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 Β. V . , A m s t e r d a m — Pr in ted in The Ne ther lands

PHYSICAL STANDARDS IN ABSORPTION AND REFLECTION SPECTROPHOTOMETRY

J F VERRILL

Division of Quantum Metrology, National Physical Laboratory

Teddington, Middx, TW11 OLW, UK

ABSTRACT

In recent years there has been a significant growth in the availability of different types of reference materials and transfer standards for the calibration of spectrophotometers and colorimeters. The quantities that need to be checked are the photometric linearity of the absorbance/ transmittance/reflectance scale, the accuracy of the wavelength scale and the stray radiation performance, together with the accuracy of several possible colour specification scales used on instruments dedicated to colour measurement. The principal areas of interest are regularly transmitting standards for use by analytical chemists and diffusely reflecting standards for use in colour measurement, solar reflectance and other areas. Transfer standards for specular reflectance are also available but the whole area of diffuse transmittance spectrophotometry has been less well researched, possibly due to the lack of a well defined need.

INTRODUCTION

When optical radiation falls on any material object it will in part be

reflected and in part absorbed. In some cases it may also be in part

transmitted. Spectrophotometry can be divided into two main areas, reflectance

and transmittance. Transmission and reflection can be of two kinds, regular

(specular) and diffuse. Many samples have both regular and diffuse properties.

The measurement of spectral reflectance and transmittance is known as

spectrophotometry, a misleading word since it is not confined to the visible

region of the spectrum as the syllable "phot" would imply. The term

"spectrometry" is preferable, though not endorsed by the CIE.

As transmittance and reflectance are both dimensionless ratios there is no

need for a fundamental physical standard such as is needed for time or mass.

However, the accuracy of any measurement will depend greatly on the design of

the instrument and possibly on the quality of the sample itself. Measurements

should be traceable to a reference instrument in which systematic uncertainties

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have all been carefully evaluated. The need thus arises for transfer standards

through which the scales of instruments used in science and industry can be

made traceable to reference instruments in the national laboratories.

The major areas of the use of spectrophotometry are analytical chemistry and

colorimetry. This raises the question of whether solid standards are necessary

for chemistry since chemists do the majority of their measurements on liquids.

Where medium or low accuracies are needed liquids may well be preferable.

However, for high accuracy work solids are preferable to liquids, firstly

because they are more stable and secondly because the calibration values are

not, as in the case of liquids, dependent on the expertise and skill of whoever

prepares the solutions in any given laboratory.

There are three particular quantities in spectrophotometry for which

physical standards are needed, calibration of the wavelength scale, calibration

of the radiometric ratio scale and measurement of the stray radiation

properties. Adjustments can be made to the scales after instrument calibration

but if the stray radiation performance is inadequate there is often little that

can be done other than to get a better instrument. Calibration of the

wavelength scale requires known spectral lines from discharge lamps or filters

with narrow absorption peaks that can be inserted into the sample holder.

Calibration of the radiometric ratio scale should ideally be with filters whose

transmittance is independent of wavelength. This enables ratio errors to be

decoupled from wavelength errors. Several filters of differing degrees of

attenuation are needed to cover the full working range of the instrument. The

evaluation of stray radiation is a difficult subject and the complete

characterisation of a single monchromator requires radiation from a double

monochromator or a laser to be used as the source. Double monochromators

require laser lines for a full evaluation. For most users, however, these

methods are not practicable but a guide to the stray radiation performance can,

however, be obtained by means of cut-off filters. Several of these covering the

full wavelength range are desirable.

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( α )

(b)

(Ο transmittance

or reflectance

wavelength

Fig .1 Ideal spectral profiles: a) neutral density b) absorption peak c) cut off.

Figure 1 indicates schematically the three types of spectral profile that

are required, the neutral density filter, the narrow absorption peak and the

cut-off filter. For instruments dedicated to colour measurement there is the

added requirement that physical transfer standards should cover a range of

highly saturated colours and, if numbers permit, a range of low saturation

colours. Physical standards should be stable with time, durable, of low

temperature coefficient, and readily available.

A summary of currently available reference materials and transfer standards

for testing the performance of spectrophotometers and colorimeters has recently

been prepared by the CIE Committee 2 - 1 3 (ref 1 ) . This paper concentrates on the

question of how well these materials meet current requirements and indicates

areas where further development is needed.

