Astm stp 478 1970

These are: Attribute and Product Reason Measurements are Needed Luster, and in many cases, color of both interior and exterior archi-tectural metals Brightness of bright automotive trim,

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APPEARANCE OF

METALLIC SURFACES

A symposium

presented at the

Seventy-first Annual Meeting

AMERICAN SOCIETY FOR

TESTING AND MATERIALS

San Francisco, Calif., 23-28 June 1968

ASTM SPECIAL TECHNICAL PUBLICATION 478

List price $7.00

AMERICAN SOCIETY FOR TESTING AND MATERIALS

1916 Race Street, Philadelphia, Pa 19103

Downloaded/printed by

University of Washington (University of Washington) pursuant to License Agreement No further reproductions authorized.

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Library of Congress Catalog Card Number: 77-132803

ISBN 0-8031-0065-5

NOTEThe Society is not responsible, as a body,

for the statements and opinipnsadvanced in this publication

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The Symposium on Appearance of Metallic Surfaces was presented

at the Seventy-first Annual Meeting of the American Society for Testing

and Materials held in San Francisco, Calif., 23-28 June 1968 The sponsor

of this symposium was Committee E-12 on Appearance of Materials

E F Barkman, Reynolds Metals Co., presided as symposium chairman

In addition to the four papers presented at the symposium a paper by

B W Robinson has been included in this publication

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Related ASTM Publications

Stainless Steel for Architectural Use, STP 454

(1969), $9.75 Nomenclature and Definitions Applicable to Radiometric

and Photometric Characteristics of Matter,

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Introduction 1

Appearance Attributes of Metallic Surfaces—R s HUNTER 3

Measurement of Appearance Characteristics of Stainless Steel—A E GLUBISH 22

Specular and Diffuse Reflectance Measurements of Aluminum Surfaces—

E F BARKMAN 46

Instruments for the Measurement of Metallic Appearance—J s CHRISTIE 59

Proposed Method of Measurement for Appearance of Rolled Aluminum

Sur-faces Using Distinctness of Image and Diffuse Reflectance Measuring

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For the past several years, ASTM Committee E-12 on Appearance ofMaterials has been studying the appearance of metallic surfaces Thiswork has been conducted by Task Group 4 The participants in this worksoon realized that there existed no adequate and comprehensive compila-tion of techniques for describing the appearance attributes of metallicsurfaces Other materials have long been studied with respect to theirappearance For example, the color, texture, and visual appearance ofwhite and colored textiles, papers, and paints have been well defined, andASTM test methods for measuring chromatic attributes and gloss can be

found in the Book of Standards.

The metallic surfaces, however, presented a uniquely different problem

of measuring the appearance Most metallic surfaces have very little or

no chromatic attributes Or at least the chromatic features are not ofmajor importance to describe the visual properties of metals Instead,metals exhibit the features which the consumer generally refers to as

"brightness." This term, which is somewhat of a misnomer for describingwhat the eye first notices about metals, is one of the problems of accuratedefinitions confronting the workers in this field

In the past several years the metallic appearance of bare and coatedmetal products has developed an increasingly larger importance to con-sumer products such as automotive and appliances Other product areas,such as jewelry and other decorative items, are examples where the metallicappearance is a primary characteristic of consumer appeal of the product.The space age also introduced needs for measuring and defining the opticalproperties of metallic surfaces as related to reflectors, surfaces to providethermal balance in space vehicles, and the characteristics of metallic sur-faces with respect to electromagnetic radiation and reflection outside thevisible spectrum This publication, however, will present results andfindings of workers who are primarily concerned with the materials mostcommonly used for consumer products such as aluminum and stainlesssteel It should be emphasized, however, that the discussions on the ap-pearance and measurement of metallic surfaces, though giving heavy em-phasis to aluminum and stainless steel can be applied usually to othermetals such as plated coatings (whether on metal or plastic substrates)including chromium, nickel, and silver, and any other metal surface where

it is desirable to describe the metallic appearance

Chromatic attributes of metallic surfaces are not described in this

publi-1

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cation First, the techniques for measuring chromatic attributes using

such well-developed systems as the CIE and others can be regarded as

applicable to metallic surfaces with one very important qualification If

the metallic surface is nearly totally mat or diffuse, such as a mat-gold

surface, the methods for measuring chromatic attributes or color on other

materials can be used If, however, the metallic surface has some measure

of distinctness of image or mirror qualities, the question whether the

chro-matic attributes should be measured on the specular or the diffused

por-tions must be answered An extreme approach to this has been proposed,

including the application of a diffuser filter over the specular metallic

surface and measuring the chromatic attributes on the reflected and fully

diffuse portion of the light

This publication represents the experiences and findings of some of the

country's foremost authorities in the field of reflectance measurements on

metallic surfaces No other publication has covered this subject as

thor-oughly in these areas as does this compilation of papers The original

symposium contained four papers to which has been added the paper by

B W Robinson These papers cover the use of the principal instrumental

techniques that have proven useful in measuring metallic surfaces in the

research laboratories and in production environments This book should

prove extremely valuable to those who have a need for measuring or

specifying products where the metallic finish is of primary importance

E F Barkman

Director, Applied Chemistryand Mathematics, MetallurgicalResearch Division, ReynoldsMetals Co., Richmond, Va

