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NCAR/TN-340+STRNCAR TECHNICAL NOTE
October 1989
Calibration of the HAO Standard Opal Filter Set
J.L. Streete
HIGH ALTITUDE OBSERVATORY
NATIONAL CENTER FOR ATMOSPHERIC RESEARCHBOULDER, COLORADO
I -
. - - ;
iii
TABLE OF CONTENTS
Page
LIST OF FIGURES v
PREFACE vii
I. INTRODUCTION1
II. INSTRUMENTATION 3
Sources 3
Receiver 4
Data Collection 6
III. MEASUREMENT PROCEDURE 6
IV. RESULTS 7
V. SUMMARY AND SUGGESTED IMPROVEMENTS 10
Suggestions 11
VI. ACKNOWLEDGMENTS 12
VII. REFERENCES 14
v
LIST OF FIGURES
Figure 1. Tranmsittance Curve-450 nm filter
Figure 2. Transmittance Curve-550 nm filter
Figure 3. Transmittance Curve-650 nm filter
Figure 4. Transmittance Curve-800 nm filter
Figure 5. Transmittance Curve-All Filters
Figure 6. Schematic Diagram of Opal Calibration System
Figure 7. Opal Transmittance at 550 nm
Figure 8. Opal Transmittance at 650 nm
Figure 9. Opal Transmittance at 450 nm
Figure 10. Opal Transmittance at 800 nm
Figure 11. Calculated Optical Densities of the Neutral Filters
Figure 12. Transmittance of 2B, 2D, 2E, 2F Opals at 450, 550, 650and 800 nm
Figure 13. Transmittance of 2G, 2H, 21, 2K Opals at 450, 550, 650and 800 nm
Figure 14. Transmittance of 2L, 20, 2P, 2Q Opals at 450, 550, 650and 800 nm
vi i
PREFACE
New measurements of the transmittance of the HAO "fundamental
standard set" of opal filters were begun in September, 1988. The
instrumentation used in the measurements was provided by Rhodes
College, Memphis, Tennessee and the High Altitude Observatory.
Since nearly two decades had passed from the time of the last
calibration measurements, there was concern that the transmittance of
the filters might have changed. Another motivation for recalibrating the
opal filters was the desire to know the transmittance values in spectral
regions above and below those used in 1970. The spectral regions chosen
were 450 nm, 550 nm, 650 nm and 800 nm.
Comparisons of transmittance are made with the previous
values in Table 1, and new transmittance values are given Table 2. The
new values agree with the 1970 measurements to within about 10 per
cent (usually considerably better than this) except for a few filters. The
transmittance of filter 2F at 650 nm shows the greatest discrepancy with
the old results, with the new value being a factor of 1.6 higher.
It is felt that the use of the direct radiation from the sun,
thereby insuring the proper geometry for the measurements, and
viii
employing a silicon photodiode detector,whose response is extremely
linear over a wide dynamic range, have significantly improved the
accuracy of the transmittance measurements.
Jack Streete*Scientific VisitorHigh Altitude Observatory
*On leave from Rhodes College, Memphis Tennessee.
1
I. INTRODUCTION
Solar radiation attenuators called "opal filters" have been used for
several decades to calibrate photographic film in instruments used to
measure the brightness of the solar corona (see for example 1,2). The
primary component used in fabricating these filters is opal glass. This
glass contains a suspension of minute colloidal particles which produce
multiple scattering of the incident radiation. The diffusion thus produced
is very close to Lambertian, i.e.,the scattered intensity varies as the
cosine of the angle between the normal to the scattering surface and the
direction of measurement. As an example, opal glass illuminated with a
collimated light beam normal to its surface is seen to have, at 45 degrees
from normal, about 90 per cent the brightness predicted for a perfect
diffuser.3 The size of the suspended particles is such as to produce a
slight enhancement in the scattering of shorter visible wavelengths.
Transmittance measurements made with collimated laser beams,
however, show a variation of only about a factor of 1.02 in transmittance
from 400 nm to 800 nm.4
The "fundamental standard set" of opals produced and used at the
High Altitude Observatory contains twelve filters, beginning with a bare
2
opal whose attenuation factor is about 105. The rest of the filters of the
set were constructed by joining an opal glass filter with a neutral density
filter of a particular density. The last filter of the set has an attenuation
factor of about 109. In the field the filters are used by placing them in
front of the objective lens of the telescope which is pointed at the sun so
as to introduce a known attenuation to the photospheric radiation striking
the film.