REFERENCE MATERIALS AND TRANSFER STANDARDS CURRENTLY AVAILABLE

A brief summary of available materials is listed as follows. Full details

will be found in the report of CIE Committee 2 - 1 3 . Note that chemicals

requiring preparation are not listed here.

Wavelength

1 . Spectral emission lines from the following elements: deuterium, cadmium,

caesium, helium, neon, argon, krypton, mercury, potassium, zinc and rubidium

2. Absorption filters of didymium and holmium glasses.

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Regular transmittance

3. Neutral density glass filters

4. Metal on fused silica neutral density filters

Regular reflectance

5. First surface aluminium mirror.

6. Second surface aluminium mirrors with or without wedge.

7. First surface gold mirror.

Diffuse transmittance

No reference materials for spectral diffuse transmittance are as yet

available.

Diffuse reflectance

8. Barium sulphate

9. Halon

10. Russian Opal, Ever White

11. White ceramic tile

12. Vitrolite

13. Black glazed ceramic tile

14. Black porcelain enamel

15. Ever Black

16. Ceramic Colour Standards

17. Enamel Colour Standards

Stray light

18. Cut-off filters.

The list excludes materials not yet commercially available such as coloured

opals and fluorescent standards. It also excludes printed or painted papers and

cards of which there are many types.

WAVELENGTH STANDARDS

The wavelength scales of reference spectrophotometers are calibrated against

well known spectral lines. A list of those most frequently used is given by

Clarke (ref. 2 ) . An uncertainty of .01 nm is adequate for almost all

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requirements in analytical chemistry and colorimetry. Many spectral lines are

known to higher accuracies than this. In principle, spectral lines could be

used with any spectrophotometer but in practice there may be major problems.

Most commercial instruments are not designed to permit arbitrary sources to be

focussed on the entrance slit of the monochromator.

Fig. 2 Wavelength standards; a) holmium glass b) didymium glass.

percentage transmittance as a function of wavelength in nm.

The big advantage of glasses with absorption peaks is that they can be

inserted directly into the sample holder. The most widely used materials for

wavelength absorption peaks are holmium and didymium oxides in a glass matrix

(ref. 3). The wavelength of the absorption peaks is, for practical purposes,

independent of temperature but the transmittance values at the peaks show

significant changes with temperature. Transmittance curves for the ultraviolet

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and visible regions are shown in figure 2. Modern spectrophotometers coupled to

data stations are able to locate the peaks to •+ 0.2 nm. While this is adequate

for routine work, it is a much higher uncertainty than can be achieved with

spectral emission lines. There is clearly a need for materials with narrower

absorption peaks to be used as wavelength standards.

REGULAR TRANSMITTANCE STANDARDS

Fig. 3 illustrates some of the systematic errors that can arise within the

sample compartment of a spectrophotometer. For simplicity only two rays are

shown in (a) and (b) but the cross section of the beam will have a finite area

at the sample and a different finite area at the detector. Now consider what

happens when a sample is placed in the beam. Because the refractive index of

the sample is greater than unity the optical path length within the sample

compartment is increased which means that the cross section of the beam at the

detector is changed. If the detector sensitivity is not uniform across its area

a systematic error will arise. A similar error arises if the sample has a wedge

so that the beam is deflected at the detector.

Fig.3 Systematic errors arising in the sample compartment of a

spectrophotometer :

a) change of beam cross section at detector on insertion

of sample

b) sample wedge

c) parasitic beams from interreflections.

Generally the rays within the sample compartment are not normal to the

sample. Departures from normal incidence must be limited to a few degrees or

significant errors will arise. These can be either as a result of variation of

reflectance with angle of incidence or increased path length within the sample

(ref. 4 ) .

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All materials reflect a percentage of the incident radiation. Some of the

radiation reflected from the sample may be reflected back from a component of

the spectrophotometer to pass through the sample and reach the detector.

Likewise radiation may be reflected from the detector and be returned after

reflection at the sample. The parasitic beams give an error in the measured

value of transmittance. Interreflection errors can be avoided by careful design

with components suitably angled so that parasitic beams do not reach the

detector. For most instruments interreflection errors are negligibly small for

non-metallic samples but they often become significant for metallic samples

where the reflectance is higher.

If there is a significant component of diffuse transmittance then the

instrumental reading will be dependent on the solid angle of collection. Total

transmittance of samples with a significant component of diffuse transmittance

should be measured with the sample at the entrance port of an integrating

sphere.

