Ultraviolet filter of thorium fluoride and cryolite on quartz bas

Holmes, S. J.

English

NASA Technical Reports Server

The invention 
researchinaeronaut  
programs of the Nati 
The invention is own 
licensing in accorda 
lation (14 Code of F 
To encourage c -owned inven- 
tions, it is NASA p 
revocable license 
the invention whi l  
Patent Office and 
after issuance of 
invention does no 
limited exclusive 
tive to further en 
desiring to make 
obtain a royalty- 
Address inqu 
General Counsel 
Aeronautics and 
hin a specified period, presently two years, 
Any company 
NASA-HQ 
https://ntrs.nasa.gov/search.jsp?R=19690014954 2020-03-16T18:23:50+00:00Z
Feb. 11, 1969 JAMES E. WEBB 3,427,089 
ADMINISTRATOR OF THE NATIONAL AERONAUTICS 
A N D  SPACE ADMINISTRATION 
ULTRAVIOLET FILTER 
F i l ed  March 12, 1965 .- I 
" '  
.-: - 
s a  9 *7 
*_r 9 a  3 
3,427,519 United Statest Patent Office patented Fe.. 11, 1g6g 
I 
1 2 l i 
is inefficient in that the metallic films a sorb a large part 
of the ultraviolet radiations. Also, it is &ikult and costly 
fhc National A ~ ~ ~ .  to fabricate the metallic films. Conventional coating 
na,llirs and space Ad,,,inistrnfion with respect to in- methods used in the interference filler art, such as vacuum 
sention of Samuel J. IIolmes, Norfh Ilollywood, Calif. 6 deposition, cannot be used. 
This invention teaches making the filter out of alter- 
6 Claims nate layers of thin films of the materials thorium fluoride 
and cryolite. It has been discovered that these two ma- 
teriale cooperate to provide a very effleient alter for trnns- 
mission of a band of ultraviolet, while rejecting unwanted 
side bands. In addition these films can be applied by con- 
ventional techniques. 
I t  is therefore an object of this invention to provide a 
highly efficient filter that will transmit a high percentage 
of a band of ultraviolet radiation while rejecting unwanted 
side band radiations. 
It is, therefore, an object of this invention to provide a 
highly efficient filter that will transmit a band of radiation 
in the region between 2050 to 2250 angstroms. 
It is another object of this invention to provide a filter 
that transmits a narrow band of radiation in the region 
of 2150 angstroms while rejecting side band radiations 
of longer and shorter wave lengths. 
8 I t  is also an object of this invention to  provide a highly 
25 efficient ultraviolet filter that can be fabricated using Origin of the invention 
The invention described herein was made in the per- standard techniques. 
formance of work under a NASA contract and is subject Other objects and advantages will appear from the 
to the provisions of Section 305 of the National Aero- specification and claims taken in connection with the ac- 
nautics and Space Act of 1958, Public Law 85-568 (72 companying sheet of drawing wherein: 
Stat. 435; 42 USC 2457). FIGURE 1 is a cross section of a filter showing the 
This invention relates to optical devices and, more par- coating and base; 
ticularly, to a coating for an interference type filter that FIGURE 2 is an exploded portion of the cross sectional 
iS Capable Of transmitting a narrow bandwidth Of radia- view of FIGURE 1, to j)lustrab the various individual thin 
tion in the ultraviolet region. film layers that make up the mating; and 
FIGURE 3 is a graph showing the light transmission 
type filters, are well knowns They naPmally aampriwa @ ~ h ~ ~ a ~ ~ ~ F i ~ ~ i ~ ~  aC the filter lllustratod in FIGURE 1, 
base and a ooatlng made up QL many layers of thin films Referring to FIGURE 1, there is shown a filter that 
applied to the base. includes an optical base 4 of quartz or other ultraviolet 
With respect to these coatings, in some cases they may transparent material, and a filter coating 6 deposited on 
contain as many as forty or more thin film layers. The 40 the ,base. Coating 6 is made up of a plurality of thin film 
many thin film layers are selected SO as to have alternately layers applied on top of each other. 
