iter . Representative curves for the certified transmittance values and associated expanded uncertainties in transmittance are shown in Figures 1 (a) and (b). The certified values and uncertainties are listed in the data file on the accompanying floppy disk.

INSTRUCTIONS FOR USE

Storage and Handling: The SRM is quite fragile and should be handled with care when mounting and dismounting

it from the measurement apparatus. The filter may be handled by wearing clean plastic or thin rubber gloves and

holding it gently by the edge. The surfaces should never be touched. Dust can be removed by gently blowing the surface with clean, dry air. When not in use, SRM 2055 should always be kept in its accompanying protective box,

which has the SRM serial number engraved on the bottom. It is recommended that the SRM be stored in a dessicator cabinet, if available.

Test Measurements: The spectrophotometer system should be set up under the following conditions. The spectral

resolution should ideally be 8cm" 1 in order to match that used in the SRM calibration measurements. Lower

(i.e., larger cm"

1 value) resolution may be used and the certified values averaged to match the resolution of the

Certain commercial equipment, instruments, or materials are identified in this certificate in order to adequately specify the experimental procedure. Such identification does not imply recommendation 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 this purpose.

SRM 2055

36

Page 2 of 5
measurement. At resolutions higher than 8 cm"

1

, Fabry-Perot fringes will appear in the spectrum due to interference

in the silicon substrate. In addition, the depth and width of the prominent silicon absorption features near 9 urn and

16 um wavelengths, and weaker structures at intervening wavelengths, will be affected by choice of resolution. It is recommended that transmittance levels away from these features be used to compare the user's data with the SRM

calibration data if the user's resolution is different from 8 cm"

1

.

The IR beam parallelism may range from collimated up to f/3 focusing geometry. The beam should be centered on

the filter with a spot size less than 10 mm in diameter. For most FT-IR spectrophotometers, a spot size in the 1 mm

to 5 mm diameter range will be most convenient. The average angle of incidence at the filter surface should be less than 6°. The filter should be reproducibly moved in and out of the beam path and the spectra, with and without the

filter in place, ratioed to produce a transmittance spectrum. The measurements should be repeated several times in order to assess their reproducibility.

Steps should be taken to reduce the effects of detector nonlinearity, ambient thermal emission, and inter-reflections involving the filter, spectrophotometer, and detector. The magnitude of each of these effects will depend on the user's system and may be reduced by using additional filters and/or stops in the beam path, tilting the filter slightly or blocking half of the beam, and performing additional measurements to test for the effects of ambient thermal emission not coming from the source. A measurement should be made with the beam path at the filter position

blocked with an opaque object in order to test for stray light or electronic offset which could produce a false transmittance signal. If a significant signal is found, it should be subtracted from both the filter and reference measurements before ratioing them to obtain the filter transmittance.

Comparison with Certified Values: The primary purpose of this SRM is to check the accuracy of the transmittance (ordinate) scale of the user's spectrophotometer system. In conjunction with SRMs 2053, 2054, and

2056, transmittance measurements of this filter can be used to assess the linearity of the instrument response scale. The user's transmittance spectrum should be compared with the certified values at the wavelength(s) of interest. If the user's values differ from the certified values by less than the quadrature sum of the certified expanded

uncertainties and the expanded uncertainty of the mean of the user's values, then the user's values are accurate to

this level and no correction should be attempted on the basis of comparison with this SRM.

If the user's measured values differ significantly from the certified values, then additional steps along the lines suggested in the previous section may be taken in order to attempt to improve the transmittance (ordinate) scale accuracy of the instrument. Because of the wide variety of possible spectrophotometer systems and the number and

complexity of the various sources of error that may be present in different systems, it is not possible to give a general correction algorithm that will work for all systems. The user of FT-IR systems is referred to the literature on

the accuracy of measurements with these instruments [3]. In particular, it should be noted that different types of

filters will introduce very different sources of error into the measurements. This SRM is highly reflective and has a transmittance significantly less than 1, so it will tend to reveal errors due to detector nonlinearity and inter- reflections. However, it is optically quite thin and will not yield much difference in the beam geometry, with and

without the filter in place, which can be a significant source of error for thicker filters.

REFERENCES

[1] Kaplan, S.G. and Hanssen, L.M., "Standard Reference Materials: Infrared Transmittance Standards - SRMs

2053, 2054, 2055, and 2056," NIST Special Publication 260-123, In Preparation (2000).