There are, of course, many other sources of error that lie outside the

sample compartment but the preceding summary indicates that errors are

dependent on both the quality of the sample and the quality of the

spectrophotometer.

Currently available physical regular transmittance standards are of two

types, neutral density glass filters and metal film on silica (fused quartz).

The spectral transmittance curves of four filters of each type of nominal

transmittance 92%, 56%, 32% and 10% are shown in fig. 4. The advantages of the

glass filters are that they are very stable and the surface reflectance is low

and similar to that of cuvettes. However, they absorb strongly below 400 nm and

so cannot be used in the ultraviolet. Metal film filters consisting of a thin

layer of a nickel-chromium alloy on a silica (quartz plate) were developed to

overcome this problem and can be used down to 200 nm. They are also more

neutral than glass filters in the near infrared. But, the higher reflectance of

the metal film does cause problems in some spectrophotometers (refs. 5,6). What

is needed is a material which is approximately neutral down to 200 nm with a

reflectance similar to that of silica. At the present time there are no obvious

candidates.

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2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 800nnr

W a v e l e n g t h

§ 5 0 %

n e u t r a l dens i ty m e t a l f i l m on s i l i c a f i l t e r s

4 0 0 5 0 0 6 0 0

W a v e l e n g t h

Fig. 4 Transmittance curves of four neutral density glass filters and four metal film on silica filters.

REGULAR REFLECTANCE STANDARDS

Regular reflectance is usually measured with a special attachment to a

spectrophotometer built for regular transmittance measurements. There is no

need for a series of neutral mirrors of different transmittances because the

linearity of the radiometric ratio can be checked with the same filters as are

used for transmittance. However, with regular reflectance attachments the path

of the beam may be very different for the reference ( 1 0 0 % ) and sample readings.

Take, for example the VW type of reflectance accessory, fig. 5. For the

reference reading the beam follows the V path and for the sample reading the

beam follows the W path. The method gives the square of the spectral

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reflectance as the beam is incident twice on the sample. If the mirrors are not

perfectly aligned then the beam will not fall on the same patch of the detector

for the reference and sample readings. If the detector sensitivity is not

uniform over its area then an error will result. Thus the need arises for

spectrally calibrated regular reflectance standards with a high neutral

reflectance. It is important that the reference standard and the sample are

mounted in the same plane. Therefore a front surface mirror should be used as

the standard where front surfaces are to be measured. Aluminium and gold films

are both used for reflectance standards. Aluminium is neutral in the visible

region whereas gold is not, but gold is neutral in the infrared and has a

higher reflectance in that region than aluminium. Back surface mirrors are also

available and are more stable because the metal surface is protected by the

substrate. However they should only be used in the same plane as the sample

surface as many reflectance attachments give readings that are a function of

sample position. Back surface mirrors with a wedge are also available and have

the advantage that the front and rear surface reflected beams are not

coincident. They cannot, of course, be used with reflectance attachments where

the front surface is used for location.

Fig. 5 VW regular reflectance attachment. Misalignment of the sample causes a displacement of the beam at the detector.

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DIFFUSE REFLECTANCE STANDARDS

Fig. 6 Variation of radiance factor with angle for a glossy

and a matt Russian opal.

Diffuse reflectance standards are required primarily for colorimetry but in

recent years other areas such as integrated solar reflectance have become

important. A major consideration is whether such standards should be matt or

glossy. The big advantage of glossy standards is that they are much easier to

keep clean than matt standards. However, glossy standards have several

disadvantages. Firstly they are a less good approximation to a Lambertian

diffuser than a matt standard (refs. 7,8). This is illustrated in fig. 6 which

gives the variation of luminance factor with angle for a glossy and a matt

Russian opal. A perfect diffuser would have a luminance factor of unity

independent of angle. Secondly the specular component may not be collected with

the same efficiency as the diffused light in the integrating sphere giving rise

to a systematic error (ref. 9). In fig. 7 the diffuse radiation is screened

from the detector but the specular component is not and will therefore be

collected with a higher efficiency. If a gloss trap is used to exclude the

specular component it may well not be perfectly efficient. Thirdly the radiance

factors for ρ and s polarized light are much more different for glossy samples

than for matt samples as shown in fig. 8 (refs. 7,8). This point is of great

importance in instruments with a 0°/45° (or 45°/0°) measuring head. If the

state of polarization of the incident radiation is uncertain then the radiance

factor for the glossy sample of figure 8 could be anywhere between 0.93 and

1.01 but for the matt sample it will lie between 0.98 and 0.99. In spite of all

these disadvantages those laboratories issuing transfer standards of diffuse

reflectance have opted predominantly for glossy rather than matt materials

because of the much greater durability and ease of keeping clean.