high and low indexes of refraction. These films control the Referring to FIGURE 2, there is shown an exp!oded 
reflection and transmission of light in selected regions of view of a portion of the filter in FIGURE 1. The film 
the spectrum by causing interferences among light waves layers that make up the coating are shown individually. 
generated at the many interfaces. The thin films coact 46 Actually, the thickness of the coating is quite thin and the 
to  block passage of unwanted wavelengths on both sides separate layers can not be distinguished. As an item of 
of the selected narrow band of wavelengths that is being interest, in the article ‘ m e  New Optics,” previously cited, 
transmitted. it is mentioned that it is practically impossible to analyze 
A more detailed explanation of filters of this type can be the coating formula on a filter. As the article sets forth, 
found for example in the magazine, International Science 60 “imagine peeling off and analyzing each of forty or so 
and Technology, April 1962, in the article entitled, “The layers a fiw hundred angstroms thick.” 
New Optics,” by Harold S. Stewart, page 16; also in It has Been discovered that when a coating is made of 
U.S. Patent 2,852,980, FIGURE 2, there is shown a filter alternate thin fjlm layers of thorium fluoride and cryolite 
having a geometry of the interference type. [(NaAIF6) sodium aluminum fluoride], it has a high abil- 
A filter that will transmit ultraviolet radiation has high 66 ity to pass a band of ul@aviolet radiation while blocking 
utility due to the many uses for UltraViOkt radiation. Some adjacent side bands. 
of these uses are: in mineral detection, in research-such At a wavelength of approximately 2 150 angstroms, 
as mapping the heavens, for fluorescent lighting, and in thorium fluoride has an index of refraction of approxi- 
sterilization of food and medical instruments: mately 1.7 and is used as a high index material, while 
However, one problem in transmitting ultraviolet radia- 60 cryolite has an index of refraction of 1.3 and is used 
tion is that it is rapidly absorbed by most forms of matter, as the low index material. These two materials cooperate 
including gases and air. Even ordinary window glass will to  transmit the ultraviolet while blocking the side bands. 
absorb ultraviolet radiation. Referring to FIGURE 2, as is conventional with inter- 
Due to the high absorption of ultraviolet radiation, it ference type filters, the coating is made with alternate 
has been difficult to  construct an efficient filter capable of 66 layers 8, 10 of one-quarter wavelength thickness of ma- 
transmitting selected wavelengths of ultraviolet radiation terial. The thorium fluoride is the first layer applied direct- 
while filtering out unwanted wavelengths. Most materials ly to base 4. Then the cryolite layer is applied in a one- 
suitable for use as a filter coating material absorb the quarter wavelength thickness on top of the layer of 
ultraviolet and thus cannot be used, or if they can be thorium fluoride. The thorium fluoride layers 8 and 
used, are very inefficient. 70 cryolite layers 10 are then alternately applied lo form 
An example of one type of ultraviolet filter presently coating 6. In the embodiment disclosed, the layers are 
used is one made with metallic films. However, this type applied to form a twenty-one layer sandwich for optimum 
3,427,089 
ULTRAVIOLET FILTER 
J : , ~ ~ ~ ~ ~  E. Webb, Adtninistrafor 
Filed Mar. 12, 1965, Ser. No. 439,490 
U S  CI. 350-1 
Int. Cl. G02b 5/20,5/28,13/14 
30 
Interference type filters, also known as Fabry-Perot 36 
3 i427.089 
3 
results, as will be explained Mer. Also as is conventional 
with this type filter, the micldlc layer of thorium fluoride 
81 is normally made one-half wavelength thick. 
It is known io usc both thorium fluoride and cryolite in 
optical filtcrs. However, previously, these two materials 
haw bcen used as substitutes for one another. They have 
bcen uscd as low index of refraction materials. They have 
not been used together wherein one #provides the high, 
and one the low index of refraction. 
As mcntioned previously both cryolite and thorium 
fluoride are well known in the optical field. Many tech- 
niques have been developed for applying these materials. 
Qno aE the advantages in itsing thase materials over use 
of metallic films is that conventional vacuum evaporation 
can be used to apply the individual films. 
