NBS

Reference Publications

NATL INST. OF STAND & TECH

Allies T7q570 ^ w. ^ *

5 NBS TECHNICAL NOTE 1175

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

Extension of a

Reference

Spectrophotometer

into the Near

Infrared

—yc100

, U5753

1175

1983
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

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'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. ^Some divisions within the center are located at Boulder, CO 80303.
Extension of a Reference Spectrophotometer £-f^^^

into the Near Infrared ^^Ji

Kenneth L. Eckerle, Victor R. Weidner, Jack J. Hsia

Radiometric Physics Division

National Bureau of Standards

Washington, DC 20234

and

Zu Wen Chao

Precision Instrument Development Center

Taiwan National Science Council

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued April 1983
National Bureau of Standards Technical Note 1175

Natl. Bur. Stand. (U.S.), Tech.Note 1175, 34 pages (April 1983)

U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 1983

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price $4.50

(Add 25 percent for other than U.S. mailing)
CONTENTS

Page

1Introduction 1

2. Modifications for Extension into the Near Infrared 2

A. Sources and Source Optics 2

B. Wavelength Selection 4

C. Sample Compartment 4

D. Detectors and Spheres 4

E. Electronics 5

F. Microcomputer and Interface Boards 5

3. Characterization 6

A. Instrument Sensitivity Function 6

B. Stability--Drift and Noise 6

C. Uniformity of Receivers 13

D. Uniformity of Light Beam 17

E. Linearity 17

F. Stray Radiant Energy 19

(1) Photoconductive PbS Detector 20

(2) Photovoltaic Silicon Photodiode 21

(3) Extended S-20 Photomultipl ier 22

G. Wavelength Calibration 24

4. Transmittance Measurements 26

5. References 28

m
LIST OF TABLES

Page

1. Drift and noise using the PbS detector and the

lock- in amplifier at several sensitivity settings 13

2. The elements and wavelengths of the atomic lines used

for the wavelength calibration and the published values

for the minima of the absorption bands 24

LIST OF FIGURES

1. A schematic of the instrument with the PbS detector 3

2. Photoconductive PbS detector circuitry 5

3. Instrument sensitity function using the cooled PbS

detector. All counts in this and following figures

refer to accumulation over a 10-second time interval.

The maximum count level in this figure is 50,000 . 7

4. Instrument sensitity function using the silicon photodiode 8

5. Instrument sensitivity function using the extended

S-20 photomultiplier tube 9

6. Drift and standard deviation of the noise for the

cooled PbS detector 10

7. Drift and standard deviation of the noise for the

silicon photodiode 11

8. Drift and standard deviation of the noise for the

extended S-20 photomultiplier tube 12

9. Horizontal uniformity of the PbS detector-averaging

sphere combination 14

10. Vertical uniformity of the PbS detector-averaging

sphere combination 15

11. Uniformity of the light beam in the sample

compartment. The abscissa refers to positions

in units of millimeters 16

12. Wavelength correction curve 25

IV
Extension of a Reference Spectrophotometer

into the Near Infrared

Kenneth L. Eckerle, Zu Wen Chao*, Victor R. Weidner, and Jack J. Hsia

The purpose of this paper is to document adaptation of an existing reference spectrophotometer to the near infrared. Its

previous wavelength range was from approximately 200 to 800 nm. The present adaptation increases its wavelength range to 2500 nm. The hardware and software necessary to achieve this were implemented along with tests necessary to characterize the instrument

when it is operated in the near infrared (800 to 2500 nm). The

accuracy of the transmittance measurements ranges from approximately 0.0002 to 0.012 transmittance units depending on instrument

configuration, wavelength, and the sample itself. Incidental to

the success of this work was conversion to a dedicated microcomputer with interface boards. Many of the latter were designed at

NBS.

Key words: lead sulfide detector; near infrared; photomultipl ier;

reference spectrophotometer; silicon photodiode; spectrophotometry;

transmittance; wavelength.

1. Introduction

A high accuracy spectrophotometer [1], [2], [3]^ was adapted in order to

perform measurements in the wavelength range from 800 to 2500 nm. We maintained

a small systematic error. Many of the desirable features of the original design

depend on using circular apertures instead of slits in the monochromator as well

as off-axis parabolic mirrors to produce a collimated beam with no interreflection

between sample and optics. It was decided to keep these features but in the

adaptions some compromises had to be made. The bandpass was increased to approximately 6 nm from its previous value of approximately 0.75 to 1.5 nm. Since current practice is to use neutral filters (whose transmittance is not sensitive to

bandpass) for transmittance standards, this compromise is not serious. Also, many

absorption bands in the near infrared are broad. The sources and detectors which

are currently available provide a smaller signal to noise ratio in the near IR

than that which we have in the visible. This leads to larger random errors.

