C. Burgess and K.D. Mie lenz (Edi tors) , Advances in Standards and Methodology in Spectrophotometry

1 9 8 7 Elsevier Sc ience Publ ishers B.V., A m s t e r d a m — Pr in ted in The Ne ther lands

THE NEW AUTOMATED REFERENCE SPECTROPHOTOMETER AT NPL

G H C FREEMAN

Division of Quantum Metrology, National Physical Laboratory, Teddington, Middlesex TW11 OLW, UK

ABSTRACT

The new NPL reference spectrophotometer uses reflecting optics and a grating-double monochromator. These will allow it to be used from 200 nm to 4000 nm. The input optics comprise the light source (a deuterium and a tungsten ribbon lamp), focussing mirror, second order wavelength blocking filters and a chopper for IR work. Off-axis parabolic mirrors are used in the measurement beam (four of them if a focussed beam is used) with beam defining apertures, shutter, sheet polaroid and linearity measuring system. For the visible and UV the detector is a photomultiplier (S20 cathode) behind a 100 mm diameter, barium sulphate coated, integrating sphere. The monochromator exit aperture has a working range of 2.0 to 0.1 mm in width and 10 to 1 mm in height. For work of £he highest accuracy the beam has to be parallel and the aperture 1 x 1 mm or smaller. The maximum beam size is 30 mm, square or circular.

-k

In the visible the total uncertainty can be as good as 10 of the value but is limited by the sample quality. The largest correction (< 0.03 %) is from the photomultiplier nonlinearity. Measurements can be extended to 1000 nm with silicon detectors and 1600 nm with germanium. Lead sulphide detectors are being investigated for use out to 2500 nm.

INTRODUCTION

National laboratories have the responsibility of establishing and

disseminating national standards, and for carrying out international

intercomparisons. In spectrophotometry there are no fundamental standards,

only a measuring technique and instruments where all the conceivable errors

can be investigated and either eliminated or measured.

The National Physical Laboratory (NPL) in the UK and the National Bureau of

Standards (NBS) in USA have had a long history in this area. A brief and over

simplified history of spectrophotometry at NPL starts with Guild's visual

spectrophotometer in the 1920's (ref 1). Much research at NPL in the early I93O's in collaboration with GEC Ltd., Wembly, and Cintel Ltd. led to

successive photoelectric instruments with single-stage vacuum photo-emissive

detectors, the very slight nonlinearity being corrected using an early form of

the double-aperture device. Preston, Donaldson and Harding took the lead at

different times through to the late 1950's (ref 2 & 3 ) . The Hilger-Muller

UVISIR quartz double monochromator was used from 19̂ 6 onwards as the

70

dispersive element in a manual instrument that was continually developed over

the years. In i960 Clarke began investigating photomultipliers which were then thought to be too unstable and nonlinear for the best class of photometric

accuracy. He showed that by selecting the tube and carefully controlling the

operating conditions the EMI 9558 tube could be used to achieve uncertainties of only 2 parts in 10 of the photometric ratio under the best conditions in

the visible. He and Anne Compton developed the NPL instrument in various ways

and improved the the technique for measuring the linearity (ref k & 5)·

The NBS also has a long history starting with visual spectrophotometers but it

was not until the early 1970's when Mavrodineanu visited NPL to study techniques that NBS built their latest instruments. Both were superior to the

old NPL instrument as they used the latest engineering and optical components

(ref 6 to 9).

Clarke and Mielenz have published the science behind the measuring techniques

(ref 5. 9 & 10). Both laboratories have developed transmittance transfer

standards, based on neutral density glasses, and they have been used to

demonstrate that the scales of the two laboratories agree to a few parts in

The NPL is now developing a more modern spectrophotometer that will cover a

larger spectral range than the earlier manual instrument. It carries out the

measurement automatically and so eliminates operator fatigue and errors.

