PUBLICATIONS A1110H 7 b b 0 5

NBSIR 84-2961

A Procedure for Tristimulus Color

Measurements on Building Stone

Lawrence I. Knab

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

October 1984

"S. DEPARTMENT OF COMMERCE

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100

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84-2961

1984

MIONAL BUREAU OF STANDARDS

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NBSIR 84-2961

A PROCEDURE FOR TRISTIMULUS COLOR

MEASUREMENTS ON BUILDING STONE

no.

I J < /

Cx

Lawrence I. Knab

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

October 1 984

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

NATIONAL BUREAU OF STANDARDS. Emect Ambler. Director

INTRODUCTION

As part of the National Acid Precipitation Assessment Program, the research

goal of the Ifeterials Effects Task Group is to assess the damage caused by acid

deposition on selected building materials, including building stone. One of

the ways this damage is being assessed is by exposing building stone at various

outdoor exposure sites for ten or more years. The changes in properties,

including color, of the exposed stone as compared to the unexposed stone will

be measured and analyzed. Additional information, including the rationale for

selection and a description of the stone briquettes used for the different

stone types, is given in other reports (e.g. Reference 1 for Salem limestone).

Although there is an ASTM method (Reference 2) for instrument evaluation of

color differences of opaque materials, there was no specific procedure for

measuring color differences on exposed building stone. This report describes

a procedure developed to measure color and color differences on building stone

surfaces using a specific diffuse reflectance, tristimulus colorimeter. The

colorimeter had three filters, was based on CIE standard illuminant C, had an

angle of illumination of 0° from the perpendicular (with light striking the

sample at 90° relative to the sample surface), and had an angle of viewing of

45° from the perpendicular (with light returning at 45° relative to the sample

surface). The development of the procedure was based on color measurements

taken on the surfaces of Salem limestone and Shelbourne marble prior to their

exposure. Consultations with a statistician and a color metrologist were used

as input in developing the procedure.

COLORIMETER AND STANDARD PANELS

The colorimeter used was a Reflection Mater 670* (figure 1) used in conjunction

with a Search Unit 610T* (figure 2), both manufactured by Seragen* (Photovolt*).

Reasons for choosing this colorimeter include its availability and portability.

Prior to using, the colorimeter was serviced and calibrated by its manufacturer

and the standard reflectance enamel panels, NBS Sets Nos. 72 and 6, were calibrated with Gardner* colorimeters, 2005 and 805 series respectively. After

servicing, preliminary measurements comparing the readings of the Seragen

colorimeter and a colorimeter from a different manufacturer indicated that the

Seragen colorimeter was performing with a satisfactory accuracy (closeness to

the true answer). Preliminary measurements indicated that the precision

(repeatability about a given base line) of the Seragen colorimeter was within

the limits of the manufacturer's specification, when the procedure in this

report was followed.

*Certain manufacturer names, commercial equipment, instruments, or materials inidentified in this report in order to adequately specify the experimental

procedure. Such identification does not imply recommendation or endorsement by

the National Bureau of Standards., nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

1
It is recommended that, as the color data on both the unexposed and exposed

stone are collected and analyzed, the Seragen colorimeter accuracy and precision

be periodically determined. Also the standard reflectance panels should be

recalibrated during the study, or at least at the end of the study. The procedure in this report should be evaluated and, if necessary, updated based on subsequent data analyses.

To insure consistency of results over the ten year or longer duration of the

study, it is recommended that the same colorimeter (Seragen) be used for all

measurements taken. The differing characteristics of different colorimeters

which all reputedly measure the same quantities, precluded the development of a procedure which would apply to all suitable colorimeters. The procedure described in this report is specific to the colorimeter used. Procedures for

other suitable colorimeters could also be developed. The details of those

procedures for the other colorimeters, however, would most likely differ from

the procedure given in this report.

