J O H N S T R O N G , C a li f o r n ia I n s t i t u t e o f T e c h n o lo g y , P a s a d e n a , C a lif.
B ASICALLY new instruments depend on the development of new materials by chemists as well as the discovery of new properties, effects, and techniques by physicists.
Tables I and II list some materials and physical effects which have been important in instrumental development in the past or from which we may expect future developments.
New instruments will undoubtedly come from the ingenious combination of effects and materials already well known.
Examples of this type of development are listed in Table III.
Finally, we have those developments which may be aptly described by the phrase “good engineering” . The present article is concerned chiefly with the fundamental optical developments in astronomical and meteorological instruments with which the author is most familiar. These should be of interest to the reader because parallel developments in some chemical instruments may be expected. These develop
ments, together with "good engineering” , will increase the usefulness to chemists of all optical instruments now limited by their inadequate efficiency and sensitivity. Examples are monochromators (or spectrometers) used with either a photo
cell or thermopile in the visible, ultraviolet, or infrared part
564 INDUSTRIAL AND EN G IN EERIN G CHEMISTRY VOL. 11, NO. 10
Credit, Prentice-H all, In c . Fi g u r e 1. Re f l e c t i v i t i e s o f Me t a l s Us e d f o r Mi r r o r s i n In s t r u m e n t s
S u r fa ce F ilm s b y E vap o ratio n
Glass is used for astronomical mirrors and lenses. The reflection coefficient of a glass surface is not ideally suited for either of these applications, being too low for the former and too high for the latter. But the reflection coefficient of the surfaces of mirrors and lenses is altered by thin surface films deposited by evaporation. This increases the efficiency in both cases; a thin film of aluminum (Figure 1) gives a glass mirror a reflectivity of approximately 90 per cent throughout the entire useful spectrum (27); a thin film of calcium fluorite on the surfaces of a lens eliminates 90 per cent of the transmis
sion losses arising at the surfaces owing to reflection (29).
These surface films have been applied to increase the effi
ciency of monochromators. The use of evaporated aluminum films has eliminated the expensive roof prisms
used in the early models of one double-quartz monochromator (see Figure 2). The use of evaporated fluorite films on this same double monochromator will more than double its transmission. The high gain resulting from the application of fluorite films is achieved be
cause the light beam in this monochromator penetrates twenty quartz-air surfaces.
D o u b le -Q u a r tz M o n o c h r o m a to r for O zon e a n d W ater D e te r m in a tio n s The double-quartz monochromator, shown in Figure 2, is in use with the sun as a light source, to make routine analyses of the entire atmosphere for ozone and water vapor con
tent.
For the ozone determination, which takes only 2 minutes, a sodium photocell in an evacuated quartz-glass envelope is used as a receiver. The photocell current is amplified w ith an electrom
eter tube (34). The intensity of the sunlight a t 3050 A. and 3110 A. is measured. F or the water vapor determination, which takes 5 minutes, the intensity of the <£-band a t 1.15 ¡i is measured with a thermopile (10). I t is noteworthy th a t the slits of the double monochrom ator remain the same for both determinations (0.075 mm., 0.003 inch).
D eterm inations have been m ade of the ozone in an absorption cell using th e relative absorp
tion a t 3050 A. to 3110 A. Here a tungsten- filament lamp in a quartz-glass envelope serves
as th e source of continuous radiation. (Tung
sten lamps in quartz envelopes are supplied by th e Phillips Laboratory, Eindhoven, Holland.) This source is useful from 2800 A. to 3 fi. The voltage across th e filament is regulated by a Raytheon regulator, the performance of which is expressed as follows (data taken with the output power adjusted to full load):
A p p lied v o lta g e 70 80 90 100
O u tp u t v o lta g e 114 116 1 1 6 .0 1 1 6 .0 A p p lied v o lta g e 110 120 130 150 O u tp u t v o lta g e 1 1 6 .0 1 1 6 .1 1 1 6 .1 116.1
W ith th e slits 5 A. wide the response of the galvanometer a t 3050 A. is about 100 divisions.