The "standard set" was last calibrated in 19705 ,and there is concern
that the attenuation might well have changed with time due to changes in
the neutral density filters or the cement between them and the opals.The
purpose of the present measurements was to recalibrate the set, using the
sun as the radiation source, if possible.
For a perfect diffuser, a Lambertian surface, the attenuation
produced, except for the reflectance, is approximately the solid angle
subtended by the source at the opal divided by 2x(pi). In other words, the
attenuation is directly proportional to the angular size of the source.
Therefore, since the opal glass provides such a Lambertian surface, it is
important, when calibrating the opal filters to use a source which
subtends the same solid angle at the opal as will the source used in the
field. For this reason, the sun (or a source which geometrically simulates
3
the sun). must be used in measurements of transmittance.
Although the attenuation of the filters (opal glass and neutral
density filter) is not strongly dependent on wavelength, there is a slight
wavelength dependence, mainly due to the neutral density filters. For this
reason, the transmittance of the filters was measured in four spectral
regions centered at 450 nm, 550 nm, 650 nm and 800 nm. These spectral
regions were isolated with broadband interference filters each having a
bandpass of approximately 75 nm. Figure 1 through Figure 4 are
transmittance curves for these four filters, and Figure 5 shows the
combined transmittances for the filters.
Even though the calibration of the opal filters is relatively
straightforward, a great deal care must be taken to eliminate scattered
light when the opal is in the light beam. This is especially true when
measuring the most dense filters with attenuations of about 109.
II. INSTRUMENTATION
Sources.
Initially, two radiation sources were used in the calibration
measurements, a quartz halogen lamp with optics to simulate the sun
geometrically, and the sun itself. The final measurements were made
using only the sun as the source of radiation. An important advantage of
4
using the sun is that the irradiance produced in the plane of the detector
is great enough to measure directly even the densest filters. This was not
the case in 1969 when it was necessary to move the tungsten source
closer to the detector in order to have sufficient signal for filters of
attenuation greater than 108. The High Altitude Observatory's heliostat
was used to direct a solar beam into the laboratory. The tracking accuracy
was such that only one or two times during a measurement session of
about five hours was it necessary to realign the heliostat.
Receiver.
As shown in Figure 6, the receiver end of the system was comprised
of the interference filter, F1, whose holder was located in front of a
Geneva motion driven, four position aperture wheel which held the opal
filter, F2. Also the aperture wheel held, in the other three positions, a
calibrated neutral density filter required in the open position when using
the sun, and two opaque metal inserts used to block the radiation and to
provide an optical zero. The neutral density filter was measured to have
transmittances of 0.111, 0.105, 0.104, 0.103 respectively at the spectral
positions 450 nm, 550 nm, 650 nm, and 800 nm.
A 676 mm focal length off-axis parabolic mirror, M4, imaged the
solar disk onto the detector. An aperture over the mirror was slightly
5
smaller than the solar beam passed by the filter wheel aperture and was
used to insure that radiation was collected over the same solid angle for
the opal-in and opal-out positions.
The detector was a silicon photodiode, model UDT-111A made by
United Detector Technology, Inc. An important feature of this detector is
that it has a 1 percent linear response over the range of its sensitivity
which is from 10-2 to 10-11 watt. There was provision for changing the
horizontal position of the detector slightly by means of a micrometer
drive. An adjustable aperture was installed in front of the detector to
provide a field stop slightly smaller than the solar image. Inserted
between the filter aperture and mirror M4 and between mirror M4 and the
detector was a set of baffles, B1 and B2, used to eliminate scattered
light.
A light tight box was installed over the instrument, so that the
room would not need to be completely dark when carrying out the
measurements. This was necessary since other projects were sometimes
going on in the same room. However, these projects did not require that
the overhead lights be on. The top of the box was hinged so that the optical
alignment could be checked regularly.
6
Data Collection.
The detector was interfaced with an Integrated Solutions computer
and software was written to provide output voltages, standard deviations
of the voltages and calculations of transmittances for each of the twelve
opal filters in the four spectral regions provided by the interference
filters. Options were available to designate the aperture port, gain of the
detector, and number of measurements to be made at each position. The
data point sampling rate was one per second. Copies of the data were
available from a laser printer.
