Labsphere RSA-PE-20 User manual

AQ-00073-000, Rev. 7
RSA-PE-20
Introduction .................................................................................. 1
Unpacking and Inspection ........................................................... 2
Installation and Assembly ........................................................... 3
Installing the Accessory PC Board Kit ................................ 3
Installing the Accessory in Your Sample Compartment ...... 6
Accessory Alignment ........................................................... 6
Diagnostic Scans .................................................................. 8
System Description and Operation ........................................... 11
Integrating Sphere .............................................................. 11
Transfer Optics .................................................................. 12
Loading the Sample Holders ............................................. 12
Theory of Operation .......................................................... 13
Operating Procedures for the RSA-PE-20 Accessory ....... 21
Maintenance ............................................................................... 26
General Information ........................................................... 26
Mirror Cleaning Procedure ................................................ 26
Spectralon Cleaning Instructions ....................................... 26
Appendix A Specifications ........................................................ 27
Appendix B Typical Spectralon Reflectance Values ............... 28
Appendix C Substitution Error Sample Scans ....................... 29
Appendix D Reflectance Glossary ............................................ 31
Appendix E Recommended Reading ....................................... 33

AQ-00073-000, Rev. 7 1
Introduction
The Labsphere RSA-PE-20 is a diffuse reflectance and transmittance accessory designed for a
number of PerkinElmer UV-VIS spectrometers. The list of compatible spectrometers includes
the Lambda 25/35/45 series as well as the older models Lambda 2/12/14/20/40.
The RSA-PE-20 accessory essentially is an optical bench, including transfer optics and an inte-
grating sphere, that fits neatly into the sample compartment of the spectrometer instrument. An
illustration of the accessory is provided in Figure 1. Since the reference beam passes through the
accessory unimpeded, the RSA-PE-20 accommodates either single or double beam configura-
tion spectrometer. The accessory, however, is a single beam device. The accessory is ideal for
measuring diffuse reflectance or transmittance for scientific and industrial applications including
color, thermochromism, composition of opaque materials such as biological or geological speci-
mens. The geometric measurement capabilities of the accessory include 8°/hemispherical
reflectance, 0°/diffuse reflectance, 0°/hemispherical transmittance and 0°/diffuse transmittance.
Sample holders for the test and reference samples are supplied with the accessory.
This manual provides the installation and operating procedures for the RSA-PE-20. The opera-
tion of your spectrometer system with the integrating sphere accessory installed, however, essen-
tially is unchanged. Since the procedures may vary from one spectrometer to the next, you
should consult your PerkinElmer instruction manual when taking reflectance and transmittance
measurements.
Figure 1. The RSA-PE-20 fits into the sample compartment of
your PerkinElmer spectrometer.

AQ-00073-000, Rev. 7 2
Unpacking and Inspection
The RSA-PE-20 was thoroughly inspected and calibrated before shipping and should be ready to
operate after completing the set-up instructions. All Labsphere instrumentation is packaged and
shipped in reinforced shipping containers. The RSA-PE-20 is sold in two different configura-
tions, depending on the source of your order and the model number of your PerkinElmer UV/
VIS spectrometer. Carefully check the components after unpacking for any damage that may
have occurred during shipping. If there is any such damage, file a claim immediately with the
freight carrier and contact the Labsphere Customer Service Department at
![phone].
Standard components accompanying all orders:
• RSA-PE-20 Accessory
• One 8°and two 0°sample holders
• Accessory cable assembly
• Cuvette Holder
• Tool Kit
• USRS-99-010 Uncalibrated Spectralon Reflectance Standard
• Instruction Manual
If your system was ordered directly from Labsphere, an instrument modification kit will accom-
pany the accessory and the components listed below will be labeled by Labsphere part number. If
your RSA-PE-20 was ordered through your PerkinElmer representative, the modification kit will
be provided directly from PerkinElmer and installed by their qualified technician.
• Accessory PC Board Kit part No. AS-00820-000 for use with Lambda 2 and
Lambda 12 model spectrometers
• RSAPE20 wiring harness Part No. AS-01803-000 for use with the Lambda 14
spectrometer
Optional Components
• Calibrated SRS-99-010 Spectralon Reflectance Standard
• Custom sample holders