The ratio of the emission a t 3050 A. to th a t a t 3110 A., for the em pty cell, is reproducible to one p a rt in 1500. W ith this inherent reproduci
bility one can get an accurate determ ination of th e ozone in the absorption cell.
The monochromator shown in Figure 2 and its accessories have been used for measuring the reflection of mirrors, the transmission of optical glass, the absorption of filters, and the spectral response of photoelectric cells. The use of a double monochromator is required for highly selective effects like the photoelectric effect, and its use in colorimetric work is recommended when highly selective receivers are employed.
Ta b l e I V . In d e x o p Re f r a c t i o n o p Sy n t h e t i c Ma t e r i a l s
M a te ria l C D e F 0
6563 5893 5461 4861 4358
F u se d q u a rtz 1.4 5 6 7 1 .4 5 8 7 1.4 6 0 4 1 .4 6 3 4 1 .4 6 6 9
C aFs 1 .4 3 2 5 1 .4 3 3 8 1 .4 3 4 9 1 .4 3 6 9 1 .4 3 9 5
L iF 1 .3 9 0 6 1.3 9 2 2 1 .3 9 3 0 1.3 9 4 3 1.3 9 7
KC1 1 .4 8 7 0 1.4901 1.4 9 2 9 1.4981 1 .5 0 4 3
K B r 1 .5 5 4 4 1.5 5 9 0 1.5631 1 .5 7 0 9 1 .5 8 0 6
ICI 1 .6 5 6 9 1.6 6 5 5 1.6721 1.6 8 5 3 1 .7 0 2 5
M gO 1.7 3 3 7 1 .7 3 7 8 1 .7 4 1 2 1.7 4 7 5 1 .7 5 5 0
Plexiglas 1.4 8 5 6 1.4881 1.4902 1 .4 9 3 8 1 .4 9 9 2
L u c ite 1.4 9 1 6 1 .4 9 4 5 1 .4 9 6 7 1 .5 0 0 8 1 .5 0 6 4
q u a r t z p r i s m s a l u m i n i z e d o n b a c h s c r e w t o m a i n
t a i n f o c u s t h r o u g h o u t t h e . s p e c t r u m
d r u m c a l i b r a t e d in w a v e le n g th s
q u a r t z p r i s m s w i t h f r o n t f a c e s g r o u n d t o f o r m l e n s e s
q u a r t z r e f l e c t i n g i n t e r m e d i a t e
s l i t
m g s c r e w
A p a r t i t i o n ( n o t s h o w n ) p r e v e n t s s t r a y l i g h t f r o m o n e s y s t e m e n t e r i n g t h e o t h e r .
Thi s e ar n s l i d i n g o n t h e p i n r o t a t e s t h e r e a r p r i s m t a b l e m a i n t a i n i n g m i n i m u m d e v i a t i o n t h r o u g h o u t t h e s p e c t r u m . All Lhc s l i t s a r e s e p a r a t e l y a d j u s t a b l e . The e n t r a n c e a n d e x i t s l i t s a r e c u r v e d t o c o m p e n s a t e f o r p r i s m a t i c d i s t o r t i o n .
Credit, Prentice-H all, In c . Fi g u r e 2 . Hi l g e r- Mu l l e r Do u b l e Mo n o c h r o m a t o r w i t h Qu a r t z Op t i c s T w o p rism a a t le ft, sh o w n h e re b a c k e d w ith reflectin g c o a ts of a lu m in u m , w ere fo rm erly
b a c k e d w ith q u a rtz roof p rism s
565
, hum an eye
L > • •
/ + t h r o u g h ^
—<< glass
3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0 7 0 0 0
w a v e - l e n g t h in A n g s t r o m u n i t s
u l t r a v i o l e t b l u e , g r e e n o r a n g e r e d
v i o l e t y e l l o w
C redit, P ren ticc-IIa ll, In c . Fi g u r e 3 . Sp e c t r a l Se n s i t i v i t y o p Ph o t o v o l t a i c Ce l l
N o te t h a t s e n s itiv ity goes t o zero on lo n g w av e le n g th sid e n t a p p ro x im a te ly 7200 A.
