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Optronic Laboratories, Inc.
Sphere Standards andStandard SpheresDr. Richard Young
Optronic Laboratories, Inc.
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Optronic Laboratories, Inc.
Sphere ApplicationsStandard Spheres
Total flux measurements All the light from a lamp is measured
Irradiance measurements Light at a surface is measured
Sphere standards
Large uniform source is used for calibrationof instruments
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Optronic Laboratories, Inc.
Total FluxAn integrating sphere has several
interesting properties:
Any part of the sphere surface sees allother parts of the sphere surface equally. This means a detector at any point on the surface can measure the total
power in the entire sphere.
Reflections from the sphere wall add to the
lamp power, giving more power inside thesphere than the lamp is generating.
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Optronic Laboratories, Inc.
Total FluxThe lamp is placedin the center.
A baffle preventsdirect light hitting
the detector.
The sphere wallsand baffle are
highly reflective.
Lamp
Baffle
Detector
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Optronic Laboratories, Inc.
Total FluxLight from thelamp hits the
sphere wall equallyin almost all
directions
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Optronic Laboratories, Inc.
Total Flux
Light from thelamp hits thesphere wall equallyin almost all
directions ...but there are
variations insphere response.
Shadow
area
Partial Shadow
area
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Total Flux
In these shadowareas, the firststrike (lightdirectly from the
lamp) is not fullymeasured.
A sphere cannothave PERFECTresponse.
Shadow
area
Partial Shadow
area
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Total FluxAlthough perfect response is not
attainable with this design, practicalspheres can come very close.
How close they come depends onattention to small details of design.
An expert is someone who knows some of the
worst mistakes that can be made in his subjectand how to avoid them.
- W. Heisenberg
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Total Flux
Response is bestviewed on a radargraph.
Response varies
with sphere sizeand reflectivity.
If a reflectivity of
95% is used
0
0.2
0.4
0.6
0.8
1
0.5 m sphere
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Total Flux
Response is bestviewed on a radargraph.
Response varies
with sphere sizeand reflectivity.
If a reflectivity of
95% is used
0
0.2
0.4
0.6
0.8
1
1.0 m sphere
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Total Flux
Response is bestviewed on a radargraph.
Response varies
with sphere sizeand reflectivity.
If a reflectivity of
95% is used
0
0.2
0.4
0.6
0.8
1
2.0 m sphere
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Total Flux
A reflectivity of98% or more ismore common forUS manufactured
spheres.0
0.2
0.4
0.6
0.8
1
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Total Flux
Some Europeanstandardsrecommend 80%reflectivity.
But this giveslarge geometricerrors.
0
0.2
0.4
0.6
0.8
1
Note the higher
response in places
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Total Flux
This high responseis caused byreflections fromthe detector side
of the baffle.
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Total Flux
This high responseis caused byreflections fromthe detector side
of the baffle. It is present in all
spheres, but someare much worsethan others.
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Total Flux
All the prior sphereresponses had onething in common:
The detector had a
cosine collector on it. If we remove the
cosine collector
Radial Response Map
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1
2 3 45 6 7
89 1 01 1 1 2