*Guest Worker from the Precision Instrument Development Center of the Taiwan

National Science Council. P.O. Box 2-2, Hsinchu, Taiwan.

^Figures in brackets indicate literature references at the end of this paper,
However, by repeating measurements and using the mean value, the standard deviation of the mean becomes less with the number of measurements. Care was taken in

the choice of components and characterization of these components so that we can make measurements with a good estimate of the error in transmittance measurements.

2. Modifications for Extension into the Near Infrared

A schematic showing how the instrument is used over most of the wavelength

range of interest is shown in figure 1. (Two other detectors used in the DC mode

are for wavelengths from about 750 to 1100 nm, as will be described below.)

Although many of the components shown in figure 1 have previously been described

in other publications, they will be described in this paper briefly, and in some cases, the reader will be referred to other publications for details.

A. Sources and Source Optics

The He-Ne laser is used to define the optic axis, align the optics, and

also to align the samples to be measured. Atomic line sources are used to calibrate the wavelength scale. A 500 watt coil filament lamp is used as an infrared

source. It is aligned prior to measurements by rotation about a vertical axis so that the filaments approximate a spatially continuous source. A water-cooled

jacket surrounds the lamp primarily to keep the surrounding space cool and reduce

thermal effects on the rest of the apparatus. A 25.4 mm diameter flat mirror

reflects the light beam onto a 51 mm diameter off-axis parabolic mirror of focal

length 177 mm which collimates the light beam. Two lenses, one on either side of

the chopper blade, focus the source on the chopper blade and then re-col limate the

beam. These lenses are of high grade fused quartz whose index of refraction is

constant with wavelength in the near infrared. The purpose of having an image of

the filament on the chopper blade [4] is to provide illumination which increases

and decreases uniformly in the sample compartment. Therefore, the use of two

apertures for the linearity measurements will produce similar shaped waveforms

with different amplitudes. These lenses are 37 mm in diameter with focal lengths

of 200 mm. A Glan-Taylor prism polarizer produces a well-defined polarized beam.

Normally, measurements are made with two polarizations--one with the electric

vector parallel to the grating grooves (90°) and one with the electric vector

perpendicular to the grating grooves (0°). A 51 mm square flat mirror then

diverts the light onto a 51 mm diameter off-axis parabolic mirror of 195 mm focal

length which produces an image of the source on the entrance slit of the predisperser.
o

4-> O

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CU -a

CO.n

Q.

(U

4-> cE

S-

-!-> I/) C

<uoa

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O)o

00

<u
B. Wavelength Selection

An Ebert prism predisperser with f-number f/6 preselects the approximate

wavelength band of interest. A one meter Czerny-Turner monochromator equipped

with circular apertures at the entrance and exit ports and off-axis parabolic

mirrors produces an approximate "point source" for the sample compartment optics.

Its f-number is f/8.7. The grating is 102 mm by 102 mm with blaze wavelength at

800 nm and 150 grooves per mm. Normally, 1 mm diameter circular apertures are used in the monochromator which produce a bandpass of approximately 6 nm.

C. Sample Compartment

A 25.4 mm diameter flat mirror diverts the light onto a 51 mm diameter

off-axis parabolic mirror of 195 mm focal length. These optics produce a collimated beam in the sample compartment of size 20 mm by 20 mm. A limiting aperture

produces a beam of size and shape desired for a measurement. The sample holder is

either a filter wheel or a sliding holder. The filter wheel allows up to 3 samples

to be measured sequentially. The slide allows measurement of different size

filters one at a time. Both holders are automated. A black cavity is used for

measurement of the dark signal from the detector. A double aperture permits

measurement of the linearity of the detection system [5].

D. Detectors and Spheres

The instrument sensitivity function, which includes the source, output

efficiency of the optical system, and response of the detector, determines the

useful wavelength range for a given detector. For our system, a photomultiplier

with an extended S-20 cathode 50 mm in diameter will be used over the wavelength

range of approximately 750 to 900 nm. A silicon photodiode can be used from

approximately 900 to 1060 nm. A lead sulfide detector which is thermoelectrically

cooled to -80 °C can be used from about 900 to 2500 nm. The spheres chosen for

the PbS detector and the silicon photodiode are approximately 90 mm in diameter

and have a diffusing target at their center. The sphere used for the photomultiplier does not have a target and is described in reference [6]. All three spheres

have a pressed polytetrafluorethylene powder for their coatings [7]. The silicon

photodiode (100 mm surface area) and the photomultiplier are used in the direct

current mode without the chopper and lock-in amplifier. The cooled PbS detector

(10 mm by 10 mm) is used with the chopper and lock-in amplifier.
E. Electronics

The photoconductive PbS detector is connected to the preamplifier of a lock-in amplifier via the circuit shown in figure 2. The voltage output of the

lock-in is applied across a thermally stable resistor of value 3.3 MQ. (voltage to

current converter). The current in this circuit is measured by a current-tofrequency converter whose output pulses are counted by a counter which is read by

a microcomputer. The lock- in amplifier is synchronized with a two-sector chopper

blade with frequency of about 102 Hertz.

