Heat Rejuvenation of SW Filters 6-27-16 · A Study Concerning the Heat Rejuvenation of Short Wave...

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White spots on a filter. Note that it is very hard to photograph a black filter since almost all visible light is absorbed by the filter. However, if

you photograph the surface reflection of a uniformly illuminated wall, then you can see the white spots actually showing

up as black and the filter shows up as white. Figure A

Figure A

A Study Concerning the Heat Rejuvenation of Short Wave Ultraviolet Filters By Don Newsome #14, Renton, WA and Jim Forsyth #1988, College Place, WA

ABSTRACT The users of short wave ultraviolet lights have observed that over an extended period of use, there is a general

decrease in the amount of ultraviolet output through the filter that is used to block the visible light from the UV

lamp. This is called solarization and results in a much lower useful UV light output and shortened effective life

of the UV light. This study describes the attempts to restore the UV output of the light through rejuvenation of

the filter by a heating process.

Explanation of Terms Before we start we need to define some terms.

Lamp = We use the engineering term for what some may call a “bulb” or “tube”. It is the part that

generates the UV wavelengths. A lamp is NOT a light assembly.

Light = A light assembly, usually a UV light assembly. Some people call this a “lamp”, but that is

technically incorrect.

SW = Short wave ultraviolet, specifically 253.7 nm, in the UV-C range.

254 nm = Actually 253.7 nm, just rounded up for easy reading.

UV filter = This paper always refers to a SW ultraviolet-transmitting visible-absorbing filter, also called SW

filter. Long Wave filters do not solarize so they are not discussed in this paper.

Introduction UV filters that are used in all SW UV lights will solarize with use

1, 2. Solarization is a chemical process that

occurs within the filter when exposed to SW UV light. The exact process that causes solarization is beyond the

scope of this paper, some of the references in1 go into more details about the process. The stronger the SW UV,

the more the solarization and the longer the exposure time the more the solarization. Solarization takes place

more rapidly during the first few minutes of exposure, and the rate of solarization decreases over prolonged UV

exposure time. Solarization reduces the amount of UV that is transmitted through the filter; however, the small

amount of visible light that is transmitted does not change as a filter solarizes. The visible light transmitted

through a used filter and the visible light through a new filter appear the same. Therefore we cannot use our

eyes to detect a solarized filter. Nevertheless, if we have two identical SW UV lights one with a heavily

solarized filter and one with a minimally solarized filter and we fluoresce the same minerals our eyes can detect

subjectively the difference in fluorescent brightness between those two UV lights. However, to determine

objectively the amount of solarization of a filter one must measure its SW transmission. We did that using a

stable 254 nm UV source and an accurate linear SW UV radiometer for all of these tests.

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There is another condition that effects the transmission of SW filters, it is a white film or coating that can

appear on the surface of a filter. See Figure A. The SW filter is not a typical glass but a phosphate glass, and as

such it can react with water and humidity3. (See the Appendix A). Over time when water and/or humidity react

with SW filters, they can form a very thin –almost colorless –film over part of the surface of the filter.

However, it is more common to see white dots or a white coating where a compound has formed from reaction

with the chemicals in the filter and the moisture in the air. In almost all cases when those white dots have been

cleaned off you can see tiny concave pits in the surface of the filter where those dots were, indicating that

indeed they used some of the material in the filter to form the white dots.

The majority of the white dots and or white coating can be cleaned off by heavy scrubbing the surfaces of the

filters with a household cleaner such as Comet® or Bon Ami®. Don gets the best results using an S.O.S.®

scrubbing pad with water. Neither the cleaning materials nor the scrubbing pad will scratch the filters. Very

important: you must scrub hard, so the filter(s) must be removed from the SW light and placed on a firm flat

surface so that you can press hard. These precautions are to avoid breaking the filter or cutting your hand.

Background Corning Glass Works (now Corning) developed the first SW filter (their #9863) in the mid 1930’s. For years

the Corning literature stated “The original transmittance characteristics may be restored essentially by heat

treatment at a temperature which will not deform the glass3.” Also Jack DeMent stated in his book, “Handbook

of Fluorescent Gems & Minerals4”, careful heating to about 200º C will restore the filter. H.C. Dake in his

booklet “The Uranium and Fluorescent Minerals, Third edition5”, says the same thing “When such a glass has

its efficiency reduced by constant irradiation with intense short wave length ultraviolet light its original

condition can be restored by carefully heating to about 200º C.” Sterling Gleason in his book “Ultraviolet

Guide to Minerals6” says “Some filters can be rejuvenated by proper heating and annealing but the process

requires heating to just under the melting point (550ºF) and maintaining that temperature for several hours,

then gradually cooling so as not to set up strains in the ‘glass’.” 200º C is 392ºF (you will see later that that

temperature is much too low to have any significant SW rejuvenation effects). It is Don’s guess that one person

referenced the 200º C and then others just copied that same temperature without testing to see if that was

correct. Maybe the same thing happened for the 550°F reference, since no SW filter that we tested showed any

melting or deformity at that temperature. For those effects, the temperature has to be 950°F or higher.