III. MEASUREMENT PROCEDURE
The measurements were carried out only when the sky was clear
and the winds relatively calm. After setting up the heliostat and
directing the radiation into the laboratory, a beam-directing mirror, M3
in Figure 6, was adjusted to center the solar radiation on the aperture
port of the filter wheel and off-axis parabolic mirror. Adjustment of the
micrometer drive was then made to center the solar image on the detector
aperture.
The opal filter being measured was clamped in the filter wheel in
one of the four available positions and behind one of the interference
7
filters. A particular opal was measured with each of the four interference
filters before beginning measurements on the next opal filter. The
measurements were done at least twice with each opal/interference
filter combination and the results checked before inserting the next opal.
Only when there was good agreement between two transmittance values
was the next spectral filter inserted. Before beginning measurements
with a new interference filter the optical alignment was checked and
adjusted if necessary.
IV. RESULTS
Table 1 shows the results of the new transmittance measurements
for each of the twelve opal filters in each of the four spectral regions.
Also listed are the 1970 values for the two spectral regions measured at
that time. Statistical analysis of the results shows that the random error
for each of the separate measurements produced a standard deviation of
0.01 to 0.02 for filters 2B through 2L and about 0.08 for filters 20, 2P and
2Q. The systematic error, an measure of which can be gotten from the
third column varies from a fraction on a per cent to about six per cent for
the first nine filters to as high as 12 per cent for the 20 filter at 800 nm.
This systematic error results primarily from from image motion and
probably from stray light in the instrument, which is more of a problem at
8
extremely low transmittances.
For the two wavelength regions where comparison can be made
between the 1970 and 1989 results, namely 550 nm and 650 nm, the
agreement is seen to be mainly better than 10 per cent for most cases.
These results are given in the fifth column for the 550 nm and 650 nm
data. Table 2 is included to give at a glance the most recently measured
transmittances of the opal filters.
Figures 7 and 8 are the transmittance curves for the twelve filters
for 550 and 650 nm, respectively. Graphs of the present measurements
(NEW VALUES) and 1970 (OLD VALUES) are shown. The main significant
difference is seen to be the 2F filter in the 650 nm region (Figure 8),
where, as is seen in Table 1, there is a difference of about 38 per cent. In
reference 5 this value is in parenthesis, possibly indicating (although not
specifically stated) that the value is questionable. Also, Figure 8 shows a
discrepancy in the transmittance of filter 2Q. Again Table 1 indicated a 27
per cent difference in the two values. Figures 9 and 10 show the
transmittance values at 450 and 800 nm respectively.
Calculations were made to determine the densities of the
neutral density filter part of the opal filter that would be required to
produce the measured transmittances. Assuming that the overall
9
transmittance, T, is given by
T - K x (Topal x TND), where
K is a constant taken for this purpose as 1 since for this
calculation we are interested only in relative values,
Topal is the measured transmitance of the bare opal, filter 2B,
and,
TND is the transmittance of the neutral filter part of
filters 2D through 2Q and is given by,
TND = T/Topal 10-D, or
D log(Topai/T), and
D is the optical density of the neutral filter.
Figure 11 shows the resulting calculated neutral filter
densities for filters 2D through 2Q. It can be seen that the density at 800
nm is always higher than for the shorter wavelength regions. Although the
original density curves for the neutral filters can not be located, with
typical neutral density filters the density does indeed increase with
wavelength. Also, for typical neutral density filters, the rate of change of
slope of the density curves usually increases with the density of the
filter. This effect is also seen in Figure 11.
Figures 12 through 14 are plots of transmittances of the
10
separate opal filters at the four wavelengths used. It can be seen that,
typically, the transmittance is slightly higher at 550 and 650 nm and
lower at 450 and 800 nm. However, in Figure 14, the transmittance of
filters 2P and 2Q is seen to be highest at in the 450 nm region. Since the
signal-to-noise was least in the 450 nm region and for filters 2P and 2Q
it is likely that these two values are too high by possibly as much as 50
per cent.
V. SUMMARY AND SUGGESTED IMPROVEMENTS
Measurements of the transmittance of the HAO "fundamental
standard set" of opal filters were begun in the fall of 1988. The
motivation behind this effort was concern that in the almost twenty years
since the last calibration of these filters, their transmittance might have
changed. Also there was interest in knowing the transmittances of the
opal filters in spectral regions outside those used in the 1970
measurements. Thus measurements were extended to include the 450 nm
and 800 nm spectral regions.