N ew O p tica l M ateria ls
R a d ia tio n R e ceiv in g D ev ices Photocells are not treated here (they are adequately discussed elsewhere, IS) except to point out that the sensitivity of the photovoj- taic type vanishes completely at about 7200 A.
(see Figure 3).
Improvement of the sensitivity of thermo
piles and other radiometric devices has been the object of many investigations (33) and work is now being done to improve their perform
ance. There seems at present no possibility of making “order of magnitude” improvements in the sensitivity of this type of instrument, which measures a flux of radiant energy, be
cause the delicacy of the measurements is now limited by the effects of Brownian motion. Re
cently, however, a novel method has appeared for photographing in the infrared spectrum out to 9 / i ( 5 - 7 , 17, 37). Because, in principle, this new method integrates the flux of energy over the time of exposure, it is not inherently limited by the effect of Brownian motion and we may reasonably expect that it or some modi
fication of it may be developed which will yield an instrument of great sensitivity.
But without radical developments in radio-The infrared spectrum is being used now
by chemists to identify compounds and to estimate the proportions in which they are present (1).
The growing of large synthetic crystals of the alkali halides was undertaken in order to supply prisms of new optical materials for infrared spectroscopy. Synthetic crystals of potassium chloride, potassium bromide, and potassium iodide were grown at the University of Michigan in cylinders 12.5 cm. (5 inches) in diameter by 12.5 cm. (5 inches) long (30).
To this list have been added two synthetic crystals, sodium fluoride and lithium fluoride, which are of importance in the ultraviolet and visible part of the spectrum (25). These two materials are important (4) because they simu
late the properties of fluorite (see Table IV).
The occurrence of fluorite in large size and quality suitable for optical usage is now very rare.
Another synthetic optical material now avail
able (from the Norton Company, Chippawa, Canada) is magnesium oxide, which is useful in infrared spectroscopy for making shutters.
Its transmission limit in the infrared is inter
mediate between th at of quartz and fluorite (about 7.5 fj.) and in the ultraviolet is 2300 A.
Magnesium oxide has a high index of refraction and a low dispersion (see Table IV). Its inert
ness toward the alkali metal vapors suits it for use as a window material for absorption cells to contain these vapors (35).
S y n th e tic th e rm o p la s tic s (L u c ite and Plexiglas) are now used for making unbreak
able spectacle lenses and sun glasses and may eventually find extensive use in optical instru
ments (36). Their optical surfaces are formed by heat and pressure in polished molds without any of the customary optical working with abrasives and rouge.
s l o p e t o o d r c a t
l e a s t g l a s s t o r e m o v e
( e a s i e s t t o m a k e )
H = 1 5
l e a s t c h r o m a t i c a b e r r a t i o n
g l a s s t o o t h i c h
T h e s e c u r v c s m a y b e p u t o n e i t h e r o r b o t h s i d e s o f t h e l e n s p r o v i d e d t h e v a l u e o f ( t - A t ) i s m a i n t a i n e d .
