NBSIR 74-553

Preparation and Calibration of

Phosphor Standards

M. L. Greenough

Electronic-Optical Development Section

Measurement Engineering Division

and

H. K. Hammond, III

Laboratory Performance Section

Engineering and Product Standards Division

Institute for Applied Technology

October 1974

Final Report

Prepared for U. S. Postal Service

Equipment Development Division

Office of Letter Mail Systems Development

Planning and New Development Department

Letter Agreement 72-2^2701

NBSIR 74-553

PREPARATION AND CALIBRATION OF

PHOSPHOR STANDARDS

M. L. Greenough

Electronic-Optical Development Section

Measurement Engineering Division

and

H. K. Hammond, III

Laboratory Performance Section

Engineering and Product Standards Division

Institute for Applied Technology

October 1974

Final Report

Prepared for U. S, Postal Service

Equipment Development Division

Office of Letter Mail Systems Development

Planning and New Development Department

Letter Agreement 72-2-02701

\ 1TM /

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Robaru. Director
v

1' '

I
Preface

This report, tof^ether with its associated "Technical manual

for phosphor standards calibrator, TIBS Report 7^-552 and the

Drawing; Package, constitute the final report on the project

sponsored by the US Postal Service, Luminescence Calibration

Equipment and Standards.

The authors wish to acknowledge the participation and

contributions of H. S. Parker, R. A, Forman and C. A. Harding

in the preparation of the luminescent wafers used in the phosphor

standards, and of J. Cohen in the conduct of tests on phosphor

standards

.
Table of Contents

Pap:e

1.0 Introduction 1

1.1 Scope 1

1.2 Backpround 1

2.0 Fabrication technique 3

2.1 Fabrication, f':eneral . 3

2.2 Fabrication, detailed H 2.2a Machining; of holders k

2.2b Fabrication of wafers 5

2.2c Assembly of standards 9

2.3 Mechanical modifications 10

2. ^ IJotes on fabrication procedure 11

3.0 Specified test characteristics 13

3.1 Accelerated ap;inp; tests 13

3.1a Procedure, phosphorescent standards 13

3.1b Test results, phosphorescent standards ... 14

3.1c Procedure, fluorescent standards l6

3. Id Test results, fluorescent standards If)

3.2 Hlph temperature tests 17

3.2a Procedure 17

3.2b Test results l8

3.3 Temperature coefficient tests 19

3.3a Procedure 19

3.3a(l) Definition of ten[)erature coefficient ... 20

3.3b Test results 21

3. ^ Immersion test 23

3.'j Shock test 2 4

4.0 Othier characteristics 24

4.1 Effect of setback 24

4.2 Effect of surface finish 26

4.2a Surface roughness 26

4.2b Anisotropy 27

4.2c Summary of effect of surface finish 28

4.3 Spectral distribution of luminescent emission ... 29

4.4 Rise and decay times 33

4.5 Intrinsic phospliorescence 35

5.0 Calibration of standards 37

5.1 Procedure, phosphorescent standards 38

5.2 Procedure, fluorescent standards 39

5.3 Assigned measured values of phosphor standards . . 4l

5.3a Phosfihorescent standards 4l

5.3b Fluorescent standards 4 3

6.0 Recommendations 4 4
List of Tables

Table 1 List of original drawings and corrections

Table 2 Average percentage change in observed phosphorescence

at 50°C

Table 3 Temperature coefficient of phosphorescent standards

Table 4 Immersion test data

Table 5 Spectral distribution of luminescent materials

Table 6 Components of phosphorescent decay characteristics

Table 7 Assigned values of new phosphorescent standards

Table 8 Assigned values of new fluorescent standards
List of Illustrations

Flp. Mo. Title

1 Modifications to USPS Dwg. flo. C39f'76, Base Plate,

Phosphorescent Standard

2 Modifications to USPS Dwc No. Rase Plate,

Fluorescent Standard

- - 3 . Modifications to USPS Dwg. IJo. 039^53 for Back Plate,

Phosphorescent Standard

, 4' Modifications to USPS Dwp:. Uo. 039^53 for Back Plate,

Fluorescent Standard

.5 ' Replacement for USPS Dwf;. No. B39659, Label

. 6 Measured PMU as a function of firinp: temperature,

p:reen phosphorescent wafers

7 kplnp- effect, preen phosphorescent standards

., 8 Apinp; effect, red phosphorescent standards

9 Phospfiormet er measurements, aplnp, green phosphorescent

standards

10 Phosphormeter measurements, aging, red phosphorescent

standards

11 Aging effects, fluorescent standards, phosphormeter

12 Aging effects, fluorescent standards, NRS calibrator

13 Effect of temperature on phosphor standards, green

iH Effect of temperature on [)hosphor standards, red

15 Effect of surface finish upon phosphiorescent activity,

green

16 Effect of surface finish upon phosphorescent activity,

red

17 Relation between orientation of surface grinding

and measured phosphorescent activity

18 . Decay characteristics of green and red phosphor

standards in NBS calibrator
1.0 Introduction

1 . 1 Scope

This report covers the fabrication and test of a

group of standards which are employed as the basic

calibrating elements in the U.S. Postal Service Model

Phosphormeter.

1.2 Background

In the Model 4A Phosphormeter, readings of

phosphorescent activity of stamps, test cards or other

material are derived from comparison with the measured

luminescent emission of designated phosphor standards. In

operation the electrical gain of the Instrument is adjusted

so that the Indicated reading agrees v/ith the value marked

on the standard. Since effective operating levels of

luminescent activity (phosphorescent and fluorescent) are

thereby established throughout the entire USPS system, it

is essential that the phosphor standards provide a high

degree of permanence in terms of short and long term

stability. Stabilization is accomplished through:

1) Fabrication procedures which optimize the

probability of stability, and

2) Periodic recallbration, particularly using the NBS

Standard Calibrator, an Instrument which refers

luminescent activity to the calibration from a standardized source of radiant energy at NBS.

The work program for the project called for the

fabrication and test of approximately 60 phosphor

standards. While the primary USPS goal was the acquisition

of these standards, there was also the secondary objective

of testing out the specified fabrication procedure. This

procedure had been prepared by the previous, commercial

source, and was supplied to NBS by the USPS. The

expectation was that deficiencies in the specifications

would be noted and corrected by NBS, and that improvements

might also be Introduced. However the authorization was

strictly limited to standards prepared by mixing the

designated phosphorescent materials with a listed ceramic

slip, firing at an elevated temperature, and mounting the

polished wafers behind quartz windows in aluminum holders

of specified dimensions. While it later became evident

that further research would be desirable on the choice of

the ceramic, because of an intrinsic phosphorescence in the

ceramic itself, such exploration was outside the scope of

the presently reported project.
Concurrently with the preparation of phosphor

standards, the development of a laboratory calibration

instrument for the phosphor standards was also sponsored by

the USPS. This has resulted in the completion of the NBS.

Phosphor Standards Calibrator, which Is covered in a separate report entitled "Technical Manual for Phosphor

Standards Calibrator, NBS Report 7^1-552". This instrument

responds to the luminescent emission from the phosphor

standards in basically the same manner as the Model 4A

Phosphormeter . There is however one point of difference,

in that the NBS instrument is designed to reject the

emission from the Intrinsic phosphorescence of the ceramic

base. This rejection of an essentially unwanted phenomenon

is obtained by confining the spectral acceptance to the

visible portion of the spectrum, so as to avoid dependence

upon the primarily Infrared intrinsic phosphorescence of

the ceramic. The result of the difference between the

measurement systems is that an additive correction constant

is required to supplement the normal scaling for conversion

of values between the systems.

-2-
2 . 0 Fabrication technique

This section covers the process by vihlch phosphor

standards were made at the National Bureau of Standards. It

Includes a general outline and an Instructional step-by-step

procedure for wafer fabrication, concluding with notes,

precautions and suggestions for modifications for future workers.

2 . 1 Fabrication, general

The Important steps In the fabrication process are:

a) Machining of holders

b) Fabrication of wafers

1) Preparation of the Ingredients of wafers

for mixing to obtain the desired level

of phosphorescence

2) Mixing, ball milling

3) Firing in a high-temperature furnace

^) Surface finishing and dimensioning of the

fired ceramic wafer

c) Assembly of windov/s and wafers into the aluminum

body of the standard holder

Testing and evaluation of the finished phosphor standards

are discussed later in Sections 3.0 and ^.0 of this report.

The complete fabrication procedure was followed

for green and red phosphorescent wafers. For fluorescent

standards the modified project authorization called for the

procurement of unmounted finishec;! wafers. Other standards

were prepared from black glass windows and from zerophosphor wafers. All of these materials viere mounted into

aluminum holders which were made in the NBS shops using

nume-^1 cal-control tapes for automatic milling machinery.

These tapes were prepared from the sponsor-supplied

drawings

.

Drawings were supplied by the USPS covering the

mechanical dimensions and composition of the wafers for

phosphor standards.

-3-
1.

The complete drawinp; list is given below in Table

USPS Drawing No . Assembly

D39665

Body

C39652

C39653

C39676

Title Corrections

Standard Assembly-Phosphormeter Section 2.2

Base Plate - Standard

Back Plate - Standard

Base Plate - Standard

Fip-. 2

Figs. 3,4

Fig. 1

Windows

A39658

A39657

Wafers

VJindow - Standard

V/indow - Zero Adjust

Commercial grade

and finish

No change

C39656 Wafer, Photoluminescent Thickness

0.11 +0.01

C 2. 8+0. 25mm)

inch

Small

A39654

A39655

B39659

A39660

Gasket - Standard

Back-up Plate

Plate - I.D.

Tab Blank - PMU

Not used

Not used

Fig. 5

Not used

2 . 2 Fabrication, detailed

2 . 2a Machining of holders

Certain modifications were made to the

original machining plan indicated in the USPS-supplled

drawings. Since these changes are essentially minor,

they are given in the form of correction drawings,

with notations in the Drav/ing List to show the updated

portions. Except as noted the basic assembly drawing

D-39665 remains applicable.