Since the transmittance of these filters is directly
proportional to the angular size of the source being observed, and since
the sun is the source used in the field, the decision was made to try to use
the sun as the source of radiation for the measurements. Previous
11
measurements used incandescent sources which geometrically simulated
the sun. The use of the sun also permitted the direct measurement of
transmittance to levels of about 10-9.
The dynamic range over which the measurements must be
made is approximately 109. Therefore a silicon photodiode detector was
chosen as the detector since its response is linear to 1 per cent over a
range of 109.
The new transmittance values agree with the 1970
measurements to within approximately 10 per cent except for the 2F and
2Q filters at 650 nm and the 2P filter at 550 nm. The discrepancy is
greatest for the 2F filter at 650 nm where the new value is 38 per cent
higher (a factor of 1.6 higher) than the 1970 value. It is recommended that
researchers employing this filter in the future use the new value for
transmittance. It might also be important to check the literature to
determine whether or not this filter has been used in the past in
calibrating systems used in measuring coronal brightness.
Suagestions
Probably the least satisfactory part of the experimental setup
was the lack of tracking accuracy of the heliostat. Although the heliostat
itself had to be realigned only a few times during several hours of
12
measurement, the beam directing mirror had to be readjusted quite often
to keep the solar image centered on the detector aperture. Lack of
centering could severely affect the value of transmittance obtained.
Correction of this problem would greatly improve the ease of making the
measurements as well as the accuracy of the transmittance values.
VI. ACKNOWLEDGMENTS
The author gratefully acknowledges the considerable assistance of
several people at HAO who helped with various phases of this project.
Among other things, David Elmore had the detector interfaced with his
computer and wrote the software necessary to provide data collection and
analysis. Lee Lacey helped with suggestions and well as design and
construction of components needed to perform the measurements. Dick
Fisher's help, encouragement and suggestions as well as manual
assistance in setting up the heliostat made my work much easier than it
otherwise would have been. Paula Rubins and Rick Sheffer actually did the
wiring and electronic fabrication necessary for the interfacing.
Others who helped set up the heliostat were Terry Leach, Charlie
Miller and Steve Tomsczyk. Several others including Chris Abato and
Charlie Miller assisted with the alignment of the optical system. The
author is indeed grateful to Art Hundhausen for providing the funds
13
necessary to carry out the measurements.
14
VI. REFERENCES
1. Poland, A.; MacQueen, R.; Munro, R.; Gosling, J.; "Radiance Calibration of
the High Altitude Observatory White-Light Coronagraph on Skylab", Applied
Optics, 16, (1977).
2. Newkirk, G.,Jr.; Dupree,R.; Schmahl,E.;"Magnetic Fields and the Structure
of the Solar Corona, II." Solar Physics, 15, (1970).
3. Smith, W., "Modern Optical Engineering", McGraw-Hill, 1966.
4. Boivin, L.,"Diffusers in Silicon-Photodiode Radiometers", Applied
Optics, 21, No. 5, (1982).
5. Elmore, D. F.; Streete, J. L.; Eddy, J. A.; "Calibration of Opal Glass
Attenuators", Astro-Geophysical Memorandum No. 178, (Feb. 27, 1970).
/U
Tr 60a
P n 50e sr m 40c ie t 30n tt a 2 020
nc
10e
0390 410 430 450 460 480 500 520 540 560 580 600
Wavelength (nm)
Figure 1. Transmittance Curve-450 nm filterci1
=I A
mU
Tr 70
p a 60n
e s 50rm
c i 40e tn t 30ta
20nc 10e
0460 480 500 520 540 560 580 600 620 640 660 680 700
Wavelength (nm)
Figure 2. Transmittance Curve-550 nm filter0)
ft0
80T
r70
ap a 60n
e s 50rmc i 40etn t 30t a
nc 10e
0580 600 620 640 660 680 700 720 740 760 780 800
Wavelength (nm)
Figure 3. Transmittance Curve-650 nm filter __z-A,,
70Tr 60an 50
e sr m 40c i
e t 30n t
a 20nc 10e
0700 720 740 760 780 800 820 840 860 880 900
Wavelength (nm)
Figure 4. Transmittance Curve-800 nm filter
iTiI 1111111 111 111111111111111111111111111111111111*- : :ua~u~uuuau~uhu *uuu0 u 0 u1*uu*a a a I A mmmla I I I i u
.a *IN * * m I m m -m 1 .ml !u * M u.n. u u * mm Ma I U I · I I Ia
+- I 0 m0 mm 0m I I U * i * i * * i u u u u u a 1 i ua i u I i mm 1 1i 1 i 1 1 u ! 1u
.roi · · · ..1 1! · · · i · m u f m h · m m m
400 _ 50400 450
~lpwrwgm~as$*jTIdIr F Ii i ii ULr 1 F'
500 550 600 650 700 750Wavelength (nm)
800 850 900
Figure 5. Transmittance Curves-All Filters--A
cD
80
percent
70
60
50
40
30
20
Transmittance
10
- 450 nm filter
- 550 nm filter
- 650 nm filter
-- 800 nm filter
a& m m Emmi a a r ·1 1 oil .I 0 am ·oil · a I A I I I11I IIls __I ME I IIII IIII I .