AQ-00073-000, Rev. 7 3
Installation and Assembly
In all cases, the RSA-PE-20 Accessory PC Board Kit installation should be accomplished by a
qualified PerkinElmer technician. A technician is available through your local PerkinElmer rep-
resentative. Once the accessory is installed, you can disconnect and reconnect the accessory as
often as desired using the accessory installation procedure below. The RSA-PE-20 is compatible
with a number of older Lambda spectrometer models as well as the current Lambda 25/35/45
series. The RSA-PE-20 installation requirements for each spectrometer are different.
Installing the Accessory PC Board Kit
If your PerkinElmer spectrometer is one of the current Lambda Series, i.e. Lambda 25/35/45, no
kit installation is required - the instrument is already configured to accept the RSA-PE-20 acces-
sory. If your spectrometer is an older Lambda series model, an accessory PC board kit will need
to be installed into the spectrometer instrument before installation of the RSA-PE-20 accessory.
Kit Modifications to the Lambda 2 and Lambda 12 Spectrometers
The Accessory PC Board Kit for the Lambda 2 and Lambda 12 spectrometers includes two com-
ponents: a ribbon cable, Part No. AS-01803-100, and the EC-12201-000 accessory side plate.
1. Disconnect electric power from the spectrometer. Remove the sample compartment
J18
J8
Figure 2. Part No. EC-12201-000 and AS-01803-100 of the accessory PC board kit used in modifications to
the Lambda 2 or Lambda 12 instrument.

AQ-00073-000, Rev. 7 4
access door and open the top cover to gain access to the instrument.
2. Loosen the two side plate mounting fasteners on the top, inside end of the plate.
One of these screws fastens the cover leash - its removal will permit the spectrome-
ter cover to fall back.
3. Remove the large ribbon cable connector from the header on the existing side plate
circuit board and lift the plate out of the instrument.
4. Install the new side plate, Part No. EC-12201-000, into the instrument in the same
orientation as the original plate.
5. Reconnect the large ribbon cable into the same header connector located on the
newly installed side plate.
6. Unplug the detector cable that runs from the detector module to the motherboard at
connector P8. Route the same end of this cable to header P8 on the newly installed
side plate. Note that the header is keyed.
7. Plug the gray connector on Part No. AS-01803-100 into the side plate header
labeled P18. Note that this connector is keyed.
8. Route the black connector of Part No. AS-01803-100 to the instrument mother-
board at P8. This connector is keyed.
9. The individual wires attached to Part No. AS-01803-100, labeled +5V and GND,
should be plugged into the motherboard electrical studs labeled 5V/REL1 and
GND respectively.
10. Tighten the two mounting screws that secure the side plate to the instrument enclo-
sure.
11. Proceed to the accessory installation procedure.
Kit Modifications to the Lambda 14 Spectrometer
The Accessory PC Board Kit for the Lambda 14 spectrometer includes three components: a rib-
bon cable, Part No. AS-01803-000, and the two components listed above, Part No. AS-01803-
100 and EC-12201-000.
1. Disconnect electric power from the spectrometer. Remove the sample compartment
access door and open the top cover to gain access to the instrument.
2. Loosen the two side plate mounting fasteners on the top, inside end of the plate.
One of these screws fastens the cover leash - its removal will permit the spectrome-
ter cover to fall back.
3. Remove the large ribbon cable connector from the header on the existing side plate
P1
P3
Figure 3. Part No. AS-01803-000 used in modification of the Lambda
14 spectrometer.

AQ-00073-000, Rev. 7 5
circuit board and lift the plate out of the instrument.
4. Install the new side plate, Part No. EC-12201-000, into the instrument in the same
orientation as the original plate.
5. Reconnect the large ribbon cable into the same header connector located on the
newly installed side plate.
6. Remove the cable running from the detector module to the motherboard header at
P9. This cable will not be used.
7. Plug the single connector end of Part No. AS-01803-000 into the header at the
detector module.
8. Connect the gray 16-pin connector on the opposite end of Part No. AS-01803-000
into the P9 header on the motherboard.
9. Connect the remaining black connector end of Part No. AS-01803-000 into the
newly installed accessory side plate header labeled P8 as shown in Figure 4.
10. Plug the gray connector on Part No. AS-01803-100 into the side plate header
labeled P18. Note that this connector is keyed.
11. Route the black connector of Part No. AS-01803-100 to the instrument mother-
board at P8. This connector is keyed.
12. The individual wires attached to Part No. AS-01803-100, labeled +5V and GND,
should be plugged into the motherboard electrical studs labeled +5V and GNDA
respectively.
13. Tighten the two mounting screws that secure the side plate to the instrument enclo-
sure.
14. Proceed to the accessory installation procedure.
Kit Modifications to the Lambda 20 and Lambda 40 Spectrometers
The accessory kit and associated cables for modification to the Lambda 20 and Lambda 40 spec-
trometers are provided by PerkinElmer. Consult your PerkinElmer technician for installing the
accessory kit for these instruments.
Figure 4. Making the detector cable hookup to the side plate.