THE SPECTROMETER

Fig. 1. Schematic diagram of the new reference spectrometer. L - alignment laser; S - source; F - filter wheel for second-order blocking; DM - double monochromator; Ρ - sheet polarizer mount; A - aperture and shutter; M - removable plane mirrors; C - sample carriage; D - integrating sphere and detector.

10

L 5

DM

71

Layout

Figure 1 is a schematic diagram of the instrument. The monochromator is a

commercial item. The rest of the system has been developed at NPL. The input

optics represent the intial arrangement where no allowance has been made for

adding more components such as polarizing prisms. This area will probably be

modified in the future.

Input Optics

a Description

Sources of radiation are imaged via a concave mirror and a flat mirror at ^5°

onto the entrance slit of the monochromator. For IR measurements an optical

chopper is placed close to the source. Second-order wavelength blocking

filters are in a filter wheel mounted on the entrance slit housing.

b Sources

i For a visible and IR spectral continuum, the lamp used is a tungsten

ribbon one specially developed by NPL at GEC for high stability (type

25/G). The ribbon is 1.6 mm wide, 0.070 mm thick and 30 m m long. With a

Spectrosil WF window it can be used in the UV to 300 nm and in the IR to

kOOO nm. It is used at currents between l8 A and 25 A depending on the

intensity required. (25 A corresponds to 200 W and 2800 K ) .

Its power supply is a constant current switched mode one which is

5 regulated to a few parts in 10 over our measuring period (Vinculum

Services SP011).

ii The UV continuum is from a deuterium lamp with an emitting area of 1 mm

diameter (Cathodeon C70, now type ROT). It is run at currents between 200

and 5OO mA (30 W) and its constant current supply is stable to better

than O.O5 %.

iii Spectral lines for wavelength calibration are from low pressure

discharges in mercury, argon, neon and iodine. The latter is also used

for transmittance measurements as its intensity at 206.2 nm is about ten times that from the deuterium lamp at 1 nm bandwidth.

72

c Mirrors

The focussing mirror has a radius of curvature of 500 mm and a diameter of

I50 mm. This and the small flat mirror are coated with aluminium and magnesium

fluoride for high reflectance at 200 nm. Because these two mirrors receive

high levels of irradiation and degrade quickly, silica coatings are being

tried as these can be cleaned with solvents and mild abrasives.

d Wavelength blocking filters

To stop second-order wavelengths coming from the monochromator, short

wavelength absorbing filters can be inserted in the beam. They are changed

when the wavelength becomes longer than kOO nm (GG22), 600 nm (0G2), 900 nm

(GaAs), 1200 nm (Si), l800 nm (Ge) and 3000 nm (interference filter on Ge).

Monochromator

A double grating monochromator (Jobin-Yvon HRD1), with additive dispersion

from a Czerny-Turner mounting, is used with three sets of gratings to cover

the spectral region 200 - 1000 nm, 500 - 2000 nm and 1000 - kOOO nm. The dispersions in the middle of these ranges are 1.2, 2Λ and 4.8 mm (nm) ^ and

the blaze wavelengths are 250 nm, 1000 nm and 2000 nm respectively.

The internal optics restrict the maximum cone of radiation leaving the

monochromator to 7·6° χ 10.5° (80 χ 110 mm mirror with a focal length of 600 mm).

It is fitted with a Moire fringe system which measures the position of the nut

on the lead screw which drives the sine-bar correction system. The output is

the wavelength in nanometers and reads to - 0.002 nm (Heidenhain VRZ 38l). The leadscrew is not uniform over its length but corrections for this are made by

the control program.

The slits are continuously adjustable from 3 mm wide to zero. The height limit

on the exit slit has been altered to give fixed heights of 1, 2, 3, 5, 7, 10

and 30 mm with square ends.

The drive is a stepping motor controlled by the microcomputer and can be used

at speeds up to 12 000 steps per second (30 revolutions per second or 12 nm

per second).