[2] Guide to the Expression of Uncertainty in Measurement, ISBN 92-67-10188-9, 1st Ed. ISO, Geneva,

Switzerland, (1993); see also Taylor, B.N. and Kuyatt, C.E., "Guidelines for Evaluating and Expressing

Uncertainty of NIST Measurement Results," NIST Technical Note 1297, U.S. Government Printing Office,

Washington DC, (1994); available at http://phvsics.nist.gov/Pubs/ .

[3] Kaplan, S.G., Hanssen, L.M., andDatla, R.U., "Testing the radiometric accuracy of Fourier transform infrared transmittance measurements," Appl. Opt. 36, 8896 (1997), and references therein.

Users of this SRM should ensure that the certificate in their possession is current. This can be accomplished by

contacting the SRM Program at: telephone (301) 975-6776; fax (301) 926-4751; e-mail srminfo@nist.gov; or via the Internet http://www. nist. sov/srm.

SRM 2055 37

Page 3 of 5
Table 1 . Standard uncertainty components, in percent of measured transmittance value,

for systematic uncertainty sources as well as reproducibility in the measured values.

Uncertainty source Value (%)

Type B

Inter-reflections 0.30 Detector nonlinearity 0.05 Detector nonequivalence 0.10 Non-source emission 0.15 Beam nonuniformity 0.20 Beam displacement, deviation, focus shift 0.10 Beam geometry, polarization 0.20

Filter vignetting 0.03

Filter scattering 0.02 Phase errors 0.10

Filter nonuniformity 2.89

Filter temperature 0.20

Quadrature sum, Sb 2.93

Type A (Reproducibility, Sdays ) Daily variation 2 um to 4 um 2.70 Daily variation 4 um to 12 um 2.20 Daily variation 12 um to 20 um 3.00 Daily variation 20 um to 25 um 3.50

SRM 2055

38

Page 4 of 5
SRM 2055

Serial No. 1-1

0.0011 —

0. 001

o

I

0.0009

E

£ 0.0008

(0

0.0007 --

0.0006 -r

ro t

0)u

c

"D

0) T5C

roo,X

12

0.00011

:: (b)

0.0001 --

910' 5 - L

10 15

Wavelength (\im)

H 1 1—I

1—I

1—I

1—I1—|1—I—I1—| 1—

h

510-5 --

4 1

0

5 "

H——

h

H 1—I

1—I

1—|

1 1—I1—|1—I t- 10 15

Wavelength (jxm)

20 25

Figure 1. (a) Certified transmittance versus wavelength, and (b) expanded uncertainty in transmittance of the neutral-density filter. The graphs should not be scaled to produce data for the filter. Data should be taken from the

file on the accompanying floppy disk.

SRM 2055

39

Page 5 of 5

Standard Reference Material 2056

Infrared Transmittance Standard

Serial No.: 1-1

This Standard Reference Material (SRM) is intended for use in checking the accuracy of the transmittance (ordinate)

scale of spectrophotometers in the infrared (IR) spectral region from 2um to 20 um (500 cm"

1 to 5000cm" 1 ). SRM 2056 is a neutral-density transmitting filter with an optical density near 4.0 (OD = -logio(T), where T is the

spectral transmittance). It consists of an approximately 97 nm thick copper-nickel film on a 250 um thick, 25 mm

diameter, high-resistivity, single-crystal <100> silicon substrate. The metal film is overcoated with 'a thin (30nm)

layer of silicon oxide to protect it against oxidation from the atmosphere. SRM 2056 also includes a floppy disk that contains the certified transmittance values and associated expanded uncertainties.

Certified Transmittance Values: The transmittance of this filter was measured at equally spaced wavenumber

values from 500 cm"

1 to 5000 cm"1 (2 um to 20 um) with an apodized resolution of 8 cm"

1 and a data spacing interval of approximately 4 cm"

1

. The transmittance was measured at the center of the filter using a 5 mm diameter spot size with the filter at the focus of an f/4 optical beam in the spectrophotometer. The certified transmittance values and

associated expanded uncertainties at each wavelength are listed in a text file on the accompanying floppy disk and

plotted in Figure 1. These values and their associated uncertainties are valid for normal incidence transmittance

within a 5 mm radius of the center of the filter and at temperatures from 20 °C to 30 °C.