-4 I I 1 r 1 I I r r Il I I I I ml I I IF I I 0Ia I I I m 1 v w I I · · I AL I I I · a I I 4
M1, I
M3: B
M4: C
F1- if
1
FECOERF2: 4-Posltlon Filer Wheel (Opal Placed at This Position)
B1, B2: Baffes
Ught Path
: Electrical Connections TERMINAL COMPUTER
o
Figure 6. Schematic Diagram of Opal Calibration System
TABLE 1. TRANSMITTANCE OF OPAL FILTERS AND COMPARISON WITH PREVIOUS RESULTS
* Per cent difernces between successve measurements
550 nm 450 nm
Opel 1 99 % DIFFERENCE 1969 % DIFFERENCEFilter VALUES 1989 VALUES' VALUES NEW/OLD VALUES
29 9.42E-06 2.5% 1.03E-05 -9.3%2D 1.05E-06 1.1% 1.1OE-06 -4.3%2E 6.87E-07 1.2% 5.74E-07 2.2%2F 3.10E-07 0.1% 3.00E-07 3.2%20 1.81E-07 1.4% 1.72E-07 5.2%2H 1.25E-07 2.0% 1.17E-07 6.2%21 7.96E-08 0.1% 8.83E-08 -10.9%2K 3.63E-08 1.3% 3.92E-08 -8.0%2L 1.44E-08 0.1% 1.61E-08 -12.1%20 8.95E-09 3.1% 1.00E-08 -11.7%2P 2.66E-09 10.3% 3.19E-09 -20.1%20 1.81E-09 9.0% 1.71E-09 5.5%
Opal 19 89 % DIFFERENCEFilter VALUES 1989 VALUES*
2B 8.37E-06 0.8%20 9.48E-07 0.1%2E 6.17E-07 3.3%2F 2.92E-07 2.3%2G 1.65E-07 0.3%2H 1.18E-07 1.3%21 6.88E-08 3.7%2K 3.22E-08 4.1%2L 1.15E-08 0.1%20 8.48E-09 5.3%2P 3.17E-09 3.6%2Q 2.71 E-09 2.0%
i50 nm
Opel 19 % DIFFERENCE 199 % DIFFERENCEFilter VALUES 19 VALUES' VALUES NEWIOLD VALUES
2B 1.00E-06 1.1% 1.01E-05 -0.7%2D 1.08E-06 0.5% 9.95E-07 8.1%2E 5.74E-07 0.5% 5.26E-07 8.4%2F 2.95E-07 2.0% 1.83E-07 38.0%20 1.87E-07 0.4% 1.47E-07 12.0%2H 1.14E-07 0.5% 1.19E-07 -4.7%21 7.68E-08 0.2% 7.99E-08 -4.0%2K 3.49E-08 2.3% 3.49E-08 -0.1%2L 1.34E-08 2.0% 1.39E-08 -3.8%20 8.45E-09 9.4% 7.92E-09 6.3%2P 2.66E-09 6.3% 2.49E-09 6.4%20 1.70E-09 3.8% 1.25E-09 26.5%
2 800 nm
Opll 1 9 8 % DIFFERENCEFilter VALUES 1989 VALUES*
20 1.02E-05 1.7%2D 1.01E-06 0.3%2E 4.87E-07 0.6%2F 2.41 E-07 1.4%2G 1.45E-07 6.4%2H 1.03E-07 0.4%21 5.66E-08 0.4%2K 2.78E-08 4.0%2 L 1.05E-08 3.0%20 6.69E-09 12.5%2 P 2.00E-09 5.8%20 1.34E-09 2.5%
-A
TABLE 2. TRANSMITTANCE OF OPAL FILTERS
T-R AN S MIT T A N C E
- - ------ -WAVELENGTH REGION (NM) - - ----- -
450 550 650 800
8.37E-06 9.42E-06 1.00E-05 1.02E-059.48E-07 1.05E-06 1.08E-06 1.01E-066.17E-07 5.87E-07 5.74E-07 4.87E-072.92E-07 3.10E-07 2.95E-07 2.41E-071.65E-07 1.81E-07 1.67E-07 1.45E-071.18E-07 1.25E-07 1.14E-07 1.03E-076.88E-08 7.96E-08 7.68E-08 5.66E-083.22E-08 3.63E-08 3.49E-08 2.78E-081.15E-08 1.44E-08 1.34E-08 1.05E-088.48E09 8.95E-09 8.45E-09 6.69E-093.17E-09 2.66E-09 2.66E-09 2.00E-092.71 E-09 1.81E-09 1.70E-09 1.34E-09