The photovoltaic silicon photodiode output current is measured directly by

the current-to-frequency converter as is the output current from the photomultiplier.

1.7 MA

Pf

250V

PbS DETECTOR

(3.6Mn) 295 K

IIOKA

10 MA

rru 22

4*-4

TO

PREAMP

Figure 2. Photoconductive PbS detector circuitry.

F. Microcomputer and Interface Boards

The instrument is completely automated. This is achieved with a 64 Kbyte memory dedicated microcomputer with an S-100 bus. A CRT display and printer

are interfaced with the computer. Special relay boards, stepper motor control

boards, and input/output boards enable automation of lock-in amplifier sensitivity

setting, opening and closing of pneumatically controlled black cavity and shutter,

and sample holder control, as well as setting of the wavelength of the monochromator and data acquisition.
3. Characterization

A. Instrument Sensitivity Function

The instrument sensitivity function is obtained by measuring the signal

without a sample in the light beam. Since the spectrophotometer is a single beam

instrument, this measurement as a function of wavelength reveals the wavelength

region over which a given detector is useful. Since the centroid of the pass

band also depends on this function, the function must be known for an accurate

wavelength calibration. The instrument sensitivity functions using the cooled

PbS detector, the silicon photodiode, and the extended S-20 photomultiplier are shown in figures 3, 4, and 5, respectively. The predominant features of the

function for the PbS detector are caused by a decreasing sensitivity of the

detector itself at long wavelengths, the wavelength dependence of the efficiency

of the grating blazed at 800 nm, and absorption by the mirror optics near 800 nm. The silicon photodiode is similarly affected except that its responsivity drops

at much shorter wavelengths. The photomultiplier responsivity is essentially

zero at 950 nm.

B. Stability--Drift and Noise

The stability of the source-detector combinations using the 500 watt

source and three detectors was determined by alternately measuring the signal and

dark signal at equal time intervals for 30 minutes. These measurements determine

the system drift which can be eliminated from measurements by taking data in such

a fashion as to determine the drift each time a transmittance measurement is

made. Superimposed on the drift are fluctuations which must be regarded as noise

and cannot be eliminated from a measurement--thus leading to an uncertainty.

Typical curves illustrating these two effects are shown in figures 6, 7, and 8

for the three detectors along with values of the drift and coefficient of variation of the residual noise y~ ^^^ ^ 30-minute period.

We see that the drift is fairly linear and can be taken into account. The

noise depends on the signal level and is given only for those tests used to

obtain the figure. The noise for other signal levels for the PbS detector is

given in table 1

.
POLARIZATION

750 1000 1500 2000

WAVELENGTH (nm)

2500

Figure 3. Instrument sensitivity function using the cooled

PbS detector. All counts in this and following

figures refer to accumulation over a 10-second

time interval. The maximum count level in this

figure is 50,000.
5000

4000

3000-

co

o

u

2000

iOOO

J a_ I \ I I I L

800 900 IOOO

WAVELENGTH (nm)

1100

Figure 4. Instrument sensitivity function using the silicon photodiode,
y oo

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12
Table 1. Drift and noise using the PbS detector and the

lock-in amplifier at several sensitivity settings

Lock-in Average Drift

Amplifier

Sensitivity

for 30 min f (%)

1 mV 0.5189 0.4752

2 mV 0.6988 0.2985

5 mV 0.5146 0.1576

10 mV 0.2241 0.0794

C. Uniformity of Receivers

Our receivers consist of an averaging sphere and detector. Since the

detectors are non-uniform across their surface, a sphere tends to average this

effect. Non-uniformity of the receiver must be taken into account because when

a sample which is slightly wedge shaped is inserted into the light beam it

deflects the beam slightly. Even if the surfaces of the sample are plane parallel

and it is inserted into the light beam at an angle, the sample will also deflect

the beam.

Tests carried out on the averaging sphere used for the photomultiplier tube

have been discussed previously [6], [3]. Results of those tests indicate an efficiency of approximately 50 percent and that a beam deflection of 1 mm (a

large deflection) would cause an error of approximately 0.01 percent.

