Physical Standards in Absorption and Reflection Spectrophotometry

C. B u r g e s s a n d K.D. M i e l e n z ( E d i t o r s ) , Advances

in Standards

and Methodology

in

Spectrophotometry

1 9 8 7 Elsevier S c i e n c e P u b l i s h e r s Β. V . , A m s t e r d a m — P r i n t e d in T h e N e t h e r l a n d s

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 reflected

and

radiation in

part

falls

on any material

absorbed.

In

some

transmitted. Spectrophotometry can be divided and

transmittance. Transmission

and

object

cases

it

it

may

will also

in

part

be

in

be

part

into two main areas, reflectance

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

spectrophotometry, a misleading word since it is not region

of

the

spectrum

as

the

syllable

"phot"

confined would

is to

imply.

known the

as

visible

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

112 have

all been

through made

carefully

which

the

traceable

The major

to

of

since

Where

or

medium for

because

they

not,

in

as

prepares

the

This raises

for chemistry

However,

low

high are

the

standards

of

radiometric

in

but can

i f the be

stray

done

wavelength with

Calibration

transmittance decoupled

requires

is

from

calibration

known

independent wavelength

are needed

to

of

radiation

characterisation

of

monochromator

or

require

lines

methods however,

laser are not

a

single

laser for

range

are

is

be

but

by m e a n s

lines be

scale

firstly

values

skill

of

are

whoever

This

full

working

a

difficult

used

as

a guide

of c u t - o f f

to

is o f t e n

of

range

the

be with

of

subject

most

stray

the

Double users,

radiation

filters. Several

the

filters holder. whose

errors

to

degrees

instrument.

and

radiation

or

filters

ratio

that

of

sample

differing

source. For

little

lamps

the

enables

requires the

into

ideally

radiation calibration

Calibration

discharge

which

calibration

stray

instrument

there

filters

for

scale,

the

after

from

should

evaluation.

desirable.

of

inserted

Several

the

full

liquids.

preferable.

liquids,

and

instrument.

monchromator

to a

practicable

be o b t a i n e d

full wavelength

a

to

be

and

necessary

on

spectrophotometry

scales

wavelength.

cover

are

calibration

the w a v e l e n g t h

better

can

errors.

evaluation

the

is i n a d e q u a t e

a

ratio

attenuation

well

expertise

in

of

the

spectral

of

standards

may

measurement

to

that

radiometric

stray

and

get

peaks

be

laboratory.

scale

to

can

chemistry

measurements

preferable

the

standards

industry

laboratories.

solid their

because on

transfer

and

are analytical

liquids

quantities

performance

than

absorption of the

given

of

are

dependent

c a n be m a d e

radiation

scale

narrow

solids secondly

for

science

the n a t i o n a l

needed

and

are needed,

other

arises

in

whether

work

any

ratio

of

are

particular

properties. Adjustments

in

the m a j o r i t y

of liquids,

physical the

the q u e s t i o n

stable

three

thus

used

of spectrophotometry

accuracy

case

are

instruments

use

accuracies

more

The need

instruments

chemists do

the s o l u t i o n s

There

of

reference

areas

colorimetry.

evaluated.

scales

the from

be of The

complete a

double

monochromators however,

these

performance

of these

covering

can, the

113

(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 are required, the neutral density

the

three

types

of spectral

filter, the narrow

absorption

cut-off filter. For instruments dedicated

to colour

measurement

added

standards

should

requirement

highly

saturated

colours.

Physical

that

physical

colours

and,

standards

transfer

if numbers should

be

with

peak there

cover

permit, a range stable

profile

of

time,

a

low

that

and

the

is

the

range

of

saturation

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 will

be

found

in

the

report

of

CIE

Committee

as

follows. Full

2-13.

Note

that

details

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.

114 Regular

transmittance

3.

Neutral

4.

Metal

Regular

First

6.

Second

7.

First

No

fused

glass

filters

silica neutral

density

filters

reflectance

5.

Diffuse

density

on

surface

aluminium

surface

mirror.

aluminium

surface gold

mirrors

with or without

wedge.

mirror.

transmittance reference

materials

for

spectral

diffuse

transmittance

are

as

yet

available.

Diffuse 8.

reflectance

Barium

sulphate

9.

Halon

10.

Russian Opal, Ever

11.

White

12.

Vitrolite

13.

Black

ceramic

glazed

14.

Black

15.

Ever

16.

Ceramic

17.

Enamel

Stray 18.

ceramic

porcelain

tile

enamel

Black

Colour Colour

Standards Standards

light Cut-off

The opals

list and

filters.

excludes

WAVELENGTH

Clarke

not

standards.

there are many

yet

It

commercially

also

excludes

available

printed

such

as

or painted

coloured

papers

and

types.