For an improved seal of the base and back

plates of the standards, and for demountability , an 0-

-4-
ring pasket was installed around the wafer in the

holder. This necessitated the machining of an

accommodatinp; groove in the base plate, plus repositioning of the screws which clamp the back plate.

The resultant changes are shown in Figure 1

(phosphorescent, small window) and Figure ?. (fluorescent, large window). relocation of the holes

in the plates is shown in Figures 3 and 'I respectively. Drawing A-3965'f is no longer needed.

2.2b Fabrication of wafers

The following is the step-by-step

instructional procedure for producing phosphorescent

wafers. As needed, additional explanatory material

will be found in Section 2 .

>\ - Motes on fabrication

procedure

.

2.2b(l) Preparation

(1.1) Dry the white body material

Spread out the contents of the

container of white slip as received

onto an evaporating dish. Allow to

air-dry for several weeks if

necessary. Stir occasionally and

break up clumps as needed. Cover as

needed to keep atmospheric dust from

settling out on the material.

(1.2) Weigh the dry white body and the phosphor

Using the proportion of phosphor to

total dry ingredients (white body plus

phosphor) indicated below according to

the level of phosphorescence desired,

make up a batch of 1^00 grams.

Phosphor Color Phosphor Concentration*

& Nominal PMU (% by weight)

G 22 ^.0

8.0

88 12.5

R 22 3.0

6.0

88 13.0

-5-
*riote - Per cent by v/eight is computed from ratio of weight of

phosphor divided by weight of white body plus phosphor. This

total Is always 1^00 grams.

While these concentrations were used In wafers fabricated

at NBS, modification may be necessary for specific conditions of

manufacture such as surface roughness and firing temperature.

(1.3) Add measured amount of dintllled water

Add 600 ml of water.

(1.4) Load ball mill

Add the moistened batch and about 4200

grams of grinding media, high-density

alumina (burundum) cylinders

approximately 20 mm diameter and 22 mm

' long, in a 4.5 litre, high-alumina jar

made for use on a ball mill.

(1.5) Ball mill for four hours

Operate mill at 64 rpm, or at

equivalent speed and time to

accumulate 15,000 revolutions.

2.2b(2) Wafer formation

(2.1) Pour out slip on plaster bat

Use a large flat plaster bat, such as one foot (300mm) square, with 3/l6

(5rnm) inch raised border. Level

slip at this thickness.

(2.2) Allow slip to dry slightly

Walt for a few minutes (maximum 15)

until the slip is sufficiently stiff

to allov; cutting of the wafers in 2.3

below

.

(2.3) Cut out wafers

Use cookie-like cutter whose inside

dimensions are 15/16 x 1 3/l6 inches

(24 X 30 mm) with 1/8 (3mm) inch

-6-
radius at the corners. Press cutter

nearly to bottom of wafer (1/64 to

1/32 inch) (Ojl to 0.8 mm) and then

remove cutter carefully. If clay is

sufficiently stiff and cutter is not

pressed too deep, wafer will remain on

bat and cut will not be filled in by

flow of clay.

(2.4) Dry the wafers

Permit sheet to dry on bat for 24

hours, then lift off sheet and break

off excess material from wafers.

2. 2b (3) Firinp: of wafers

(3.1) Load wafers in furnace

Place wafers on flat sheets of S0%

platinum and 20% rhodium and insert

into furnace. The furnace used at NBS

is an electrically-heated tube-type,

2-1/2 inches (63mm) and 14 inches

(350mm) long, with programmable heat

control

.

(3.2) Fire wafers

Raise temperature in furnace at rate

of 100°C per hour to a peak

temperature of 920°C for green and

1000°C for red phosphor wafers. Soak

15 minutes at peak temperature.

Return to room temperature at rate of

155°C per hour. Heat must be uniform

within 5°C in firing zone if

phosphorescence of fired wafer is to

be acceptably uniform.

2.2b(4) Finishing to size

(4.1) Grind faces of wafer

Grind wafer face down on aluminum

oxide abrasive paper placed face up on

a sheet of plate glass. Hold wafer at

edges with thumb and fingers. Using a

random circular motion, grind first

-7-
with 2^0 grit paper. Then use finer

grit, 23 um on plastic sheet with the

same motion until scratches from the

coarser grinding have been removed.

Grind the back of the wafer with 150

or 2^0 grit paper so that the

thickness is 0.11 + .01 inch (2.8

+0 , 25mm)

.

(^.2) Clean off the front face of wafer

Dust off the ground surface with a camel *s hair brush, using separate

brushes for green and red phosphors.

Drop wafer from height of 6 inches

(150mm) onto glass plate three times

to insure absence of cracks. Blow off

surface with compressed air (oil-less)

or gas jet with pressure of 30 psi (20

N/cm 2)

.

(^1.3) Visual examination

Examine v/afer carefully for uniformity

of fluorescence under shortwave

ultraviolet radiation.

2. 2b (5) Evaluation of phosphorescence

(5.1) Measurement of PMU

Place the wafer temporarily in a holder with an installed quartz windov/

and measure on a phosphormeter , If

the Pr-lU value is within acceptable

tolerance omit the next step and go on

with (5.3) below.

(5.2) Trimming of PFIJ value

If the wafer is not within tolerance,

but for example is too high in PMU

value, it may be reduced by grinding

the surface with a finer grit, such as

8pm plastic sheet. PMU values may

alternatively be raised by grinding

with a coarser grit. After each

grinding operation, perform steps

-8-
(4.2) and (4.3) before remeasurement

in (5.1) above.

(5.3) Test for uniformity

Mask off the lower half of a phosphor

standard holder and load temporarily

with the wafer. Measure on

phosphormeter . Reverse the wafer and

measure the previously-masked half.

If difference is more than 10%,

discard wafer

2. 2b (6) Storage

(6.1) Label wafer

Identify wafer on back by marking with

lead pencil. Prefix green wafers with

letter G, and red with letter R.

Follow with the nominal percentage of

phosphor in the wafer, then the serial

number of the wafer for the given

phosphor concentration.

(6.2) Storage condition

Keep fingers off finished surface of

wafer. Place accepted vrafers face

down in clear plastic compartmented

parts boxes until ready for mounting

' in standard holders.

2.2c Assembly of standards

Quartz windows were cemented into the

anodized aluminum holders by first coating the edges

of each v/indow and holder bezel with the prescribed

primer, a clear red liquid that dries relatively

quickly. The prescribed silicone adhesive sealant was

placed in the metal bezel in sufficient quantity to

over flow both inside and out vfhen the window was

inserted. Each holder was placed on a flat metal jig

having four screws protruding to a height such that

the window was held exactly 0.010 in. (0.25mm) above

the bezel while the sealant cured. Use of a jig is

imperative if the window is to be positioned parallel

-9-
to face of holder and at a prescribed distance from

it.

The silicone adhesive cures without heat by

absorption of moisture. After curing for at least a day in a room with 50% relative humidity, the excess

cement was removed with a laboratory knife. Windows

were cleaned mechanically with a section of a razor

blade. They v;ere then washed with a laboratory

detergent to which a small amount of ammonium

hydroxide had been added. A piece of lens tissue was

used as a scrubber. The windows were rinsed with hot

water and dried with lens tissue. Each window was

examined visually for cleanliness in a darkroom by

observing light scattered at the surface when a beam

from a flashlight was incident on the window. Windows

were washed three times to remove all traces of smear.

V/afers and v;lniov:s v/ero dusted with a camel's hair

brush before assembly. The wafer was then placed in

its holder, in the case of phosphorescent standards,

the one it had previously occupied. Holders were

placed, three or four at a time, on the top shelf of a laboratory oven operated at 107°C (225°P) where they

remained for at least 30 minutes. Air in the oven was

moved rapidly by means of a fan provided with the

oven. The wafers were placed in the holders to

prevent the possibility of dust collecting on their

surfaces while moisture was being driven off in the

oven. On removal from the oven, wafers were

immediately sealed in the holder bases with silicone

sealant worked in around all four edges and covering

the back of the wafer. After curing at least

overnight, excess sealant was removed with a microtome

knife. The 0-ring seal was inserted, the wafer back

covered with a small amount of fresh sealant and the

back screwed in place to complete the assembly.

2.3 Mechanical modifications

In addition to the revised sealing technique for

the standard holders as mentioned in Section 2.2a above,

several other minor changes were incorporated. These are

described below and referenced in the Drawing List, Table

1.

For simplicity, the wafers were not cemented to

the stainless steel back plate as indicated in drawing no.

A39655. Instead the wafer was made thicker, by not

- 10-
removing as much material during grinding. Therefore the

wafer thickness shown in drawing no. C39656 should be

revised to 0.11 +.01 inch, (2.8 +0.25mm) and drawing no.

A39655 is no longer needed.

As an economy measure the quartz windov;s

purchased for these standards were specified to be of

commercial grade and finish, as being entirely adequate in

our opinion for the intended service. This revised

specification applies to drawing no. A-3965B.

In order to provide an enlarged area for the

calibration labels on the phosphor standard, the previous

arrangement was modified to include a larger window,

relocated at the botton of the label. The resultant

changes, together with minor textual revisions on the

labels, are shown in Figure 5> which replaces drawing no.

B-39659 completely. The new style features a thin bottom

plate carrying the calibration data regarding PMU(T) and

the date, which is visible through the window in the

superimposed top plate. This top plate contains the fixed

information (Red Phosphor Standard, etc.) plus the engraved

serial number. Temporarily the bottom plate is a selfadhesive paper label with typewritten data and a lacquer

protective lacquer over-coating. On the bottom plate,

there is also marked the NBS-PMU value, to which is added

the correction constant for phosphormeter response to

surface reflectance and intrinsic phosphorescence. These

markings may be seen v;hen the top plate is removed, but

with the latter in place, only the sum is visible.- This is

the reading to which the phosphormeter gain should be set.