AQ-00073-000, Rev. 7 6
Installing the Accessory in Your Sample Compartment
1. Unplug the instrument and remove the sample compartment cover and any accesso-
ries within the sample compartment.
2. Mount the 8°sample holder against the reflectance port of the integrating sphere, tight-
ening the two set screws using a 0.035" hex wrench.
3. Mount a 0°sample holder against the transmittance port of the integrating sphere, tight-
ening the two set screws with the 0.035" hex wrench.
4. String the accessory connector cable through the sample compartment wall cable
run leading to the side plate. Attach the 9-pin connector to the 9-pin port marked
INTEGRATING SPHERE (J88) on the side of the instrument.
5. Plug the remaining end of the detector cable into the mating plug located in the
preamp board enclosure underneath the base of the accessory.
6. Carefully lower the accessory into the sample compartment and slide it horizontally
until the four self-retaining mounting screws on the accessory base mate with the
sample compartment base. Tighten the screws with a 2.5 mm hex wrench.
7. Lower the instrument cover, replace the sample compartment door and reconnect
the power plug to the instrument.
Accessory Alignment
The accessory was aligned precisely at the Labsphere factory before shipping and should be
ready for use upon installation. Check the alignment of the accessory before using and, if neces-
sary follow this procedure to adjust the optical alignment.
1. Load the Spectralon reflectance standard at the reflectance port.
2. Close the sample compartment, turn on the instrument and boot your UV Winlab
software application.
Accessory Cable Accessory Preamp Board
Figure 5. The accessory cable connects the sphere detector
to the instrument electronics.

AQ-00073-000, Rev. 7 7
WARNING: Do not touch the mirror surfaces with your bare fingers.
3. Configure the instrument for white light operation by entering 0 nm in the Instru-
ment section of the Lambda Manual Control Display.
4. Dim the room lights.
5. Place a piece of white translucent tissue paper in front of mirror M2. Adjust the
thumbscrews on M1 so the beam is centered.
6. Place the white paper in front of mirror M3. Adjust the thumbscrews of M2 so that
the beam is centered on mirror M3.
7. Place the white paper in front of the transmittance port of the sphere. Adjust the
thumbscrews of M3 so that the beam enters the transmittance port without clipping
the edges of the port. The transmittance port can be viewed by looking into the
sphere through the front reflectance port.
8. Place a piece of lens paper at the reflectance port. Adjust the thumbscrews of mir-
ror M3 so that the beam is centered on the reflectance port. If the beam can not be
centered, repeat the previous steps making minor adjustments in either direction so
the beam is centered on the reflectance port and is not clipped at the transmittance
port.
9. The accessory is now properly aligned. You can now proceed to the next section on
diagnostic scans.
M1
M2
M3
Sample Beam
Reference Beam
Transmittance
Port
Reflectance
Port
Integrating Sphere
Figure 6. Optical setup of the RSA-PE-20 accessory. Make sure
your UV Winlab software is configured for the sample beam at
the front port.

AQ-00073-000, Rev. 7 8
Diagnostic Scans
This section of the instruction manual contains the procedures for a series of diagnostic scans on
your accessory. These scans were performed on your accessory and our instrument before we
shipped the RSA-PE-20 to you.You should perform these scans again on your instrument after
installing the accessory to validate proper operation of the accessory. The scan results should be
retained for future use. If problems develop with your accessory in the future, repeat the diagnos-
tic scans and send us copies for analysis.
The quality assurance documentation includes a copy of the diagnostic scans we performed at
Labsphere. The instrument parameters used for each diagnostic scan are listed under the test
results. You should use these same parameters for your diagnostic scans. The user should note
that a background correction is executed as part of the 100% Baseline Scan and used for all sub-
sequent diagnostic scans.
Sample Beam Energy Scan
The energy scan is an indicator of accessory throughput.
1. Set up instrument parameters to conform to those listed in Table 1. These parame-
ters settings should be identical to those listed in the quality scans accompanying
your shipment.
2. Load the reflectance standard at the sample reflectance port. The 8° wedge should
be installed.
3. Close the sample compartment door. Execute a scan from your UV Winlab soft-
ware and save the results.
Instrument Parameter Setting
Lamps UV/Vis ON
Beam Single Beam
Method Scan
Data Interval 1 nm
Abscissa Start 250
Abscissa End 1100
Slit Mode Fix
Slit 2 nm
Integration Time 0.08 s
Ordinate Mode E1
Sample Beam Front
Attenuators 100%
Table 1. Instrument setup for sample beam energy scan.