It should be pointed out that when those books were written the only practical SW filter available was the

Corning Glass Works #9863 from Corning, NY. Sometime in the 1960’s Schott Glass Technologies, Inc. in

Mainz, Germany, started manufacturing their UG 5 SW filter. Then in 1978, Hoya Corporation USA, Optics

Division in Japan developed their U-325C filter. Sometime in the early 1980’s Corning Glass Works stopped

making color glass filters and they sold their process for the #9863 SW filters to Kopp Glass Co. in Swissvale,

PA. In the last 4 or 5 years Shijiazhuang Zeyuan Optics Material Co. in China has been making some SW

filters, but these have not been tested by us.

Because of the superior resistance to solarization1, 2

, the majority of UV light assemblies manufactured and sold

in the USA the last couple of decades have been using the Hoya U-325C filter7. Consequently the majority of

the SW filters that were heat treated in these tests were the Hoya U-325C filters.

Over the years several FMS members (including Don) tried to heat up their old solarized SW filters in an

attempt to rejuvenate them. Several of those attempts were made in home ovens, which are usually limited to a

maximum of about 550ºF or about 930ºF in the self-cleaning mode. However, none of those attempts measured

the actual SW transmission before and after heating so those did not provide that data. Without measuring the

transmission both before and after heating you can only guess as to the effect of any possible rejuvenation

method.

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International Light Technology

ILT1700 UV Radiometer

Jim Forsyth beside the

Barber-Colman oven with the

controller on top

Test Equipment The test equipment consists

of the following items:

1. UV Radiometer International Light

Technology ILT1700

radiometer. Serial number

ILT17005548.

2. UV Detector International Light

Technology UV-C detector

SED240 # SED2406561, with

NS254 #29126 filter, and

W#13356 top diffuser.

3. SW Light Source SuperBright 2000SW UV light with LS-16X SW lamp, special black felt reflector,

metal stand, but without cover or filter.

4. Oven Wheelco Instruments Division of Barber-Colman Company model 293 oven. [Used with

permission by the Materials Laboratory of the School of Engineering at Walla Walla University, College

Place, Washington].

5. Computer Desk top HP Pavilion Elite HPE computer using Microsoft Word and Microsoft Excel.

Some observations about the white film or coating on old filters. The white coating or white dots that appear on used filters after they have been exposed to humidity obviously

effects the transmission of the filter. The thicker the film or the more dots on the filters the more the reduction

in transmission. We made an effort to try to categorize the percent transmission improvement possible when

the white film coating or white dots are cleaned off. We did this by measuring (before heating) the transmission

with the coating and then after the coating (or dots) were cleaned off. Obviously there is no constant percentage

improvement, it would depend on how much coating or dots are on the filter in the first place. Some of the

measurements were as follows in the table below:

Filter ID

Manufacturer

Degree of white

coating on

solarize filter

254 nm

transmission

with coating

254 nm

transmission

with coating

removed

Percent

improvement

E Hoya U-325C Very mild 35.5% 36.5% 1%

F Unknown Severe 33.7% 50.2% 16.5%

7-89 Hoya U-325C Moderate 26.5% 35.5% 9%

G Hoya U-325C Mild 31.3% 37.3% 6%

V4

Raytech

(from “Solarization

Experiment 1985”) [they

used a Hoya filter]

Severe

37.0%

43.7%

18.1%

Because of the variation that a white coating or white dots can cause in any transmission measurements all

subsequence transmission measurements were made only on filters that had been thoroughly cleaned (the white

coating removed). The bottom line for any mineral collector with a UV light: it helps the SW transmission if

the white film or white dots have been cleaned off.

What started these tests? One of the authors (Jim) had purchased two used UV lights on eBay and he sent Don the SW filters (two filters

in each light) so Don could measure the SW transmission. The UV lights were Ultra-Violet Products, Inc. (now

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Detector being Zeroed

Filter being measured

Close up view of

where an ID is

marked on a filter

UVP, Inc.) model UVS-54 SW lights. Three things about those SW filters fascinated Don; 1. In more than 20

years of measuring the transmission of SW filters he had never seen filters that had such a low SW

transmission, 6.8%, 6.6%, 1.8%, and 1.5%. (the previous lowest

transmission he had seen was 2.5% and those filters were almost completely

covered with a white coating). 2. The filters looked almost pristine; there

were no visible film or white dots on the filters, nothing. It is hard to

imagine how those filters could have been so severally solarized which

takes time. Yet, apparently they were not exposed to moisture or humidity

which would have caused a white film or white dots on those filters. But

their surfaces were immaculate. 3. From the physical appearance of the

filters (wavy surface) and knowing how old the UVS-54 UV lights were, it

was obvious that the filters were the Corning Glass Works #9863 filters.