If another colorimeter must be used, then a modification of the procedure in

this report would be needed to provide at least a comparable level of accuracy

and precision. Also a correlation between the Seragen colorimeter and any

colorimeter replacing it would need to be established with both standard

reflectance enamel panels and standard test stone surfaces (see discussion in

last section - Equipment Repair or Modification or the Use of an Alternate

Colorimeter ) . The procedure in this report is for color readings taken in a laboratory environment. Readings taken on stone briquettes in another environment, such as in their field exposure seats, would require modification to the

procedure. For example, the stone surfaces in their field seats are at an angle relative to the horizontal as compared to the horizontal position in the

laboratory. This difference in angle could affect the response of the 610T

Search Unit

The National Park Service has information on the handling, packaging, shipping,

and characterization of the stone briquettes prior to their being measured for

color, including subjecting the briquettes to a short term, constant, environment. After subjecting the briquettes to the short terra environment, they

were wrapped in aluminum foil, placed in sealed plastic bags, carefully packaged

to prevent breakage and sent to the National Bureau of Standards (NBS). The

briquettes were stored in an NBS laboratory in their sealed plastic bags.

Prior to taking color readings, the briquettes were removed from their plastic

bags and the aluminum foil was removed from the briquettes. After completing

the color readings, each briquette was returned to the same plastic bag it was shipped in and the bag was then sealed.

PREPARATION, WARMJP, AND SELECTION OF COLOR-STONE TYPE

Follow the general operating instructions given in "Operating Instructions for

Reflection Mater 670" - Reference 3, including the instructions in paragraphs

1.1 through 1.5 given in 1. SETTING UP THE INSTRUMENT on page 1241. Use the

standard scale of 100 units full scale (Normal Sensitivity, not High Sensitivity)

on the 670 Reflection Meter. The reflectance measurements are taken with the

green, amber, and blue color filters. The plug which connects the 610T Search

2
Unit and the 670 Galvanometer is to be snugly connected so that there is no change in reading when the cord of the 610T Search Unit is moved, causing a force to be exerted on the plug. In this study, a weight was placed on the

cord of the 610T Search Unit so that no force was exerted on the plug as the

cord was moved while taking color readings

Select the filter color (green, amber, or blue) and stone type (e.g., Salem

limestone or Shelbourne Vermont marble) and use this color-stone type combination

throughout the procedure in Steps 1 through 4 of this report. After completing

the procedure for the first color-stone type combination on at most twelve stone

briquettes which are to be exposed or have been exposed, follow the procedure

of Steps 1 through 4 for the next color-stone type combination of at most twelve

briquettes. The procedure is repeated until all color-stone type combinations

are completed; that is, every stone briquette will have been measured with the

green, amber, and blue filters. The order in which the color-stone type

combinations are measured should be randomized as much as possible. The order

of measuring the twelve or fewer briquettes within a color-stone type combination

should also be randomized as much as possible. A statisician should be consulted

when developing these randomization schemes.

When taking color readings, wear clean (washed) rubber gloves and be careful

not to touch the exposed stone surface (arrow points to this surface - figure

2). Never place fingers or thumbs on color filters (figure 2). If a finger or thumb print does get on a filter, remove it by gently and carefully rubbing the

filter surface with a lint-free, soft, disposable wipe, such as a Kimwipes*

wipe. (The slight amount of moisture on the filter caused by "breathing" on the

filter just prior to wiping may help remove the finger or thumb print). Be sure all particles (e.g., dust) are first removed from the filters. Small

particles can be removed by using a stream of purified compressed air, being

careful not to cool down the filter, which could damage the filter or its

mounting. When changing the lamp for the 610T Search Unit, follow directions

on page 1245-46, paragraph 12.1 in Reference 3 and be sure all finger and thumb

prints and foreign particles are removed from the lamp surface using a lintfree, soft, disposable wipe.

Be sure the 610T Search Unit and 670 Reflection Meter are warmed up at least 30

minutes with the 610T Search Unit vertical on a flat, clean, surface and with

the light pointing down on the flat surface (face down). The color of the

warm-up surface is not to be significantly different than the surface being

measured. For example, do not use a black warm-up surface if measurements are

to be taken on lighter colored stone. (A brown surface was used as a warm-up

surface for the measurements taken in this report). Do not use standard

reflectance panels as warm-up surfaces because of the possible damage due to

heat build-up on the panel surfaces

* Refer to footnote on page 1.

3
COLOR MEASUREMENT STEPS

There is one equipment calibration step and two equipment check steps. The

calibration step is to calibrate the equipment prior to taking color measurements,

using standard reflectance panels. The first equipment check is to insure that

the equipment is performing properly on standard reflectance panels and to

detect a change that may have occurred in the calibration panel (e.g., fading).

The second equipment check is to insure that the equipment is performing properly

on the standard stone surfaces.