73

Measuring Optics

a Description

Mirrors produce a beam of parallel radiation in which measurements can be

made. Additional mirrors can be inserted to produce a focussed image of the

exit slit if convergent radiation is required. The parallel beam can be

restricted in size and this is used to adjust the cone angle if focussed 2

radiation is used. The maximum beam size is 30 x 30 mm which gives a cone

angle of 11.5°. The reduced beam can be square or circular. The shutter is on

the same mount as these apertures.

b Polarizer

For the initial measurements sheet polaroid is used in a filter wheel mounted

on the exit slit housing. The material is HNPDx006 for 275 to 500 nm, HN32x030

for 5̂0 to 8OO nm and HRx015 for 800 to 2000 nm. Prism polarizers may be used

in the future to extend the range down to 200 nm and out to 4 urn.

c Mirrors

The off-axis parabolic mirrors used for focussing are 150 mm focal length,

50 mm diameter, 31-5° off-axis angle (S0RL type 06-024-02). If measurements are required with slits longer than 3 m m

. a 100 mm diameter mirror (06-01-04)

has to be used otherwise not all the radiation is collected.

All the off-axis parabolic mirrors are fixed in position. To obtain an image

of the slit, two plane mirrors can be inserted in the parallel beam to deflect

it away from and back into the parallel path. These are on kinematic mounts

and can be placed in position and removed without having to realign the

system.

The coatings are aluminium with a magnesium fluoride overcoat,

d The sample carriage

This is a stepping motor driven, optical quality (linear ball races) slide

with 200 mm of movement. It is normally used at speeds of about 50 mm per

second although it can be driven at ten times this speed. Up to four filters

can be mounted on the carriage at one time if they are cuvette-sized or less

than 51 mm across. For focussed radiation, adjustment is available to place

the face of the filter at the focus. Tilt, etc is set while mounting the

sample in the beam (white light from the zero order of the monochromator is

74

used). Temperature controlled enclosures can be placed around the filters if

control of better than - 1 Κ about 25 °C, or use at other temperatures, is

required. These are controlled from a constant temperature water bath and

restrict the number of holders in use to two.

The detector

For use in the UV and visible out to 800 nm a photomultiplier with an

S20 cathode (EMI 9558QA; 50 mm cathode w i t h 11 V e n e t i a n b l i n d dynodes) is used

behind a 100 mm diameter integrating sphere. The sphere's target is at its

centre and the monochromator exit slit is imaged on it.

The dynode resistor chain comprises eleven 100 kohm resistors with a 150 volt

Zener diode between the cathode and first dynode. There is no load resistor as

the anode current is taken to the current input of a high quality operational

amplifier. The feedback resistor determines the output voltage. It is 10 ohms

and the typical maximum signal is 2 volts (2 uA).

A high quality silicon cell (Centronic OSD300-5)is used for wavelengths

between 600 nm and 1000 nm and a germanium cell between 1000 nm and l600 nm.

The uniformity and linearity of these can be very good and allow them to be

used without integrating spheres. The same amplifier is used but with a

100 Mohms feedback resistor and with zero voltage bias on the current input

terminal.

BEHAVIOUR

The measurements

a Measurement cycle

The normal cycle is dark reading with shutter closed ( Ι π) , reference ( I ), D η

[test (IRP) , reference,] η times, dark reading; where I represents the

1 D

electrical background, I D the signal received with no filter or a reference η

filter in the beam and 1 ^ the signal measured with the test filter in the

beam, n, the number of times the cycle is repeated can be up to 19, usually 9

is adequate for good precision. The two values of I_. are averaged and this is LJ

subtracted from I and I . η values of the transmittance are calculated by Η 1

averaging the corrected I_. values on each side of 1 ^ and dividing into the

η 1

corrected 1^. The final value of the transmittance is the mean of the η values

and the random uncertainty is the standard error of this mean. If the transmittance is low then a more complex cycle is used where I is

75

measured more often: I ß (I R, ± T, I R, I ß t) η times. The values are treated as

above.

The values of I D, I D and Ι„, are the mean of 70 readings of the digital

voltmeter (Solartron 7060) taking 10 seconds. When the system is stable, I D is Β

typically 2 uV and the difference between the first and last value is usually

less than 0.3 uV. This is for EHT values of 900 V and less.

b Stability

Variations in the signal can be from one or more of three possible causes; the

source, the detector system or the mechanical/optical components.