Ultra-Violet Products did not start using their UVG filters (equivalent to the

Hoya U-325C filters) until sometime in 1981.

Because those four filters were so clean and Jim had access to commercial ovens we both thought we could try

a truly scientific heat rejuvenation experiment. Don has access to a laboratory grade UV radiometer to measure

the filter transmissions and Jim has access to laboratory grade temperature controlled ovens to bake the filters.

Procedure We started with just the four filters that Jim

had. Don measured their physical size,

length, width, thickness, and noted their

surface condition (pristine). He then

marked with a diamond scribe a small

unique identification letter in the corner of

each filter (always on the smoothest side of

the filter). This procedure was followed for

each of the filters subsequently tested (note that for some filters the thickness

was measured after the heating sequence).

The SW transmission was always measured before and after any heating of

the filters.

The UV detector was connected to the UV radiometer. They were turned

“on” and allowed to warm up. The SW Light Source was turned “on” for several minutes and warmed up to be

sure that it was stable. The radiometer and detector was “zeroed” by putting a light-tight cap over the detector

and pressing the Zero button, which sets the reference reading at zero. Then

after the zero cap was removed the Set 100% button is pressed, this sets the

radiometer to read 100.0. Then the SW filter was put on top of the detector and

the reading in percent transmission was recorded. This measurement procedure

was repeated several times for each filter. Each time the filter was placed in a

different location on the detector so as to get an average percent transmission

for the whole filter. Before each measurement the radiometer was checked to

make sure that it still reads 100.0. If the UV lamp output has drifted, then the

Set 100% button was pressed to ensure that the correct percent transmission was

being measured when the filter was placed on the detector.

Detector under

SW UV source

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Some measurement details In a typical hand-held UV light that uses a linear lamp the SW arc is only about 3/8 inch wide. That means if

the lamp is in the center of a 3 inch wide filter one would expect that the solarization would be more at the

center line of the filter rather than the edges (because the lamp is closest to the center line). However, the UV

detector has an active area of one inch in diameter, which means it is integrating everything within that one-inch

diameter and cannot measure only the 3/8 inch center of a filter. Therefore to get an average transmission of

any filter several measurements are made on various locations on the filter including the center line and near the

edges of the filter. A minimum of five to a maximum of twelve readings were made for each filter

measurement in order to arrive at an average transmission value.

There is obviously some variation in measuring any filter transmission. Those variations are due to

instrumentation uncertainty; UV output drift of the UV light source, but most likely the biggest variation is due

to the actual solarization of any specific filter. As pointed out above, many SW filters are most likely not

uniformly solarized, but they may be more solarized along a center line. That is why multiple measurements

were made to arrive at an average. However, if we repeat those measurements at a later time and take the same

number of readings on that same filter we most likely will get a different average value (as measured to 3

significate numbers). And that is what happened to several filters that were heated several times. The filters

were always measured when they came back from the oven, and then measured later before they were sent back

to the oven again. From the time they were returned from baking and later when they were sent out again they

were just stored at room temperature and away from any UV light so the UV transmission value should not

have changed. But the measurement average coming back from the oven and the measurement average going

out to the oven were not always in agreement. But the differences were usually only within + or – 1%, which

are well within acceptable tolerances.

Heating the filters After the average percent transmission

is calculated the filters to be heated

are carefully packaged and shipped to

Jim to be heated in the ovens at the

Materials Laboratory of the School of

Engineering at Walla Walla

University, College Place, WA. Jim

places the filters in a metal pan half

filled with clean sand. He then completely covers the filters in sand. The sand acts like a heat sink to reduce

any thermal shock that would crack the filters if they heat up or cool down too quickly.

The pan with the filters and sand is then placed in the oven, turned “on” to the testing temperature, and left “on”

until the specified bake time has been reached. The oven controller keeps the oven at the constant temperature

set on the controller. Once the bake time is over the oven is turned “off” and allowed to cool down slowly.

Usually the oven cools down overnight so the next morning the pan is taken out and the filters removed.

What transmission does a

new unused filter have at

254 nm? If we are going to rejuvenate

filters one of the things we

need to know is what is the

SW transmission of brand new

unused filters? So nine Hoya

Corporation USA, Optics

Division U-325C new FS-20

Sand pan in open oven

Oven

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60.559.6

60.1 60.0

61.062.1

54.4

59.7

61.8

50.0

52.0

54.0

56.0

58.0

60.0

62.0

1 2 3 4 5 6 7 8 9

Per

cen

t tr

an

smis

sio

n a

t 2

54

nm

Individual SW filters

New unused Hoya Optics,

U-325C filtersAverage for all

9 filters is 59.92

Figure 1

6.8 6.8

22.6

41.6

6.6

14.2

24.9

43.2

1.5 1.5

12

45.6

1.8

5.5

10.9

48.2

0

10

20

30

40

50

60

Per

cen

t tr

an

smis

sion

at

254 n

m

Heating Conditions

Solarized Corning 9863 filters

before and after heating (before starts on the left and goes to the right)

Before A & C 400 F for 4 hr. 760 F for 6 hr. 925 F for 5 hr.