Ultimately it is planned that the bottom paper. labels will

be replaced with etched aluminum plates.

2 . ^ Notes on fabrication procedure

The following notes on the wafer fabrication

procedure are referenced to the items listed in Section

2.2b - Fabrication of wafers. Comments and other

explanatory material are included as experience has

Indicated

.

(1.3) The quantity of water specified, 600 ml, was

found to be the correct amount which will

produce a slip (after four hours of milling as

in (1.5) that flows conveniently onto the

plaster bat.

-11-
(3.1) As a precaution to prevent contalnlnation , the

wafers when fired at NBS v/ere placed in a

completely covering structure. A folded-metal

tiered assembly was constructed of 0.01 inch

thick, (0.25mm) 80% platinum, 20% rhodium

sheet. Wafers were placed inside the assembly,

and fired in batches of six. Similar or

equivalent measures may be necessary in order

to pass the uniformity test as specified in

step (5.3) of the procedure.

(3.2) In the course of the work it has become evident

that phosphorescent activity has a rather

critical dependence upon the temperature of

firing, A plot of measured PMU for green

wafers fired at temperatures from 85O to 1000°C

is shown in Figure 6. It can be seen that the

resultant activity drops from 55 to ^ PMU over

this region. In view of this steep function of

temperature, it v/as felt desirable to revise

the specified firing temperature from the

original 1000° downward to 920°C. At this

temperature, it was found that the required

percentage of luminescent materials in the slip

spproximately matched those in the original

specifications.

Because of this critical temperature

dependence, uniformity of temperature inside

the furnace is important. For this reason only

a small number of wafers (six) were, loaded at

one time, with these placed in the center where

the temperature was presumably most uniform.

(5.2) Trimming of PMU value by means of surface

finishing should not be used unless necessary

to bring wafers within the acceptance range.

Until more is known about the effects of

surface finish, it appears desirable to perform

final grinding with the 23um grit paper, since

the PMU characteristic seems to be relatively

independent of grit size in this region. The

results of a short investigation of the subject

are given in Section 4.2- Effects of surface

finish.

-12-
3.0 Specified test characteristics

In the authorizing Work Statement, as modified by

mutual agreement, the following tests on phosphor standards are

specified:

1) Accelerated aging

2) High temperature

3) Temperature coefficient

k) Immersion (substituted for humidity test)

5) Shock test

The specific procedures which were followed are given

in detail below, together with the test results.

3 . 1 Accelerated aging tests

3.1a Procedure, phosphorescent standards

These tests were conducted in two

categories, which were 1) aging exposure for

approximately 100 hours on all standards with

measurement every 24 hours, and 2) exposure up to

approximately 800 hours on two sets v;ith measurements

every two hours for the first 24 hours, followed by

less frequent measurement for the remainder of the

time. The selected sets for the latter were the

primary sets to be kept at NBS , v/hich include v/afers

from both NBS and commercial sources.

Aging v/as conducted by mounting a bare

mercury lamp of the type used in Postal Service

equipment in a vertical position and then positioning

st?^ndards around it at such a distance that the level

of irradiation is comparable to the average level of a phosphormeter (approximately 2 mW per square

centimeter). Two lamps were used and seven standards

were positioned around each lamp. In addition to two

phosphorescent standards for each of the three levels

of intensity in both red and green there was one

standard containing a ceramic wafer without phosphor

and one standard of black glass. The standards were

irradiated continuously except for the time required

for periodic measurement. The quartz windows were

-13-
cleaned before start of aginp: tests but not during

aging.

The detailed procedure was to read the

reference standard, the control standard, the two aged

standards, and then re-read the reference standard.

The average value of the two readings on the reference

; standard was used as the reference. All green

' standards were evaluated in rapid succession by

measuring their intensity relative to that of Green

Standard No. 25^ supplied with Phosphormeter 4a8 as

reference. All red standards were similarly evaluated

.relative to Red Standard Mo. 458.

One control standard at each level

containing a commercially procured (COM) wafer was not

irradiated but was measured each time the irradiated

standards of that level were measured.

Data from the aging tests v/ere corrected by

subtracting the reading for the blank ceramic wafer

(mounted in a standard holder) from each reading for a phosphor standard.

Measurements on all standards were

subsequently reduced to a percentage of the initial

reading in order to illustrate most clearly any aging

effect of ultraviolet radiation.

The data represent only one reading on each

standard for each time interval except for the initial

data (unexposed) which represent the average of two

readings made by re-reading each standard in reverse

order.

Each measurement on the NBS calibrator was

follov/ed by one on the phosphormeter.

3.1b Test results, phosphorescent standards

The relative aging effect of green and red

phosphorescent standards is plotted in Figures 7 and 8

for data obtained from the NBS calibrator and in

Figures 9 and 10 for data obtained from the

phosphormeter. Readings on the unirradiated (control)

standard at each level are plotted as points in

circles. Irradiated standards containing wafers of

commercial manufacture are plotted as points in

-111-
squares; standards contalninp- wafers of NBS

manufacture are shown as points in triRnp;les.

Examination of the plots reveals the

following: 1) Little change is observed in the

relative intensity of green or red phosphor standards

after the first few hours of aging. 2) Plots of data

from the NBS calibrator (Figures 7 and 8) appear to be

somewhat different than those for data from the

phosphormet er (Figures 9 and 10), This difference may

be due in part to the effect of subtraction of

different amounts for the intrinsic phosphorescence of

the ceramic wafer, which is particularly large for red

standards measured on the phosphormeter.

Ultraviolet irradiation of the red and green

phosphorescent standards previously aged for 100 hours

was continued to determine hov; much exposure beyond

100 hours might be required to produce . significant

degradation in phosphorescence. Irradiation at a

level equivalent to the average in a phosphormeter was

continued to 760 hours for green standards and 828

hours for red. Measurements were made on irradiated

standards and on a control standard of similar type

relative to a reference standard for each color, using

both the MBS calibrator and the phosphormeter.

Measurements on the green standards with

increasing time of irradiation indicated that there

was no certain change in the phosphorescence of the

low, intermediate or high level standards.

Measurements on the red standards however indicated

that the phosphorescence of the tvio higher levels of

standards increased slowly as irradiation was

continued, the final values being about 5% higher than

the values at the start of the aging test. While an

increase in the indicated values of irradiated

standards could be caused by a decrease in the

phosphorescence of the reference standard,

measurements made each time on an unirradiated control

standard at each level indicated no changes in values

of the control standards. Thus the indicated rise of

5% during the aging tests to 828 hours for red

phosphor standards must be regarded as real.

From these data the precision of a single

measurement for the NBS instrument as a comparator was

shown to be of the order of 0,H%, When measurement

-15-
requirements are more stringent than this, it will be

necessary to make multiple readings.

3.1c Procedure a fluorescent standards

Accelerated aging tests were conducted on

the designated Red Fluorescent and Balance Adjust

standards, using both the NBS calibrator and the

phosphormeter , In the phosphormeter, evaluation

involves simultaneous, two-color sensing of

fluorescent activity, wherein the signal derived from

a photocell with a yellow filter partially cancels out

the main signal obtained from a photocell with a red

filter. This procedure is used to minimize the effect

of fluorescent whitened paper. To accomplish the

desired degree of cancellation, a Balance Adjust

standard is normally employed. With this standard in

place, approximate equality is first set into the

signal processing channels. Then introduction of the

Red Fluorescent standard causes an up-scale reading on

the red channel, with the magnitude of this reading

being the indicator of PMU value. From our separate

measurements of red and yellow fluorescent activity,

it appears that the yellow channel is designed to

cancel approximately 11 percent of the red activity on

a nominally-rated standard of 22 PMU, Thus the true

measure of the PMU scale for fluorescent evaluation

requires measurement of both the Balance Adjust

Standard and the fluorescent standard under test, plus

a knowledge of the relative weighting assigned by the

phosphormeter to the red and yellow sensing channels.

In view of this complex mode of operation we

have chosen for completeness to measure both red and

yellow output, on Balance Adjust as well as Red

Fluorescent standards. All of these data may not be

usable without further knowledge of the relative

weighting given them in the phosphormeter. In the

absence of specification information regarding

weighting, we have elected to give primary attention

to the major factor, the red-channel fluorescence. In

the discussions which follow, it can be assumed that

this is the parameter under consideration unless

otherwise indicated.

3 . Id Test results, fluorescent standards

Readings of overall PMU indicated by the

phosphormeter are plotted in Figure 11, and of red-

-16-
channel fluorescent activity measured on the NBS

calibrator, in Figure 12. Irradiation of a Red

Fluorescent and a Balance Adjust (fluorescent)

standard for 117 hours revealed no certain change due

to radiation for either standard when measured on a

phosphormeter and on the NBS calibrator relative to a

corresponding non-irradiated control standard.

Instrument, readings on the irradiated red fluorescent

standard suggest that during the first few hours of

irradiation the fluorescent output was reduced and

then as radiation was continued the output increased

again to approximately the original value at the end

of 117 hours. A phenomenon of this type has been

previously encountered when a green phosphorescent

(dummy) stamp was irradiated. In any case it must be

assumed that the change took place, since it was

observed with both instruments and was absent on the

non-irradiated control standard.

3 . 2 High Temperature Test

The purposes of this test were to determine

qualitatively the effect of elevated operating temperatures

upon the phosphor standards, and to note any residual

permanent changes in phosphorescent activity. This test

was performed on phosphorescent standards only.

3.2a Procedure

Standards were placed in an oven at 50°C for

about 2 hours and then removed for measurement of

phosphorescence on a phosphormeter and the NBS

calibrator. They were returned to the oven and

heating was continued for H additional hours. The

standards v;ere again removed and measured for

phosphorescence on the two instruments. Measurements

were made within about one minute after removal from

the oven.