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Appearance Attributes of Metallic Surfaces

REFERENCE: Hunter, R S., "Appearance Attributes of Metallic

Surfaces," Appearance of Metallic Surfaces, ASTM STP 478, American

Society for Testing and Materials, 1970, pp 3-21

ABSTRACT: The appearance of bare and finished metal surfaces is becoming

more important, especially to the automotive and architectural fields Themetals industry, however, is behind the paint, plastics, and paper industries

in the development of methods for instrumental appearance measurements

of metal products Metals are optically different from nonmetals because of freeelectron movement, and thus nonmetal measurement techniques are notgenerally applicable A complete analysis of metallic appearance requirescomplex combinations of spectrophotometric and goniophotometric curves

By isolating attributes which can be related directly to observed appearanceand correlated with physical properties, simpler instrumental measurementtechniques can be developed These attributes can be divided arbitrarily intocolor and geometric attributes As described in this paper, the color attributes

of metallic appearance are shininess, hue, and saturation The geometricattributes are haze, distinctness of image, luster, surface texture, anddirectionality

KEY WORDS: metallic appearance, gloss, metals, brightness, surface

properties, goniophotometers, reflection, evaluation, tests

Bare metal surfaces are harder to measure for specific appearances thanare nonmetal surfaces of paint, paper, plastic, and the like This is because

of the variety of geometric distributions of reflected light associated withmetallic appearance Although measurements are not in general use, ap-pearance is commercially important for metallic surfaces used in automo-tive trim, bus and truck paneling, architectural metals, and decorative andcostume jewelry The present paper identifies six different methods ofevaluating reflected light for correlation with six geometric attributes ofappearance

Appearance and the Metals Industry

Whenever a product has a metallic surface exposed to observation bythe human eye, the appearance of that surface has economic importance

1 President, Hunter Associates Laboratory, Inc., Fairfax, Va 22030

3

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TABLE 1—Metals, areas of use, and typical industry designations of the important

appearance attributes.

Bright Finishes

of Appearance Use Attributes

Diffuse Finishes Areas Important

of Appearance Use Attributes Aluminum (uncolored)

Aluminum (colored)

Stainless steel

Chromium, nickel and cadmium

on steel, zinc, brass etc

Silver, plating and sterling

Yellow metals and alloys such as

copper, brass, and gold

Evaporated metal films on

plastics

auto homewares jewelry

auto homewares

auto building homewares

building homewares jewelry homewares jewelry

brightness uniformity color brightness brightness uniformity color

brightness uniformity color brightness uniformity color brightness uniformity color brightness uniformity

auto building homewares building

auto building truck and railroad homewares auto building homewares

building homewares jewelry homewares

luster uniformity luster color uniformity luster uniformity color

luster uniformity color luster uniformity color luster uniformity color luster uniformity

Examples of metallic products where appearance is important include:

1 Automotive trim and decorative items

2 Truck, trailer, and railroad-car bodies; boats and buses

3 Siding panels and trim for building and architectural uses

4 Homewares and household appliances

5 Costume jewelry, silverware, and other decorative metals

Table 1 lists various metals and the areas where use requires that

ap-pearance receive special attention This table also sets forth the common

industry terms for appearance attributes that are important for each area

of use It should be noted that the major appearance distinction separates

diffuse from bright or shiny surfaces

Some metal finishes reach the market in the same form as physically

processed from the production line; that is, rolled, cast, or extruded Other

metals are subject to special mechanical or chemical finishing processes

The mechanical process may be buffing, wire brushing, tumbling,

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blasting, or shot blasting Chemical methods of finishing include etching,

electroplating, anodizing, and special chemical finishes

Sheet aluminum and stainless steel finishes receive greater attention

than do the finishes of some other metallic surfaces Both these materials

FIG 1—Building faced with dark anodized aluminum in which the luster of the finish

on the small area of the building to the right is much lower than that of the main building

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are used in large volume in the automotive and architectural fields where

their appearance is subjected to close scrutiny and where the public places

a high degree of importance on superior esthetic appearance Both sheet

aluminum and stainless steel are subject to impairment of appearance by

inadequate or improper processing and use

In many cases, special architectural finishes are used on these metals,

and they are very durable; however, these special finishes have optical

properties of color and luster that are difficult to control during

prepara-tion Often the large surface areas to be covered or the structural design

involved requires the use of a large number of panels or pieces When these

panels or pieces are placed immediately adjacent to each other in the

struc-ture, uniform appearance normally is desired Deviations from uniform

appearance, such as those shown in Fig 1, are identified easily by the viewer

and cause an unfavorable reaction and customer dissatisfaction

Need for Instrumental Methods of Measurement

In order to quantify appearance information for design, production,

marketing, and research, the metals industry should use measurements

much more widely than at the present time Existing appearance test

methods require improvement; new methods need to be developed

There are at least five specific areas in the use of metals where new or

improved measurements of appearance attributes are needed These are:

Attribute and Product Reason Measurements are Needed

Luster, and in many cases, color

of both interior and exterior

archi-tectural metals

Brightness of bright automotive

trim, especially electrolytically

brightened aluminum

Luster of diffuse-finish auto trim in

driver's field of view

Color and luster of diffuse-finish

aluminum siding for buses

Brightness and color of bright

stainless steel sheet

Architect, producer, and builderneed to assess appearance uniform-ity of different pieces and conform-ity to architect's standard

Component fabricators and automanufacturers need to identifybrightness and conformity to spe-ification specimens

Auto manufacturers must assurethat luster is below glare limit set

by Government Auto SafetyStandards

Aluminum siding applied to busesmust be suitably uniform in appear-ance and close to manufacturer'sstandard panel

Stainless steel producer must assurethat polishing and heat treatingprocesses are adjusted to give finishdesired

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The metals industry is well behind the paint, plastic, and paper industries

in the development of methods for the measurement of appearance

Al-though the economic value of metal products where appearance is

import-ant is large enough to command attention, three features of the problem

have hindered the development of optical testing:

1 Metals are optically different from nonmetals; therefore, existing

procedures suitable for other materials are not directly adaptable to metals

2 There is great variety in the appearances available in metal products,

largely because of the variety in geometric distribution of reflected light

3 The attributes which require measurement are not well denned In

the absence of adequate identification, methods for measurement have not

beeni developed

Although the metals industry now utilizes some instrumental techniques

to assess and control product appearance, most of these evaluations are

accomplished visually There is widespread use of standard specimens

which are used visually to compare with the finished product On large

building projects, or purchase contracts for metals, standard specimens

are divided between producers, purchasers, architects, inspectors, and

others wrho are concerned with the project Metals are optically so complex

that this practice of exchanging specimens representing desired product

appearance should continue, even though instrumental methods are

de-veloped to handle the most critical features of appearance

However, standard specimens are not completely adequate for

appear-ance control They are not always stable and may be subject to change

with use and exposure It is often not feasible to obtain specimens

repre-senting tolerances in addition to those reprerepre-senting the desired final

ap-pearance Even with standard specimens, numerical values based on

in-strumental measurements are useful to identify acceptable ranges of

product quality

Physical Bases for Appearance of Metals

A quick look at some of the physical differences between metals and

non-metals shows why instrumentation for measuring attributes of metallic

appearance lags behind that for nonmetals

Metals are optically different from nonmetals because the electrons in

metals are free to move about, whereas in nonmetals they are not This

freedom of electron movement contributes high specular reflecting power

and hi^h opacity Thus a metal reflects, on initial impact, a high percentage

of incident light The electrons in a nonmetal are not free to move about

and offset the electrical field of light Thus incident light can enter the

non-metal through its surface, or skin, and into its body In the nonnon-metal body,

the light may be transmitted, absorbed, or scattered within the subsurface

structure The light which returns through the skin leaves the reflecting

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surface quite uniformly in all directions This internal scattering causes

the uniform diffusion typical of the light reflected by paint, plastic, paper,

textile, and other nonmetallic products

A metal surface will reflect at the point of contact 50 to 90 percent of any

incident light in the direction of mirror reflection By contrast, a nonmetal

surface reflects only 4 to 10 percent in the same manner; the rest of the

light passes through the skin into the nonmetal body The Fresnel law of

specular reflectance for perpendicular incidence of any smooth surface

demonstrates the difference between the reflecting power of a metal and

that of a nonmetal surface:

where :

R s o° = fraction of a perpendicular incident beam that is specularly

re-flected by the surface,

7i = refractive index of the material, and

k = extinction coefficient, which is related to the presence of free

elec-trons and to the electrical conductivity of the material

The electrical coefficient k is the key to the difference between metals

and nonmetals Metals, since they possess free electrons, have finite values

of k, while in a nonmetal, k is zero or substantially zero.

In considering what happens to light at first contact with a metal surface,

it has been noted that all the incident light is either reflected at the first

surface, or is absorbed — leaving none to enter into the subsurface of the

material Metals vary, however, in the ratio of their reflecting and absorbing

capacities For one given metal, this ratio will vary with wavelength These

variations in reflectance by wavelength are shown by spectrophotometric

curves A spectrophotometric curve indicates the properties of the metal

which are responsible for its color Some typical spectrophotometric curves

for metals are shown in Fig 2

Sometimes, metal objects owe their color not to optical constants of the

metals, but to transparent colored films applied to the metal Whenever a

transparent colored film of dyed lacquer, anodic coating, metal oxide, or

glass (as in a mirror) covers the base metal, it affects the color of the metal

object Spectrophotometric curves and the corresponding color are then

attributable to absorption in the film plus absorption by the base metal

The geometric distribution of reflected light by a metal is determined

primarily by the contour of its surface A smooth surface reflects light only

in the specular or mirror direction A rough surface spreads reflected light

in directions which normally center in the specular direction The amount

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FIG 2—Spectrophotometric curves for five metals prepared from the reflectance data

pub-lished in the Handbook of Physics and Chemistry.