B & D 550 F for 5 hr.

A B C D

Figure 2

(2” x 5.5”) and FS-60 (2.65” x 9.25”) filters from the UV SYSTEMS, Inc. stock were measured. These nine

were measured to determine what the average transmission of a typical Hoya U-325C filter should be. (See

Figure 1). Filters 1-3 are FS-20 filters and all were received at the same time, and most likely from the same

batch. Filters 4-5 are also FS-20 but

were received at a different date from

1-3. Filters 7-9 are FS-60 filters and

were received at yet another date, and

are most likely from the same batch.

The apparent discrepancy in the

transmission of #7 could be due to the

thickness. The nominal thickness of

those U-325C filters is 4.5 mm plus or

minus 1 mm. Filter #7 could have been

thicker than the rest resulting in less

transmission, unfortunately the

thickness of those filters were not

measured and those specific filters are

not available now because they were

put back in stock before the results

were graphed. Also note that the Y axis started at 50% (instead of 0) to emphasis the difference between the

filters.

Results In this study we conducted six tests during an approximately 10 month period, each time using different baking

times and oven temperatures. Not all of the individual tests are documented here (that would make this report

too long). However, the majority of the important tests are graphed, and the transmission before and the final

transmission after all of the heating tests is shown for each filter in Table 1 and Table 2.

First Filter Test Series The first four filters tested were from Jim marked A, B, C, and D. It could be that A & B came from one of the

UVS-54 lights and C & D from the other, but we do not know for sure. Figure 2 shows the results of four

different heating times on those filters. First heating at 400ºF has no effect on the filters, 550ºF had a slight

improvement, 760ºF had

an even better

improvement, and

finally 925ºF had the

biggest improvement.

Since the four filters

were all Corning #9683

filters (which are now

made by Kopp Glass)

we wanted to try other

filters, other

temperatures, and

different heating

durations to see if we

could determine the best

rejuvenation procedure.

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36.5

50.2

35.5 37.3

49.4

65.4

54.751.6

0

10

20

30

40

50

60

70

E F 7-89 G

Per

cen

t tr

an

smis

sio

n a

t 2

54

nm

Filter ID

Hoya and Corning filters before and after

heating at 760 F for 6 hours

(before is the left column and after is the right column)

Hoya

(5.01 mm

thick)

Corning

(polished

to only

3.21 mm

thick)

Hoya

(5.21 mm

thick)

Hoya

(4.91 mm

thick)

Figure 3

21.9 19.3 22.2

66.2

37.0

49.2

57.5

37.7

73.7

52.9

0

10

20

30

40

50

60

70

80

K J V2 T6 V4

Per

cen

t tr

an

smis

sio

n a

t 2

54

nm

Filter ID

Before and After Heating at 760 F

for 6 hours (before in the left column and after is the right column)

Un

kn

ow

n

Sch

ott

Ho

ya

po

lish

ed

2.7

mm

th

in

Ra

yte

ch

Ho

ya

Figure 4

Second Filter Test Series We did not get much change in the A, B, C, and D filters so we tried a higher temperature. We heated filters A,

B, C, and D at 760ºF we also included some additional filters that had not been heated before. (See Figures 3 &

4). In the 760ºF heat tests the used unheated filters E, F, 7-89, and G were some old filters UV SYSTEMS had

saved. Filters J, K, were also old filters, while V2, V4, and T6 were from the references 1 and 2, “Solarization

of Short-Wave Filters”, also called “Solarization Experiment 1985” tests and as such were the only filters where

we know the intensity and exposure time to SW that caused their solarization. Those three filters (V2, V4, and

T6) were exposed to an average of 3.6mW/cm2 of 253.7 nm for 98.2 hours in 1987. Since then they had been

stored in a garage in the shipping box that was used for those 1987 tests. As with all these heat rejuvenation

tests any white coating or dots were cleaned off before any transmission measurements were taken.

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17.9 15.6

30.937.737.8

33.5

54.6

35.2

0

20

40

60

M4 N2 P V2Per

cen

t tr

an

smis

sio

n a

t 2

54

nm

Filter ID letter

Before and after heating

at 925 F for 3/4 hr (before is the

left column and after is the right)

Figure 5

18.2 16.49.1

49.2

38.3 38.130.6

48.1

0

20

40

60

M3 N3 Q-B K

Per

cen

t tr

an

smis

sio

n a

t 2

54

nm

Filter ID letter


at 925 F for 1.5 hr. (before is the


Figure 6

Third Filter Test Series In the third series of tests, we tried higher temperatures at different heating durations. (See Figures 5 and 6).