Two standards at each of the three phosphor

levels were tested. Initially standards were

withdrawn from the oven in pairs, but the aluminum

holders cooled quickly, especially while in contact

with the metal head of the phosphormeter or the NBS

calibrator. To minimize cooling, standards were

subsequently withdrawn one at a time and measured

first on the phosphormeter and then on the NBS

calibrator. Readings on the phosphormeter could be

made more quickly than on the NBS calibrator and so

-17-
these data should be more representative because the

temperature of the standard has had less time to

change.

For measurement control, a standard similar

to each of the heated pairs was kept at room

temperature and measured each time a pair of heated

standards was measured. In addition, two standards

with blank ceramic wafers were measured; one was

heated, the other served as control. The instrument

reading for the heated blank wafer standard was

subtracted from the readings for the other heated

standards. The reading for the blank standard kept at

room temperature was subtracted from the readings on

- the control standards.

3.2b Test results

The data show that the percentage change in

phosphorescence on heating is about the same for the

six standards of a given color measured on a given

instrument, but the change appears to be dependent on

the time of heating. It is significantly larger for 4

hours than for 2 hours. Thus we conclude that

equilibrium is not reached in 2 hours, as shown in

Table 2.

Table 2

Average Percentage Change in Observed Phosphorescence

After 2 and H Hours at 50°C Relative to 24°C

Measuring Instrument

NBS Calibrator Phosphormeter ^A

Standard Hours at 50°C Hours at 50°C ~2~ HI

Green Phos. -7.6 -8.7 -3.7 -5.9

Red Phos. +1.7 +2.7 +0.7 +1.7

The percentage change also appears to be

dependent on the instrument used. While the data

agree in direction, it can be seen that there are

consistently larger shifts on the NBS calibrator than

on the phosphormeter.

-18-
It is possible that the source of difference

lies in the intrinsic phosphorescence of the ceramic

wafer, the effect of which has been subtracted before

listing in Table 2. The question raised is whether

simple subtraction of measured values on blank wafer

standards is an adequate technique for the

phosphormeter

.

More important, however, is the fact that

our data show that the phosphorescence of red

standards increases with increase in temperature while

the phosphorescence of green standards decreases and

at a much faster rate.

Remeasurement of the standards after return

to room temperature indicated that this heat treatment

caused no change in the values of the standards at

room temperature,

3 . 3 Temperature coefficient tests

Phosphorescent standards were tested to determine

the extent to which PMU value are dependent upon ambient

temperature.

3.3a Procedure

Details of the temperature coefficient tests

were established as a result of the conference with

the USPS project engineer on April l6, 1973.

In essence the standards were maintained at

constant temperature for at least one hour, then

withdrawn and measured as quikcly as possible. The

desired temperatures for measurement were 32, 50, 75,

100, 125, and 1^I0°F. (0, 10, 24, 38, 52, 60°C).

It is recognized that the procedure

introduces a degree of approximation in the derived

temperature coefficient. The actual temperature at

the instant of measurement is necessarily somewhat

different from that in the oven, being influenced by

the thermal conductivity of the aluminum holder, the

temperature of the measuring head and the elapsed time

between removal and measurement. To determine the

temperature at the measurement time would require

instrumentation of unjustifiably high sophistication.

-19-
These difficulties were evident durlnp the

tests. At hlph temperatures, the standard cooled

rapidly during measurement when the hot aluminum case

contacted the cooler metal of the optical head of the

Phosphormeter . The converse was true at lov;

temperatures. At temperatures below the de\i point for

the laboratory atmosphere, v;ater vapor condensed on

the cold quartz windows. Even though the window was

wiped with lens tissue immediately before measurement,

it was impossible to be certain that no moisture had

condensed on the windov; by the time the reading was

made

,

For measurements belov; room temperature, the

standards were placed in a cardboard box in a refrigerator freezer with the bulb of a mercury

thermometer centered in the box. After the prescribed

time interval, the box v/as removed from, the freezer

and taken to the phosphormeter where the standards

were removed for measurement one at a time. The

temperature in the box rose gradually during the 15 to

20 minutes required to measure all the standards on

the two instruments. An attempt was made to obtain

more precise temperature control near 0°C by placing

standards in individual plastic envelopes and

immersing them in ice water. Some of the plastic bags

leaked however, and the standards harl to be dried

before measurement. In any case it appeared that

temperatures of standards at time of measurement are

probably no more accurately knov/n when liquid cooled

than when air cooled.

3.3a(l) Definition of temperature coefficient

The formula which has been adopted for

expressing the phosphorescent activity at

other than the reference temperature of 75°P

(2^I°C) is

PMU at = [Pf^n at 75°F] x [1+ a(At)],

desired

operating

temperature

where

:

At = temperature difference in °F

between operating temperature and

75°F, and

ais the temperature coefficient for

the phosphor (different each type)

-20-
A nef:ative value for a signifies that

phosphorescent activity decreases v;ith

elevation of temperature. When a is

large, there is a large dependence upon

the operating temperature of the standard.

3.3b Test results

In Figures 13 and are plotted the data

from which temperature coefficients v/ere determined,

for the green and red phosphorescent standards

respectively. All values indicated have been

corrected for offset as described earlier, hence the

data represent the behavior of visual phosphorescence

only. It is quickly apparent that the green

phosphorescence drops rather rapidly with temperature

increase, while the red phosphorescence increases at a lower rate.

In more detail. Figures 13 and 1^ show

phosphormet er readings after the correction for offset

has been applied, for high, medium and low level

standards. Each plotted point is the mean between two

wafers from two sources, commercial and NBS.

Additionally the plots shov; the relative output of the

NBS calibrator when scaled to an empiricallydetermined fit with the phosphormeter data. This

scaling permits preliminary conversion of the

calibrator output in terms of PMU. At each of the

three levels a single straight line has been

constructed, chosen to represent estimated system

performance, with allowance for measurement

uncertainty. In general the data at each temperature

agreed well between the phosphormeter and the NBS

calibrator. V/here the two instruments disagree, more

weight was given to the indications from the NBS

instrument, since the fit to a straight line was

usually better. The slope of the drawn straight lines

was used to determine values of the temperature

coefficient in the formula given above. The derived

values a are listed in Table 3 below:

-21-
Table 3

Temperature coefficients of phosrjhorescent standards

Color and

level of

standard

Temperature

coefficients , (per °F)

Temperature

coefficients

,

(per °F)

Green, hi^h

" med.

" low

-0. 0019

- .0017

- .0021

-0. 0011

- .0009

- .0012

Red, high

" Med.

" low

+0.00038

+ .0003^

+ .0003^

+0.00021

+ .00019

+ .00019

From the values of temperature coefficient in the

table it can be seen that each color of standard might

well be represented by an average coefficient

regardless of level. Thus v/e can say that the

phosphorescent emission of green standards at all

levels decreases by approximately 1,9% for each 10°F

rise in temperature. The emission of red standards

containing red phosphor increases by ,35% for the same

temperature rise. The precision of the coefficient

determination is of the order of 10^ according to

these data. It should l^e emphasized that v/e are

considering the behavior of true visual phosphorescent

emission only; all contributions whether minor or

major from the blank ceramic wafer have been

subtracted.

The correlation of observed luminescent activity

as a function of temperature was also checked against

the manufacturer-supplied data listed in Appendix A of

the V/ork Statement. When these data are converted to

equivalent temperature coefficient, the resultant is a value of -.0012, applicable to both green and red.

This of course is at variance with the adopted -.0019

and +.0035 from Table 3, where the coefficient for red

phosphors is even slightly positive rather than

negative. The explanation appears to lie in the

behavior of the offset due to the intrinsic

phosphorescence of the blank ceramic wafer itself.

When the raw data from phosphormeter readings are

taken, without correction for the offset, coefficients

of -.0017 and -.0007 are found. The manufacturer's

coefficients correlate better with those data, because
the overriding effects of the ceramic wafer provide

agreement in the direction of change.

The offset itself appears to display a temperature

coefficient of -.00^2. This value is obtained from

phosphormeter data when the instrument is calibrated

with the red phosphorescent standard supplied with it.

Because some proportion of the phosphormeter response

is contributed by intrinsic phosphorescence, it is to

be expected that the temperature behavior of the

composite phosphor standard will reflect this

proportion. It is particularly evident in the case of

red phosphorescent standards. Thus it is not

surprising that the observed small positive

coefficient representing the true visual

phosphorescence is masked when the offset generates

about one-third of the phosphormeter ' s response.

To summarize the results of the temperature

coefficient tests on phosphorescent standards, the

measurements show for visual phosphorescence a

negative coefficient of 0.0019 per °F for green

phosphor standards, and a positive coefficient of

0.00035 per°F for red standards. Both coefficients

can be considered as constant for low, medium and high

levels of phosphorescent activity, and applicable to

the temperature range from 32°to 1^I0°F.

3 . Immersion test

Six green and six red phosphorescent standards

that had been subjected previously to aging, high

temperature and temperature coefficient tests were given

the immersion test. This latter test is designed to shovr

the effectiveness of the sealing technique rather than the

susceptibility of the wafer to mojsture.

The standards were first measured before the test

on Phosphormeter Ha8 calibrated with the standards supplied

with the instrument. To conduct the immersion test, the

standards were placed face dovm on the bottom of a laboratory sink at the end of the day. The sink was filled

with water at 50°C to a level 8 inches -above the standard.

The next morning the standards were removed and dried. The

windows were cleaned, and the standards were examined for

water penetration. Mo penetration was observed. The green

standard in holder No. 24 showed a dark spot in the upper

left-hand corner of the wafer, but this had been noted

-23-
previously. It was due to silicone adhesive that ran under

the corner when the wafer was sealed in the holder.

The standards were then remeasured on the

phosphormeter . No significant difference in

phosphorescence was observed after immersion. the

measurement data obtained before and after immersion are

tabulated below. The first holder in each pair contains a wafer of commercial manufacture, the second holder a wafer

made at HBS.

Table 4

Immersion Test Data

Cxreen Phosphor Standards Red Phosphor Standards

Holder

No.