STANDARDS

The wavelength known

materials

fluorescent

c a r d s of w h i c h

well

White

tile

scales

spectral

(ref. 2 ) .

of

lines. An

reference A

list

uncertainty

of of

spectrophotometers those

most

.01 nm

is

are

calibrated

frequently

used

adequate

for

is

against given

almost

by all

115 requirements in analytical chemistry and colorimetry. Many spectral lines known to higher

accuracies

than

used with any spectrophotometer

this. In

principle, spectral

but in practice there may

lines

be major

are

could

be

problems.

Most commercial instruments are not designed to permit arbitrary sources to be focussed on the entrance slit of the monochromator.

Fig. 2

The

Wavelength standards; a) holmium glass b) didymium glass. percentage transmittance as a function of wavelength in nm. big

advantage

inserted directly

of glasses

with

absorption

peaks

is

into the sample holder. The most widely

that used

they

can

materials

be for

wavelength absorption peaks are holmium and didymium oxides in a glass matrix (ref. 3 ) . The wavelength of the absorption independent

of

temperature

but

the

peaks

is, for practical

transmittance

significant changes with temperature. Transmittance

values

at

the

purposes, peaks

show

curves for the ultraviolet

116 and

visible

data for

regions

stations routine

spectral

are able

work,

it

emission

absorption

are

peaks

shown

in

to l o c a t e is

a much

lines.

to be u s e d

is

TRANSMITTANCE

STANDARDS

3 illustrates

some

at

compartment in

the

sample

happens the

(a) and

when

sample

compartment detector a so

that

Fig.3

sample

is

increased

the

means

is d e f l e c t e d

at t h e

the

rays

Departures

significant

errors

reflectance

with

(ref. 4 ) .

within

from will

angle

normal

this

can

be

is

materials

with

errors For

at

that

can

simplicity beam

the

arise

with

narrower

only

will

have

detector.

two a

Now

within

what

the

refractive

length

within

cross

error

section

is n o t

arises

of

the

uniform

if the

are

consider

path

the

rays

area

Because

sensitivity

the

finite

beam.

that

to

adequate

achieved

optical

arise. A similar

c h a n g e of b e a m c r o s s of sample sample wedge p a r a s i t i c b e a m s from

Generally

the

While

than for

of the

area

the

If the d e t e c t o r

Systematic errors arising spectrophotometer :

b) c)

in

nm.

coupled

standards.

section

unity

which

need

systematic

finite

placed

•+ 0.2

a

spectrophotometer.

than

will

to

spectrophotometers

uncertainty

clearly

the c r o s s

is

greater

error

of

different

is

the beam

a)

sample.

a

is c h a n g e d .

systematic

a

(b) but

and a

of

peaks

as w a v e l e n g t h

Fig.

shown

the

2. M o d e r n

higher

There

REGULAR

sample

figure

index

the beam

across

sample

of

sample at

its

has a

the area

wedge

detector.

in t h e

sample

section

compartment

at d e t e c t o r

on

of a

insertion

interreflections. the

sample

incidence

arise. These of incidence

can or

compartment must

be

limited

be e i t h e r

increased

are

as

path

a

not to

a

result

length

normal few of

to

the

degrees

or

variation

of

within

the

sample

117 All materials reflect a percentage of the incident radiation reflected from the sample may be reflected the

spectrophotometer

to

Likewise radiation may

pass

through

be reflected

reflection at the sample. The

the

from

parasitic

sample

the

radiation.

and

detector

beams

give

Some

of

the

back from a component of reach and

an

the

detector.

be returned

error

in

the

after

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 should

be measured

with

the

sample

at

component of diffuse

the

entrance

port

of

transmittance

an

integrating

sphere. There are, of course, many sample

compartment

dependent

on

both

other

but

the

the

quality

sources

preceding of

of

summary

the

error

that

indicates

sample

and

lie

outside

the

that

errors

are

the

quality

of

the

of

two

spectrophotometer. Currently

available

physical

regular

transmittance

standards

types, neutral density glass filters and metal film on silica The

spectral

transmittance

curves

of

four

filters

of

each

are

(fused type

quartz).

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 overcome

this

problem

and

can

be

used

(quartz plate) were developed

down

to

200 nm.

They

are

also

to

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.

118

200

400

300

500

600

700

800nnr

Wavelength

neutral density m e t a l

f i l m on s i l i c a f i l t e r s

§ 50%

400

Fig. 4

500 Wavelength

600

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

REGULAR REFLECTANCE STANDARDS Regular

reflectance

spectrophotometer need

built

is

usually

for regular

measured

with

a

transmittance

for a series of neutral mirrors of

special

attachment

measurements.

different

There

transmittances

to is

because

a no

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 % ) Take,

for

example

the

VW

type

of

reflectance

and sample readings.

accessory,

fig. 5.

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

follows

the

W

path.