to be diffuse if it spreads reflected light so much that mirror images cannot

be seen in it

Geometric distributions of reflected light are given by goniophotometric

curves The examples in Fig 3 show typical distributions of light reflected

by metal surfaces Here, with light incident on the material at 45 deg, are

shown the amounts of light reflected, at angles near and at the angle of

specular reflection, by four specimens of aluminum foil (A, B, C, D) A

curve for depolished white opal glass, labelled E, has been added to

demon-strate a distribution characteristic of nonmetals The most diffuse of the

metal surfaces spreads light much less uniformly than does E, the diffuse

nonmetal surface Not only do the widths of the peaks of the

goniophoto-metric curves vary with surface roughness, but their shapes vary with

change of surface structure and contour There is an infinite variety in the

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FIG 3—Goniophotometric curves for angles near the ^5 deg direction of specular

reflec-tion for four aluminum foils with different finishes (A, B, C, D, and a white ground vilrolite

glass, E).

Appearance Attributes of Metals

In finishing metals, the mill technologist does not think in terms of

ex-tinction coefficients, spectrophotometric curves, or goniophotometric

curves He thinks in terms of adjustments he must make to the finishing

process, the ingredients, or the equipment to bring about the appearance

which his customer desires When he communicates with customers and

other technologists about the appearance of his product, he uses terms such

as brightness, color, luster, graininess, and sparkle Through such usage,

these terms have become so well established in the industry, that it is

possible to categorize them in a table such as Table 2

The appearance attributes of the metals divide, as they do for all other

products, into color attributes associated with the spectral distribution of

light and geometric attributes associated with goniophotometric

distributions

The color attributes are much the same as those encountered with other

materials Color in metals is important for gold, silver, and for alloys used

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Hunter 1937 "six types of gloss"

(Footnote 4)

Tingle and George (between haze

distinctness of image, Tingle

George identify an attribute they

Hunter 1970 metals proposal

Metals Industry Terms Brightness of Bright Finishes Luster of Uniformity of Both

Diffuse Finishes Types of Finishes freedom

from haze and and call freedom from haze whiteness

freedom from haze

distinct- specular gloss contrast gloss ness-of-

image gloss

freedom from diffuseness or image blur matteness image clarity

distinct- specular gloss luster ness-of- as indication image of glare gloss

distinct- specular gloss contrast gloss ness-of-

image gloss

absence of absence of surface directionality texture

microlinearity macrolinearity

directionality

absence of directionality surface texture

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FIG 4—Traditional color solid showing the arrangement of the attributes of hue, satura

tion, and shininess for metals.

ing as well as with content Heat treatment, solution chemicals, and

atmosphere chemicals are some of the process variables that may affect

color

The color attributes of appearance are, as with other materials, hue,

saturation, and lightness (or luminance) Figure 4 shows how these

at-tributes relate to each other in the traditional color solid The dimensions

of this solid are qualitatively the same as those for the diffuse-surface color

solid, although if one is thinking in terms of visually uniform scales, they

are quantitatively different

Geometric attributes of surface appearance are explained most easily by the

assumption that reflected light can be divided into diffuse and specular

components Color characteristically is associated with the specular

com-ponent of reflection The diffuse comcom-ponent then is called:

Haze when applied to bright finishes, or

Diffuseness when applied to diffuse finishes

The complementary and better known terms are:

Brightness, the lack of haze on bright surfaces, or

Luster, the lack of diffuseness on diffuse finishes

Barkman2 built a system for the evaluation of metallic appearance based

on the geometric separation of specular and diffuse reflection He used the

instrumental procedure which treats as diffuse light everything reflected

more than 4 deg from the direction of mirror reflection Using data obtained

with a hazemeter,3 Barkman prepared the triangle shown in Fig 5 with

2 Barkman, E F., "Specular and Diffuse Reflecting Characteristics," Metallurgical

Research Report 571-13A, Reynolds Metals Co., April 1959.