AQ-00073-000, Rev. 7 9
100% Baseline Scan
This scan provides the direct comparison of a sample scan to the baseline at the same sphere
configuration. The 100% baseline scan is used for general accessory troubleshooting purposes.
1. Set up instrument parameters as listed in Table 2.
2. Load the reflectance standard at the sample reflectance port.
3. Close the sample door and perform a background correction scan.
4. Perform a sample scan with the same sphere configuration and save the results.
Blocked Beam Scan
The blocked beam or zeroline scan records the detector noise level from the accessory. You
should use the baseline recorded during the 100% baseline scan in the previous procedure, or
execute a new baseline if it is not available.
1. Set up instrument parameters as listed in Table 2.
2. Load a metal plate or other non-transmitting sample into the transmittance port.
The sample should be absorbent such that reflected light from the sample beam
does is not sensed by the reference beam detector. Leave the reflectance standard at
the sample reflectance port.
3. Close the sample compartment door. Execute a sample scan and save the results.
Light Trap Zeroline
The light trap scan determines if the sample beam is correctly centered on the sample reflectance
port. You should use the baseline recorded during the 100% baseline scan, or execute a new
baseline if it is not available.
Instrument Parameter Setting
Lamps UV/Vis ON
Beam Double Beam
Method Scan
Data Interval 1 nm
Abscissa Start 250
Abscissa End 1100
Slit Mode Fix
Slit 2 nm
Integration Time 0.08 s
Ordinate Mode %R
Sample Beam Front
Attenuators 100%/100%
Table 2. Instrument setup for the 100% Baseline Scan.

AQ-00073-000, Rev. 7 10
1. Set up instrument parameters as listed in Table 2.
2. Leave the transmittance port empty and install a light trap over the sample reflec-
tance port.
3. Close the sample compartment door. Execute a sample scan and save the results.
Time Drive Scan
The time drive scan indicates the stability of the light source and the sphere detector.
1. Set up instrument parameters as listed in Table 3.
2. Load a reflectance standard at the sample reflectance port and close the sample
compartment door.
3. Perform an Autozero scan at 500 nm.
4. Run a time drive at 500 nm and save the results.
Instrument Parameter Setting
Lamps UV/Vis ON
Beam Double Beam
Method Time Drive
Total Time 180s
Wavelength 500 nm
Slit Mode Fix
Slit 2 nm
Integration Time 0.24 s
Ordinate Mode %R
Sample Beam Front
Attenuators 100%/100%
Table 3. Instrument setup for the Time Drive Scan.

AQ-00073-000, Rev. 7 11
System Description and Operation
The RSA-PE-20 accessory is specifically designed to measure the reflectance or transmittance of
solids, liquids, powders, or other small objects that can fit in the transmittance or reflectance
ports. Basic components of the accessory include the integrating sphere, transfer optics and
detector preamplification module. Although the RSA-PE-20 is a single beam construction, the
accessory is compatible with the double beam configuration of the Lambda Series spectrometers
and your UV Winlab software.
Integrating Sphere
The integrating sphere assembly, shown in Figure 7, is 50 mm in diameter and constructed of
Spectralon. Spectralon is the same material used on the reflectance standard supplied with the
accessory. Typical reflectance values for Spectralon are provided in Appendix B.
The RSA-PE-20 integrating sphere features two ports: a transmittance port where the beam
enters the sphere, and a sample reflectance port. The illustration in Figure 7 shows the accessory
integrating sphere along with the transfer optics and preamplification board. The silicon detector
is mounted directly to the preamp board underneath. Two Spectralon baffles inside the sphere
shield the detector sensor from direct illumination by the ports.
The sample holders, mounted either at the transmittance or sample reflectance port, clamp the
Sample Reflectance Port
Port Frame Port Frame
Transmittance
Port
Preamp Board
Detector Baffles
Spectralon Sphere
Detector
Figure 7. Construction of the integrating sphere.