Step 1

Equipment Calibration Using Standard Reflectance Panels . Use a standard

reflectance enamel panel from NBS Set No. 72 and use the color calibration

values on the label affixed to the back of each panel. The color calibration

values of the standard calibration panel chosen should match as closely

as possible the stone color readings to be taken*. It is noted that the

calibration panel used for the standard briquette may differ from that

used for the briquettes to be exposed or the briquettes which have been

exposed. In the case where more than one calibration panel is used, the

order of using the panels is given in Step 2b.

Place the standard panel on a flat surface, coated side up (figure 3).

Place the centering template (see figure 4) on the standard panel. The

purpose of the centering template is to insure that the 610T Search Unit

light strikes the center of the standard panel. Select the filter color

to be used - green, amber, or blue. Next place the 610T Search Unit in

the hole in the centering template (figure 5). Using the sensitivity

controls F and G (paragraph 2.2, pg. 1241 - Reference 3), adjust the 670

Reflection Meter to the calibration value on the back of the panel for

the filter color being used. Interpolate the needle reading to the nearest

0.1 unit (e.g., 61.j2). To check for instrument drift**, record repeated

readings one minute apart until at least four readings have been taken

and the last two readings agree to within 0.2 units (e.g., 39.3, 39.2,

39.0, 39.2). If any drift has occurred, adjust the F and G sensitivity

controls such that the 670 Reflection Mater matches the calibration value

on the panel back (see table 1). Resetting the instrument to the calibration

value is done to keep the instrument calibrated as closely as possible to

the correct value when taking subsequent readings on other surfaces (i.e.,

panels from set No. 6, standard briquettes, and briquettes which are to

be exposed or have been exposed)

* Trial color readings on stone briquetts should be taken - see discussion

In Steps 3b and 4b. ** The drift criteria were empirically determined to be satisfactory. Compliance with these criteria indicates that the instrument has stabilized.

4
Ste p

2-

Equipment Check Using Standard Reflectance Panels *

Step 2a. The "check" panels from NBS Set No. 6 are generally similar to

the calibration panels in NBS Set No. 72. The panels in Set No. 6 are used to:

(i) insure that the equipment is performing properly on standard

reflectance enamel panels

(ii) detect a change (e.g., fading) in the calibration panels

(iii) serve as a backup set of calibration panels throughout the study.

After completing the calibration process with a panel from Set No. 72,

select for the chosen color, the "check" panel from Set No. 6 with a calibration value which most closely agrees with the calibration value

from the panel used from Set No. 72. Note that for both Set Nos. 72 and 6,

usually the panel number (e.g., 40) corresponds roughly to the calibration

values on the back of the panels for the three colors. For example, panel

40-72 (40 is the panel number, 72 is the set number) and panel 40-6 (40 is

panel number, 6 is set number) agree most closely. Place the centering

template on the "check" panel and then place and center the 610T Search Unit

on the "check" panel, as when using the standard reflectance panels shown

in figures 3 to 5.

Take at least two repeated readings one minute apart until the last two

readings agree within 0.2 units. If the average of these last two readings

agrees within 1.5 units with the calibration value on the back of the "check"

panel, then the equipment is considered to be performing satisfactorily

(see table 1). If a difference of more than 1.5 units exists, then repeat

the entire procedure beginning at Step 1.

Step 2b. Return the 610T Search Unit to the calibration panel used in

Step 1, using the centering template. Take at least two repeated readings

one minute apart until the last two readings agree within 0.2 units.

If the difference between the average of the last two readings and the

calibration value on the back of the calibration panel exceeds 1.5 units

(see table 1) repeat the entire procedure, beginning at Step 1. If drift

has occurred (less than or equal to 1.5 units) from the calibration value

and readings are about to be taken on a standard briquette, adjust the F

and G sensitivity controls such that the 670 Reflection Meter matches the

calibration value on the back of the panel (table 1). Resetting the

instrument to the calibration value is done to keep the instrument calibrated

as close as possible to the correct value when taking subsequent readings

on the standard briquette.

If for a given color, it is required to use more than one calibration

panel, repeat Steps 1 and 2 for each calibration panel used. As an example,

for briquettes to be exposed or which have been exposed, if panel 20-72

was used as the calibration panel in Step 1, then 20-6 would be used as the

"check" panel in Step 2. If for the standard briquette, panel 40-72 was

5
used as the calibration panel in Step 1, then 40-6 would be used as the

"check" panel in Step 2. Note that Steps 1 and 2 should be performed

last for the panels used with the standard briquette so that the equipment

has been calibrated on the proper panel prior to taking color readings on

the standard briquette in Step 3.