3 Hazemeter, ASTM Method D 1003-61, 1968 Book of ASTM Standards, Part 27,

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REFLECTANCE LIMITS FOR UNCOATED ALUMINUM SURFACES

FIG o—Diagram used by Barkman to characterize metals by diffuse and total reflectances

(specular = total minus diffuse).

which to distinguish aluminum finishes of different types In his work,

Barkman was not concerned with the color attributes of aluminum finishes

but only with their lightness and degree of specularity or diffuseness

In reality, the complete division of reflected light into specular and diffuse

components is not possible; nor is it possible for two numbers resulting

from such a distinction to completely characterize the appearance of a

metal A consideration of the variety of shapes of the goniophotometric

curves in Fig 3 and a close study of the different metallic surfaces seen in

everyday life make it obvious that no limited number of attributes of

ap-pearance can provide complete analyses of metals The variety possible in

both geometric curves of metallic surfaces and in surface markings and

texture is infinite Since the metals technologist deals with products which

are optically complex, he must look for specific features of the appearance

of metal surfaces which he can recognize and describe in understandable

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In his 1937 study, Hunter4 found that technologists working with

non-metals observed several geometric aspects of specular reflection as they

rated commercial products visually for gloss The aspects of importance

were not always the same For example, with porcelain enamel refrigerator

and stove finishes, distinctness-of- reflected images was important With house

paint, on the other hand, it was merely the shininess or intensity of light

reflected in the specular direction that mattered

In the case of textiles and papers, the grading for gloss seemed often to

be based on visual contrast between reflected highlights and of surface

areas adjacent to these highlights The term contrast gloss was used in

1937 for this type of gloss The term luster today is used widely for the same

sort of appearance quality In the 1937 study, these six different geometric

aspects of reflectance were called "types of gloss," and this classification

has been in use since then by those concerned with instrumentation in this

field

In daily work, however, technologists do not think in terms of the

dis-tinctions between these "types of gloss." A materials technologist examines

a product to determine whether, in his opinion, the final gloss quality will

be satisfactory to the customer The attributes that are important to this

technologist are the ones which he knows to be important to the customer

He tends to relate them to factors he can control If these attributes

in-volve specular reflection, he calls them "gloss" without differentiating

shininess, luster, and distinctness of images This is the basis for the

defini-tion of gloss as any geometrically selective reflectance that causes shiny or

lustrous appearance

In any specific case, the important gloss attribute is normally the one

most likely to be objectionable to the customer Thus, in furniture finishing,

it is "depth of finish" (or freedom from surface texture) which is important

In automotive polishes, on the other hand, freedom from haze receives

major attention

Turning from products in general to bare metals, there have been two

recent studies of aluminum appearance wrhich are quite similar in method

to the 1937 gloss study In 1965, Tingle and George5 identified attributes

used to describe aluminum automotive trim A paper by Robinson6 on

description of aluminum finishes is included in this publication The terms

used in these various studies are not the same as those resulting from the

Hunter 1937 study, yet the appearance attributes seem generally to be

4 Hunter, R S., "Methods of Determining Gloss," Journal of Research, National

Bureau of Standards, RP 958, Vol 18, No 1, Jan 1937.

5 Tingle, W H and George, D J., "Measuring Appearance Characteristics of

Ano-dized Aluminum Automotive Trim," Report 650513, Society of Automotive Engineers,

May 1965.

6 Robinson, B W., "Proposed Method of Measurement for Appearance of Rolled

Aluminum Surfaces Using Distinctness of Image and Diffuse Reflectance Measuring

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the same These terms for what seem to be the same attributes are shown

in Table 2

In the top line of this table, it is noted that the metals industry uses

FIG 6—Modified desk lamp used to demonstrate gloss or geometric differences in the

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"brightness" when speaking of bright finishes and "luster" when speaking

of diffuse finishes The metallurgists appear to mean by luster almost

ex-actly what was called contrast gloss However, the relation between the

metallurgists' "brightness" and the optical "freedom from haze" is not so

straightforward The presence of haze seems to be normally the major

factor in detracting from what the metallurgists call brightness However,

poor distinctness-of-reflected images also detracts from what the

metal-lurgists call brightness In practice surface scratches, grain structure, and

other types of visible surface texture also detract from brightness

A number of photographs of pairs of metal panels have been taken with

the lamp unit pictured in Fig 6 in order to illustrate differences in the six

attributes described below and listed in Table 2 This lamp, as can be

seen, is a desk reading lamp with fluorescent tubes It has been modified

by inserting black velvet behind the tubes and a piece of coarse (34 in.)

wire screen in front The wire screen reveals detail in the quality of images

reflected by the surfaces The black velvet behind the tubes provides a

dark target The reflected image of this dark target may be used by an

observer to identify near-specular haze

Freedom From Haze

In the brighter metals, technologists usually will say that brightness is

the most important appearance attribute What is brightness?

Metal-lurgists are generally uncertain However, workers with optical training

FIG 7—Comparison of reflection from modified desk lamp by hazy metal (right) and

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have been unanimous in considering "metallic brightness" to be

synon-ymous with "freedom from haze." Haze is considered to be present on an

otherwise shiny bright metal when there appears to be a cloudy film on the

surface This film scatters light and thus detracts from mirror quality

Figure 7 shows on the right a hazy panel compared with a bright panel

on the left

Metallic brightness certainly does not correlate with specular reflectance

Because of its freedom from haze, polished bright chrome plating with a

specular reflectance at about 65 percent always is called "brighter" than

bright anodized aluminum at 75 to 80 percent specular reflectance

Bright-ness is the complement of diffuse reflectance

Distinctness-of-Reflected-Im age Gloss

Distinctness-of-reflected-image gloss is, as its name implies, an attribute

of mirror quality Metallurgists seem to associate it with the brightness of

bright finishes It is not associated necessarily with haze, for a metal

surface can be free of haze and yet be wavy enough to reflect distorted

images

Distinctness-of-image gloss is chiefly a matter of how closely the

specu-larly reflected light is restricted exactly to the direction of mirror reflection

Spreading of reflected light beams by only 0.1 or 0.2 deg is enough to visibly

impair images Figure 8 illustrates a difference between two metal panels

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FIG 9—Two panels of diffuse aluminum used for bus siding which show a difference

in luster.