AQ-00073-000, Rev. 7 12
reflectance standard or sample against the respective port. Three sample holders are provided
with the accessory for mounting a sample either at 0° or 8° angles of incidence. The sample
holders mount directly to the port frame at each end of the sphere.
The sphere detector is a silicon photodiode installed at the detector port at the bottom of the inte-
grating sphere. The photodiode operates in photovoltaic mode; the detector output is conditioned
by the preamplifier board for compatibility with the instrument electronics. Detailed information
on the photodiode is provided in Appendix A.
Transfer Optics
The transfer optics of the RSA-PE-20 accessory direct the spectrometer sample beam to the
transmittance port on the integrating sphere. The sample beam is the front beam in the accessory.
As shown in Figure 6, mirrors labeled M1, M2 and M3 guide the sample beam through the sam-
ple transmittance port and onto the sample reflectance port at normal incidence. All mirrors are
AlMgF2coated. Mirrors M1 and M2 are flat. Mirror M3 is a spherical mirror that condenses the
sample beam onto the target sample. The reference beam passes through the accessory unim-
peded.
Loading the Sample Holders
Both transmittance and sample reflectance ports are 5/8-inch holes at either end of the accessory
sphere. The transmittance port is closest to mirror M3. A sample holder clamp fits over a dove-
tail extension located underneath each port, securing the reflectance standard or sample tightly
against the port opening.
The sample holder with the 8° wedge should be installed at the sample reflectance port when
taking 8° hemispherical measurements. The 0° sample holder should be installed at the sample
reflectance port when taking diffuse measurements with the specular component excluded. Fig-
ure 8 describes the proper way to load the sample holder. When mounting a sample or reflec-
tance standard at any port, you should make sure the reflecting surface lies flat against the wedge
and completely fills the port surface area.
To remove the sample holder at the sample reflectance port, loosen the set screws along the side
of the respective wedge and remove the entire sample holder assembly.

AQ-00073-000, Rev. 7 13
In some instances, you may need to load a light trap at the sample reflectance port. The light trap
can be loaded using either wedge and fits into the sample holder in the same manner as a reflec-
tance standard.
Theory of Operation
During the traditional measurement of sample absorption by a spectrometer, the relationship
between absorptance and transmittance of the sample beam is described by the Kirchoff equa-
tion:
where:
A = absorptance and
T = transmittance.
When measuring the absorptance of a sample in the sample compartment, the detector signal of
the spectrometer represents the portion of the sample beam that is not absorbed or scattered by
the sample.
When using the RSA-PE-20 accessory, it is convenient to use the Kirchoff relationship:
Sample or Standard
Clam
p
8 Wedge
Figure 8. Loading a standard or sample into the sample
holder.
AT+1
,
=

AQ-00073-000, Rev. 7 14
where R = reflectance of the sample beam at the sample surface.
During reflectance measurements, the reflected component of the sample beam is collected by
the integrating sphere and detected by the sphere detector. The detector signal represents the part
of the sample beam that is not transmitted and not absorbed by the sample substance.
Double Beam Spectroscopy
In a double-beam, ratio-recording spectrometer, the measurement of reflectance factor and trans-
mittance involves the calculation of a baseline based upon values recorded in a background cor-
rection scan. Background correction is used to compensate for changes in sphere efficiency due
to the introduction of the sample into the system and for any imbalance in the energy of the sam-
ple and reference beams. This correction is performed automatically by the instrument and is the
same routine used during spectroscopy evolutions without the accessory. The theoretical basis
for the background correction, as it applies to simple reflectance and transmittance measure-
ments, is explained briefly in this section.
Radiation from the instrument illumination sources is split into two different beams: the sample
beam and the reference beam. Each beam is interrupted periodically by means of an optical
chopper such that the integrating sphere is illuminated alternately by the two beams. At any
given wavelength, the instrument records the ratio of the signal produced by the detector when
the sphere is illuminated by the sample beam to that when the reference beam is measured.
Therefore, when a background correction scan is performed, for either reflectance or transmit-
tance measurement, the background correction value B recorded by the instrument may be
expressed as:
where:
Esis the energy of the sample beam,
Eris the energy of the reference beam,
ρs is the reflectance factor of the sample at the sample reflectance port
ρr is the reflectance factor of the reference port, and
κsand κrare the efficiency with which the energy reflected from the sample and
reference beams is captured by the sphere and converted into a signal by the
detector.
ATR++ 1
,
=
BEsρsκs
Erρrκr
----------------=Eq. 2