(i) The room is stabilised at 25 - 1 °C so temperature variations should not

cause any through-put variations caused by thermal expansion, either at

the slits, component mounts or changes in focus.

(ii) To test the detector system (EHT supply, photomultiplier and

electronics), tritium activated phosphors (Beta lights) were used. At

I I 5 0 V EHT with a signal of 2 V, the drift is less than 2 mV for

1 0 minutes, a typical measurement time. As the transmittance

measurements are based on a time symmetrical sequence, the effect of a

linear drift is removed and only variations in the drift will be

important. Thus the detector should contribute much less than 1 χ 10

to the uncertainty.

The noise on these stability measurements is averaged over one second by

the computer and the remaining, lower frequency, noise is 1 mV

peak-to-peak. If the integration period is 1 0 seconds then the expected

noise is 0.3 uV. Both the drift and noise are less at lower EHT values.

The temperature coefficient of the photomultiplier gain is up to 4 % Κ 1

in the UV and at the red end of the spectrum. Therefore care has to be

taken to keep the detector at constant temperature if low drifts are to

be achieved.

In practice we find that the system must be "on" and at the working EHT 5

for 3 hours or more if random uncertainties of a few parts in 10 are

required. After changing the EHT voltage the signal has settled down to

within 0 . 1 % of its new value in 15 minutes.

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(iii) Most of the observed drifts of the complete system are caused by the

sources. With the tungsten lamp the system settles to less than 0.1 %

per 10 minutes after kO minutes. After being on for 5 hours the system

can be as good as 0.1 % per hour.

The deuterium lamp is not as stable and can be worse than 0.5 % per

10 minutes after one hour. After running for 12 hours or more this lamp

will also be better than 0.1 % per 10 minutes (at a wavelength of

250 nm).

Variation in wavelength can be caused by wear within the monochromator and by

variations in the temperature. There is some evidence to suggest the

temperature coefficient is about 0.02 nm Κ ^.

c Precision of measurement

The total uncertainty of a transmittance measurement results from combining in

quadrature various measurement uncertainties. The first of these is the

precision of measurement (random uncertainty of a measurement). The

repeatability of measurement and the uncertainty associated with linearity

corrections are two that are always considered. Other uncertainties come from

corrections for inter-reflection errors, temperature coefficient, angle of

incidence, cone angle, etc. and must be included if such corrections are made.

The uncertainties are stated at 95 % confidence level, i.e. twice the standard

error of the mean.

The random uncertainty of a series of nine transmittance measurements is

usually better than 2 in 10 , the actual value depending on the signal and the

transmittance of the filter. Table 1 shows these aspects and is based on part

of a set of measurements made on some NPL in-house metal-on-silica standards.

Except at the short wavelengths, where the signal is less than 0.2 V, a

precision (random uncertainy) of better than 1 χ 10 of the value is

achieved. It is often considerably better especially in the visible where it -4

is 1 χ 10 or better.

77

TABLE 1 PRECISION OF MEASUREMENT

UV - deuterium lamp Wavelength (nm) 200 210 225 250 300 350 Transmittance

90 % .12 .06 .032 018 .014 .014

30 % .06 .04 .016 012 .008 .008 10 % .06 .014 .010 008 .004 .004

3 % .06 .010 .006 002 .002 .002

1 % .06 .008 .002 0008 .0008 .0010

Signal (uA) .011 .057 .21 2.1 2.2 1.3

VISIBLE - tungsten lamp Wavelength (nm) 400 450 550 65Ο 700

Transmittance

90 % .008 .002 .002 002 .002

30 % .004 .001 .001 001 .001

10 % .0016 .0010 .0008 0008 .0014

3 % .0010 .0008 .0004 0004 .0004

1 % .0004 .0002 .0002 0002 .0002

Signal (uA) .8 1.9 2.5 1.1 .88

All precision values are % of 100 % and are stated at 95 % confidence level Conditions^ focussed radiation, 10 cone, 0.7 nm bandwidth, 3 χ 0.8 mm patch, 10 second integration period, 9 transmittance values, D lamp - 300 mA, EHT 1000 V; W lamp - 22 A, EHT 750 V.