15

24

16

28

22

19

Phosphormeter Reading Holder Phosphormeter Reading

Before After No. Before After

24.0 24.0 5 23.4 23.1

23.2 23.0 4 22.6 22.2

48.6 48.3 6 47.0 46.8

48.3 48.6 21 43.1 42.8

95.2 95.4 14 103.0 102.5

97.4 97.6 13 103.8 102.3

3 . 5 Shock test

An abbreviated shock test was performed on one

standard. It was dropped several times from a height of

approximately l8 inches (45 cm) onto an asphalt tile over

concrete floor. No visible damage was sustained, nor was

there any measurable change in PMJ value as indicated on

the phosphormeter.

0 Other characteristics

4.1 Effect of setback

The term setback refers to the displacement of

the irradiated, emitting surface of the luminescent

material as moved away from its "normal" position. Setback

is a problem because the NBS calibrator is substantially

independent of the displacement, while the phosphormeter

and the facer-canceler are quite dependent upon it. The

latter two, it should be mentioned, are understood to be

-24-
similar in geometry, hence should behave alike regarding

setback.

The tv/o conditions where setback is a particular

concern are 1) variation among the phosphor standards of

the thickness of the shoulder against which the quartz

windows are mounted, and 2) definition of the PMU-efficacy

relationship _at the time of conversion computation. In

regard to the former, which is the variation among

standards, the problem is considerably aggravated by the

method of specifying tolerance contained in the original

mechanical drav;ing, wherein the shoulder thickness must be

indirectly derived. This leads to possible summation of

three or more individual tolerances. In the machining of

the sample holders in the NBS shops using the supplied

numerical control tape, it appears that an appreciable

range of tolerance sums was encountered. Specifically, the

distance from the front plane to the outer quartz surface

ranged from 0.029 to 0.05^ inch (0.7^< to 1.^ mm). On the

phosphormeter such a displacement range causes a ^.5

percent change in indicated PMU value. At the nominal 22-

Pmu level, the resultant potential correction is

approximately +0.5 PMU.

Trial application of corrections, based upon the

individual measured shoulder thickness, appeared to reduce

by a factor of about one-half the unexplained differences

in the correlation between PMU values indicated by the

phosphormeter and the NBS instrument. Thus eventually it

may be possible to apply corrections for setback, and to

this end, setback measurements have been recorded for all

standards. For the present however, the correction step

was omitted, primarily because this imprecision is masked

by the greater uncertainty regarding intrinsic

phosphorescence. Details of the latter are covered in

Section ^.5 - Intrinsic phosphorescence.

The second area of concern involving the setback

dependence of the phosphormeter arises because the stamps

and test cards are located in a plane v;hlch is much closer

to the irradiating and sensing elements, compared to that

of the active luminescent wafer in the phosphor standards.

Experiments have shown that this displacement causes a test

card v/hich gives a reading of 15 PMTi at the wafer plane to

Increase to over 21 PMU at the normal stamp plane, the

front face of the phosphormeter. The decision v/hich this

imposes is whether NBS-Phosphormeter conversion should be

based upon phosphor standards or upon stamps and test

cards. Agreement on phosphor standards was chosen as more

-25-
consistent with the Intended service of the NRS calibrator,

where stamps and test cards are only of secondary Interest.

The consequence of this choice is of course that the

phosphormeter will display about per cent greater

readings on stamps than will the NRS instrument. Test

cards may show up to as much as this, depending upon their

prevailing setback.

The Inconsistency regarding the measurement of

stamps and test cards may be eliminated with the adoption

of a single plane for positioning during measurement. This

would require that a spacer plate of approximately 0.10

inch (2.5mm) be placed over the phosphormeter face when

measuring stamps. Appropriate modification would be

necessary for wlndov/ed test cards. With this technique it

should be possible to obtain inter-instrument agreement on

all the luminescent materials of interest,

4,2 Effect of surface finish

Experiments were made to determine the influence

of surface grinding on phosphorescence; specifically (1)

the effect of surface roughness on the magnitude of the

phosphorescence, and (2) the effect of grinding technique

(circular or linear grinding motion) on the magnitude and

the anistropy (directional effects) of the phosphorescence.

Three green and three red phosphor ceramic samples, each

with different amounts of phosphor additive, and hence

phosphorescent intensity, were used. A zero phosphor

sample v;as also prepared. Samples were ground by hand

using abrasive paper placed on flat plate glass, and

considerable care was taken to prevent contamination

between different samples. After grinding, the debris

remaining on the samples was brushed away v;ith a camel hair

brush, then blown with pressurized dry tank nitrogen.

Samples were mounted for measurement in a special holder

with an open 1 cm square aperture.

4 , 2a Surface roughness

For the surface roughness experiments, the

samples were all circularly ground v/ith 59m m gr'it (240

grade aluminum oxide) and the phosphorescence was

measured with both the phosphormeter and the NBS

meter. Four measurements of phosphorescence were made

on each sample, corresponding to 90° intervals of

rotation through the central axis normal to the ground

surface. The procedure was repeated next with 23^ m - and finally 8 ym grit. Determinations of surface

-26-
roughness using a stylus instrument were made of the

various samples, but were inconclusive as to the

finish actually secured.

The results of the surface roughness

experiments are shown in Figure 15, plots of the

percentage of the initial value (after grinding with

59ym grit) vs. the grit size of the abrasive, for

green phosphorescent wafers. Taking the

phosphorescent activity with the coarsest grind as the

reference, the activity of green phosphors appeared to

increase by a few percent for medium grit (23pm), then

decrease by 4 to l6 percent for the finest grit (Sym).

Because the larger percentage effects were observed

for the higher levels of phosphorescence, the PMU

range due to surface finish could be as much as 15

PMU.

Red phosphors displayed the same general

behavior, but on a larger scale, as can be seen in

Figure l6. The overall range of finish-influenced

effect was as much as 30 to '^0 percent, with an

absolute change as large as 20 PMU. Similarly to the

performance of the green phosphors, the greater part

of the observed change appeared to occur between the

23 and 8m m grit

.

A comparison of the measured effects with

regard to the phosphormeter and the IIBS instrument

showed no significant difference. Although in the

latter the usual correction for intrinsic

phosphorescent activity was applied so as to yield

solely the visual phosphorescent activity the results

are in approximate agreement, as is evident in figures

l6 and 17. This suggests that effects due t© surface

roughness apply to the intrinsic phosphorescence of

the ceramic base as well as the phosphorescence due to

the Introduction of luminescent material.

4 . 2b Anisotropy

For the anisotropy experiments, red phosphor

samples were used; the samples v/ere ground first vrith

a specific grit size using circular motion and the

phosphorescence measured, again at 90° intervals of

sample rotation, using the NRo instrument. Then the

sample was ground using a linear motion against

abrasive of the same specific grit size and

phosphorescence measurements were made as before.

-27-
This last step was taken to exaggerate any possible

orientation effects for ease of measurement.

Figure iB shov/s plots of the percentage

change of NBS-PMU from the average value of the

circularly ground sample vs. sample orientation;

samples ground with three different abrasive grit

sizes are shown. The orientation of the sample

relative to the grinding direction and instrument setup is depicted in the circular inset. It may be noted

that in all cases, the circularly ground sample has a

higher value of PKU than the linearly ground sample.

In general, the circularly ground samples are quite

isotropic; with a single exception the noted change

with orientation was less than one percent. As

expected, some anisotropy was found for the linearly

ground wafers, amounting to about tv;o percent. In

view of the deliberate effort to maximize the effect,

such a change is small. The anomaly shown by the 8 y m

sample at 270° is believed due to inhonogeneity of the

phosphor at the surface.

It may be noted that in all cases the

circularly ground sample has a higher value of PMU

than when the same sample was linearly ground. An

explanation for this is lacking.

^ . 2c Summary of effect of surface finish

From the results above, it would appear that

anisotropy is not a problem and that merely nominal

attempts at random circular grinding should be quite

adequate

.

A considerable problem does appear to exist,

however, regarding surface roughness. Phosphorescent

activity can be seriously affected by the grit size

used in finishing. While grinding with the 8 ^ m grit

always reduced the PPIU value compared to that for 23

Mm repeating the experiment would frequently produce

different values. Thus, reproducibility of absolute

readings was found to be rather poor. It is our

conclusion from these measurements that uniform

surface preparation, such as the use of the 2%m grit,

is preferable to attempts at "trimming" using variable

grit size.

The samples per se are exceedingly complex

from a materials point of view, consisting largely of

-28-
sintered ceramic with a small percentage of phosphor

embedded, and probably inhomogenously distributed.

Thus, grinding not only affects surface roughness, but

may also affect surface concentration of the phosphor.

Other complexities may also exist. Consequently, the

results of these experiments cannot be well explained,

and a great deal of systematic experimentation would

probably be required for a definitive model.

i| . 3 Spectral distribution of luminescent emission

In this section the known information regarding

the spectral emittance of the luminescent materials has

been assembled. The significant characteristics are

discussed, along with the limitations on available data.

As will be seen, there are problem areas where more data

would be desirable.

Four luminescent materials are involved:

1) green phosphorescence

2) red "

3) red fluorescence

^) yellow "

Ideally it would be desirable to know the spectral

emittance of these materials under the various conditions

of use, in the facer canceller, the Model HA phosphormeter

and the HBS calibrator. The term "conditions of use" here

refers to the manner in which the phosphors are excited and

measured. These are different in the three systems of

implementation, as is covered more fully later in iSection

H.k - Rise and decay times.

The apparent presence of several different time

factors in the measured function of luminescent decay

suggests that several components are present in the

phosphors. Each component can conceivably be expected to

possess individual spectral characterict ics . The

contribution of these components would be superimposed

under any given conditions of excitation and measurement

according to the proportionality of the relative efficiency

excitation and measurement. One example would be when the

measurement delay is so great that the phosphorescence from

a rapid-response component has decayed virtually

completely. Then this portion of the spectral emittance

-29-
would not be evident In the measured characteristic.