The type of sphere used for the photovoltaic silicon detector and the

photoconductive PbS detector has been shown to have a high efficiency if the

detector is properly coupled to the sphere [8]. The uniformity was measured by

moving the sphere and detector ±0.5, ±1.0, and ±1.5 mm about the central position

both horizontally and vertically. (See figures 9 and 10.) Since the differences

are small, the noise (the error bars indicate 3 standard deviations) obscures any

real effect. However, there seems to be a trend in the horizontal scan. The

scans were made in a sequence such that the drift in the data was eliminated,

i.e., the scans were made at equal time intervals in the sequence 0, .5, 1, 1.5,

1.5, 1.0, 0.5, in positive and negative directions and the resulting data

averaged, assuming a linear drift.

13
L0005

O

:: 1.0000

0.9995 —

1.0 -0.5 0.5 1.0

DISPLACEMENT (mm)

Figure 9. Horizontal uniformity of the PbS

detector-averaging sphere combination

14
1.00051—

o

1.0000 —

0.9995

JI 1 I 1

-1.5 -1.0 -0.5 0,5 1.0

DISPLACEMENT (mm)

1.5

Figure 10. Vertical uniformity of the PbS

detector-averaging sphere combination

15
5000r-

, VERTICAL

4000-

S 3000^-

z

o

u

M

HORIZONTAL

2000-

1000-

-12 -10 -8 -6 - •2 2

(CENTER)

POSITION

8 10 12

Figure 11. Uniformity of the light beam in the sample compartment.

The abscissa refers to positions in units of millimeters.

16
D. Uniformity of Light Beam

The uniformity of the light beam is not yery critical since most samples

to be measured must be quite uniform to qualify as a transmittance standard. A

5 mm diameter aperture was scanned horizontally and vertically across the light

beam. Typical results are shown in figure 11. It is seen that in the worst case

the signal differs by 40 percent from the signal obtained at the center position

for the useful portion of the beam.

E. Linearity

All spectrophotometers are subject to a non-linearity of the photometric

scale to some degree. This non-linearity may be expressed as an additive correction, AT, to the measured transmittance, T^ , so that the real transmittance, T,

a is

T = T + AT.

a

The quantity, AT, was determined by using the double aperture method as described

in reference [5]. If the signal current for the detectors can be expressed by

the equation

I((f)) = nG(4))(})

then the relevant derivations in reference [5] apply here. ^ is the incident

flux, n is the quantum efficiency, and G is the effective gain. This is true for

the PbS detector because the signal current may be expressed in terms of the gain

of the lock-in amplifier, g{^) , the signal voltage from the PbS detector, V , and

the voltage to current resistance, Ry^, [9] such that

iM = g^y.

^vi

gM

^vi

*nA^

1+CO T

where

= riG((j))({)

17
E = bias voltage (250 V in figure 2)

R = resistance of the PbS detector (3.6 Mfi in figure 2)

R| = load resistor (1.7 Mfi in figure 2)

(j) = radiant flux

^ = quantum efficiency of the PbS detector

A = cross sectional area of PbS detector

perpendicular to current flow

T = lifetime of charge carrier in PbS detector

n = number of free charge carriers in PbS detector

0) = angular frequency of modulation of the chopper.

The signal current I for the silicon photodiode can be expressed by the following

expression [10]

where Q{(^) is the external quantum efficiency. The external quantum efficiency

is [10]

Q((|)) = (1-p) r]FM

where p is the reflectance of the photodiode, and F((j)) is the collection efficiency for our geometry. Thus, the signal current for the silicon photodiode can also be expressed as

I((j)) = nG((j))({).

The linearity correction AT for the photomultipl ier tube, PbS detector, and

silicon photodiodes and the standard deviation of AT, 6(AT) are listed below for

transmittance T. The scale indicated is that for the current-to-frequency

converter, and the sensitivity indicated for the PbS detector is the lock-inamplifier sensitivity setting. We found that the useful sensitivity settings for

the lock-in amplifier range from 1 to 50 mV.

18
Photomultiplier

7

PbS

,-6

Si photodiode

-8 3x10' amp scale 3 x lo amp scale 3 x lo "" amp scale

10 mV sensitivity

(Measured at 850 nm) (Measured at 1250 nm) (Measured at 1060 nm)

T AT

xlO"^

MAT)

xlO"^

AT

xlO"^

6(AT)

xlO"^

AT

xlO"^

6(AT)

xlO"^

0.1 1.10 2.03 -0.41 0.65 0.97 0.71

0.2 2.05 3.10 -0.40 0.90 1.28 1.00

0.3 2.80 3.98 -0.10 0.92 1.12 1.09

0.4 3.32 5.13 0.37 0.92 0.63 1.18

0.5 3.59 6.49 0.90 1.06 -0.02 1.41

0.6 3.58 7.61 1.35 1.31 -0.07 1.68

0.7 3.24 8.01 1.61 1.47 -1.15 1.84

0.8 2.55 7.20 1.55 1.40 -1.30 1.70

0.9 1.48 4.68 1.06 0.96 -0.98 1.14

1.0 0.00 0.00 0.00 0.00 0.00 0.00

It is seen that only for the photomultiplier tube is the standard deviation of

AT, 6(AT), significantly less than AT. For all three detectors, we regard 6(AT) as a systematic error in the measurement of transmittance, T, which will vary

depending on detector and sensitivity scale used.