Inter-reflection errors

The largest optical error is that caused by the various beams that are

reflected by the sample surfaces, see Mielenz (ref 10). The variation of the

transmittance with angle can be measured using a rotating filter holder

developed by Clarke in 1970. Most of the variation of transmittance with angle

is due to variations in the absorptance and reflectance. Any inter-reflection

will cause distortions in the measured parabolic curve.

With a filter of about 4 % reflectance we find that the variation of

transmittance with angle is less than 0.002 %. With metal-on-silica filters of

10 % transmittance in focussed radiation, the variation with angle is

observable, see figure 2, but is small enough to choose an angle and to make

corrections. Figure 2 is typical of the measurements made between the

wavelengths of 350 nm and 700 nm, indicating that there is no wavelength

dependency. So far, with parallel radiation we have not found any

inter-reflection error with either type of filter.

78

c 16.60

Ε

Angle ( d e g r e e s )

Fig. 2. Inter-reflection error. The transmittance of a filter of nichrome-on-silica at 300 nm as a function of angle for the two planes of polarization.

Other optical errors

a Shutter position, electrical and optical background

At present the shutter is mounted on the same support as the apertures which

define the beam size. The electrical background I D measured with the shutter Β

closed is taken as the optical background. This assumption can cause errors.

The light not absorbed by the shutter blades is reflected and can be scattered

around the cabinet onto the photomultiplier. This obviously is not present

when the shutter is open. In the infrared the slight heating of the blades may

cause enough emission to alter the background.

In the UV and visible there is no background error from this cause. Putting

the shutter in the input beam before the slit produces exactly the same I to Β

within the precision of measurement.

The use of a light trap, which totally absorbs the radiation, in place of the

sample could be used to test for such errors but a more realistic optical

background measure would be to use a totally absorbing sample of the same size

and same reflectance as the one under test as this would correct for light

scattered within the cabinet from the reflected beam. Both techniques are

used.

79

b Retro-reflection

Tests were made for retro-reflection of the matt black absorbing surfaces on

the back of the apertures. This is where the beam reflected from the filter

usually gets absorbed and scattered. Transmittance measurements were made with

a filter at 4° so that all the beam falls on the aperture mount. For some

measurements a polished black glass was used to reflect the beam away from the

aperture and onto the cabinet walls. No difference between the two types of

measurements was found. This is not surprising as the inter-reflection error

is so low and would have shown an effect if retro-reflection was present.

However, it may be important in the IR as it is suggested those

retro-reflection coefficients may be several percent compared with 0.1 % or

less in the visible (ref 12).

c Low angle scatter

No slit or aperture jaws are perfect. Thus, even if the optical aberrations

are zero, there will be some radiation outside the beam defined by the

apertures and slit. It is therefore important to have identical sample mounts

in the reference and test positions. With parallel radiation, up to OA % of

the signal is outside the main beam as seen by eye but this is all within 1 mm

of the edge of the beam. With two similar holders in the reference and test

positions their measured transmittance is identical to better than 0.001 %.

d Heterochromatic scatter

Scatter within the monochromator is less than the precision of measurement.

Placing absorbing filters, such as WG 295. in front of the entrance slit

indicates the scatter at short wavelength settings is less than 0.001 %. If

these filters are placed in the filter holder after the monochromator,

fluorescence gives false readings corresponding to densities of between 2

and 4. For such tests it is therefore recommended that the filters are placed

before the monochromator.

Sample corrections

Corrections to the measured transmittance of glass filters for cone angle and

angle of incidence have been published by Mielenz (ref 10). For our in-house

standards work we aim to measure the filters under the same optical conditions

as they are used and so no corrections are required.