Step 3

Equipment Check Using A Standard Stone Briquette . This step is to insure

that the equipment is performing properly on the surface of a standard

briquette of the stone type (e.g., Salem Limestone or Shelbourne Vermont

marble) being used.

Readings on the standard briquette are taken after the equipment calibration*

and check steps are completed (Steps 1 and 2). The procedure is as follows,

using the color-stone type combination selected and using a standard

briquette of the same stone type selected.

Step 3a. The color readings are taken at four positions on the exposed

stone surface as shown in figure 6. A briquette template is used to

position the 610T Search Unit on the stone surface. Place the standard

briquette on a flat, clean, horizontal surface and orient the standard

briquette as shown in figure 2, with the finished (exposed) surface facing

up (arrow points to finished surface) and with the label on the bevelled

surface facing the operator. Attach the briquette template to the 610T

Search Unit as shown in figure 7 by hand tightening the two knurled screws while keeping the 610T Search Unit vertical with the light pointing down.

Attaching the template should take about 15 to 20 seconds. Locate the

610T Search Unit on position 1 by gently placing the flat 1 in. by 2 in.

template surface (figure 7) against the non-bevelled stone surface (figure

2) so that the 1 inch edge (figure 7) of the template coincides with the 2

inch corner edge formed by the intersection of the bevelled and non-bevelled

stone surfaces as shown in figures 2 and 6. When the 610T Search Unit is

positioned, record repeated readings one minute apart until at least four

readings have been taken and the last two readings agree within 0.2 unit

(table 2). Then move directly to position 2, maintaining the 610T Search

Unit vertical - the total time to move should be about 5 seconds. Position

2 is found similarly to position 1, by matching the appropriate 1 inch

template edge (figure 7) with the 2 inch stone corner edge (see figures 6

and 8) . Take at least two repeated readings one minute apart until the

last two readings agree within 0.2 unit (e.g., 60.5, 60.3 or 60.5, 60.2,

60.0) and then move directly to position 3 (figures 6 and 9). Position 3

is found similarly to positions 1 and 2 - by matching the appropriate 1

inch template edge with the 2 inch stone corner edge. Use the same reading

* Be sure the equipment was calibrated using the calibration panel from Set

No. 72 with a calibration value which most closely agrees with the standard

briquette color readings (see discussion in Step 3b.)

6
process at position 3 as used in position 2. Then move directly to position

4 (figures 6 and 10), again using the reading process used in positions 2

and 3. Position 4 is found similarly to the other three positions, that

is, by matching the appropriate 1 inch template edge with the 2 inch stone

corner edge. After position 4, move directly back to position 1 on the

standard briquette, again using the reading process of positions 2, 3 and

4. The average of the final (taken after position 4) last two readings at

position 1 must agree within 1.0 unit of the average of the initial (taken

prior to position 2) last two readings at position 1 (see table 2). If more than 1.0 unit difference exists, the standard briquette readings must

be retaken, by repeating the entire procedure starting at Step 1. If no more than 1.0 unit of difference exists and the color readings agree

satisfactorily* with previous** readings (table 3), then the color readings

for the standard briquette using the selected filter color are completed

(see table 2), provided the instrument drift is acceptable as given in

Step 3b below.

Step 3b. The drift is to be checked by removing, in about 10 seconds or less, the briquette template from the 610T Search Unit, while keeping the

unit vertical with the light pointing down. After removal of the template,

immediately place the 610T Search Unit on the calibration panel used to

calibrate the equipment, using the centering template. Record repeated

readings one minute apart until at least two readings have been taken and

the last two readings agree within 0.2 units (table 2). If the average

of these last two readings with the 610T Search Unit on the calibration

panel does not differ by more than 1.5 units from the calibration value on

the back of the calibration panel, the standard briquette readings are considered satisfactory (see table 2). If, however, there is more than

1.5 units difference, the standard briquette readings are not satisfactory

and must be redone by repeating the entire procedure beginning at Step 1.