Contrast Gloss

Contrast gloss, sometimes called luster, is important for diffuse finishes

(see Table 2) Only metal surfaces which reflect light diffusely are said to

exhibit luster The necessary diffuseness can be produced mechanically,

chemically, or by fabricating techniques Visually, luster is observed in

surfaces having no visible reflected images by the rate of change of surface

luminance from one area to another Figure 9 illustrates a difference in

luster between two low-gloss metallic surfaces The panel on the right shows

a greater change in surface luminance from top to bottom and is therefore

more lustrous

Since they both involve reflection in directions adjacent to the direction

of mirror reflection, haze and luster are more or less different degrees of

the same thing Haze is the property of bright metals, and luster is a

prop-erty of diffuse finishes

Texture or Surface Uniformity

Visible surface texture on a metal is not related to the average reflectance

of a surface area Structure, or texture in a surface is a matter of

point-to-point change in reflecting characteristics Texture can make the position

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FIG 10—Reflection from two aluminum panels comparing a textured surface on the

left with a smooth surface on the right.

the left compared with a smoother panel on the right This aspect of

sur-face visibility in the furniture and paint industries has been called "depth

of finish." When an observer can see no markings such as he would use

ordinarily to recognize the existence of a surface (that is, waviness,

granu-larity, dirt, scratches, or dust, he says that the finish has "depth." What he

means is that there is no structure on which to focus, and, thus, no clues

by which he can recognize the position of the surface in space

Visible markings in metal sheets frequently are associated with a crystal

structure which is distorted during cold rolling to give visible texture

patterns Sometimes surface waviness is produced deliberately by using

rolls with textured surfaces (as in the left hand panel in Fig 10) At other

times waviness appears when it is not wanted

Directionality

Optical reflectance by metals almost always varies with orientation of

the reflecting surface with respect to an incident beam of light This change

of reflectance with direction of incidence can result from any one, or from

combinations of the following causes:

1 Orientation of crystal structure as a result of rolling

2 Markings on the rolls used

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FIG 11—Reflection of bright incandescent lamp in two panels; (left) free from

direc-tionality; (right) highly directional because of buffing marks.

4 Lines on extrusions as a result of passage through uneven dies

5 Markings on casting as a result of markings on the molds used

Figure 11 shows reflection of a bright incandescent lamp on two panels—

one of which is highly directional because of polishing marks; the other is

free of such directionality Directionality is an important appearance

characteristic of all types of metal finishes It is not one of the Hunter

1937 "six types of gloss."

Specular Gloss

For nonmetals, this attribute usually is listed first Probably 95 percent

of all gloss measurements of nonmetal surfaces are for specular gloss

Because of its popularity for nonmetals, specular gloss frequently has

been tried for measurements of metals Such metal measurements have

seldom been found to give vital information Measurements of specular

gloss are recommended for tests of lighting-unit reflector efficiency They

also can provide sensitive measures of the presence of light absorbing films

on metals, or of the degree of smoothness or polish when a surface is not

very smooth

It is for these applications that in Table 2 specular gloss is suggested as a

significant attribute of both diffuse and specular finishes Figure 12

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FIG 12—A difference in specular gloss The stainless steel mill finish on the left has a

brown light-absorbing oxide film Specular gloss is therefore less in this panel than in the

one on the right.

in specular gloss because the one on the left has a brown light-absorbing

oxide film

Conclusion

The appearance of metal surfaces is becoming more and more important

as decorative applications continue to increase Fortunately, the attributes

of metallic appearance can be described, separated, and analyzed The color

attributes are hue, saturation, and lightness The geometric attributes are

freedom from haze, distinctness-of-image gloss, contrast gloss (luster),

texture or surface uniformity, directionality, and specular gloss The

definition of these attributes, particularly the geometric ones, has been

largely responsible for the progress made in the measurement of metallic

appearance Although these attributes cannot provide a complete

descrip-tion of an object, they allow the isoladescrip-tion of particular features and the

design of instruments to measure them By having a clear understanding

of appearance factors and combining instrumental measurement with

visual judgment, the metals technologist will be able to achieve better

control of the appearance of metallic surfaces

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Measurement of Appearance Characteristics

of Stainless Steel

REFERENCE: Glubish, A E., Measurement of Appearance

Character-istics of Stainless Steel," Appearance of Metallic Surfaces, ASTM STP 478,

American Society for Testing and Materials, 1970, pp 22-4o.