The

method

gives

the

square

of

For

the

reading

the

the

spectral

119 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

uniform

over

spectrally

and

sample

readings.

its area

then an

calibrated

regular

reflectance. It is

important

error

If

will

reflectance

that

the

the

detector

result. Thus standards

reference

sensitivity the

with

standard

need a

and

is

not

arises

for

high the

neutral

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 are both used for reflectance standards. Aluminium is neutral region whereas

gold

is not, but gold

is neutral

in the

in

infrared

the

films

visible

and

has

a

higher reflectance in that region than aluminium. Back surface mirrors are also available and are more stable

because

the metal

substrate. However they should only be used

in

surface the

same

is protected plane

as

surface as many reflectance attachments give readings that are a

by

the

the

sample

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.

120 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

important. A major consideration is whether such standards

have

should

become

be matt

or

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

clean

than

disadvantages.

matt

standards.

Firstly

they

are

However,

a

less

glossy

good

standards

approximation

to

diffuser than a matt standard (refs. 7 , 8 ) . This is illustrated gives the variation Russian

opal.

A

of

luminance

perfect

factor with angle

diffuser

would

have

a

have a

Lambertian

in fig. 6 which

for a glossy

luminance

several

and

factor

a matt

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 from

error

the detector

collected with

but

(ref. 9 ) . In

fig. 7 the diffuse

the

component

a higher

specular

efficiency.

is

If a gloss

not trap

radiation and

will

is used

is

screened

therefore

to exclude

be 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 importance

in

instruments with

a

0°/45°

(or 45°/0°) measuring

head.

great

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 reflectance

have

those

opted

laboratories

predominantly

issuing for

transfer

glossy

rather

standards than

matt

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

of

diffuse

materials

121

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

(ref. 1 0 ) and pressed

used

matt

PTFE powder

reflectance (halon)

standards

are

(ref. 1 1 ) . Barium

either as a pressing or with a binder as a

barium

sulphate

sulphate

is used

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 will be different

for the reference and sample

the

sphere

(ref. 9 ) . Halon is widely used

122 in North America but less so in Europe

possibly

because

historically

opal has been more readily available in Europe. Halon has a higher than barium sulphate at around

2000

Russian

reflectance

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%,

materials generally have a refractive index of around

Because

solid

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 reflectance. Reflectances below H% can be achieved but where a very low reflectance

is

required

with

a trap

a

in

into a diffuse

structured

the

form

surface

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

standards. Unfortunately diffusely reflecting materials

with

for

sharp

transfer

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

calibration. The difficulty

here

dependent so one must always standard

of

the

is that

be certain

slope

can

the mid that

the

be

point

used

for

wavelength

value

is

temperature

surface

temperature

of

the

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

unambiguously distinguished

from a thermochromic shift of the spectral

Generally,

below

the

reflectance

the

evaluating stray radiation performance.

steep

slope

is

too

high

for

slope. use

in

123 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

rays entering the sphere are detected with equal evidence

to

uncertainty

justify of

Calibrated

this

less

opal

assumption

than

about

diffusers

are

2%

it

is

where

frequently

this assumes

efficiency.

not

possible

absolute used

Unless to

values

as

ascribe

are

transfer

that

all

there

is an

required.

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 many

of

these

fluoresce.

fluorescence, the

cut

by Schott. It should, however, be noted

Unless

it

is

off filter must

known

that

be placed

there

between

is the

no

that

significant

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

need

where

improvements

are

needed.

transmittance standards of low down

to

peaks,

200 and

nm,

transmitting

diffuse

reflectance wavelength

reflectance

better cut off properties.

In

standards

particular with

a wavelength

standards with

there

with

narrow

is

a

range

narrower

absorption

for

extending absorption peaks

and

124

REFERENCES

1

CIE

(in

preparation).

Survey

of

reference

materials

for

testing

the

performance of spectrophotometers and colorimeters. Report of CIE Committee TC2-13. 2

Commission Internationale de l'Eclairage, Paris.

Clarke F J J absorption

(l°/8l).

Reduction

spectrometry.

UV

of

the

uncertainties

Spectrometry

Group

of

standards No 9 .

Bulletin,

in

Part 2 ,

8I-9O.

3

Dodd

4

Mielenz K D ( 1 9 7 2 ) ,

C X

and

West

Τ W

(I96I).

Spectral

transmittance

properties

of

rare

earth glasses. J. Opt. Soc. Am, Vol 5 1 , 9 1 5 - 9 1 6 .

J.

5

Res.

NBS.

Mielenz

Vol

Κ D

Physical parameters in high accuracy

76A,

and

spectrophotometry.

455-^67·

(1973)·

Mavrodineanu R

Reflection

correction

for

high-accuracy transmittance measurements on filter glasses. J. Res. NBS. Vol 77A

699-703.

6

Verrill J F ( 1 9 8 3 ) .

7

Clarke F J J,

A re-evaluation of metal film on silica neutral density

filters. UV Spectrometry Group Bulletin, No 1 1 , 3 0 - 3 8 . Garforth

F A

and

Parry D J

(1977).

Goniophotometric

and

polarization properties of the common white reflection standards. NPL Report MOM 2 6 . 8

Clarke F J J, polarization

9

Garforth F A properties

of

and

Parry D J

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