Expiration of Certification: The certification of this SRM is deemed to be valid until 31 December 2001,

provided the SRM is stored and handled in accordance with the Storage and Handling section of this certificate. Certification will be nullified if the SRM is damaged, contaminated, or exposed to excessive temperatures or humidity.

Maintenance of SRM Certification: The neutral-density filter has been measured over a period of approximately one year, and no significant change in transmittance has been noted. A reference filter is kept at NIST and will be monitored over time. Users will be notified if a significant change occurs before the stated expiration date.

The technical measurements leading to certification were performed by S.G. Kaplan and L.M. Hanssen of the NIST

Optical Technology Division. The overall direction and coordination of the technical measurements were performed

by R.U. Datla of the NIST Optical Technology Division.

Statistical consultation was provided by M.C. Croarkin of the NIST Statistical Engineering Division.

The support aspects involved in the preparation, certification, and issuance of this SRM were coordinated through

the NIST Standard Reference Materials Program by J. W.L. Thomas.

Albert C. Parr, Chief

Optical Technology Division

Gaithersburg, MD 20899

Certificate Issue Date: 09 August 2000

Nancy M. Trahey, Chief

Standard Reference Materials Program

SRM 2056 Page 1 of 5

41
Source of Material: The silicon substrates used in making the filters were obtained from Virginia Semiconductor

Corporation 1 of Fredericksburg, VA. The metal films were deposited on the substrates by Luxel Corporation 1 of

Friday Harbor, WA.

CERTIFICATION ANALYSIS

Measurement Conditions: The calibration measurements were made using a Bomem DA31 Fourier transform

infrared (FT-IR) spectrophotometer. The temperature in the filter compartment of the instrument was approximately

24 °C, and the spectrophotometer was purged with dry, carbon dioxide free air. A neutral-density filter with optical

density near 2 (SRM 2054) was used as the reference in measuring the transmittance of this SRM. The

transmittance of the filter under test was calculated from

Tfilter ~ Trelative X Tre ference ( 1 )

Details of the measurement and analysis can be found in Reference [1].

Determination of Uncertainty in Transmittance: The combined uncertainty includes "Type A" uncertainties,

which are evaluated by statistical methods, and "Type B" uncertainties, which are determined by other means. Type A

uncertainties include the reproducibility, listed in Table 1, and the repeatability, listed on the accompanying floppy disk. Each filter was measured three times in one day, then removed from the spectrophotometer. A subset of the filters were measured again twice approximately six months after the first set of measurements. Each set of measurements was evaluated for repeatability variance, sr2

. The day-to-day variance was also computed for each filter, and no

evidence of long-term drift in the measurements was found. The maximum daily variance of all the filter measurements was used as the value of reproducibility variance, Sdays

2

, which was averaged over several different wavelength bands. Values for a number of systematic (Type B) sources of uncertainty were also estimated (Table 1) and added in quadrature to produce a variance component s b

2

- The variance in transmittance of the OD 2 reference

is s re ference

2

- The combined uncertainty for the filter was calculated from

s

2 = Is 2 +s

2 +s2

15 meas ^ r T s

days B

2 2 (2)

2 _ / S mea5 S

| V | L

. IVI | LL> 2 s filter - V 2 2 ' fllter ^filter Tre ference

The expanded uncertainty (coverage factor of 2) is 2sf;iter- Representative curves for the certified transmittance values and associated expanded uncertainties in transmittance are shown in Figures 1 (a) and (b). The certified values and uncertainties are listed in the data file on the accompanying floppy disk. INSTRUCTIONS FOR USE

Storage and Handling: The SRM is quite fragile and should be handled with care when mounting and dismounting

it from the measurement apparatus. The filter may be handled by wearing clean plastic or thin rubber gloves and

holding it gently by the edge. The surfaces should never be touched. Dust can be removed by gently blowing the surface with clean, dry air. When not in use, SRM 2056 should always be kept in its accompanying protective box,

which has the SRM serial number engraved on the bottom. It is recommended that the SRM be stored in a

dessicator cabinet, if available.

Test Measurements: The spectrophotometer system should be set up under the following conditions. The spectral resolution should ideally be 8cm" 1 in order to match that used in the SRM calibration measurements. Lower

(i.e., larger cm'

1 value) resolution may be used and the certified values averaged to match the resolution of the measurement. At resolutions higher than 8 cm"

1

, Fabry-Perot fringes will appear in the spectrum due to interference

1 Certain commercial equipment, instruments, or materials are identified in this certificate in order to adequately specify the experimental procedure. Such identification does not imply recommendation 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 this purpose.