ABSTRACT: One of the primary reasons for the selection of stainless steel for

architectural and automotive uses is its inherent ability to retain its original beauty for long periods of time Appearance, therefore, is important for matching the alloy or finish to a particular application.

The physical characteristics that affect the appearance of a metal surface include many factors such as color, gloss, haze, texture, directionality, shape, and several others not yet categorized Present day visual methods consider all of these factors simultaneously but are incapable of determining which attribute is the most influential to a particular appearance This paper describes instrumental methods now available in the laboratory to measure some of the attributes mentioned above but which are not used widely to measure and control appearance during production.

This paper consists of color, gloss, haze, and directionality data, and pictorial views of three bright and three diffuse stainless steel finishes selected to best show the surface variations usually obtained High magnification electron micrographs of each surface are included also The paper shows that there are certain measurements and photographic techniques that can be useful for evaluating the appearance of ferritic and austenitic steel surfaces if they are applied carefully.

KEY WORDS: appearance, stainless steel, gloss, color, haze, orientation,

electron microscopes, characteristics, evaluation, tests

The ability of stainless steel to retain its original appearance over theyears is one of this metal's most important characteristics The physicalcharacteristics that control its appearance to the eye are complex anddifficult to determine The visual appearance of a metal's surface dependsupon many factors such as color, gloss, haze, shape, texture, and othercharacteristics not yet categorized All of these factors combine in a com-plex manner to make description and measurement very difficult Presentday visual methods usually take into consideration all of these factors atonce without separation because the human eye is an exceedingly agile

1 Allegheny Ludlum Industries, Inc., Research Center, Brackenridge, Pa 15014 22

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and sensitive organ In the long run evaluations made visually do not

con-stitute a satisfactory quality control method, but at the moment in many

areas of the metal industry there are no other methods available

There-fore, we must combine the visual methods with the available instrumental

methods with the hope that eventually specific instruments will be able

to measure some of the more important attributes of appearance enabling

that attribute to be checked against what was produced before This

re-quires a close working arrangement between instrument designers and the

metal industries to better understand the problems of appearance we are

trying to resolve

You are all probably aware that there are a large variety of metal

com-positions that are all called stainless steels, such as Types 302, 201, 211,

304, and 434 Each of these alloys have a slight color variation depending

upon composition, but this natural alloy color can be altered drastically

by processing Since more color variation can be introduced by processing

than by composition and evaluating, variations in processing can become

complicated unnecessarily, I decided in this introductory effort to keep

the discussion general and simplified and concern myself with the general

appearance of material usually produced and how its appearance relates

to ordinary gloss and color measurements

The approach in this paper will be to separate the material first of all

into two classes, bright and dull stainless; the bright being highly reflective

and approaching a mirror-like condition and the dull having little specular

brightness and approaching a perfect diffuser There will be two classes of

alloys included, ferritic Type 434 containing 18 percent chromium and

little nickel and austenitic Type 304 having 18 percent chromium and 8

percent nickel This alloy difference causes some variation in processing

which results in some basic differences in appearance which will be pointed

out as the individual figures are discussed

Before going to the first group of figures, I must explain briefly the

photo-graphic techniques used to demonstrate the appearance variations

en-countered To show the mirror-like quality of a bright stainless surface at

normal magnification it was necessary to photograph a reflected image,

then visually judge its reproduction Using a Polaroid CU-5 closeup

camera, the reflected image is the ring shaped flash lamp The first figure

in each of the bright metal specimens was taken in this manner The figures

at X30 magnification were taken using dark field illumination to best

show surface defects on very reflective material Dark field illumination is a

result of the light striking the specimens at an oblique angle and the

camera perpendicular to the specimen The X100 and X500 magnification

figures were taken with bright field illumination on an ordinary

metal-lograph Bright field is when the light and camera are perpendicular to

the specimen The X7500 magnification was made using the electron

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The gloss values were obtained using a specially designed multipurpose

glossmeter that enabled me to obtain 45 and 20-deg specular gloss and 20

and 0-deg diffuse reflectivity values with incident light at 45 deg to the

specimen

Color and haze values were obtained using Hunter's sphere color

differ-ence meter

The entire set of bright material consists of a view of a rhodium mirror,

buffed Type 304, bright annealed Type 434, and bright annealed Type

304 The data mentioned in the figures will be summarized in Tables 3

and 4

Fig 1

This figure shows the very good reflection that the rhodium mirror

standard produces The edges of the ring shaped flash should be noted

because the sharpness of these edges is indicative of a very good surface

T^he degree of blackness also indicates a good surface There is no

magnifi-cation of view of this specimen The specular gloss at 45 deg is 73.4 units

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WITH ACROSS

FIG 2—Front faced rhodium mirror, dark field illumination.