A t = y t _ ^ r a y 3
A (.n - l) R 3
K = a c o n s t a n t b e t w e e n O a n d 4
n ■= i n d e x o f r e f r a c t i o n o f l e n s m a t e r i a l
C redit, P rentice-H all, In c . Di a g r a mo p Sc h m i d t Pr i n c i p l e a s Ap p l i e dt oa n As t r o n o m i c a l
Te l e s c o p e
A bove. P a ra lle l lig h t fro m th e rig h t is m odified b y th e co rre c tin g len s so t h a t th e sp h erica l m irro r a t th e le ft foouses i t to a p o in t. B elow . V ario u s sh ap e s t h a t m a y be g iv en th e c o r
re c tin g lens a n d fo rm u la d escrib in g th e ir co n to u rs Fi g u r e 4 .
s p h e r i c a l m i r r o r f o c a l r a t i o = -i-d
c o r r e c t i n g l e n s
INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 11, NO. 10
For example, to measure with a thermopile most effectively the energy emergent from the slit of a monochromator (especially as applied to infrared spectroscopy) it is necessary to form a reduced image of the exit slit on the thermocouple and at the same time increase the solid angle within which the thermocouple is irradiated. At present an ellip
tical mirror is used, giving a fivefold reduction in the image size and an increase of angle of irradia
tion to a cone of 100° diameter (9, 20). The thermopile is at one focus of the elliptical mirror and the exit slit of the monochromator is at the other focus. The elliptical mirror is not completely suited for this application, for although the image is definite on the optical axis the imaged ends of the slit which lie off the optical axis exhibit coma (§).
This coma necessitates the use of an oversized
thermopile receiver. The technique may be improved by apply
ing the Schmidt principle instead of an elliptical mirror. Thus, without any sacrifice in the solid angle of irradiation, the area of the thermopile receiver may be reduced three- or fourfold with an attendant doubling of the thermopile sensitivity.
T h e S c h m id t P r in c ip le in O p tica l D e sig n The Schmidt principle in optical design is described in a recent issue of the Scientific American (11). The Schmidt principle involves the use of a lens positioned at the center of curvature of a spherical mirror (Figure 4). The lens is figured to introduce a compensating spherical aberration in a parallel beam of light, so th at the spherical mirror can focus the beam to a point. As the effect of the lens is not critical in respect to the angle at which the beam passes through the correcting lens, the combination of mirror and lens exhibits a large field.
The Schmidt lens introduces but very little achromatism and because it is thin it introduces but little absorption. Many applications of the Schmidt principle have been discussed (11, S3, 26).
Although the making of Schmidt correcting plates is now an optical task of considerable difficulty, the principle may, in the future, be used in the construction of an inexpensive but effective double monochromator. Perhaps the Schmidt correcting plates will be formed from methyl methacrylate polymer by pressing, in the same manner that spectacle lenses are now made.
R e p lica G ratin g s
Excellent replica gratings (both reflection and transmission types) which throw a large fraction of the incident light in a single order of the spectrum, are now available. [Replica gratings produced from matrices made by R. W. Wood are obtainable from the W. M. Welch Scientific Co., 1518 Sedg
wick St., Chicago, 111. Transmission replicas can be trans
formed into reflection replicas by aluminizing the cellulose film (27).] Combined with a fore-prism monochromator to eliminate overlapping orders and stray radiation, these replica gratings may be used in certain types of monochromators to obtain a Unear wave-length scale and a greater dispersion than a prism affords.
O th er A p p lic a tio n s o f N o n re flectin g S u r fa ce F ilm s Nonreflecting surface films of fluorite and other substances are deposited by evaporation on optical surfaces primarily to make optical instruments more efficient. In addition, these nonreflecting films eliminate halations and the thin veil of background light produced in a lens by internal reflections.
Many applications of these films are possible, wherever sur
face reflections from a transparent optical body are to be elimi
nated. Promising among them are the application of non- reflecting surfaces of fluorite to decrease the reflectivity of residual-ray crystals in the near infrared and the application 566
of paraffin layers 2 0 ¡x thick to quartz to increase the trans
mission of quartz lenses in the far infrared.
R e sid u a l-R a y F ilte r M e th o d in t h e F ar In fra red The residual-ray filter method uses successive reflections from crystals to obtain monochromatic bands of infrared ra
diation. The method affords, in effect, a monochromator.