All neutral glass filters have a relatively large temperature coefficient, see

figure 3, and corrections of a few parts in 10 have to be applied if they are

used at temperatures only 1 Κ different from that at which they were measured.

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Λ00 500 600

Wavelength (nm)

700 800

Fig. 3· Temperature coefficient of a 50 % glass filter. The coefficient is the measured change in transmittance for a 1 Κ change in temperature near 20 C.

Linearity

The double aperture system of measuring linearity is used (ref 4, 5 & 13)· If

the signals measured with half apertures A and Β are I. and I , and with both

A u

apertures in the beam is i^+g» the linearity L is

L = h+B

/ ( IA + V

Thus, if doubling the intensity produces more than twice the signal

> 1 ^ + Ig] then L is greater than unity and the detector is said to be

superlinear.

With the photomultiplier used as described above, it is superlinear at 2 V by -4

2 χ 10 (L = 1.0002) and becomes slightly sublinear at very low signals. This

superlinearity is close to what is calculated from the change in dynode

voltages produced by the change in the current in the dynode resistor chain

caused by the current flowing to the anode.

Many ways have been tried over the years to reduce the nonlinearity but the

simplest one is to place a -resistor in the lead between the anode and the

current input of the amplifier. This causes the anode to become negative when

current flows and spoils the collecting efficiency of the anode. By varying

the resistor, any value of the linearity can be obtained from the above

superlinear value to a large sublinear one. With a resistor of 500 kohms the

nonlinearity is reduced to about one tenth of its value. The linearity varies

81

with EHT as shown in figure 4. These are some early results where the scatter

of points is large. With care the scatter at 1000 V can be as small as that

for the 600 V shown in the figure.

Fig. 4. Linearity as a function Fig. 5· The correction to the of signal at different EHTs. measured transmittance.

A. Original data. B. The calculated correction.

Taking the curve for 900 V EHT and summing the values, the correction to the

transmittance as a function of the transmittance is shown in figure 5 for

reference (or 100 %) signals of 4.4 volts and 2.2 volts. The high value was

chosen to accentuate the correction curve. In practice signals above 2.0 volts

are rarely used and a computer program is used to calculate the appropriate

curve for each reference signal measured.

To process the data, the original data points in figures 4 and 5 are fitted to

a straight line by a least squares method with weighting of the points

according to the standard error of the mean of the values. The slope and

intercept are used to calculate the linearity factors. The appropriate

variances are then used to calculate the uncertainty of the correction, by

summing the calculated standard deviations, and this is then added to the

random uncertainty of the measurements in quadrature.

The uncertainties associated with the correction to the transmittance varies

with the value and depends on the number of divide-by-two steps that are used.

Each step has its own uncertainty and these must be added linearly as the

uncertainties could be systematic and it is the. largest possible total

82

uncertainty that is important. For measurements made with 1000 V on the

photomultiplier a 10 % filter would have an uncertainty of 6 χ 10 of its

value arising from the correction and a 1 % filter an uncertainty of

-5 14 χ 10 . At lower EHTs the uncertainties are smaller.

AUTOMATION

In order to achieve reliable and repeatable measurements all movements of the

components are controlled by the computer (CBM 8032), either from within the

measurement sequence or from the keyboard. This latter allows the components

to be positioned and the performance of each component to be checked

independently before measurements are started. The shutter, filter wheel,

polarizer, wavelength drive and readout, and the position of the sample

carriage are thus controlled. The digital voltmeter is read when required and

results are printed and can be stored on floppy disc.

Setting the position of the filters and the beam size is carried out in white

light at zero wavelength (grating zero order). The EHT used is determined by

the maximum signal and this is found simply by scanning the wavelength range

to be measured and watching the DVM. A manual shutter in front of the detector

is used to prevent the high intensity zero-order white light from damaging it.