Note that M1, M2, M3, M4, were all from one solarized Hoya U-325C filter that was cut into 4 pieces and N1,

N2, N3, and N4, were all from one Corning 9863 thinner filter that was cut into 4 pieces. From the limited

research we were able to do we thought that the melting temperature of those filters was in the 975ºF range. So

we ran four tests at 925ºF (to make sure we were below the melting point) all at different heating durations, 0.75

hour, 1.5 hours, 3 hours, and 5 hours. And we ran only one test at 975ºF (in case the filters melted at that

temperature), however, no filter melted or deformed significantly [in that test].

We do not know why V2 (a Schott UG 5 filter) has some negative changes instead of positive changes. When

we first heated V2 at 760°F for 6 hours it improved from 22.2% to 37.7%. But when we reheated it at 925ºF for

0.75 hours it decreased from 37.5% to 35.2%, and then when we reheated at again 1000ºF for 5 hours it

decreased again from 34.8% to 28%. V2 was the only confirmed Schott filter in our tests, however we had

some unknown filters that could have been made by Schott, but we have no way to confirm that.

K (a Hoya U-325C filter) also had some negative changes. When we first heated K at 760ºF for 6 hours it

improved from 21.9% to 49.2%. But when we reheated it at 925ºF for 1.5 hours it decreased slightly from

49.2% to 48.1%. But when it was heated at 1000ºF for 5 hours it increased from 48% to 55.8%!! Sometimes

things just do not come out the way you expect.

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20.3

12.2 12.2

52.946.9

42.9

0

20

40

60

M1 R-B RPer

cen

t tr

an

smis

sio

n a

t 2

45

nm

Filter ID letter


at 975 F for 3 hr.(before is the left column and after is the

right)

Figure 7

24.3 22.1 24.2

45.0 47.4 45.2

0

20

40

60

S3 T3 U3Per

cen

t tr

an

smis

sion

at

25

4 n

m

Filter ID letter


at 975 F for 5 hr. (before is the


Figure 8

24.2 24.1 24.3 25.426.6

39.9

49.145.0

37.3

47.8

0

10

20

30

40

50

60

S1 S2 S3 S4 S5

Per

cen

t tr

an

smis

sio

n a

t 2

54

nm

Filter ID

Hoya S1-S5 (before is the left column and after is the right)

10

00°F

for

1.5

hr.

10

00°F

fo

r 5

hr.

97

5°F

for

5 h

r.

97

5°F

fo

r 3

hr.

10

00°F

fo

r 5

hr.

wit

hs

low

co

ol

Figure 9

22.3 21.8 22.1 22.2 22.8

44.0

50.347.4 48.8

43.8

0

10

20

30

40

50

60

T1 T2 T3 T4 T5Per

cen

t tr

an

smis

sio

n a

t 2

56

4 n

m

Filter ID

Hoya T1-T5 (before is the left column and after is the right)

1000°F

for

1.5

hr.

10

00°F

fo

r 5

hr.

97

5°F

for

5 h

r.

97

5°F

fo

r 3

hr.

10

00°F

fo

r 5

hr.

wit

hs

low

co

ol

10

00°F

for

1.5

hr.

Figure 10

Fourth and Fifth heating tests After examining the results of the fourth test, we think we know approximately the optimum temperature and

heating time for the best rejuvenations results. Therefore, we decided to duplicate some of the tests but with

filters of approximately the same amount of solarization (about the same transmissions). So Don took three FS-

60 sized Hoya filters and cut them up into five equal parts S1-S5, T1-T5, and U1-U5. The five S series had an

average transmission before heating of 24.9%. The five T series averaged 22.2%. The five U series averaged

25.0%. The heating temperatures and

durations were 975ºF for 3 hours and

975 ºF for 5 hours. (See Figures 9, 10,

& 11). We also wanted to run some

tests at 1000ºF just to see if there were

some melting or deforming effects at

that high temperature (and there was).

We ran tests at 1000ºF for 1.5 hours,

1000 ºF for 5 hours and 1000 ºF for 5

hours but with a slow cool down time.

(See Figures 9, 10, 11, & 12). The

melting or deforming results are

shown in Table 1 and in the photos in

Appendix B.

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24.1 21.8 25.3

48.0

34.8

49.1 50.3 47.4

55.8

28.0

0

20

40

60

S2 T2 U2 K V2

Per

cen

t tr

an

smis

sio

n a

t 2

54

nm

Filter ID letter


for 1000 F for 5 hr. (before is the left column and after is the right)

Figure 12

27.3 25.3 24.2 24.1 23.9

45.547.4

45.2 44.5

54.3

0

10

20

30

40

50

60

U1 U2 U3 U4 U5

Per

cen

t tr

an

smis

sio

n a

t 2

54

nm

Filter ID

Hoya U1-U5 (before is the left column and after is the right)

10

00°F

fo

r 1

.5 h

r.

10

00°F

fo

r 5

hr.

97

5°F

fo

r 5

hr.

97

5°F

fo

r 3

hr.