F. Stray Radiant Energy

New methods for measuring stray radiant energy and the stray light ratio

have recently been published [11], [12]. The stray radiant energy is defined by

ASTM Method E387 [13]. It generally underestimates the effect of stray light on a spectrophotometric measurement. The stray light ratio as defined by Kaye some- times allows a correction to be made. However, for a general characterization of our instrument, we chose to measure stray radiant energy using filters and a liquid, whose properties are well known, to obtain order of magnitude estimates

of stray light. If we ever desire to make measurements where the stray light

will become a problem, we will attempt, using appropriate blocking filters, to

measure the stray light ratio. This seems reasonable, since as Kaye points out,

the sample itself can influence the stray light ratio. Further, except for the

19
extended S-20 photomultipl ier, our measurements of stray light are noise-limited.

That is, the mean of 100 measurements of stray radiant energy expressed as apparent transmittance is usually about the same magnitude as the standard error

(standard deviation of the mean) for that measurement.

Samples of chloroform, selenium filter, germanium windows, and silicon

wafers have been shown to be useful for measuring stray radiant energy [14].

Also, two broad-band interference filters which transmit almost the whole visible

spectrum were used to estimate stray light for the silicon photodiode and the

photomultipl ier.

The silicon wafers cut off wavelengths below about 900 nm and transmit more

than 52 percent at wavelengths out to 2500 nm. The selenium glass has a cut-off

at approximately 800 nm and transmits approximately 88 percent out to 2500 nm. The 4 mm germanium window starts cutting off at approximately 1950 nm and transmits approximately 46 percent out to 2500 nm. The combination of 3 mm and 4 mm germanium windows transmits approximately 27 percent between 1950 and 2500 nm. The first interference filter (no. 1) cuts off at about 700 nm and transmits more

than 70 percent over most of the visible spectrum. The other interference filter

(no. 2) cuts off at about 850 nm and has a transmittance exceeding 80 percent

over the visible spectrum and has a low transmittance in the near infrared.

Chloroform has a strong absorption band at about 1690 nm.

Consequently, the silicon, germanium, and silicon filters test for long

wavelength stray radiant energy. The interference filters in combination test

for short wavelength stray radiant energy and the chloroform tests for both short

and long wavelength stray radiant energy.

(1) Photoconductive PbS Detector

Two measurements were made at 1690 nm with a 10 mm thick cuvette

containing chloroform. The measurements were repeated 100 times with a 10-second

integration time by first measuring the dark signal and then the signal with the

chloroform in the beam. The clear space signal was measured at the beginning and

end of each 100 measurements. The results are shown below as apparent transmittance, T . a

20
Approximate

Wavelength

Run 1 1690 nm

Run 2 1690 nm

Mean x^ ± Standard Error

a

3.76 X 10"^ ±0.92 X 10"^

4.06 X 10"^ ±0.91 X 10"^,

If we take 3 standard errors to be the uncertainty of the measurement which is

-4

less than the mean t,, the stray light is of the order of 10 . a

In a similar manner, a 3 mm thick germanium window was placed in the beam,

and then the combination of 3 mm and 4 mm windows was placed in the beam for 100

measurements each. The results follow:

Mean x^ ± Standard Error

a

4.85 X 10"^ ±7.26 x 10"^

8.45 X 10"^ ±7.72 X 10"^

2.23 X 10"^ ±2.01 X 10"^

1.53 X 10"^ ±2.22 X 10"^

3.67 X 10"^ ±3.30 X 10"^

-2.79 X lO"'' ±3.41 X 10"^

Again we see that, taking 3 standard errors as the error in the measurement, the

-4

stray light is of the order of 10 or less.