After taking the color readings on the standard briquette, the readings

should be checked to see that they generally agree with the color reading

of the standard reflectance enamel panel (Set No. 72) used to calibrate the

equipment in Step 1. That is, the standard panel having a calibration

color reading which most closely agrees with the standard briquette color

readings is to be used. For example, the green color readings in table 2

* If questions arise as to what is "satisfactory", a statistician should

be consulted. If satisfactory agreement with table 3 is not obtained,

then it needs to be determined whether the lack of agreement is due to

(i) the equipment performing improperly, (ii) a change In stone color

(which could occur, for example, during long term storage), or (iii) a change in the calibration panel (e.g., fading).

** Initial baseline readings need to be taken on standard briquettes for

each stone type prior to taking readings on exposed briquettes of the same stone type. See, for example, table 3.

7
ranged from 45 to 46, generally agreeing with the 39.96 value on the back of

the calibration panel, 40-72. (There is no 50-72 panel, so 40-72 was the

appropriate panel choice). To help match the color value of the calibration

panel with that of the color readings of the stone, trial readings on the

standard briquette should be taken prior to beginning the procedure.

Step 4

Exposed Briquette Color Readings . Color readings are taken on stone

briquettes which either have been exposed or are to be exposed at the

field sites - these briquettes are called "exposed". After completing the

equipment calibration and equipment checks on a standard panel(s) and a standard briquette, the color readings on the exposed briquettes are taken

as follows:

Step 4a. Start by calibrating the equipment using the calibration panel

from Set No. 72 which has a calibration color value which most closely

agrees with the exposed briquette color readings. (Repeat only Step 1 - see table 4). Color readings on the first exposed briquette of the color - stone type combination (e.g., green - Salem limestone) selected are taken

in the same way as for the standard briquette (follow Step 3a, except

ignore the requirement that the readings are to agree with previous standard

briquette readings). That is, the orientation, location of the finished

(exposed) surface, the 610T Search Unit positions 1 through 4, and the

process and acceptance criteria of taking color readings at positions

1,2, 3, 4, and then again at position 1, are the same for the first exposed

briquette measured and the standard briquette. In particular, be sure

that (a) the last two readings at a given position agree within 0.2 units

and (b) the average of the final (taken after position 4) last two readings

at position 1 agree within 1.0 unit of the average of the initial (taken

prior to position 2) last two readings at position 1 (table 4). If more

than 1.0 unit of difference exists, the exposed briquette readings must

be retaken by repeating all steps of the procedure, beginning at Step 1.

If no more than 1.0 unit of difference exists, the readings on the first

exposed briquette are satisfactory. Table 4 shows sample readings for

the first exposed briquette.

To measure the second exposed briquette of the same color-stone type

combination, move directly from position 1 of the first (measured) exposed

briquette to position 1 of the second (unmeasured) exposed briquette and

repeat the process of taking at least two readings one minute apart and

moving to the next higher numbered position when the last two repeated

readings agree within 0.2 units. On the second exposed briquette, move from

position 1 to 2 to 3 to 4 and back to 1 (table 4). As with the first

exposed briquette, the average of the final (taken after position 4) last

two readings at position 1 must agree within 1.0 unit of the average of the

initial (taken prior to position 2) last two readings at position 1

(table 4). If a difference of more than 1.0 unit exists, the readings are

not acceptable and must be retaken, repeating all steps of the procedure,

starting at Step 1. The measurement procedure and acceptance criteria for

8
the third exposed briquette of the same color-stone type combination are

the same as for the second exposed briquette (see table 4).

Step 4b. At the end of measuring at most three exposed briquettes, or when 45 minutes of time has elapsed from when the 610T Search Unit was last

on the standard calibration panel (Set No. 72), whichever occurs first,

remove the 610T Search Unit from the briquette template (keeping the unit

vertical with the light pointing down and taking about 10 seconds or

less) and place the 610T Search Unit, using the centering template, on

the standard panel used to calibrate the equipment (see Step 4a). To

check for instrument drift, record repeated readings one minute apart

until at least four readings have been taken and the last two readings

agree to within 0.2 units. If the average of these last two readings

with the 610T Search Unit on the calibration panel does not differ by more than 1.5 units from the value on the back of the calibration panel,

the exposure briquette readings are considered satisfactory (table 4). If,

however, there is more than 1.5 units difference, the readings on the

exposure briquettes (at most three briquettes) are not satisfactory and

must be redone by repeating the entire procedure, beginning at Step 1

with the first briquette. If drift of less than or equal to 1.5 units

has occurred, adjust the F and G sensitivity controls such that the 670

Reflection Meter reads the correct calibration value (see table 4) .