AQ-00073-000, Rev. 7 15
The sphere efficiency factors (κ) for a given sample are a function of many variables including
the spatial distribution of the energy reflected from the sample, the reflectance of the sphere
wall, the geometry of the sphere (location of ports, baffles, etc.) and the efficiency of the detector
itself. Proper measurement procedures make it possible to greatly reduce or eliminate the effect
of such factors on transmittance and reflectance factor measurements.
During the sample scan of a transmittance measurement, the transmittance (Ts) of a non-scatter-
ing sample placed in the path of the sample beam affects the amount of energy entering the
sphere and reaching the sample reflectance port. The sample scan value STof the ratio recorded
by the instrument is:
The value DTdisplayed by the instrument, however, is STdivided by the value B recorded in the
background correction curve:
Therefore, in a transmittance measurement of a non-scattering sample, the value displayed by
the instrument is simply equal to the transmittance of the sample.
In reflectance measurement for a double beam spectrometer, the standard positioned at the sam-
ple reflectance port during the uncorrected background measurement is replaced by another
sample of unknown reflectance ρu. The value SRof the ratio recorded by the instrument during
the sample scan is:
The spectral data displayed by the instrument is:
STTsEsρsκs
Erρrκr
----------------------=Eq. 3a
DTST
B
------TsEsρsκsErρrκr
ErρrκrEsρsκs
---------------------------------------T
s
== = Eq. 3b
SREsρuκu
Erρrκr
-----------------=Eq. 4a

AQ-00073-000, Rev. 7 16
Assuming that the spatial distribution of energy reflected from this unknown sample is the same
as that of the original standard, the efficiency factor (κ) will be the same for both materials:
Therefore, the value displayed by the instrument will simply be equal to the ratio of the reflec-
tance factor of the unknown sample to that of the original standard:
If the reflectance factor of this original standard is known, it can be used as a reference standard
and the reflectance factor of the unknown sample can be derived as follows:
This operation is known as the reference correction to distinguish it from background correction
described in this manual.
The foregoing account involves two simplifying assumptions that bear further discussion. First,
the efficiencies of the integrating sphere, represented by κr,κsand κu, are not necessarily con-
stant under changes in the system, as suggested by Equations 3a and 4a. The introduction of a
reflectance or transmittance sample into the system may change the sphere efficiency. It is com-
monly assumed, however, that the sphere efficiencies will change by the same factor. This factor
is eliminated when the instrument records its ratio, so that the foregoing analysis remains valid.
The second assumption applies to reflectance factor measurements and is represented by Equa-
tion 5. Here it is assumed that the geometric scattering properties of the sample and the reference
DRSR
B
------=EsρuκuErρrκr
ErρrκrEsρsκs
----------------------------------ρuκu
ρsκs
-----------==
Eq. 4b
κuκs
=Eq. 5
DRρu
ρs
-----=Eq. 6a
ρuDRρs
=Eq. 6b

AQ-00073-000, Rev. 7 17
standard are identical. Therefore, the efficiency factor for radiation reflected from each sample
will be the same. If the geometric scattering properties of the sample differ greatly from those of
the reference, significant systematic errors may be introduced into the measurements. Therefore,
the geometric scattering properties of the sample and the reference should be matched as closely
as possible.
Although your PerkinElmer spectrometer may be a double beam instrument, the RSA-PE-20 is
a single beam accessory such that combined operation is in dual beam spectrometer mode. The
value of κrfor your system does not change from background correction to sample scan.
Single Beam Substitution Error Correction
The single beam substitution error, sometimes known as single beam sample absorption error, is
the systematic, predictable, and non-random error inherent in single beam integrating spheres
measuring reflectance or transmittance. The error is caused by the difference in the throughput
of the sphere when the reference makes up a portion of the sphere wall and when the sample is
substituted for the reference. In integrating spheres which are designed for dual beam spectrom-
eters, it is not possible to place both the reference beam and the sample beam into the sphere at
the same time. These dual beam spectrometers employ a beamsplitter to divide the instrument’s
beam between the sample and reference channels.
Thus, even though the dual beam spectrometer has two beams, it is still only possible to utilize a
single beam integrating sphere accessory. Unfortunately, single beam integrating spheres are
subject to substitution errors during sample measurements which must be corrected to obtain
accurate results.
In reflectance measurements the throughput and corresponding measured reflectance is usually
lower when the sample is present since a reference material of high reflectance is used. In trans-
mittance measurements the throughput and the measured transmittance is usually higher when
the sample is present since an open port is typically used as a reference. With spectrometers that
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Physical
Reference
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Reference Detector
(optional)
Sample Beam
Reference Beam
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Sample
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Reference Detector
(optional)
Sample Beam
Reference Beam
Background Scan Sample Scan
Figure 9. Operation of a dual beam spectrometer.