Another example might be a phosphor with a component of

very long time of rise and decay which would not be

stimulated appreciably by a short pulse of ultraviolet

irradiation, yet a quicker-response component might be

fully excited. This would lead to displaying the spectral

contribution primarily of the latter. But the same

material, when excited for a length of time, could then

reveal the contribution from its slower-response component.

It seems logical to expect that the resultant spectral

characteristics would be dissimilar. Unfortunately the

resources of the project did not permit pursuit of the

subject

.

The data of Section ^.^ below, wherein it

appears that three distinct time constant values are

required to match the measured decay function, cast doubt

that one common spectral characteristic can be derived for

any one phosphor. The alternative, that of determining the

characteristic for each implementation (facer canceler,

phosphormeter , NBS calibrator is difficult and costly. In

particular, for the facer canceler it would be necessary to

simulate the excitation build-up as a typical stamp-bearing

envelope approached the sensing head. For the NRS

calibrator it might be possible to interpose a variable

filter such as an interference wedge betvreen the phosphor

sample and photomultiplier tube. Because it v/ould require

considerable fabrication however, this approach has not

been attempted. Thus within the scope of this report, we

have not found it feasible to develop a quantitative model

of the spectral ernittances of the four phosphors with which

phosphor standards are involved. We have instead assembled

and reproduced data from earlier investigators, some from

the manufacturers of luminescent materials and some from

prior measurement at the UBS. By and large these are

believed to represent data on fluorescent rather than

phosphorescent emission, since the derived characteristics

is not as critically specific to the conditions of

excitation and measurement. These data are listed in Table

5 below.

-30-
Table 5

Summarized Spectral Characteristics of Luminescent Materials

Relative spectral emittance for total area equal 1.00

Green Red Yellow Red

Wavelength, Phosphorescent Phosphorescent Fluorescent Fluoresce

nm Source ( 1

j

Source (2) Source ( 3

)

Source (

450

55

60 II

65

70 0.0152

475

0 r\ 80 0.0021 .0396

85 .0701

90 .0087 .085^

95 A r> r\ .0030

500 .0371

—

05

10 .1064

15 —

20 .1911 0.0028 —

525 —

30 .2095 .0072

35

40 .1661 .0137 —

45

550 .1123 .0241 "TM "~ 55

60 .0701 .0359 .0061

65 .0366

70 .0426 n li 0 0 .213^

c cr 5 0 . 3OI8 0 . 0071

oO . 02 4 4 .0556 . 1006 , 0l4l

. UcflJ . 0177

90 .0169 .0828 .0091 .0318

95 .0061 .0318

600 .0127 .0965 .0061 .0282

05 .0061 .0141

10 .1026 .0030 . 3357

15 .1131

20 .1000 .0283

-31-
625 .0530

30 — .02Q5 .0^95

5^ n s> li 7

. u u ± 5

n n 7 T n 07 n

650 .0665 .OOlR .0106

55 .0013 .0071

DU « V J M ^ • w U J_ u n n 7 n

. u w U n 07

1

• W H U J. • w u u X

675 __ 80 — .0307 —

0!:) n n "3 r: Q n y u n ? ? n n 7 T

700 .0169 .0071

05 — .0283

-LU 1 n nil? • DPI U c X cP

. UU J5

nnvo

70 c: 30 .0031

35

40

45

Sources

(1) "Development of phosphormeter standards, measurement

techniques, and specification of solar cell characteristics"

NBS Report 10 596, Table 2a

(2) Same Table 2b

(3) Manufacturer's data sheet (plot, transcribed) as reproduced in

"Development and testinp; of ceramic wafers used in secondary

standards for the improved Model 4 Phosphormeter, Addendum I

to Work Group II", Contract RE 7-70, 22 February 1971,

Attachment VII

(4) Same Attachment VIII

-32-
^ . 4 Rise and decay times

The nature of phosphorescent materials is to

display a storage characteristic, in that when an

irradiating ultraviolet source is cut off, the visible or

other luminescent emission does not cease immediately. The

materials used in phosphor standards show this

characteristic^ requiring from 10 to 100 milliseconds for

the afterglow to decay to one per cent of the initial

emission.

The decay characteristic as a function of time

after source extinction has been measured for typical green

and red phosphor standards, and is shown in Figure l8. The

values were measured by varying the elapsed time between

the end of the exciting ultraviolet radiation and the

sampling time, using the Sample Delay provision in the

instrument. The response shown in Figure l8 is plotted on

a semi-logarithmic graph, wherein an exponential decay

characteristic would be represented by a straight line. If

a simple straight-line representation were possible, the

sloping function would be completely described by the time

for the luminescent emission to die away to 1/e or 37% of

its value at zero time. Using the customary technique of

specifying the time to this 1/e value, the green phosphor

would be considered as having a time constant of 3.2

milliseconds. For the red phosphor the time constant is 22

milliseconds

.

It is evident from Figure l8 that such singlevalue representations of the decay characteristics of green

and red phosphors produce only a very rough approximation

to the measured function. A much better match can be found

if it is assumed that there are as many as three

components. Solving by best fit for the three time

constants and their relative proportions yields a composite

curve which matches very closely the measured decay

functions. Table 6 below lists these components.

-33-
Table 6

Components of Phosphorescent Decay Characteristics

Time Constant Time Constant Time Constant

No. 1 no. 2 No. 3

Relative Relative Relative

Standard Tp Amplitude Tp Amplitude Tp Amplitude

Green phos. 1.^ ms' 0.^0 3 • 3 ms 0.32 9 . 3 ms 0.28

Red phos. 3.0 ms 0.l6 17 ms 0.^1^ HS ms 0.^0

The sum of these components matches the observed data to

better than approximately one percent of the Initial value.

The significance of the multiple tine constants

is that any measured properties of the phosphorescent

materials will depend rather strongly on how they are

measured. Both the apparent phosphorescent activity and

the spectral distribution can be affected. For an

understanding of this, it is necessary to examine the

^ build-up as well as the decay characteristics.

It is to be expected that phosphorescent

• materials acquire energy during excitation with the same

general characteristic at which energy is emitted after

cessation of excitation. For example a phosphor v/ith a

long time constant requires prolonged excitation in order

to develop maximum phosphorescent activity. The data on

relative amplitudes of the various components in Table 6

above apply to the duty cycle in the NBS calibrator, which

is 10 milliseconds on, 90 milliseconds off. If the lamp-on

time were increased, the relative amplitude v/ould increase

for the component having the longest time constant. Thus

the shape of the composite decay characteristic would be

altered, leading to a change in the percentage decay during

the sample delay period. This of course would result in a different scaling factor of apparent phosphorescent

activity, even though the sample time remained unchanged.

Furthermore there is at least a possibility that

the components responsible for the several time constants

have different spectral emittance characteristics. Their

sum, which is the overall spectral distribution, might

therefore be altered by a change in excitation conditions.

Somewhat the same effect could be introduced in the

selection of the time delay between cessation of excitation

-3^4-
and the measurement sampling time. When this delay is made

long enough to allow full decay of the shorter time

constant, any spectral contributions from these components

would thereby be eliminated.

It is known that the excitation and measurement

duty cycles are different in the three systems of interest,

which are the _facer-canceler , the phosphormeter and the NBS

calibrator. Excitation conditions in the first two are

similar (7.5 rns on, 2^.5 ms off) except for the rising

buildup as the stamp approaches the irradiating uv lamp in

the facer-canceler . This buildup function is not

duplicated in either of the other systems. Measurement

conditions however (0.5 ms delay) are similar in the facercanceler and the UBS instrument. Here the phosphormeter

differs from the others, since the sample period begins

immediately after lamp extinction.

Thus all the systems Involved have differences in

excitation and measurement conditions, and in viev; of the

observed composite of several time constants, can be

expected to show differences in response. These

differences will chiefly be evident in scaling factors,

where small variations in sample delay are likely to have

more effect in the phosphormeter, since response is large

to the short time constant section of the decay

characteristic

.

^ . 5 Intrinsic phosphorescence

The ceramic base material in the present design

of phosphor standards displays a strong phosphorescence of

its own. This can seriously interfere v/ith the calibration

process unless measures are taken to either 1) standardize

and calibrate this intrinsic phosphorescence, or 2)

eliminate the effect and calibrate only the phosphorescence

resulting from the added luminescent material. The latter

course is the one employed in the NRS calibrator. Hov/ever

a certain amount of information has been collected about

this essentially-unwanted phenomenon of intrinsic

phosphorescence, even though a detailed investigation was

considered beyond the scope of the project. Such data as

we have learned are presented here for completeness.

Zero-phosphor standards were made in the normal

process but without the inclusion of luminescent material.

Measurements on these specimens on the phosphormeter and

the NBS calibrator were then made to establish the general

degree of response to intrinsic phosphorescence. In the

-35-
phosphormeter , the response varied from low in the green

range (1 PMU) to moderate in the red phosphorescent mode (8

PMU) , There was also a variability with regard to specific

phosphorneters and zero-phosphor samples. In the NBS

calibrator there was a small response amounting to

approximately 0.2 PMU.

Response to intrinsic phosphorescence in the

facer-canceler was not measured, since there would be no

normal occasion for the device to look at phosphor

standards. The inference from other conditions, such as

the timing of the sample period (0.5 ms ) , suggest that the

facer-canceler response would be somewhat less than that of

the phosphormeter.

In order to determine at least qualitatively the

reasons for the observed response in the phosphormeter to

intrinsic phosphorescence, the following information has

been developed.

^ The spectral distribution of the emitted

intrinsic phosphorescence appears to lie in the

region of 700 to 800 nanometres. Supporting

evidence for this conclusion comes from

experiments with long-wave pass/exclusion

filters inserted into the NBS calibrator, and

from off-line experiments with a monochromator

,

the latter not fully documented. V7hen the

infrared region above 650 nanometres is

excluded by optical filters, the observed

response to intrinsic phosphorescence is

greatly reduced. A report from the original

manufacturer of the phosphormeter and standards

indicated that the infrared emission may equal

or exceed that in the visible region.