(2) Photovoltaic Silicon Photodiode

Two silicon wafers, two germanium windows, and two interference

filters (the latter transmit the visible spectrum almost entirely) were used to

obtain estimates of the stray light for this detector. The interference filters

are each known to transmit approximately one percent at 900 nm. Consequently,

neglecting interreflections (a conservative assumption) the combination should

transmit approximately 10" of the incident radiation at 900 nm. The results

are:

Approximate

Wavelength Sample

900 nm 3 mm Ge

900 nm 3 mm Ge + 4 mm Ge

1100 nm 3 rmi Ge

1100 nm 3 mm Ge + 4 mm Ge

1500 nm 3 mill Ge

1500 nm 3 mm Ge + 4 mm Ge

21
Approximate

Wavelength Sample

850 nm 1 Silicon Wafer

850 nm 2 Silicon Wafers

Mean t^ ± Standard Error

a

1.18 X 10"^ ±8.37 X 10"^

2.02 X 10"^ ±8.92 X 10"^

-4. -5

900 nm 3 mm Germanium -1.20 x lo ±3.91 x 10

900 nm 3 mm Ge + 4 mm Ge -2.01 x lo"^ ±4.11 x 10'^

900 nm Interference 1.017 x lo"^ ±3.443 x 10"^

Filter No. 1

900 nm Interference 9.138 x lo"^ ±4.890 x 10"

Filter No. 2

900 nm Combination of 2.568 x lo"^ ±3.990 x 10"^

Filters No. 1 and No. 2

We again see that we are noise limited, and the stray radiant energy measured

5

-4

with the Si wafers and the Ge windows is of the order of 10 or less. The

interference filters in combination should transmit on the order of 9 x lo

-4

whereas we measure a higher amount by 1.6 x lo with three standard errors of

-4 -4

1.2 X 10 so that again the stray radiant energy is of the order of 10 or

less.

(3) Extended S-20 Photomul tipl ier

Stray radiant energy at 725 nm as measured with the selenium

-5 -7

filter had a mean x^ of 1.44 x lo with a standard error of 6.37 x lo . The

a stray radiant energy from long wavelengths was also measured with the germanium

filters with the results:

Mean x^ ± Standard Error

a

Approximate

Wavelength Sample

900 nm 3 mm Ge

900 nm 3 mm Ge + 4 mm Ge

4.68 X 10"^ ±8.41 X 10"^

1.37 X

10"'' ±7.13 X 10"^

22
These numbers are wery small since the detector is not sensitive to wavelengths

above 950 nm. However, this detector is wery sensitive to visible radiation, and

it was necessary to use a blocking filter to absorb wavelengths below 750 nm to

obtain low stray radiant energy from the visible at 900 nm. A stray radiant

energy of 0.5 percent was observed without the blocking filter. The results with

and without the blocking filter are reported for illustration.

Without Blocking Filter

Approximate

Wavelength Sample Mean x ± Standard Error

900 nm Interference 1.671 x lo"^ ±1.746 x lo"^

Filter 1

Interfer

Filter 2

Combinat'

of 1 and 2

900 nm Interference 1.762 x lo"^ ±1.895 x lo"^

900 nm Combination 5.240 x lo"^ ±4.285 x lo"'^

With Blocking Filter

Approximate

Wavelength Sample Mean t ± Standard Error

900 nm Interference 1.039 x lo"^ ±5.840 x lo"^

Filter 1

Interfer

Filter 2

Combinat"

of 1 and 2

900 nm Interference 9.063 x lo"^ ±8.966 x 10"^

900 nm Combination 9.346 x 10"^ ±8.881 x lo"'^

The measurements of the individual interference filters show transmittances

which, when multiplied together, predict a transmittance, neglecting interreflec-

-5

tions, for the combination of 9.42 x lo . Comparison with the combination of

filters 1 and 2 leads one to predict that the stray radiant energy is 10" or

less when using the blocking filter.

23
G. Wavelength Calibration

The dial reading of the spectrophotometer on a counter (CN) may not

indicate the true wavelength. We account for this fact by calibrating the dial

to give a wavelength correction, AX,

^^ "

^-CN

" ^TRUE

where \r.^ is the dial reading and Xynnr is the published wavelength for a given

atomic line. The atomic lines that were used are given in table 2. We scanned

the instrument across the lines and chose the peak wavelength as Xp^,. Using the

photomul tipl ier and measuring the Rb lines we showed that our entrance and exit

apertures are symmetrical and the half-height bandpass is approximately 5.5 nm.

Table 2. The elements and wavelengths of the atomic lines used

for the wavelength calibration and the published values

for the minima of the absorption bands.

Element or Chemical

Wavelength

(nm)

Rb 780.02

Rb 794.76

Hg 1013.98

Hg 1128.70

Hg 1529.52

Hg 1692.1

Hg 1970.1

,2,4-trichl orobenzene 2152.5

,2,4-trichl orobenzene 2437.4

By using these lines, figure 12 was obtained by plotting the wavelength correction, A>., as a function of wavelength X, and fitting a straight line through the

resulting data using the method of least squares.