10

00

°F

fo

r5

hr.

wit

h s

low

co

ol

Figure 11

33.7 34.1

25.329.0 28.6 30.0 30.0 30.7

54.2

47.9

41.8

47.445.3 47.2

52.4 52.5

0.0

10.0

20.0

30.0

40.0

50.0

60.0

1 2 3 4 5 6 7 8

Per

cen

t tr

an

smis

sion

at

254 n

m

FS-60 filter ID

Hoya U-325C, full size FS-60 filters

all baked for 5 hours each (before is the left column and after is the right)

955°F

950°F

975°F

975°F

955°F

955°F

955°F

950°F

Post Test Results After the tests were

officially over eight

Hoya full sized FS-60

filters were baked.

See Figure 13. Filters

1 & 2 were heated

together, as was 3 &

4, etc. While no filter

melted (as did the S2

filter) the whole filter

#1 sagged (concave)

about two or three mm

in the front with a

three mm rise

of 18

(convex) in the back after baking at 975°F, even though the filter was completely covered in sand. Filter #2

also sagged (concave) about one or two mm but only on the front. Filters #3, #4, #6, and #8 had almost no

deforming. While #7 baked at 950°F had a 50 mm by 60 mm area that sagged (concave) about three mm and an

equal raise (convex) on the back. Filter #5 had only a slight one mm sagging at 955°F. None of the sagging or

deforming would hinder the use of these filters in a SW TripleBright II UV light.

Conclusions What can we say about these rejuvenation tests?

1. In our tests no amount of heating temperature or baking time can rejuvenate a solarized SW filter 100%;

some can be rejuvenated significantly but none to their original percent transmission.

2. After all of the heating some filters increase their transmission significantly and some with slightly less

improvement.(U series vs. S series)

3. 1000ºF is too high a temperature since some filters will deform or melt at that temperature (even though

some will not, you cannot know for sure which ones will melt at that temperature). See some of the photos

in the Appendix. We do not recommend heating a solarized filter to 1000ºF because it might melt, even

though that seemed to have the best rejuvenation results. In fact three filters had some slight deforming at

975°F. T4 showed some slight deforming after 3 hours and both S3 and T3 after 5 hours. However, two

filters did not show any significate deforming at 1000ºF, T5 after 5 hours and U1 after 1.5 hours.

4. There can be many inconsistencies in the heat rejuvenation process. For example some filters show good

results at a specific temperature for a short baking time, and then at a higher temperature and longer baking

time they get only modest results. (T3 vs. T5). Figure 10.

5. Generally speaking, the higher the temperature and the longer the baking time gave the best results.

Although in many cases there is not a significant difference. (U1 vs. U2 vs. U3). Figure 11.

6. Burying the filters in sand to act as a heat sink so the filter does not experience a thermal shock seems to

work very well, we only had two filters crack. Two cracks out of 51 times filters were handled we think

substantiates the use of sand heat sinks.

7. The filters with the least amount of solarization to start with (highest transmission) had the least percent

improvement after heating. That is only logical if you start with a 4.5 mm thick filter that has say 50%

transmission it cannot improve to 60% transmission (the upper limit), [a 20% increase in transmission]

because that is the maximum that a new filter would have. While a filter with 12% transmission might

improve to maybe 52% [a 333% increase in transmission].

8. Sometimes you can look at a filter and tell it is old (and therefore most likely solarized) by the amount of

white coating or white dots on the filter (as shown in Figure A). However, that is not a foolproof method

since the filters A, B, C, and D had no coating or white dots, and yet were heavily solarized.

9. Since a new Hoya U-325C filter has a transmission of about 60% any solarized filter that still has about

49% or higher transmission, is considered a good filter and should have significant useful life remaining in

it.

10. Any solarized filter that has a transmission of 30% or less should be replaced or rejuvenated.

11. SW filters are expensive; nevertheless it is hard to envision a commercial company that would heat

rejuvenate solarized filters for a profit. Besides the expense of a laboratory grade UV radiometer and

laboratory grade high temperature controlled oven, the logistics of keeping tabs on a specific customers

filter when it is put in the oven with other filters, the paper work required, and the man-hours to measure the

filters both before and after heating, the liability if a customer’s filter breaks, makes a commercial

rejuvenating process seem unlikely. However, one never knows what is commercially possible.

12. An oven temperature of between 950ºF and 975ºF with a baking time of at least 5 hours seems to

generally give the best rejuvenation results 13. These heat rejuvenation tests might not be valid for all short wave-transmitting visible-absorbing filters

since we did not have a uniform supply of solarized filters from all manufacturers.

of 18

Table 1.

Thickness of Filters and Transmission Before and After Heating

(The thickness was measured after all the heating tests were completed)

[The thickness of the filters has a direct effect on the transmission of the filter.

The thicker the filter the less the transmission.]

Filter

ID

Letter or

number

Notes Mfg. Condition Average

filter

thickness

in mm.