Fig 2

This is a dark field illumination view at X30 magnification In this kind

of illumination the more mirror-like specimen would appear closer to black

with minute defects showing us white areas due to the diffusivity of light

Diffuse reflectivity values with and across are very low at 20 deg 0.3 units,

at 0 deg, zero value The figure shows two photographs showing the view

with the incident light parallel and perpendicular to the surface scratches

If you can see no difference in the side-by-side views, the specimen has

very little directionality and would appear the same when viewed in either

direction Directionality value of the mirror is the lowest possible being

1.0 unit Directionality is determined by a simple ratio of the 20-deg

diffuse values, the across value divided by the with value

Fig 3

Figure 3 shows, at the top, two bright field photographs of the same

surface at 100 on the left and 500 times normal on the right The large

photograph at the bottom is 7500 times normal view which shows the

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FIG 3—Front faced rhodium mirror, light and electron micrographs.

The small line at the bottom indicates a tenth of a mil in length to give

some idea of the size of area This surface is very smooth

Fig 4

This next group shows the appearance of buffed Type 304 which is an

austenitic grade stainless steel having 18 percent chromium and 8 percent

nickel

The reflection shown here as you can see is very distinct indicating a

mirror-like surface It is difficult in this photograph to see much difference

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FIG 4—Buffed, Type 304, reflection of ring flash lamp of closeup camera.

FIG o—Buffed, Type 304, dark field illumination.

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gloss, however, is much lower at 60 units which is usually the maximum

value obtained on stainless steel This value is near the value obtained on

a chromium front face mirror which is usually 58 units

Fig 5

The dark field photograph at X30 magnification with the incident light

projected with and across the specimen show quite clearly the effect of

buffing on the surface The buff scratches are shown clearly on the right

FIG 6—Buffed, Type 804, light and electron micrographs.

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Both views have more white areas appearing than the mirror although the

diffuse reflectivity values are still quite low The 20-deg diffuse values with

the incident light projected with the buffing scratches is low at 0.7 units

and at 0-deg diffuse 0.4 units However, when the incident light is projected

across the buffing lines the diffuse reflectivity naturally goes up to 3.5 units

at 20 deg and 1.5 at 0 deg because of the diffusivity of the buffing lines

This in turn accounts for the higher directionality values of 3.5

Fig 6

These three views indicate how smooth the surface of the buffed

speci-men really is About the only defects visible are the shallow buffing lines

and a few inclusions The haze value is very low being 19.3 units which

compares well with the mirror which was 14 units

Fig 7

This next group shows the appearance of rolled Type 434 containing 18

percent chromium and very little nickel

This shows the reflection from Type 434 that has been bright annealed

and skin passed on very bright rolls to produce the maximum surface

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WITH ACROSS

FIG 8—Bright annealed, Type ^34-, dark field illumination.

quality obtainable by rolling As you can see it is a distinct reflection very

similar to the reflection from the Type 304 buffed specimen Specular gloss,

however, is lower at 54.0 and 55.9 units with and across the rolling

direc-tion These lower values could be due to the absence of nickel that was

present in the buffed Type 304 specimen

Fig 8

The dark field illumination shows more areas of diffusion The diffuse

reflectivity values being 2.3 units at 20 deg and 0.3 units at 0 deg when the

incident light is with the rolling direction and 4.6 units at 20 deg and 1.7

at 0 deg when the incident light is across the rolling direction The

direc-tionality of this material is 2.0 which is low

Fig 9

The bright field photographs show some of the shallow surface defects

which were the cause of the increased diffuse reflectivity shown on the

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FIG 9—Bright annealed, Type 434, light and electron micrographs.

The electron microscope view shows one of these shallow defects, but, in

general, the surface is mirror-like with a haze value of 22.3 units

Fig 10

This shows the reflection from Type 304 that has been bright annealed

and skin passed on bright rolls similar to the previous specimen The

re-flecting quality of this material is much poorer than anything shown

pre-viously The image of the flash lamp is very poor The specular gloss,

how-ever, is good and relatively high at 55.6 with and 55.0 across the rolling

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FIG 10—Bright annealed, Type 304, reflection of ring flash lamp of closeup camera.

FIG 11—Bright annealed, Type 304, dark field illumination.

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Fig 11

This dark field illumination shows the reason for the poor reflecting

quality There are many rough areas on the surface causing diffusion The

diffuse reflectivity is 8.4 units at 20 deg and 0.6 at 0 deg with the rolling

direction and 14.8 at 20 deg and 1.5 at 0 deg across the rolling direction

The directionality is low at 1.8 units which means it will appear similar

when viewed in either direction

Fig 12

The bright field illumination shows the numerous shallow surface defects

that caused the diffuse values to rise It is shown clearly at X500

magnifications

FIG 12—Bright annealed, Type 304, light and electron micrographs.

Tiêu đề	Appearance of Metallic Surfaces
Tác giả	E. F. Barkman, B. W. Robinson
Trường học	University of Washington
Chuyên ngành	Materials Science
Thể loại	Báo cáo
Năm xuất bản	1970
Thành phố	San Francisco

Định dạng
Số trang	93
Dung lượng	4,12 MB