SRM 2056 Page 2 of 5

42
in the silicon substrate. In addition, the depth and width of the prominent silicon absorption features near 9 urn and

16 um wavelengths, and weaker structures at intervening wavelengths, will be affected by choice of resolution. It is recommended that transmittance levels away from these features be used to compare the user's data with the SRM

calibration data if the user's resolution is different from 8 cm" 1

.

The IR beam parallelism may range from collimated up to f/3 focusing geometry. The beam should be centered on

the filter with a spot size less than 10 mm in diameter. For most FT-IR spectrophotometers, a spot size in the 1 mm

to 5 mm diameter range will be most convenient. The average angle of incidence at the filter surface should be less than 6°. The filter should be reproducibly moved in and out of the beam path and the spectra, with and without the

filter in place, ratioed to produce a transmittance spectrum. The measurements should be repeated several times in order to assess their reproducibility.

Steps should be taken to reduce the effects of detector nonlinearity, ambient thermal emission, and inter-reflections involving the filter, spectrophotometer, and detector. The magnitude of each of these effects will depend on the user's system and may be reduced by using additional filters and/or stops in the beam path, tilting the filter slightly or blocking half of the beam, and performing additional measurements to test for the effects of ambient thermal emission not coming from the source. A measurement should be made with the beam path at the filter position

blocked with an opaque object in order to test for stray light or electronic offset which could produce a false transmittance signal. If a significant signal is found, it should be subtracted from both the filter and reference measurements before ratioing them to obtain the filter transmittance.

Comparison with Certified Values: The primary purpose of this SRM is to check the accuracy of the transmittance (ordinate) scale of the user's spectrophotometer system. In conjunction with SRMs 2053, 2054, and

2055, transmittance measurements of this filter can be used to assess the linearity of the instrument response scale. The user's transmittance spectrum should be compared with the certified values at the wavelength(s) of interest. If the user's values differ from the certified values by less than the quadrature sum of the certified expanded

uncertainties and the expanded uncertainty of the mean of the user's values, then the user's values are accurate to

this level and no correction should be attempted on the basis of comparison with this SRM.

If the user's measured values differ significantly from the certified values, then additional steps along the lines suggested in the previous section may be taken in order to attempt to improve the transmittance (ordinate) scale accuracy of the instrument. Because of the wide variety of possible spectrophotometer systems and the number and

complexity of the various sources of error that may be present in different systems, it is not possible to give a general correction algorithm that will work for all systems. The user of FT-IR systems is referred to the literature on

the accuracy of measurements with these instruments [3]. In particular, it should be noted that different types of

filters will introduce very different sources of error into the measurements. This SRM is highly reflective and has a transmittance significantly less than 1, so it will tend to reveal errors due to detector nonlinearity and inter- reflections. However, it is optically quite thin and will not yield much difference in the beam geometry, with and

without the filter in place, which can be a significant source of error for thicker filters.

REFERENCES

[1] Kaplan, S.G. and Hanssen, L.M., "Standard Reference Materials: Infrared Transmittance Standards - SRMs

2053, 2054, 2055, and 2056," NIST Special Publication 260-123, In Preparation (2000).

[2] Guide to the Expression of Uncertainty in Measurement, ISBN 92-67-10188-9, 1st Ed. ISO, Geneva,

Switzerland, (1993); see also Taylor, B.N. and Kuyatt, C.E., "Guidelines for Evaluating and Expressing

Uncertainty of NIST Measurement Results," NIST Technical Note 1297, U.S. Government Printing Office,

Washington DC, (1994); available at http://phvsics.nist.gov/Pubs/ .

[3] Kaplan, S.G., Hanssen, L.M., and Datla, R.U., "Testing the radiometric accuracy of Fourier transform infrared transmittance measurements," Appl. Opt. 36, 8896 (1997), and references therein.

Users of this SRM should ensure that the certificate in their possession is current. This can be accomplished by

contacting the SRM Program at: telephone (301) 975-6776; fax (301) 926-4751; e-mail srminfo@nist.gov; or via the Internet http://www.nist. sov/srm.

SRM 2056 Page 3 of 5

43
Table 1 . Standard uncertainty components, in percent of measured transmittance value,

for systematic uncertainty sources as well as reproducibility in the measured values.