After completing the first set of at most three exposure briquettes for

the same color-stone type combination, measure at most three additional

sets of the same color-stone type combination, with each set containing at

most three exposure briquettes.

The same procedure and acceptance criteria are to be used for the additional

sets as for the first set. Continue until color measurements on the

exposure briquettes are completed or until at most twelve (four sets of three)

exposure briquettes are measured for the same color-stone type combination.

After completing the color readings on the last set of exposure briquettes

measured, including checking the instrument drift, an equipment check

using the panel(s) from Set No. 6 is to be performed as described in

Step 2. Finally, an equipment check using the standard briquette surface

is to be performed as described in Step 3. If the equipment checks based

on the panel(s) from Set No. 6 or the standard briquette are unsatisfactory

according to the specifications previously given, then the color readings

on all the exposed briquettes (at most twelve) taken with the same colorstone type combination are suspect and must be retaken, repeating the

entire procedure, beginning at Step 1.

As the color readings are taken on the exposed briquettes, the readings

should be checked to see that they generally agree with the color reading

of the standard reflectance enamel panel used to calibrate the equipment.

That is, the standard panel having a calibration color reading which

most closely agrees with the exposed briquette color readings is to be

used. For example, the green color readings in table 4 ranged from 41 to

45, generally agreeing with the 39.96 value on the back of the calibration

panel, 40-72. To help match the color value of the calibration panel

9
with that of the color readings of the stone, trial color readings on the exposed briquettes should be taken prior to beginning the procedure.

When Steps 1 through 4 are completed according to specification, the color

measurements for at most twelve briquettes are acceptable for the selected

color-stone type combination (e.g., green - Salem limestone).

The average of the last two readings at each of the stone briquette surface

positions 2, 3, and 4 is to be used as the color reading value at that

position. The average of the final (taken after position 4) last two

readings and the initial (taken prior to position 2) last two readings at

position 1 is to be used for the color reading value at position 1 (see

table 4)

EQUIPMENT REPAIR OR MODIFICATION OR USE OF AN ALTERNATE COLORIMETER

If any equipment repairs or modifications have been made, including a lamp or photocell replacement in the 610T Search Unit, or if another colorimeter needs

to be used, a check should be made to determine if the base line of color

measurements has changed. This determination can be made by correlating the

color measurements of the repaired or modified equipment or of the alternate

colorimeter with the original equipment using standard enamel reflectance panels

and standard briquette stone surfaces. Due to the many factors affecting the

diffuse reflectance response of a colorimeter, it is recommended that an expert

in color metrology and colorimeter performance be consulted when making this

determination.

Several potential problems are discussed below:

a. Translucence

Preliminary measurements have indicated that the Shelbourne Vermont marble

is translucent. The diffuse reflectance response of a colorimeter on a translucent surface is affected by the geometric configuration of the colorimeter.

The "light spot" (image size of light, about 5/8 in., emitted from the 610T

Search Unit on the surface being sampled) should be monitored for a change

in size and/or location throughout testing, particularly when replacing

a lamp or if the 610T Search Unit is accidentally jarred or dropped. A

major change in the "light spot" should be visually evident (see paragraph

12.1, pg. 1245-46, Reference 3) and probably would be detected by a change

in the readings of the Vermont marble standard briquette (C16-21).

The effect of translucence would also need to be taken into account when

using an alternate colorimeter with a geometric configuration different

than that of the 610T Search Unit.

b. Additional Stone Types

Standard briquettes need to be used for each additional stone type studied.

10
Fluorescence

If an additional stone type is used which is fluorescent (preliminary tests

showed that neither the Salem limestone nor the Shelbourne Vermont marble

are fluorescent), then equipment repair or modifications or the use of an alternate colorimeter which would change the equipment performance for

fluorescent stone samples would need to be taken into account.

REFERENCES

1. Ross, M. and Knab, L. "Selection, Procurement, and Description of Salem

Limestone Samples Used to Study Effects of Acid Rain," NBSIR 84-2905, July,

1984.

2. "Standard Method for Instrumental Evaluation of Color Differences of Opaque

Materials" ASTM D2244-79.

3. "Operating Instructions for Reflection Meter 670," Photovolt-Seragen

Diagnostics, P.0. Box 1210, Indianapolis, Indiana, 46206.