Referring to FIGURE 3 there is shown in graph form, 
the transmission of ultraviolet through the &Iter. The hori- 
zontal scale is wavelength in angstroms, and the vertical 
scale is in percent of transmission of ultraviolet through 
the filter. 
Curve 14 on the graph indicates that the filter shows 
an extraordinary transmission of ultraviolet in  the 2150 
angstrom area. In this area, approximately 80 percent of 
the ultraviolet is transmitted through the filter. The slopes 
on both sides of the curve are quite steep. The curve 
reaches a low at approximately 2050 and 2250 angstroms 
on each side of the 2150 angstrom line. 
The curve indicates that radiation having wavelengths 
shorter than 2050 angstroms or wavelengths above 2250 
angstroms will be rapidly attenuated, and only a small 
portion will be transmitted by the filter. It wilf be noted 
that this filter can be used with other conventional filter 
techniques to cut out the side band wavelengths as desired. 
Although the filter is disclosed as having twenty-one 
layers, this is a compromise. If more layers are provided 
there will be an increase in the efficiency of the filter in 
blocking side band wave lengths. However, this will be 
at the expense of decreasing the percent of the desired 
ultraviolet bandwidth transmission. Conversely, if less 
layers ape used, this will increase the percentage of the 
desired ultraviolet bandwidth transmitted but will also 
increase the transmission of unwanted sideband wave- 
lengths. 
In certain circumstances it is possible to use a coating 
with as little as five layers. The number of layers will 
depend on the individual purpose for which the blter is 
to be used, and for general purposes, twenty-one layers 
are treated as the optimum amount. 
In summary, a filter having an ultraviolet transparent 
base is provided with a coating made of alternate thin 
film layers of thorium fluoride and cryolite. The thorium 
fluoride has a high index of refraction of approximately 
1.7 and the cryolite has a low index of refraction of ap- 
nroximatelv 1.3. 
4 
of ultraviolet radiation, the improvement comprising: 
a base transparent to ultraviolet radiation; and 
a coating on said base of alternative layers of materials 
having indexes of refraction of approximately 1.7 
1.3 at  a wavelength of ap,woximatety 2150 atig- 
stroms. 
2. In an interference type filter for transmitting a band 
of ultraviolet radiation, the improvement comprising: 
a base transparent to ultraviolet radiation; and 
a coating on said base of altetiate layers of thorium 
fluoride and a material having an index of refraction 
of approximately 1.3 at  a uavelength of approxi- 
anatdy 2150 ongatroms, 
3. In an interference type filter for transmitting a band 
15 of ultraviolet radiation, the improvement comprising: 
a base transparent to ultraviolet radiations; and 
a coating on said base of alternate layers of a material 
having an index of refraction of approximately 1.7 
at wavelength af approximately 2150 angstroms and 
4. In an interference type filter for transmitting a band 
of ultraviolet radiation, the improvement comprising: 
a base transparent to  ultraviolet radiation; and 
a coating on said base of alternate layers of thorium 
5. In an interference type filter for transmitting a band 
of ultraviolet radiations in approximately the 2150 ang- 
5 
20 cryolite. 
25 fluoride and cryolite. 
30 
35 
40 
46 
50 
’These fiim layers are each applied in a onequarter 65 
uavc length thickness, with the exception of the center 
film layer being applied one-half wavelength thick. The 
films are applied until there are twenty-one layers. The 
resultant filter will transmit a band of radiation between 
approximately 2150 t o  2250 angstroms. There will be 60 
about 80 percent transmission of wavelengths of 2150 
angstroms while rapidly attenuating the side bands. 
Although but a single embodiment of the present in- 
vention has been illustrated and described, it will be 
apparent to one skilled in the art that changes and modi- 65 
fications may h e  made therein without departing from 
the invention ;IS defined in the appended claims. 