%

Transmission

at the start

of the tests

Number of

times

heated

%

Transmission

at the end

of all the

heating tests

A From Ultra-

Violet Products,

Inc. UVS-54 Corning

No film or

spots 3.46 6.8

4

41.6

B From Ultra-

Violet Products,

Inc. UVS-54 Corning

No film or

spots 3.55 6.6

4

43.2

C From Ultra-

Violet Products,

Inc. UVS-54 Corning

No film or

spots 3.66 1.5

4

45.6

D From Ultra-

Violet Products,

Inc. UVS-54 Corning

No film or

spots 3.54 1.8

4

48.2

E One end broken Hoya Cleaned 5 36.5 1 49.4

F Polished thin Unknown Cleaned 3.23 50.2 1 65.4

Used

7-89 Hoya Cleaned 5 35.5

1

54.7

G

Hoya Cleaned 4.89 37.3 1 51.6

I Heavily pitted

from white

coating Hoya Cleaned 4.98 50.9

1

62.9

J Unknown No film or

spots 4.08 19.3

1

57.5

K Hoya

4.67 21.9 3 55.7

M1 Hoya Cleaned 4.85 20.2 1 52.9

M2 Hoya Cleaned 4.85 19.5 1 45.2

M3 Hoya Cleaned 4.93 18.2 1 38.3

M4 Hoya Cleaned 4.97 17.9 1 37.8

N1 Filter was

molded thinner Corning Cleaned 3.76 25.9

1

52.9

N2 Filter was


1

49.0

N3 Filter was


1

38.1

N4 Filter was

molded thinner Corning

No film or

spots 3.46 15.3

1

33.5

O Polished thin Unknown Cleaned 3.29 36.7 1 49.1

of 18

P Polished thin Unknown Cleaned 3.29 30.9 1 54.6

Q Filter was


No film or

spots 3.57 9.1

1

26.2

R-B Filter was


No film or

spots 3.19 12.2

1

46.9

R Filter was


No film or

spots 3.04 12.2

1

42.9

T6

From

“Solarization

Experiment

1985”

Hoya No film or

spots 2.68 66.2

1

73.4

S1

Very slight

surface

deforming after

heating to

1000°F for

1.5 hr.

Hoya Cleaned 5.08 24.2

1

39.9

S2 Badly melted

after heated to

1000°F for 5 hr.


1

49.1

S3

Medium surface

deforming on

back after

heating to 975°F for 5 hr.


1

45.0

S4

Almost no

deforming after

heating to 975°F for 3 hr.


1

37.3

S5 Badly melted

after heated to

1000°F for 5 hr.


1

47.8

T1

Very slight

surface

deforming after

heating to 1000°F for

1.5 hr.


1

44.0

T2 Badly melted

after heated to

1000°F for 5 hr.


1

50.3

T3

Very Slight

deformed after

heated to 975°F

for 5 hr.


1

47.4

T4

Slight surface

deformed after

heated to 975°F

for 3 hr.


1

48.8

of 18

T5

Almost no

deforming after

heated at 1000°F

for 5 hr.


1

43.8

U1

Almost no

deforming after

heated at 1000°F for 5 hr.


1

45.5

U2

Very slight

surface

deforming after

heated to 1000°F

for 5 hr.


1

47.4

U3

Almost no

deforming after

heated at 975°F for 5 hr.


1

45.2

U4

No deforming

at all when

heated to 975°F

for 3 hr.


1

44.5

U5

Somewhat

melted after

heated to 1000°F

for 5 hr.


1

54.3

V2

From

“Solarization

Experiment

1985”

Schott No film or

spots 5.12 22.2

3

28.0

V4

From

“Solarization

Experiment

1985”

Hoya No film or

spots 4.53 37.0

2

52.9

The thickness of the filters was measured with a digital Vernier caliper. A minimum of 5 readings were made

and then averaged. Some of the tolerance is because some of the filters do not seem to be the same thickness

from one side to the other --at least when measured to three significate numbers.

REFERENCES

1. Don Newsome, "Solarization of Short-Wave Ultraviolet-Transmitting,

Visible-Absorbing Filters", Properties and Characteristics of Optical Glass,

Proceedings of SPIE-The International Society for Optical Engineering, Volume 970, p.

192-208, (1988).

2. Don Newsome, “Solarization of Short-Wave Filters”, Journal of the Fluorescent Mineral

Society, Volume 16, 1990.

3. Corning Color Filter Glass brochure Printed sales brochure available in the 1980’s.

4. Jack DeMent, “Handbook of Fluorescent Gems and Minerals: An Exposition and Catalog of

the Fluorescent and Phosphorescent Gems and Minerals, Including the Uses of Ultra-violet Light in the

Earth Sciences”, Mineralogist Publishing Company, 1949. 68 pages.

of 18

5. H.C. Dake, “The Uranium and Fluorescent Minerals”, Third Edition, Mineralogist

Publishing Company, 1953, 79 pages

6. Sterling Gleason, “Ultraviolet Guide to Minerals”, Van Nostrand Company, Inc., 1960,

244 pages.