Acknowledgements

This effort was funded by the National Park Service through Ms. Susan Sherwood.

The author wishes to acknowledge Dr. James Filliben of the Statistical Engineering

Division at NBS for his statistical consultation and Dr. Jack Hsia of the

Radiometric Physics Division, also at NBS, for his color metrology input.

11
TABLE 1. Equipment calibration and check using standard reflectance panels - sample readings (not to be used as data)

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12
TABLE 2. Equipment check using standard stone briquette - sample readings (not

to be used as data)

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13
Table 3a. Color Measurements Using Standard Briquette HU5-12

(Salem Limestone)

Filter

Color

Positions

2

* 4

Min* Max* Min Max Min Max MLn Max

Green 45.0 45.3 45.9 46.0 45.0 45.6 45.5 46.0

Amber 48.9 49.0 49.3 49.8 49.0 49.0 49.4 49.8

Blue 37.0 37.2 37.9 38.0 37.1 37.4 37.9 38.0

* Minimum and maximum color readings taken in May and June of 1984 when

briquettes were measured prior to installing at four exposure sites.

See original data sheets (National Park Service, c/o Ms. Susan Sherwood)

for further detail.

14
Table 3b. Color Measurements Using Standard Briquette C16-21

(Shelbourne Vermont Marble)

Filter

Color

Positions

1 2 3 4

Min* Max* Min Max Min Max Min Max

Green 67.2 67 .8 72.6 73.0 72.0 73.0 69.8 70.0

Amber 67.1 67 .3 72.9 73.2 72.2 73.0 69.6 70.0

Blue 64.5 65.0 70.2 71.8 71.1 72.0 68.1 68.8

* Minimum and maximum color readings taken in May and June of 1984 when

briquettes were measured prior to installing at four exposure sites. See

original data sheets (National Park Service, c/o Ms . Susan Sherwood) for

further detail.

15
TABLE 4. Exposed briquettes - sample readings (not to be used as data)

Clouts

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16
FIGURE 1. Reflection Meter 670, galvanometer used to take color measurements.

17
2 inch corner

edge formed

by intersection

of bevelled

and non-bevelled

stone surfaces

' ' »0

I

610 T search

unit

Color filter

FIGURE 2. 610T Search Unit resting on stone briquette in measurement

position 1. Label, HU5-12, is on a bevelled stone surface.

18
FIGURE 3. Standard reflectance enamel panel.

19
FIGURE 4. Standard reflectance enamel panel with centering template.

20
FIGURE 5. 610T Search Unit in hole in centering template,

resting on standard reflectance enamel panel.

Template

21
3

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22
Knurled

screw

Briquette

template

1 inch edge

610 T

search unit

Color filter

Bottom orifice

where light

emanates

1 inch x 2 inch 1 inch edge

flat surface

FIGURE 7. Briquette template attached to 610T Search Unit.

23
FIGURE 8. 610T Search Unit resting on stone briquette in measurement

position 2.

24
FIGURE 9. 610T Search Unit resting on stone briquette in measurement

position 3.

25
FIGURE 10. 610T Search Unit resting on stone briquette in measurement

postion 4.

26
u.5. OEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date REPORT NO.

BIBLIOGRAPHIC DATA

SHEET (See instructions) NBSIR 84-2961

4. TITLE AND SUBTITLE

A Procedure for Tristimulus Color Measurements on Building Stone

5. AUTHOR(S)

Lawrence I . Knab

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

NATIONAL bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

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

National Park Service

Contracting Policy Branch

Washington, D.C. 20240

10 . supplementary notes

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

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

bibliography or literature survey, mention it here) This report describes a procedure used to measure the color of building stone surfaces

using a specific tristimulus colorimeter with three color filters. Color changes are

to be monitored during a ten year or more outdoor exposure period to determine the

effects of acid rain on stone color. A step-by-step procedure is provided, including

equipment calibrations and checks using standard reflectance panels, equipment checks

using standard stone surfaces, and color measurements of stone surfaces which are to

be, or have been exposed at field sites.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon si

acid rain; building stone; color; colorimeter; reflectance panels: stone; tristimulus

13. AVAILABILITY

Unlimited For Official Distribution. Do Not Release to NTIS

j Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Q31 Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

26

15. Price

$ 8 .

Omm* C 41*P|0