24
IaJ q:

IZ

o

/<

I)

o.u —

2.0

A - Rb

®

1.0

O - Hg (LOW PRESSURE)

D - Hg (HIGH PRESSURE)

(3— 0.5 mm of 1-2-4 trichlorobenzene

1 1 1 700 1000 1500 2000

WAVELENGTH (nm)

2500

Figure 12. Wavelength correction curve,

25
Due to the fact that our system has a low sensitivity near 2500 nm and the

atomic lines that we attempted to measure were relatively weak, we did not

obtain any data for atomic lines beyond 1970.1 nm. There are some absorption

bands in 1 ,2,4-trichlorobenzene whose wavelengths have been published [15]. We

chose to measure two of these that are relatively symmetrical, and thus insensitive to bandpass. The uncertainty in these published values is given in refer- ence [15] as ±1 nm. The least squares fit of the atomic lines falls within 1 nm of the measured minima of the absorption bands.

Repeated measurements on the Hg lines show the noise in the measurements

obtained by scanning the instrument across the atomic lines. The range of this

error is approximately 0.15 nm. Therefore, we assign a total uncertainty as the sum of a systematic error of 0.18 (the difference obtained with two types of

lamps) plus 0.15/2 or approximately 0.26 nm.

If there is a substantial rate of change of the instrument sensitivity

function with wavelength, the effect would be to cause the actual passband

centroid to be shifted away from the geometrical passband. In such a case,

further corrections must be made. In particular, for a triangular geometrical

passband the displacement dX of the passband centroid from the center of the

geometrical passband is given by [16]

where a is the slope of the instrument sensitivity curve normalized at the

center of the triangle and w is the width at half-height of the geometrical

passband.

4. Transmittance Measurements

We have shown above that the drift with time of the signals obtained with the

detectors is essentially linear over a period of time of 10 to 20 minutes--the

time necessary to make transmittance measurements. This means that we can use a previously published method for taking and analyzing the data [3]. Our automatic

filter holders accommodate one to three filters at a time. We devised a method

of taking the data in a symmetrical fashion such that four measurements are made

for each filter, the clear space or 100 percent signal is measured five times,

and dark signals are interspersed between. Each reading takes 20 seconds--a 10

second wait and then a 10 second integration time. Differences from previous

26
data-taking techniques are required primarily because of the large dynamic range

of the instrument sensitivity functions and the noise at low signal levels

especially for the PbS detector. In order to conserve flux levels at their

highest possible values, we chose to eliminate one Glan-Taylor polarizer and to

adjust automatically the sensitivity scale on the lock-in amplifier at each

wavelength. This adjustment is made by a combination of software in the computer

and hardware in the lock- in amplifier.

Previous estimates for the systematic error for this instrument over the

wavelength range 200 to 800 nm are approximately 0.0001 transmittance units for

a filter having a transmittance of 0.1 to 0.92. The random error expressed as the standard error for four measurements varied from approximately 0.00003 to

0.0002 depending on the level of transmittance. Lower transmittances were measured using a step-down technique [1] with a corresponding propagation of

errors.

The uncertainties in transmittance in the near infrared vary by two orders

of magnitude depending on the wavelength, filter, and the detector used. It

would be possible to construct tables estimating the errors for all wavelengths

and transmittances. However, the number of measurements would be very large, and

the measurements would have to be repeated for a meaningful calibration. Consequently, we chose to sample the possibilities, and it is possible to give order

of magnitude estimates for any future measurements.

The linearity was measured for the photomultiplier, silicon photodiode, and

lead sulfide detectors. The uncertainties in these measurements are regarded as systematic errors. The standard error in the non-linearity for the photomultiplier is sufficiently small so that the systematic error estimate remains as approximately 0.0001 transmittance units. The standard error in the non-linearity

for the silicon photodiode is about 1.4 x lo' transmittance units. We therefore

-4

estimate the systematic error to be 0.0001 plus 3 times 1.4 x lo or about

0.0005 transmittance units. Non-linearity measurements with the PbS using the 10

mV and 2 mV scales have standard errors ranging from 0.00014 to 0.0007, respectively. Consequently, we have systematic errors ranging from 0.0005 to 0.003.

The random errors are determined by repeated measurements. The standard

error for 4 measurements will vary from about 0.00003 for the photomultiplier and

low transmittance filter to 0.003 for the PbS detector and medium transmittance

filter at the extreme wavelength of 2500 nm where the instrument sensitivity

function is small

27
The total uncertainty is usually conservatively estimated as the systematic

error plus three times the standard error. This gives estimates ranging from

approximately 0.0002 to 0.012 transmittance units depending on the apparatus

used, wavelength, and transmittance of the filter to be measured.

We have used state-of-the-art equipment to extend the wavelength range of

this spectrophotometer into the near infrared. We believe that the instrument

will provide a valuable measurement capability for the National Bureau of Standards.

Acknowledgment.