1 31 4 1 5
1 61 7
1 81 9
2 02 1
2 2
2 32 42 5
2 6
2 72 8
2 9
3 03 1
3 2
3 3
3 43 5
3 6
3 7
3 83 9
4 0
4 1
4 2
4 3
4 4
4 5
4 6
4 7
4 8
4 9
5 0
5 1
5 25 3
5 4
5 5
5 6
5 7
5 8
5 9
6 0
6 1
6 2
6 3
6 4
6 5
6 6
6 76 8
6 9
7 0
7 1
7 2
7 3
7 4
7 5
7 6
7 7
7 8
7 9
8 0
8 1
8 28 3
8 4
8 5
8 6
8 7
8 8
8 9
9 0
9 1
9 2
9 3
9 4
9 5
9 6
9 79 8
9 9
1 00
1 01
1 02
1 03
1 04
1 05
1 06
1 07
1 08
1 09
1 1 0
1 1 1
1 1 21 1 3
1 1 4
1 1 5
1 1 6
1 1 7
1 1 8
1 1 9
1 2 0
1 2 1
1 2 2
1 2 3
1 2 4
1 2 5
1 2 6
1 2 71 2 8
1 2 9
1 3 0
1 3 1
1 3 2
1 3 3
1 3 4
1 3 5
1 3 6
1 3 7
1 3 8
1 3 9
1 4 0
1 4 1
1 4 21 4 3
1 4 4
1 4 5
1 4 6
1 4 7
1 4 8
1 4 9
1 5 0
1 5 1
1 5 2
1 5 3
1 5 4
1 5 5
1 5 61 5 71 5 8
1 5 9
1 6 01 6 1
1 6 2
1 6 3
1 6 41 6 5
1 6 6
1 6 71 6 8
1 6 91 7 0
1 7 11 7 2
1 7 31 7 4
1 7 51 7 6
1 7 71 7 8
1 7 91 8 01 8 1
1 8 21 8 31 8 4
1 8 51 8 61 8 71 8 81 8 91 9 0
1 9 11 9 21 9 3
1 9 4
1 9 51 9 61 9 71 9 81 9 92 00
2 012 022 032 042 05
2 062 072 08
2 092 1 0
2 1 12 1 2
2 1 32 1 4
2 1 5
2 1 62 1 72 1 8
2 1 9
2 2 02 2 1
2 2 2
2 2 32 2 4
2 2 5
2 2 6
2 2 72 2 8
2 2 9
2 3 02 3 12 3 2
2 3 3
2 3 4
2 3 5
2 3 6
2 3 7
2 3 8
2 3 9
2 4 0
2 4 1
2 4 2
2 4 3
2 4 4
2 4 52 4 6
2 4 7
2 4 8
2 4 9
2 5 0
2 5 1
2 5 2
2 5 3
2 5 4
2 5 5
2 5 6
2 5 7
2 5 8
2 5 9
2 6 02 6 1
2 6 2
2 6 3
2 6 4
2 6 5
2 6 6
2 6 7
2 6 8
2 6 9
2 7 0
2 7 1
2 7 2
2 7 3
2 7 4
2 7 52 7 6
2 7 7
2 7 8
2 7 9
2 8 0
2 8 1
2 8 2
2 8 3
2 8 4
2 8 5
2 8 6
2 8 7
2 8 8
2 8 9
2 9 02 9 1
2 9 2
2 9 3
2 9 4
2 9 5
2 9 6
2 9 7
2 9 8
2 9 9
3 00
3 01
3 02
3 03
3 04
3 053 06
3 07
3 08
3 09
3 1 0
3 1 1
3 1 2
3 1 3
3 1 4
3 1 5
3 1 6
3 1 7
3 1 8
3 1 9
3 2 03 2 1
3 2 2
3 2 3
3 2 4
3 2 5
3 2 6
3 2 7
3 2 8
3 2 9
3 3 0
3 3 1
3 3 2
3 3 3
3 3 4
3 3 53 3 6
3 3 7
3 3 8
3 3 9
3 4 0
3 4 1
3 4 2
3 4 3
3 4 4
3 4 5
3 4 6
3 4 7
3 4 8
3 4 93 5 0
3 5 1
3 5 2
3 5 33 5 4
3 5 5
3 5 6
3 5 73 5 8
3 5 9
3 6 03 6 1
3 6 23 6 3
3 6 43 6 5
3 6 63 6 7
3 6 83 6 9
3 7 03 7 1
3 7 23 7 33 7 4
3 7 53 7 63 7 7
3 7 83 7 93 8 03 8 1 3 8 23 8 3
3 8 43 8 53 8 6
Cosine collectorremoved
Equatorial
Angle
...the response ofeven the bestsphere is destroyed.
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Total FluxMany Sources are highly directional
Fluorescent Lamps
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Total FluxMany Sources are highly directional
LEDs
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Total Flux
A sphere has areasof uniformresponse (green).
And non-uniform
areas (red). If the source is
highly directional,it should be
pointed at a greenarea for the bestresults.
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Total Flux
The green area isbigger for largerspheres.
The red area is
bigger for largerbaffles.
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Total FluxGeometrically, the highest
accuracies are obtained by orientinglamps so the maximum output isdirected at areas of uniformresponse.
Highly reflective coatings give muchlower geometrical errors, regardlessof orientation, than less reflectivecoatings.
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Total FluxHowever,Anything placed inside spheres,
including lamps, holders, sockets
and cables, can absorb light andchange the sphere throughput.
The higher the reflectivity of the
sphere, the bigger the change tothroughput when something isplaced inside.
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Total Flux
0.01%
0.10%
1.00%
10.00%
100.00%
0.00001% 0.0001% 0.001% 0.01% 0.1%
Object in Sphere as % of Sphere Volume
%C
hangeinSphereT
hroughput
99%
98%
97%
95%
90%
80%
Coating
Reflectivity
Here, the effective reflectivity is
changed by just 0.25%
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Total FluxThis example is for a black sphericalobject in the center of the sphere.
Actual changes will depend on the
objects reflectance, shape andposition in the sphere, and can belarger than shown.