What is claimed: 
1. In an interference type filter for transmitting a band 
I 
strom wavelength region the improvement comprising: 
a quartz base; and 
a coating on said base of alternating layers of first 
thorium fluoride and then cryolite to a depth of at  
6. In an interference filter for transmitting a band of 
ultraviolet radiation having a wavelength of approxi- 
mately 2150 angstroms, the improvement comprising: 
a quartz base; and 
a coating on said base of twenty-one layers, tlie odd 
numbered layers being of thorium fluoride having 
an indcx af mfraction of approximately 1.7, and 
the even numbered layers being cryolite, having 
an index of refraction of approximately 1.3; said 
filter having a short wavelength cut-off at approxi- 
mately 2050 angstroms, a long wavelength cut-off 
a t  approximately 2250 angstroms, and transmitting 
approximately 80 percent of the ultraviolet at wave- 
lengths of approximately 2 150 angstroms. 
least five layers. 
, 
References Cited 
UNITED STATES PATENTS ‘1 
i 3,015,253 
1/1962 Foreman et al. 
3,200,253 8/1965 Geier. 
% OTHER REFERENCES 1 i 
Schroeder, “Interference Transmission * Filters for the 
Far Ultraviolet” J.O.S.A., volume 52, pp. 1380-1386 
(December 1962). 
Smakula, Kalnajs, and Redman, “Optical Materials 
and Their Preparation” Applied Optics, volume 3, pp. 
323-328 (March 1964). . 
DAVID SCHONBERG, Primary Exantitw. 
PAWL R. MILLER, Assistant Examiner. 
350-166 
U.S. $3. X.R. 
NASA CASE’ 
ULTRAVIOLET FILTER 
This  i nven t ion  relates t o  a f i l t e r  t h a t  w i l l  
t r a n s m i t  a band o f  u l t r a v i o l e t  r a d i a t i o n ,  wh i l e  re- 
j e c t i n g  a d j a c e n t  s i d e  bands.  
One problem i n  t r a n s m i t t i n g  u l t x a v i o l e t  r a d i a -  
t ~ o n ,  t h a t  the  u l t r a v i o l e t  r a d i a t i o n  is absorbed 
3 y  m o s t  forms of m a t t e r .  A s  an example, gasses, a i r  
.:ad even o r d i n a r y  window g l a s s  w i l l  absorb u l t r a -  
violet r ad iz tyon .  A s  a r e s u l t  of the above, m o s t  
materials s u i t a b l e  for use as a filter e i t h e r  cannot 
be used, o r  are i n e f f i c i e n t  because they  absorb  a 
l a r g e  percentage  of t h e  u l t r a v i o l e t  t h e y  are t o  
t r a n s m i t .  
Re fe r r ing  t o  F igure  2 o f  t h e  drawing t h e r e  is 
shown a greatly en la rged  cross s e c t i o n  of che f i l t e r  
of t h i s  i nven t ion .  The inven to r  d i scovered  t h a t  by 
apply ing  a l t e r n a t e  l a y e r s  o f  Thorium F1ori.de and Cryo- 
l i t e  t o  a q u a r t z  base, t h a t  a ve ry  e f f i c i e n t  f i l t e r  
r e s u l t s .  Also Thorium F l o r i d e  and C r y o l i t e  can be 
app l i ed  by convent iona l  vacuum d e p o s i t i o n  techniques .  
I n  F igure  3 of  the drawings there is  shown a 
graph of t h e  pe rcen tage -o f  t r ansmiss ion  of u l t r a v i o l e t  
through t h e  f i l < & r .  r b o u t  80 p e r c e n t  ,of the u l t r a v i o l e t  
is t r a n s m i t t e d  a t  wavelengths o f  about  2150 angst,roms, 
whi le  t h e  s i d e  bands are g r e a t l y  a t t e n u a t e d .  
A f i l t e r  t h a t  w i l l  t r a n s m i t  u l t r a v i o l e t  radia- 
Zion has h igh  u l t i l i t y  due t o  the many.uses f o r  a l t r a -  
v i o l e t  r a d i a t i o n .  Some of t h e  u s e s  are: i n  researcl-., 
such a s  xapping t h e  heavens,  i n  mine ra l  d e t e c t i o n  ana  
.n  s t e r i l i z a t i o n  of  food and medical I-nstruments.  
i nventoi- : Samue L J. H o l m e s  
. E m p l o y e r :  Spectral-ab (NAS 5-519) 
=+&a=&++ 


Ultraviolet filter

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server