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Detector

Fig. 7 Integrating sphere with a single screen. The diffuse component of reflection is screened from the detector. The regular component which falls on the opposite side of the sphere is unscreened.

Fig. 8 Differences in variation of radiance factor with angle of ρ and s polarized light for glossy and matt Russian opals.

The most widely used matt reflectance standards are barium sulphate

(ref. 1 0 ) and pressed PTFE powder (halon) (ref. 1 1 ) . Barium sulphate is used

either as a pressing or with a binder as a paint. A number of manufacturers

supply painted barium sulphate reference standards recessed back into a metal

plate. Recessing prevents scuffing of the surface when placed against a port

but it introduces a major new problem because the standard will not be in the

same plane as the sample and thus the efficiency of collection by the sphere

will be different for the reference and sample (ref. 9 ) . Halon is widely used

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in North America but less so in Europe possibly because historically Russian

opal has been more readily available in Europe. Halon has a higher reflectance

than barium sulphate at around 2000 nm and is therefore to be preferred as an

integrating sphere coating for use in the infrared.

Black tiles have a total reflectance typically of 4 to 5%, Because solid

materials generally have a refractive index of around 1.5 or greater a smooth

surface will always give a glossy reflectance of about 4%. Abrading the surface

does not reduce this. It merely converts the glossy reflectance into a diffuse

reflectance. Reflectances below H% can be achieved with a structured surface

but where a very low reflectance is required a trap in the form of a glass

wedge is preferred (ref. 9) fig. 9.

Fig. 9 Black glass wedge gloss trap.

Fig. 1 depicts two other types of spectral profile required for transfer

standards. Unfortunately diffusely reflecting materials with sharp absorption

peaks are not available at the present time although there is a definite need

for them as wavelength standards. Spectral profiles with a single steep slope

are available and the mid point of the slope can be used for wavelength

calibration. The difficulty here is that the mid point value is temperature

dependent so one must always be certain that the surface temperature of the

standard is the same as that when calibrated, if a wavelength error is to be

unambiguously distinguished from a thermochromic shift of the spectral slope.

Generally, the reflectance below the steep slope is too high for use in

evaluating stray radiation performance.

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DIFFUSE TRANSMITTANCE STANDARDS

This is a neglected area. Indeed, the author has been unable to identify any

calibrated reference materials or transfer standards for spectral diffuse

transmittance. In measuring diffuse transmittance a reference reading is taken

with the sample removed so that the incident radiation falls on one small patch

of the sphere wall opposite the entrance port. A second reading is then taken

with the sample at the entrance port and the ratio of the two readings taken to

give the diffuse transmittance. The difficulty is that this assumes that all

rays entering the sphere are detected with equal efficiency. Unless there is

evidence to justify this assumption it is not possible to ascribe an

uncertainty of less than about 2% where absolute values are required.

Calibrated opal diffusers are frequently used as transfer standards in

densitometry. Systematic uncertainties are always large because of uncertainty

in the absolute value of transmittance of the standard, lack of a well defined

geometry of collection, non Lambertian diffusion and multiple reflections

between the sample and the detector.

STRAY LIGHT STANDARDS

A number of chemical standards for stray radiation measurement are available

but these lie outside the scope of this paper. The most widely used glass cut

off filters are those produced by Schott. It should, however, be noted that

many of these fluoresce. Unless it is known that there is no significant

fluorescence, the cut off filter must be placed between the source and the

monochromator rather than between the monochromator and the detector.

CONCLUSIONS

Although a wide range of physical standards for spectrophotometry is now

available, there are still several areas where there are no suitable standards

or where improvements are needed. In particular there is a need for

transmittance standards of low reflectance with a wavelength range extending

down to 200 nm, transmitting wavelength standards with narrower absorption

peaks, and diffuse reflectance standards with narrow absorption peaks and

better cut off properties.

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8 I - 9 O .

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l'Eclairage, Paris.

1 1 Weidner R and Hsia J J ( I 9 8 I ) . Reflection properties of pressed

polytetrafluorethylene powder. J. Opt. Soc. Am. Vol 7 1 . 8 5 6 - 8 6 I .