OpalFilter
2B2D2E2F2G2H212K2L202P20
1TRA 1
NSs 1
ITTANCE
12B 2D 2E 2F 2G 2H 21 2K 2L 20 2P 2Q
OPAL FILTER DESIGNATIONS
Figure 7. Opal Transmittance at 550 nm
- OLD VALUES
-- NEW VALUES II
1.00E-04
1.00E-05
1.00E-06
1.00E-07
1.00E-08
1.00E-09
N<
2B 2D 2 2E F 2G 2H 21 2K 2L 20 2P 20OPAL FILTER DESIGNATION
Figure 8. Opal Transmittance at 650 nrr
TRANSMITTANCE
-*- OLD VALUES
-- NEW VALUES
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2B 2D 2F 2G 2H 21 2KOPAL FILTER DESIGNATIONS
2L 20 2P 2Q
Figure 9. Opal Transmittance at 450 nmen
TRANSMITTANCE
1 It-4 -
1.00E-05
1.00E-06
1.00E-07
1.00E-08
1.00E-092B 2D 2E 2F 2G 2H 21 2K 2L 20 2P 2Q
OPAL FILTER DESIGNATIONS
Figure 10. Opal Transmittance at 800 nm h)
A2
TRANSMITTANCE
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Densities of Neutral Filters Requiredto Produce Measured Transmlttances
* 800 nm
* 650 nm
* 550 nm
* 450 nm
2D 2E 2F 2G 2H 21 2K 2LOpal Filter Designations
Figure 11. Calculated Optical Densities ofthe Neutral Filters
20 2P 2Q
(I)
4.00
3.50
Density
3.00
2.50-
2.00.
1.50-
1.00
0.50
0.00
--~ ~ ~ ~ ~ ~ _
| ~ _ --
I
i
ii
Ni
1.00E-04
1.00E-05
1.00E-06
1.00E-07450 550 650
BANDPASS CENTER OF INTERFERENCEFILTERS
Figure 12. Transmittance of 2B, 2D, 2E, 2FOpals at 450, 550, 650 and 800 nm
TRANSMITTANCE
--. 2B
-0- 2D
-O- 2E
-- 2F
800
I\)00
IIa
II II I
I--- iII
I I---- - i II-
i iII I i~~~~~~~~II Ii
I I
IIII
I I
WI - Ii i0 - -41I II I---
I I
I
III
I
I
I
1.IIIi -1IIIs I
I
t�----·----·--� I --
I ----� , -
�---I
I lI .--
I
I
1.00E-06
1.00E-07
i nnFt-nA
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~-J_ ----_- b
__ I
_~~~~ 1
-- 2G
-- 2H
-a- 21
-o- 2K
50 NM 550 NM 650 NMBANDPASS CENTER OF INTERFERENCE
FILTERSFigure 13. Transmittance of 2G, 2H, 21, 2K
Opals at 450, 550, 650, and 800 nm
TRANSMITTANCE
800 NM
CD
--
I 0%FVp" Ijq
4,1
1 .OOE-07
I .OOE-08
1 .OOE-09
-- 2L
-- 2P
-- 2Q
_i i i i i lii.
.... i i i i i i i i i i i i i i i i i i J i i i-ii
i- i iiii
v~~~~~~~~LIl I
7~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ i i,
450 NM
Figure14.Opals
550 NM 650 NMBANDPASS CENTER OF INTERFERENCE
FILTERSTransmittance of 2L,_ 20, 2P, 2Qat 450, 550, 650, and 800 nm
800 NM
A)CO
TRANSMITTANCE