7. Hoya U-325 Filter description http://www.hoyaoptics.com/pdf/U325C.pdf

APPENDIX A

From Hoya Corporation USA, Optics Division

In the various processes of fabricating optical components such as lenses and prisms, surface deterioration is

often encountered and recognized as dimming, staining and latent scratching. These surface defects are caused

by chemical reactions of glass constituents with water in the surrounding environment or with detergents in the

cleaning fluids.

Dimming Polished glass exposed to high humidity and rapid temperature variations may "sweat". Water vapor may

condense to form droplets on the glass surface. Some of the glass components that dissolve in the droplets may

in turn attack the glass surface and react with gaseous elements in the air (CO2, for example). Reaction

products form as white spots or a cloudy film as the glass surface dries. This phenomenon is called "dimming".

Fig. 2

The resistance of a glass to dimming is expressed in terms of "water durability by the powdered method".

Staining When damage is caused to the polished optical glass surface by moisture and acid, the reflected light of an

interference color may be seen on that surface.

This phenomenon is called "staining". Water contact causes chemical reactions (ion exchange between cations

in the glass and hydronium ions in water) that result in a silica-rich surface layer that causes an interference

color on that layer.

Fig. 3

In this catalog, this resistance is expressed as "acid durability by the powdered method" and "staining

resistance by the surface method".

of 18

Corning #9683 filters after baking, note the wavy surface

which is typical. These filters are NOT deformed.

Hoya U-325C filters after baking, this is with their flat

specular (smooth) side up but the photo does not show

that, it shows just the reflections of the wall.

Latent Scratch Fine scratches created on the glass surfaces during polishing will sometimes grow to a large size and become

visible when surfaces are exposed to corrosive ions from inorganic builders found in cleaning detergents. This

type of scratch is customarily called a "latent scratch".

Fig. 4

The inorganic builders such as Na2CO3, NaHCO3 or polymerized phosphate (mostly Na5P3O10) may attack

glass in various ways: Through hydrolysis of the builders in the solution, the builders can form corrosive ions

which attack the glass: hydroxyl ions, (OH-out of Na2CO3, NaHCO3), or polymerized phosphoric ions out of

polymerized phosphate. Corrosion resistance to hydroxyl ions is expressed in terms of "latent scratch

resistivity " and is designated as "latent scratch resistance to polymerized phosphoric ions”.

APPENDIX B

Photos of some of the filters after all of the baking tests

of 18

Addendum

Filters 1 – 8 were solarized whole FS-60 filters from SW TripleBright or SW TripleBright II UV lights. There

were baked after the heat rejuvenation tests were completed. Figure 13 shows the results of that baking.

Table 2.

Thickness of Filters and Transmission Before and After Heating

(The thickness was measured after all the heating tests were completed)

[The thickness of the filters has a direct effect on the transmission of the filter.

The thicker the filter the less the transmission.]

Hoya U-325C filters after baking, with the specular side

(smooth) side up. Note S2 and S5 have melted and deformed.

Hoya U-325C filters, back side, after all of the baking. Note S2, T2,

and S5 have melted and deformed.

of 18

Filter

ID

Letter

or

number

Notes Mfg. Condition

Average

filter

thickness

in mm.

%

Transmission

at the start

of the tests

Number

of times

heated

%

Transmission

at the end

of all the

heating tests

1

Solarized FS-60

from a

TripleBright II.

Filter sagged 2-3

mm from

heating.

Hoya No film

or spots

4.93

30.7

1

52.5

2

Solarized FS-60

from a

TripleBright.

Filter sagged

slightly from

heating.

Hoya No film

or spots

4.83

30.0

1

52.4

3

Solarized FS-60

from a

TripleBright II Hoya

No film

or spots

4.72

30.0

1

47.2

4

Solarized FS-60

from a


No film

or spots

5.00

28.6

1

45.3

5

Solarized FS-60

from a

TripleBright.

Very slight

sagging from

heating.

Hoya No film

or spots

4.89

29.0

1

47.4

6

Solarized FS-60

from a


No film

or spots

5.09

25.3

1

41.8

7

Solarized FS-60

from a

TripleBright.

Filter sagged

about 3 mm

from heating.

Hoya No film

or spots

4.87

34.1

1

47.9

8

Solarized FS-60

from a


No film

or spots

4.93

33.7

1

54.2

We wish to thank, Melissa Russell, Patricia Croteau, Alma Newsome, and some of the members of the FMS

Research Chapter, specifically Dr. Glenn Waychunas for reviewing the paper and providing much needed help.

We also want to thank Hoya Corporation USA, Optics Division, for important information, and Raytech

Industries and Kopp Glass Co. with useful information.

For more information contact Don Newsome at [email protected] or at (425) 228-9988 or Jim Forsyth at

[email protected] or at (509) 522-2756.

Heat Rejuvenation of SW Filters 6-27-16 · A Study Concerning the Heat Rejuvenation of Short Wave...

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