Presumably the heat-absorbing filter

incorporated in phosphormeters and facer

cancelers is to supress the response in this

near Infrared region as much as possible.

The time constant of intrinsic phosphorescence

is in the order of 1.0 millisecond. This value

was derived by emphasizing the response to the

near infrared in the MRS instrument through the

use of filters. Test specimens have included

those from zero to high values of phosphor

content.

-36-
The variability of Intrinsic phosphorescence is

appreciable, judglnp by readings ranging fron

7.6 to 8,7 Pr^J on a series of eight zerophosphor standards, as measured on one

phosphormeter . This variability on zerophosphor specimens suggests that there can also

be variability in the contributions of the

ceramic base in the normal phosphor standards.

It is possible that further v/ork could develop

a method for direct measurement of intrinsic

phosphorescence on the normal standards.

Correlation so far has been poor betv/een

phosphormeter readings and attempts to Isolate

intrinsic phosphorescence in the NBS

instrument

.

It should be apparent from the foregoing

discussion that intrinsic phosphorescence in the ceramic is

a serious problem which interferes with accurate evaluation

of phosphor standards. In our view, response to this

factor should be eliminated, either through improved

selections of materials for the standards, or through

minimizing phosphormeter response. For the latter there

are the dual possibilities of excluding the near Infrared

spectral region or lengthening the sample delay.

In the UBS calibrator the equivalent magnitude of

response to intrinsic phosphorescence is approximately 0.2

PMU. This amount, although small, must be removed. Two

methods were tried, 1) subtraction of the measured response

to zero phosphor standards and 2) increased sample delay to

allov/ complete decay. The results were equivalent, with

operational convenience favoring the adoption of the

subtractlve technique. The criterion for agreement was

equal ratios between low, medium and high-level standards.

In the absence of techniques for measuring the

Intrinsic phosphorescence on assembled phosphor standards,

the approximations of one (green) and 8 PMU (red) have

been adopted. The application of these corrections is

covered later in Section 5 - Calibration of standards.

5 . 0 Calibration of standards

In this section the process is described by which the

present level of luminescent activity as it exists in the Bureau

of Engraving and Printing and in the Postal Service, was related

to output indications from the NBS Instrument. Specifically the

goal v/as to determine the multiplying constants so as to convert

-37-
to current PMU values from the efficacy figures derived from the

NBS Instrument. Efficacy Is the name given to the Instrument's

output as described In the companion report "Techanical manual

for phosphor standards calibrator". Efficacy Is an Indicator of

luminescent efficiency which is in many ways analogous to

reflectance but which takes into account the wavelength shift due

to luminescence. It is equivalent to the ratio of emitted

visible luminescent power divided by the ultraviolet power which

would be reflected from a perfectly diffusing surface under the

same conditions of uv irradiation. As it is accomplished in the

NBS instrument, efficacy is substantially Independent of detailed

characteristics in the system.

5 . 1 Procedure, phosphorescent standards

In order to establish the conversion coefficient

it was first necessary to acquire a standard, or preferably

a group of standards, of known value (s). This was done by

collecting a number of the new phosphor standards (3 red

and 3 green) plus two prior reference standards, all in the

vicinity of 22 PMJ. These were measured on 5

phosphormeters in the Washington area, vrith normal

calibration based upon the reference standards for the

- Individual phosphormeters. An intercomparison of plotted

measured values yielded the group average, which could in

turn be represented by any member standard provided a

correction was applied to compensate for that standard's

departure from the group average. On this basis the two

reference standards for the ^a8 phosphormeter were

selected, and assigned the values of 23.16 and 23.76 PMU

for the standards 25^ (green) and ^^5^ (red) respectively.

The cited PMJ values Include the effects of

surface reflectance and intrinsic phosphorescence to which

the phosphormeter is known to respond. V/hen the correction

for this undeslred response was applied, so as to indicate

the visible phosphorescence (NRS-PriU) only, the two final

values became 22.16 and 15.76.

The efficacy of these two conversion standards

was then measured in the NBS Instrument approximately 15

times over a period of two weeks, establishing mean

efficacy figures of 0.002096 and O.OOI616. The probable

uncertainties in these measurements (standard deviation)

were 0.5 and 0.8 per cent for green and red respectively.

The conversion of efficacy to NBS-PMU was

established on the basis of the above data. To obtain NBS-

-38-
PMU for green phosphorescent standards, for example, it is

necessary to multiply the measured efficacy by IO583,

Similarly the conversion constant for red standards is

9771. With the completion of the one-time conversion

process, it was then possible to evaluate in terms of NBSPMU any phosphorescent standard whose efficacy was known.

Each of the newly fabricated phosphor standards

was measured for efficacy at least three times over a twoweek interval. The resultant efficacy figures were

converted to NBS-PMU values, using the two conversion

coefficients for green and red standards as mentioned

above. Mean NBS-PMU values were found for each standard,

rounded off to the nearest 0.1 unit, and assigned to the

individual standards. These are the calibrated levels of

phosphorescent activity for the new standards.

As described in detail in the Technical Manual,

the NBS-PMU values must be corrected by the addition of a constant to take into account the response in the

phosphormeter to surface reflectance and intrinsic

phosphorescence. Thus the working USPS ratings of the nev; green phorescent standards has 1.0 PMU added to the NBSderived level, in order to determine the meter indication

to which the phosphormeter must be set. Red standards

require correction by 8.0 PMU for the same purpose. These

final working values were then marked on the labels

attached to the standards, using the labelling technique

mentioned in Section 2.3 - Mechanical modifications.

5 . 2 Procedure, fluorescent standards

Fluorescent standards are instrumented in the NBS

calibrator so as to provide measurement of the emitted

luminescent output (efficacy) in two spectral ranges,

namely the red and yellow regions of the spectrum. The

same general arrangement is employed in the phosphormeter.

he two responses are combined by separately measuring red

and yellow channel efficacies, converting these to PMU with

appropriate constants, then subtracting the yellow-channel

PMU from the red. The purpose of the overall conversion

process was to establish the two scaling factors, for the

red and yellow channels respectively, so that efficacy

readings from the NBS instrument could be converted to PMU

values for the phosphormeter. Since it v;as necessary to

determine two conversion constants, data from tv;o

measurement conditions were required. The most convenient

approach v;as to take two standards, one high and one low.

-39-
and force agreement with two simultaneous equations of the

form

:

PMU value = (red channel efficacy) -K^ (yellow channel)

Data from two standards consisting of known or designated

PMU, together with instrument response, then allowed solving

for K-^ and Ko.

The first step was to determine the PMU values to

be assigned to the tv;o standards used for the conversion.

For this evaluation one group of six Red Fluorescent and

another of six Background Fluorescent standards were

assembled. The chosen standards were obtained principally

from those at the Bureau of Engraving and Printing

(Standards nos. 861, 862, 867, 868, 870, 66l, 662, 667,

668, 670) plus the calibration for Phosphormeter 4a8 (no,

858 and 658). This group was assumed to be representative

of fluorescent standards in operating use, and hence

suitable for establishing the conversion scale factor. The

marked values on each standard were taken as their

reference values, in contrast to a round-robin program of

measurement on phosphormeters used for the phosphorescent

standards. Then the 12 standards were measured for red

channel and yellow channel efficacy on the MBS instrument.

From comparison of the measured red channel

efficacy with respect to marked PMU values, it was

determined that standard no. 858 was representative of the

group average within a small fraction of one PMU. Its

marked value and indicated response were then entered into

the equation above. The same procedure was followed for

Background standard no. 658, yielding a second set of

values. Solution of the equations produced the two

conversion constants of K]_ = 8506 and K2 = 930. These are

the proportioning constants listed in the Technical Manual

for the instrument, under Section 2.2c - Conversion to PMU

values

,

It should be mentioned that the derived constants

are critically dependent upon the spectral characteristics

of the red and yellow channel isolating filters,

particularly as regards cross-channel pickup. It may be

necessary to re-examine the derivation of the conversion

constants in the event that the filters are changed.

Thus the steps described so far were to select a

group of standards whose overall activity level was chosen

for perpetuation, to select two representative samples from

-Ho-
this group and to use them for the determination of

conversion constants. The remaining step as a check was to

convert all the other standards in the group on the basis

of these constants, comparing the resultant and m.arked

values. Agreement was found to be within approximately 0.5

PMU in all cases, with a standard deviation of 0.3 PMU.

The conversion constants of 806^i (red) and 822

(yellow) were applied to the figures of efficacy as

provided by the MBS calibrator instrument v;hen measuring

the new standards. The resultant assigned values are given

in the following section.

The method of derivation of efficacy is covered

in detail in the Technical Manual under Sections 2.1b

Definition of efficacy, and 2.2 - Theory of operation,

detailed

.

5 . 3 Assigned measured values of phosphor standards

5 . 3a Phosphorescent standards

Application of the conversion constants

derived as in Section 5.1 resulted in assignment of

PMU values to the new standards as listed below. For

reference, readings on the Model ^IAB phosphormeter are

also given.

Table 7

Assigned values of nev; phosphorescent standards

Value assigned by Reading on 4a8

Standard NBS Calibrator Phosphormeter

Green phosphorescent

G20 23.^ 2h,0

21 23.3 23.8

22 23.1 23.2

23 21.8 23.0

2^1 23.^ 23.^

25 23.2 23.2

26 23.8 23.8

27 23.7 23.3

28 23.6 23.5
G40 48.8 47.9

41 49.0 47.6

42 48.5 46.1

43 48.7 48.0

44 46.0 44.7

45 46.3 46.0

46 45.4 44.2

47 45 8 45 6

48 46.6 46.3

49 43.0 41.9

G80 85.6 88.4

81 90.3 93.9

82 89.2 90.1

83 94.6 97.0

84 89.7 89.5

85 89.0 87.9

86 88.8 86.0

87 87.1 84.7

88 90.2 89.1

89 87.0 87.1

Red phosphorescent

R20 22.0 21.7

21 22.6 23.3

22 23.2 23.4

23 23.8 22.9

24 24.5 24.2

25 25.4 23.6

26 24.3 22.7

27 25.5 23.3

28 24.8 23.9

R40 51.4 50+

41 46.1 46.4

42 49.7 48.1

43 42.3 42.6

44 42.2 40.

q

45 44.0 41.9

-42-
^ D

)i 1 47 He:,'-)

r80 89.8 89.5

ol 90,1

9 / . " 9^ . 3

83 100.9 98.6

0 4 y 1 . n RR00,0R 85 92.1 88.3

86 90.8 87.9

87 90. 3 85 .9

5.3b Fluorescent standards

Application of the conversion constants

mentioned in Section 5.2 to the new standards

fabricated from commercially-procured wafers resulted

in the PMU values listed below. For reference,

readings on the 4a8 phosphormeter are also given.