AQ-00073-000, Rev. 7 18
use a chopped signal between sample and reference, this error does not occur. Double beam
spectrometers are able to accept double beam integrating spheres in which both the sample and
reference beams are placed in the sphere. Double beam spectrometers employ a chopper to
divide the instrument beam between the sample, reference, and dark channels. Although your
spectrometer is a double instrument, the RSA-PE-20 is designed as a single beam accessory to
accommodate the size of the instrument sample compartment. Since only the sample beam is
subject to sphere configuration, your RSA-PE-20 measurements are subject to substitution error.
When a sample and a reference are of similar reflectance, the substitution correction is very
small; at worst it may reach as much as 4 - 5%. In quality control applications where a threshold
value is used, this may not be a concern, as the error can simply be built into the threshold. This
is also true if only peak position information is required, as single beam correction only concerns
the photometric scale.
There are both physical and mathematical solutions to the problem of single beam substitution
error. If the sphere is made sufficiently large and the area of the sample is minimized, the error
increasingly diminishes. The use of a large sphere is usually not practical in low cost spectropho-
tometer systems due to signal-to-noise concerns as well a price considerations. The integrating
sphere on your RSA-PE-20 accessory is too small to accommodate the modifications necessary
for a physical solution.
The mathematical solution is simple in theory, but complex in practice. If a reference is used that
is very close in reflectance to the reflectance of the sample, the substitution error becomes negli-
gible. If the user can match the reference reflectance to the reflectance of the sample, or has a
large database of different level photometric reflectance standards, one can set up look-up or cor-
rection tables to correct for the substitution error.
Tables 4 and 5 show reflectance values for twelve levels of photometric gray scale. Table 4 pro-
vides data for gray Spectralon exhibiting diffuse reflectance characteristics under 50%. Table 5
provides the data for Spectralon reflectances greater than 50%. The italicized values are the
actual reflectance of samples of Labsphere SRS-XX gray scale reflectance material. These
reflectance values were determined by measuring the corrected 8°/ hemispherical reflectance
factors using a Labsphere double beam spectrometer and a double beam accessory with a 150
mm diameter integrating sphere. The second set of reflectance values in regular print were mea-
sured on a spectrometer equipped with a Labsphere 50 mm single beam integrating sphere
accessory.