680 690

Fig. 6. Automation. An example showing the transmittance of a broad band and a narrow band photometer filter. A. and C. the transmittance as a function of wavelength. B. and D. the random uncertainty of measurement (SEM). Conditions: A. and B. parallel beam 12 mm diameter; 0.6 mm slits (BW = 0.5 nm) 1 mm long; EHT 900 V, max signal 2 uA. C. and D. parallel beam 6 mm diameter 0.5 mm slits (BW = OA nm) 1 mm long; EHT 1000 V, max signal 0.3 uA.

83

Two examples of the use of this automation are shown in figure 6. Figure 6A.

shows measurements made at eighty wavelengths throughout the visible on a

high-accuracy photometer filter. One set of these measurements takes nine and

one half hours. Figure 6B. shows the precision of the measurements. It rises

in the centre where the transmittance is high indicating that the scatter is

caused by the noise in the light beam, and increases again at the ends where

the signal is low and the photomultiplier noise is becoming important. Figures

6C. and 6D. show the corresponding curves for a narrow band photometer filter.

These took about 8 hours with the short measurement cycle and about 19 hours

with the long one. Measurements of such high precision would not be possible

with any other type of instrument.

REPEATABILITY OF MEASUREMENT AND TOTAL UNCERTAINTY

The uncertainties caused by various instrumental parameters are all very low

and so the meaningfullness of the measured transmittance depends on the

quality of the sample and its stability. Mielenz (ref 10) discusses the

specifications that apply to high quality neutral density glass filters and

these can be extended to cover most semi-transparent materials. In practice it

is the short and long term repeatability that gives an indication of the

quality and stability of the sample. Repeatability is the remeasurement of the

transmittance of a sample at the same wavelength after the sample has been

removed from the holder and replaced in it and all the alignment controls

repositioned so that as nearly as possible the operator has set the instrument

to measure in the same position on the sample and at the same angle. Because

it is not possible to repeat such measurements more than a few times (four or

at the most six values being obtained) and hence carry out normal statistics

on the results, the value of the repeatability used in this paper is the

maximium deviation that any value has from the mean of the values.

Generally speaking the largest factor in the stated uncertainties is the

repeatability which is governed by sample quality rather than by the

measurement capability of the instrument. The repeatability is multiplied by 4

and is added in quadrature to the individual uncertainties of measurement.

The repeatabilities for four sets of measurements on the two examples shown in

figure 6 are + 0.04 % and + 0.01 % at their peaks. The uncertainties

associated with the linearity corrections are small compared with these and so

the total uncertainty would be quoted as 0.16 % and 0.04 % respectively.

As part of an intercomparison with NBS a set of 7 high quality neutral density

glass filters have been measured a minimium of 3 times. The repeatabilities

84

vary from 0.007 % for the 90 % filter to 0.0019 % for the 10 % filter and

0.00006 % for the 0.1 % filter. The corresponding uncertainties are + 0.030 %,

+ 0.0042 % and + 0.00028 % respectively.

COMPARISON OF THE OLD AND NEW NPL TRANSMITTANCE SCALES

The NPL visible scale is maintained by a series of neutral density glass

filters with transmittances between 50 % and 1 %. These have been measured

over many years on the old manual instrument and thus are known to be stable.

(They are used for calibrating high quality commercial instruments used for

most of NPL's calibration work.) On the manual instrument the measured

uncertainties range from 0.02 % for the less dense ones to 0.005 % for the

higher density ones. Measurements on the new instrument agree with these

results to within their uncertainties.

FUTURE

The instrument is working well from 250 nm to 800 nm. In this region only a

few minor modifications and measurements are needed to be certain there are no

unknown shifts left and that the uncertainties are as stated above.

At short wavelengths the loss of signal is serious. This is probably

associated with the sphere in which case regular cleaning/recoating may have

to be undertaken. If this does not help other techniques will have to be

investigated. It is probable that all the mirror surfaces will have to be

recoated every five years and those of the input optics more often. The use of

silica coated aluminium mirrors is being investigated as they can be cleaned

chemically with mild abrasion without damaging the surface. Unfortunately

these mirrors do not have the reflectance of magnesium fluoride coated

mirrors, 80 % compared with 90 % at 200 nm.