Uncertainty source Value (%)

Type B

Inter-reflections 0.30 Detector nonlinearity 0.05 Detector nonequivalence 0.10 Non-source emission 0.15 Beam nonuniformity 0.20 Beam displacement, deviation, focus shift 0.10 Beam geometry, polarization 0.20

Filter vignetting 0.03

Filter scattering 0.02 Phase errors 0.10

Filter nonuniformity 3.46

Filter temperature 0.20

Quadrature sum, Sb 3.50

Type A (Reproducibility, s days ) Daily variation 2 urn to 4 um 2.00 Daily variation 4 um to 12 um 4.00 Daily variation 12 um to 20 um 5.00

SRM 2056

44

Page 4 of 5
0.00012

0.0001

SRM 2056

Serial No. 1-1

H 1 1

I

1

1

1 h-

©

o

c

ml

(AC

m

4 10--

2 10"5 +

-i—|—i—i—i—|—i—i—i—|—i—i—i—|—i—i—

h

4 8 12 16

Wavelength (jim)

2 10

Q)O

c

ra

4-1 +>E 1.5 10" w

c

2cf 1 10

TO

0)o

c

T3

0) T3C

roQ.X

Hi

5 10

8 12 16

Wavelength (jim)

Figure 1. (a) Certified transmittance versus wavelength, and (b) expanded uncertainty in transmittance of the neutral-density filter. The graphs should not be scaled to produce data for the filter. Data should be taken from the

file on the accompanying floppy disk.

SRM 2056

45

Page 5 of 5



Technical Publications

Periodical

Journal of Research of the National Institute of Standards and Technology—Reports NIST research and development in those disciplines of the physical and engineering sciences in which the Institute is active. These include physics, chemistry, engineering, mathematics, and computer sciences. Papers cover a broad range of subjects, with major emphasis on measurement methodology and the basic technology

underlying standardization. Also included from time to time are survey articles on topics closely related to the Institute's technical and scientific programs. Issued six times a year.

Nonperiodicals

Monographs—Major contributions to the technical literature on various subjects related to the

Institute's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) devel- oped in cooperation with interested industries, professional organizations, and regulatory bodies. Special Publications—Include proceedings of conferences sponsored by NIST, NIST annual reports, and

other special publications appropriate to this grouping such as wall charts, pocket cards, and bibliographies.

National Standard Reference Data Series;—Provides quantitative data on the physical and chemical

properties of materials, compiled from the world's literature and critically evaluated. Developed under a worldwide program coordinated by NIST under the authority of the National Standard Data Act (Public

Law 90-396). NOTE: The Journal of Physical and Chemical Reference Data (JPCRD) is published

bimonthly for NIST by the American Institute of Physics (AIP). Subscription orders and renewals are

available from AIP, P.O. Box 503284, St. Louis, MO 63150-3284.

Building Science Series—Disseminates technical information developed at the Institute on building

materials, components, systems, and whole structures. The series presents research results, test methods, and

performance criteria related to the structural and environmental functions and the durability and safety

characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of

a subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NIST under the sponsorship of

other government agencies.

Voluntary Product Standards—Developed under procedures published by the Department of Commerce

in Part 10, Title 15, of the Code of Federal Regulations. The standards establish nationally recognized

requirements for products, and provide all concerned interests with a basis for common understanding of

the characteristics of the products. NIST administers this program in support of the efforts of private-sector

standardizing organizations.

Order the following NIST publications—FIPS and NISTIRs—from the National Technical Information

Service, Springfield, VA 22161.

Federal Information Processing Standards Publications (FIPS PUB)—Publications in this series

collectively constitute the Federal Information Processing Standards Register. The Register serves as the

official source of information in the Federal Government regarding standards issued by NIST pursuant to the Federal Property and Administrative Services Act of 1949 as amended, Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717 (38 FR 12315, dated May 11, 1973) and Part 6 of

Title 15 CFR (Code of Federal Regulations).

NIST Interagency or Internal Reports (NISTIR)—The series includes interim or final reports on work

performed by NIST for outside sponsors (both government and nongovernment). In general, initial distribution is handled by the sponsor; public distribution is handled by sales through the National Technical

Information Service, Springfield, VA 22161, in hard copy, electronic media, or microfiche form. NISTIR's

may also report results of NIST projects of transitory or limited interest, including those that will be

published subsequently in more comprehensive form.
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