This work was partially supported by the DoD Calibration and Coordination

Group (CCG) of the Departments of Air Force, Navy, and Army. We also wish to

thank Joel Fowler for designing and building many of the interface boards used to

automate the instrument.

5. References

[1] Mielenz, K. D. , Eckerle, K. L., Madden, R. P., and Reader, J., "New

Reference Spectrophotometer", Appl . Opt. 12 (7), 1630 (July 1973).

[2] Mielenz, K. D. and Eckerle, K. L., "Design, Construction, and Testing

of a New High Accuracy Spectrophotometer", Nat. Bur. Stand. (U.S.)

Tech Note 729 (June 1972).

[3] Eckerle, K. L., "Modification of an NBS Reference Spectrophotometer",

Nat. Bur. Stand. (U.S.) Tech Note 913 (July 1976).

[4] Chopping at an image of the source was suggested by Wolfgang Budde

of NRC in Canada.

[5] Mielenz, K. D. and Eckerle, K. L., "Spectrophotometer Linearity Testing

Using the Double-Aperture Method", Appl. Opt. H (10), 2294 (1972).

[6] Eckerle, K. L., Venable, W. H., and Weidner, V. R. , "Averaging Sphere

for Ultraviolet, Visible, and Near Infrared Wavelengths: A Highly

Effective Design", Appl. Opt. l^ (3), 703 (March 1976).

[7] Weidner, V. R. and Hsia, J. J., "Reflection properties of pressed

polytetrafluorethylene powder", J. Opt. Soc. Am. 71 (7), 856 (July 1981).

28
[8] Mielenz, K. D. , Mavrodineau, R. , and Cehelnek, E. D., "Efficient

averaging spheres for visible and ultraviolet wavelengths",

Appl. Opt. 14 (8), 1940 (August 1975).

[9] "Photoconductivity in Impurity Activated Germanium and Indium Antimonide",

Technical Report AFAL TR 64 343, Dept. of Physics, Syracuse University,

Syracuse, New York, April 1, 1965.

[10] Geist, J., Gladden, W. K. , and Zalewski, E. F. , "Physics of photonflux measurements with silicon photodiodes", J. Opt. Soc. Am. 11_ (8),

1068 (August 1982).

[11] Mielenz, K. D. , Weidner, V. R. , and Burke, R. W. , Heterochromatic stray

light in UV absorption spectrometry: a new test method", Appl. Opt. 21_

(18), 3354 (Sept. 1982).

[12] Kaye, W. , "Stray Light Ratio Measurements", Anal. Chem. 53^ (14), 2201

(December 1981).

[13] "Estimating Stray Radiant Energy", 1970 Annual Book of ASTM Standards,

Part 30, p. 1154 (1970).

[14] Kaye, W. , "Resolution and stray light in near infrared spectroscopy",

Appl. Opt. 14 (8), 1977 (August 1975).

[15] Acquista, N. and Phyler, E. K. , "Calibrating Wavelengths in the Region

from 0.6 to 2.6 Microns", J. Res. Nat. Bur. Stand. (U.S.). 49 (1)

(July 1952).

[16] Venable, W. H., Jr., and Eckerle, K. L., "Didymium Glass Filters for

Calibrating the Wavelength Scale of Spectrophotometers--SRM 2009, 2010,

2013, and 2014", Nat. Bur. Stand. (U.S.) Spec. Publ . 260-66 (October 1979)

29
NBS-n4A (REV. 2-8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS TN 1175

2. Performing Organ. Report No, 3. Publication Date ;^ril 1983

4. TITLE AND SUBTITLE

Extension of a Reference Spectrophotometer into the Near Infrared

5. AUTHOR(S)

Kenneth L. Eckerle, Zu Wen Chao^ Victor. R. Weidner, and Jack J. Hsia

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP)

Same as item 6.

10. SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

The purpose of this paper is to document adaptation of an existing reference spectrophotometer to the near infrared. Its

previous wavelength range was from approximately 200 to 800 nm. The present adaptation increases its wavelength range to 2500 nm. The hardware and software necessary to achieve this were implemented along with tests necessary to characterize the instrument

when it is operated in the near infrared (800 to 2500 nm). The

accuracy of the transmittance measurements ranges from approximately 0.0002 to 0.012 transmittance units depending on instrument

configuration, wavelength, and the sample itself. Incidental to

the success of this work was conversion to a dedicated microcomputer with interface boards. Many of the latter were designed at

NBS.

12. KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

lead sulfide detector; near infrared; photomultiplier; reference spectrophotaneter;

silicon photodiode; spectrophotcmetry; transmittance; v/avelength.

13. AVAILABILITY

[3 Unlimited

I I For Official Distribution. Do Not Release to NTIS

r"^ Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C

20402.

133] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

34

15. Price

$4.50

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