Wave-length bands from 6.7 n to 150 <u are thus obtained (8, 14). A new arrangement of the residual-ray apparatus using a band of radiation at 8 . 8 ¡x affords a pyrometer particularly suited for determining surface temperatures in the range
— 100° to +100° C. (28). The band of radiation used in this pyrometer as the thermometric property lies at a position in the infrared spectrum where water vapor is very transparent.
Accordingly, no corrections for absorption in the optical path of the pyrometer are ordinarily required. With the pyrome
ter one can measure the temperature of surfaces without in
terfering with heat loss by radiation and convection or with surface heating.
This instrument was intended for astronomical (sun, moon, star, and planet temperatures) and meteorological (ozone, air, tree, grass, and ground surface temperatures) applications but it is possible th at it will find other applications. Fitted with appropriate crystals, the residual radiation falls exactly on the ozone band lying in the infrared spectrum at 9.6 ju and the in
strument so arranged is being used daily to measure the ab
sorption of sunlight by the ozone of the atmosphere. Using a residual-ray band a t 6.7 /x obtained with calcite crystals, there is a linear relationship between the square root of the water vapor in the optical path, t, expressed as centimeters of pre- cipitable water, and the produced fractional absorption of the energy in the residual-ray band, A:
A = 4.4 V r
Thus one may determine, with the apparatus, the absolute humidity. The above relation on which this “chemical analysis” is based is valid only at ordinary temperatures and a t atmospheric pressure (13, 21, 38).
ANALYTICAL EDITION 567
(14) Liobisch, T ., an d R ubens, H ., Sitzber. preuss. A kad., 16, 198, 876 (1919).
(15) M a tta u ch , J., Sitzber. A kad. W iss. W ien M ath.-naturw . Klasse.
A b t.IIa , 145,461 (1936).
(16) M a tta u c h , J., and H erzog, R ., Z . P h ysik, 89, 786 (1934).
(17) M önch, G., a n d W illenbcrg, H ., Ib id ., 77, 170 (1932).
'(18) M üller, R ., I n d . E n g . C h e m ., A nal. E d . , 11, 1 (1939).
(19) P fu n d , A. H ., J . Optical Soc. A m ., 29, 291 (1939).
(20) R an d all, H . M ., and F irestone, F . A., Rev. Sei. Instrum ents, 9, 404 (1938).
(21) Schaefer, Clem ens, an d M atossi, F ran k , “ D as U ltraro te Spek
tru m ” , p. 213, B erlin, Ju liu s S pringer, 1930.
(22) S co tt, H ow ard, J . F ra n klin In st., 220, 733 (1935).
(23) Sm iley, C. H ., P opular A stronom y, 44, 415 (1936).
(24) ‘‘Spectroscopy in In d u stry , C onference” , New Y ork, Jo h n W iley
& Sons, 1939.
(25) Stockbarger, D. C., Rev. Sci. Instrum ents, 7, 133 (1936).
(26) Strom gren, B ., Viertdjahrschr. Astronomischeii Ges., 70, 65 (1935).
(27) Strong, Jo h n , Astrophys. J ., 83, 401 (1936).
(28) S trong, Jo h n , “ A N ew R a d ia tio n P y ro m e te r” , in press.
(29) Strong, Jo h n , J . Optical Soc. A m ., 26, 73 (1936).
(30) Strong, Jo h n , P hys. Rev., 36, 1663 (1930).
(31) Strong, Jo h n , ‘‘P rocedures in E x p erim en tal P h y sics” , C hap.
I l l , New Y ork, P ren tice-H all, Inc., 193S.
(32) Ibid., C hap. V.
(33) Ibid., C hap. V III.
(34) Ibid., C hap. X .
(35) Strong, Jo h n , and Brice, R . T . , J . Optical Soc. A m ., 25, 207 (1935).
(36) T illyer, E . D., Ibid., 28, 4 (193S).
(37) W illenberg, H .t Z . P h ysik, 74, 663 (1932).
(38) W im m or, M ax, A n n . P h ysik, 81, 1091 (1926).