Spectral line sources can produce higher intensity at discrete wavelengths

than the deuterium lamp in the far UV, eg I at 206 nm, As at 193 nm and Zn at

214 nm. Unfortunately many of these are not designed for photometric use and

are of poor stability. The stabilisation of the iodine lamp is being

investigated.

To extend measurements to 2500 nm some lead sulphide detectors are being

characterised so that the optimum type can be used. Integrating spheres with

barium sulphate paint can be used out to 2000 nm but because of the poorer

signal:noise ratio of lead sulphide detectors and their smaller areas compared

with photomultipliers, the measured signals are very low, approaching the

85

noise value. A compromise has to be reached among sphere size, detector size,

detector temperature, and overall signal:noise ratio. Linearity is also a

problem.

For the region 2000 to 4000 nm other detectors with liquid nitrogen cooling

will probably be needed.

For reflection work, no commercial instrument has all the different sphere

geometries that a standards laboratory requires at the precision required.

Consequently standard techniques and spheres will be developed and modified to

fit all the sphere systems on this new reference spectrometer.

CONCLUSION

A new reference spectrometer has been built and is working well. The accuracy

of the transmittance scale has been improved. The improvement in precision has

been brought about by the use of a more stable lamp, a very stable

photomultiplier and by automation. The major correction that has to be applied

(the detector nonlinearity) has been reduced with a corresponding reduction in

its uncertainty. The limiting factor now is the quality of the samples.

ACKNOWLEDGEMENTS

This project was started by J F Verrill. I am grateful to him for his

encouragement, to A C Matthews and Julie A F Taylor who assisted me with some

oi the experimental work, to A S Jones for writing some of the computer

programs and to Anne Compton and F J J Clarke for helpful discussions.

REFERENCES

1 W.D. Wright, The measurement of Colour, Adam Hilger Ltd., London, ( I 9 6 9 ) , 4th edition, p24l onwards.

2 J.S. Preston and F.W. Cuckow, Proc. Phys. S o c , 48 (1936) 869-88Ο. A Photoelectric Spectrophotometer of High Accuracy.

3 H.G.W. Harding, Photoelectron Spectrometry Group Bulletin, No 4 (October 1951). Precautions Necessary for Accurate Measurements of Optical Density Standards.

4 F.J.J. Clarke, NPL Report MC 3042 (Nov I968). High Accuracy Spectrophotometry of Three Filters.

5 F.J.J. Clarke, NBS Special Publication 378, (May 1973) 1-29- High Accuracy Spectrophotometry at the National Physical Laboratory.

6 R. Mavrodineanu, J Res NBS, 7 6 A No 5 (1972) 405-425. An Accurate Spectrophotometer for Measuring Transmittance of solid and liquid materials.

7 K.L. Eckerle, NBS Technical Note 913, (July 1976) Modification of an NBS Reference Spectrophotometer.

8 K.D. Mielenz and K.L. Eckerle, NBS Technical Note 729, (June 1972). Design, Construction and Testing of a New High Accuracy

86

Spectrophotometer. 9 K.D. Mielenz, K.L. Eckerle, R.P. Madden and J. Reader, Applied Optics 12

No 7 (1973) 1630-1641. New Reference Spectrophotometer. 10 K.D. Mielenz, J Research NBS, 76A No 5 (1972) 455-467. Physical

Parameters in High-Accuracy Spectrophotometry. 11 F.J.J. Clarke, UV Spectrometry Group Bulletin No 9 part 2, (Dec I98I)

8I-9O. Reduction of the Uncertainties of Standards in Absorption Spectrometry.

12 J.E. Martin (Private communications 1985). See also pages 100-101 of T.J. Quinn and J.E. Martin, Phil. Trans. Roy. Soc. 3l6 No 1536 (Nov 85) 85-I89.

13 K.D. Mielenz and K.L. Eckerle, Applied Optics 11 No 10 (1972) 229̂ -2303-Spectrophotometer Linearity Testing Using the Double Aperture Method.