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Total FluxThe lamp used in calibration and thelamp to be measured are rarely the
same.
Different changes in throughputbetween these lamps will meanresults will be wrong unlessthroughput changes are alsomeasured.
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Total Flux
An auxiliary lamp,which is housedpermanently in thesphere, is used to
measure changesin throughput.
Auxiliary
Lamp
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Total FluxThe auxiliary lamp
is powered upwhile the standardor test lamp is inthe sphere. But not switched on.
The ratio of signalsis the change inthroughput. This is part of the
calibrationprocedure.
Auxiliary
Lamp
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Total FluxGood total flux measurementsrequire:
A large high reflectivity sphere.
Small, well designed, baffles.A cosine collection detector at the sphere
wall.
An auxiliary lamp.
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Total FluxIt also helps to have:
Uniform measurement procedures. e.g. keep a constant time between powering up a lamp and measuring it.
Dedicated software to guide the userthrough calibration and measurement.
Accurate power supplies for lamps.
NIST traceable calibration lamps.
Heisenberg experts for support.
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Irradiance
Irradiance is thelight flux fallingonto a surface.
The light can come
from any directionand may be frommultiple sources.
The total lighthitting the surfacemust be measured.
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Irradiance
The apparent area of a surface changeswith angle.
This is called the cosine law.A measurement device that follows the
cosine law is called a cosine collector.
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Irradiance
An integratingsphere with aninput port makesan excellent cosine
collector ...provided certain
design rules arefollowed.
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IrradianceThe first rule is:
If light does notobey the cosine lawwhen entering thesphere, there is little
the sphere can do tocompensate.
This is easier saidthan done because
a sphere has 2sides the insideand the outside.
Magnify
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IrradianceThe first rule is:
If light does notobey the cosine lawwhen entering thesphere, there is little
the sphere can do tocompensate.
This is easier saidthan done because
a sphere has 2sides the insideand the outside.
Inside
Outside
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Irradiance
This creates atube effect thatstops some of thelight at higher
angles entering thesphere directly.
Some
light is
blocked
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Irradiance
So, as the angle is changed, the cosineresponse gets worse and worse.
A circle obeys the cosine law
A tube does not obey the cosine law
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Irradiance
The answer issimple.
Make the outside ofthe sphere flat to
meet the inside atthe port.
Thus eliminatingthe tube.
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Irradiance
So now the lightenters the sphereobeying the cosinelaw.
Rule #2: Add a
baffle to prevent
direct light hitting
the detector.
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Sphere Standards
If we replace thedetector with alamp, this sphereis now a source
instead of acollection optic.
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Sphere Standards
The uniformity ofthe sphere outputdepends on theway the light
enters the sphere.Cosine diffusers at
the input giveuniform output but
throw away a lot oflight.
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Sphere Standards
It is difficult toachieve high lightlevels and gooduniformity.
Especially if theoutput level alsoneeds to be variable.
One option is to
change the design.
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Sphere Standards
By making surethe input light onlyhits the baffle, theoutput light is
randomized. If the input light
level varies, sodoes the output.
To track changes,we add a monitor.
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Sphere StandardsThis monitor is
baffled so it onlysees the outputlight, not the input.
So it can becalibrated to showthe output levels
directly.
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Sphere StandardsThe amount of
light entering thesphere can becontrolled by
varying a slit.
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Sphere StandardsNon-imaging
collection mirrorscan increase thelight intensity.
Provided thebeam isapproximately
uniform and over-fills the largestslit.
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Sphere Standards
Slits can be linear (1D) The width varies
Or area (2D)
The width and height vary together
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Sphere StandardsA 2D slit provides better control over a
wide range of levels than a linear (1D) slit.
1.0E-06
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Normalized slit width
Normalizedoutput
1D slit 2D slit
With a 1D slit, the output drops
almost vertically at small slit widths
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Sphere StandardsExpressed another way, the slit resolution
needed to change the output by 1%
1.0E-08
1.0E-07
1.0E-06
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-06 1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00
Normalized output
Norm
alizedchangeinslitfor1%
changeinoutput
1D slit 2D slit
At a resolution of 10-5, the 1D slit cannot
control the output below below 3 decades
2D slits can give
up to 1000 times
better control
than 1D slits
2D slits are clearly
superior for high dynamic
range sources
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Sphere Standards
By careful design and materialselection, one can achieve:
High uniformity of output
High maximum levels of output
Very stable operation
6 or more decades of light level adjustment
Direct reading of the output
Easy adjustability and setting to any level
An almost constant spectrum at all levels
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Sphere Standards andStandard SpheresDr. Richard Young
Optronic Laboratories, Inc.
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