Table 8

Assigned values of new fluorescent standards

Value assigned by Reading on 4a8

Standard NBS Calibrator Phosphormeter

Background fluorescent

Bl 2.4 2.5

B2 2.4 2.5

B3 2.5 2.5

Red fluorescent

F21 22.2 23.2

F22 21.1 22.5

F23 21.6 23.0

Reference standards

658 1.0 1.2

858 22.5 22.5 (set)

-43-
6.0 Conclusions and recommendations

This section contains comments and recommendations

which have resulted durlnp; the progress of the work. Minor

suggestions for improvements in the fabrication procedure were

made earlier in Section 2,^ - Notes on the fabrication procedure.

The emphasis In this last section of the report Is on

inconsistencies in measurements found between the three major

elements of the overall system: the facer canceler, the

phosphormeter and the NBS calibrator. The desirability of

bringing all of these closer to similarity on critical

performance Items Is obvious. Lastly the need is pointed out for

Improvements in fluorescnet standards, in both fabrication and

calibration.

Our study has shown at least four areas of concern

relating to inter-instrument measurements on luminescent

materials. These are, in decreasing order of significance:

1) Displacement of the measurement plane (setback)

As presently conducted with the phosphormeter

the active surface is much closer to the uv

lamp for stamps than it is for phosphor

standards. This results in a H0% Increase In

apparent luminescent activity for stamps and

test cards.

2) Inclusion of intrinsic phosphorescence in

response to red standards.

This factor accounts for about 3^% of the red

phosphorescent Indication for 22 PMU standards.

It also introduces complications when measuring

high-level phosphor standards.

3) Response in the phosphormeter to surface

reflectance

.

Because of the visible components of emission

from the uv lamp in the phosphormeter, plus a

possible hang-on characteristic in recovery

from signal overload, there appears to be a 5

to 10 percent response to reflective materials

such as barium sulfate. As in (2) above, this

complicates extrapolation to high values of

luminescence

.

4) The different duty cycles of excitation and

times prevailing among the Instruments produce
arbitrary scaling factors. However It has not

been provem that this Interferes with scale

extrapolation.

The above are the four problem areas where we believe

that remedial action should be taken. Specific recommendations

on each item are given in the sections v;hich follow.

The effect of displacement of the plane of measurement

in the phosphormeter means that use of the device will over-rate

stamps and test cards by approximately ^0 percent, as compared to

measurement on a displacement-independent system. The exact

degree of over-rating will depend upon local conditions, even

such apparently trivial ones as rotation of the uv lamp, which is

the prescribed approach for Initial mid-range adjustment. We

believe that the proper solution is to make all measurements with

stamps in the same plane as the wafer faces in the phosphor

standards. Then there will be a good chance of agreement between

instruments on comparative evaluations of stamps, test cards and

phosphor standards. This agreement cannot now exist.

One possible implementation would be to add a spacer

mask to the phosphormeter when measuring stamps, which is the

technique used in the NBS calibrator. Such a mask for stamps

should be 0,10 inch (2.5 mm) in thickness to match the average

setback in phosphor standards. In the event that other

configurations of luminescent materials are to be calibrated,

compensating changes should be made in the mask thickness.

The question naturally arises as to whether PMTJ

evaluations should be based upon present levels of phosphor

standards or of stamps, with the distinction possibly accounting

for as much as a ^0 percent change in overall scale. We

recommend retention of the present scale of conversion, which is

based upon phosphor standards, as using the most stable,

reproducible elements in the sustem for conversion determination.

Adoption of this approach is also consistent with the

recommendation of a single plane for measurement.

Response to Intrinsic phosphorescence should be

eliminated if at all possible, since it is the major source of

uncertainty on red phosphorescent standards. It should be

possible to reduce the effect considerably by employing a low

pass interference filter to suppress the near infrared above

approximately 650 nanometres. Some benefit should also be

obtained by delaying the time of signal sampling to 0.5

millisecond, in view of the 1-ms time constant for Intrinsic

phosphores cence

.
It should be possible to reduce the response to

surface reflectance by either 1) delaying the time of signal

sampllnp:, or 2) adding a filter to pass only the 25^1-nm line from

the pulsed uv lamp.

Because it appears that several of the items have in

common certain potential benefits from delaying the time of

signal sampling, we are recommending that signal sampling be

changed to approximately 0.5 ms , matching the delay in both the

facer canceler and the NBS instrument.

In regard to the desirability of improvements in

fluorescent standards, the problems chiefly originate in spectral

mismatches. The present two-channel approach used in the facer

canceler and the phosphormeter and hence in the MBS calibrator,

greatly complicates the task of standardization. The spectral

characteristics of the required red and yellow isolating filters

have critical influence in the magnitude of output indications

from all of the devices. Operationally the problem is most

noticeable in the use of the phosphormeter, which must deal with

both fluorescent standards and indicia-imprinted envelopes or

test cards, where the fluorescent inks may exhibit spectral

characteristics differing from those of the standards. Under

these conditions it is not assured that standards which have been

fabricated and calibrated by the present process are adequate for

control of the activity levels of fluorescent inks.

-46-
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1 RED PHOSPHORESCENT STANDARDS

X = NBS calibrator

0 = Phosphormeter

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20 '40 60 80

FIGURE 14 Effect of Temperature on Phosphor Standards

100 120 140

TEMPERATURE °F
ABRASIVE GRIT SIZE IN

FIGURE 15 EFFECT OP SURFACE FINISH UPON PHOSPHORESCENT ACTIVITY, GREEN
ABRASIVE GRIT SIZE IN FIGURE 16 EFFECT OF SURFACE FINISH UPON PHOSPHORESCENT ACTIVITY, RED
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ORIENTATION

FIGURE 17 RELATION BETWEEN ORIENTATION OF SURFACE GRINDING AND MEASURED

PHOSPHORESCENT ACTIVITY
FIGURE 18. DECAY CHAR. OF GREEIJ & RED PHOSPHOR STANDARDS IN

NBS CALIBRATOR
NBS-n4A IRE V. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. inihl.K A HON OK KIPOR I NO.

NBSIR 74-553

2. (lov't Ar< »-s.sion No.

3. Kct ipiint 's Acic'sMon No.

4. Ti ru: AND sunrm.i; 5. Publication Date- October 1974

Preparation and Calibration of Phosphor Standards

6. Performing Organization ( (idr

7. AU rilOR(S)

M. L. Greenough, H. K. Hammond III

8. Performing Orpan. Report No. NBSIR 74^553

9. pi:ri ormin(; organization namk ano addrk.ss

national bureau of standards

department of commerce

washington, d.c. 20234

10. Projecr/TaKk/Work Unit No.

11. f.onlract/Cirani No.

12. Sponsoring Organization Name and C^omplcte Address (Slreet, City, State, ZIP)

U.S. Postal Service

Planning and New Development Dept.

11711 Parklawn Drive

Rockville, Md. 20852

13. Type of Report & Perioti (-ovcrcd

Final report 7/72-9/74

14. Sponsoring Agency Code

15. SUPPI.EMKNTARY NOTtS

A draft of this report has been reviewed and approved by the ."sponsor

16. ABSTRA(;T (A 200-word or less factual summary of most significant irt formation, if document includes a significant

bibliography or literature survey, mention it here.)

This project involved two activities, 1) the fabrication and calibration of phosphor

standards for use in the Postal Service Model 4A8 Phosphormeter and 2) the construction

of an instrument to perform the calibration function. Both of these relate to the

standardization of the phosphorescent and fluorescent activity of the luminescent coatings

applied to postage stamps by the Bureau of Engraving and Printing. The purpose of the

luminescent coatings is to facilitate detecting the orientation of envelopes in facercanceler machines during mail processing.

Work on the project entailed the fabrication of approximately 60 phosphor standards,

which are hand-sized aluminum blocks into which stamp-sized wafers of luminescent

materials are mounted. Fabrication was carried out following the specific procedures

supplied by the Postal Service, with however, authority to verify or alter the process an necessary. On the other major project effort, an instrument was designed and constructe<

following in general the basic design of an earlier breadboard device developed under a prior project. Quantitatively in the system, evaluations are ultimately referred to

calib rations at NBS of the relative irradiance of a lamp in the ultraviolet and visible

regions of the spectrum.

This report, one of two covering the project activities, describes the preparation of

luminescent wafers, their assembly into phosphor standards and the test procedures on coi»-

jleted standards. It includes a tabulation of the calibrated values of the new standards.

17. KEY 1'ORDS (six to twelve entries; alphabvtical order, capitalize only tho fir.^t letter of the first key word unletis a proper name; separated by semicolons

)

Fluorescence measurement; Instrumentation, luminescence measuring;

Luminescence measurement; Phosphorescence measurement

18. AVAILAHILITY [X Unlimited

( For Official Distribution. Do Not Release to N TIS

Order I' rom Sup. of Ooc, U.S. Government Printing Office Mashmgton, . 20402, SD ("at. No. (13

Order I rom National Technical information Service (NTI.S) Springfield, Virginia 221'jl

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(THIS REPORT)

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21. NO. OF PAGES

72

22. Price

USCOMM-DC 2*C4?-P74