AQ-00073-000, Rev. 7 19
To use the tables, look at the wavelength of interest and find the value under the column marked
L2 corresponding as close as possible to sample reflectance you measured with your single beam
accessory. These values were determined with a PerkinElmer Lambda 2 double beam spectrom-
eter and an RSA-PE-20 single beam accessory. Compare the charted L2 value and your mea-
sured reflectance to the reflectance value under the column marked L19 immediately to its left.
The value under L19 is the actual value of reflectance of the sample at that wavelength - you
may need to interpolate. These values were determined with a Lambda 19 spectrometer and
RSA-PE-19 double beam accessory. The standards used in generating these tables were spec-
trally flat over a wide wavelength range. Supplemental information is available for some materi-
als along with the slope in their reflectance curves. This technique is the same used for
measurements of standards at the National Institute of Standards and Technology (NIST), the
National Physical Laboratory (NPL) and the National Research Council Canada (NRCC). The
data is available at Labsphere on disk. Be advised that these tables should used for diffusely
reflecting materials only.
300
0.009
0.011
0.053
0.065
0.099
0.123
0.192
0.231
0.337
0.394
0.378
0.436
325
0.009
0.011
0.050
0.064
0.095
0.121
0.188
0.230
0.329
0.392
0.371
0.434
350
0.009
0.012
0.050
0.064
0.093
0.121
0.184
0.230
0.323
0.388
0.366
0.432
375
0.009
0.011
0.049
0.064
0.092
0.122
0.182
0.229
0.319
0.387
0.362
0.431
400
0.010
0.011
0.048
0.064
0.091
0.122
0.181
0.230
0.316
0.386
0.360
0.431
425
0.010
0.011
0.048
0.064
0.091
0.122
0.181
0.231
0.314
0.385
0.359
0.431
450
0.010
0.011
0.048
0.064
0.091
0.122
0.182
0.232
0.313
0.385
0.358
0.432
475
0.010
0.011
0.049
0.064
0.091
0.122
0.183
0.233
0.313
0.385
0.359
0.433
500
0.010
0.011
0.049
0.064
0.092
0.123
0.184
0.235
0.313
0.385
0.359
0.433
525
0.010
0.011
0.050
0.065
0.092
0.123
0.186
0.236
0.313
0.385
0.360
0.434
550
0.010
0.011
0.050
0.065
0.093
0.123
0.187
0.238
0.313
0.385
0.361
0.435
575
0.010
0.011
0.050
0.066
0.093
0.124
0.188
0.239
0.314
0.386
0.362
0.436
600
0.010
0.011
0.051
0.066
0.094
0.124
0.190
0.240
0.315
0.386
0.364
0.436
625
0.010
0.011
0.051
0.066
0.095
0.125
0.191
0.241
0.316
0.386
0.365
0.436
650
0.011
0.011
0.052
0.067
0.095
0.125
0.193
0.243
0.317
0.386
0.366
0.437
675
0.011
0.011
0.052
0.067
0.096
0.126
0.194
0.243
0.317
0.386
0.367
0.437
700
0.011
0.011
0.053
0.068
0.097
0.126
0.195
0.245
0.318
0.387
0.368
0.438
725
0.011
0.011
0.054
0.068
0.098
0.127
0.196
0.245
0.319
0.388
0.369
0.439
750
0.011
0.011
0.054
0.068
0.099
0.127
0.198
0.246
0.320
0.388
0.370
0.440
775
0.012
0.011
0.055
0.069
0.100
0.128
0.199
0.247
0.321
0.388
0.371
0.440
800
0.012
0.011
0.056
0.069
0.101
0.129
0.200
0.249
0.323
0.391
0.372
0.440
825
0.012
0.011
0.057
0.070
0.102
0.130
0.202
0.249
0.324
0.391
0.374
0.442
850
0.013
0.011
0.057
0.071
0.102
0.131
0.203
0.251
0.325
0.391
0.375
0.442
875
0.013
0.012
0.058
0.072
0.104
0.132
0.204
0.251
0.326
0.391
0.376
0.442
900
0.013
0.012
0.059
0.072
0.104
0.133
0.206
0.252
0.327
0.391
0.377
0.441
925
0.014
0.012
0.059
0.072
0.105
0.133
0.207
0.252
0.328
0.391
0.378
0.441
950
0.014
0.012
0.060
0.072
0.106
0.134
0.208
0.252
0.329
0.391
0.379
0.442
975
0.014
0.013
0.061
0.073
0.107
0.134
0.209
0.254
0.331
0.392
0.380
0.443
1000
0.014
0.012
0.061
0.073
0.108
0.135
0.210
0.255
0.332
0.392
0.381
0.443
1025
0.014
0.012
0.062
0.075
0.108
0.135
0.210
0.256
0.333
0.393
0.382
0.443
1050
0.014
0.012
0.062
0.075
0.109
0.135
0.211
0.256
0.334
0.392
0.383
0.444
1075
0.014
0.012
0.062
0.075
0.110
0.136
0.212
0.257
0.334
0.394
0.383
0.444
1100
0.015
0.012
0.063
0.076
0.110
0.138
0.213
0.257
0.336
0.396
0.385
0.445
Wavelength Gray0/L2 Gray0/L19 Gray4/L2 Gray4/L19 Gray
5
/L2 Gray
5
/L19 Gray7/L2 Gray7/L19 Gray10/L2 Gray10/L19 Gray11/L2 Gray11/L1
9
Table 4. Reflectance values for gray Spectralon under 50%R.