Spectra Collection Protocol Visible, Near Infrared, and Short Wave Infrared Using the Cary 500 Spectrometer
Introduction
Spectral measurements of materials in the Visible (VIS), Near Infrared (NIR), and Short Wave Infrared (SWIR) with a high quality laboratory instrument are made with relative ease. The Cary 500 has a very stable internal light source and a dual-beam optical design that results in a greatly simplified measurement process, which results in high measurement accuracy and instrument stability.
While the measurement methodology is simple, it is imperative that those using the Cary 500 understand the measurement phenomenology and the operational aspects of the instrument. Much of this is covered in the accompanying Traceability protocol document. It is repeated below in a form appropriate for making measurements with the Cary 500. For those with spectral measurement experience in a chemical analytical laboratory, many of the methods described here will be familiar.
This document will primarily focus on the Varian Cary 500 Spectrophotometer. However, the principles covered in this document also apply to V/NIR/SWIR measurements in general. It is assumed the operators of the Cary 500 (or other similar laboratory spectrometer) will have sufficient training to properly operate the instrument according to the manufactures instructions. This document is by no means meant to replace these instructions. Instead, it is intended to provide guidance on laboratory usage to insure high quality and consistent data.
To that end, this document is a “living” work. One can never anticipate all the conditions under which you will collect spectra. Since this is the case, this document needs to serve as a basis of measurement methodology from which investigators will start when they make VNIR/SWIR laboratory measurements. For a class of targets that has been measured before, this document serves as a set of guidelines. For a new class of targets, it serves to educate and remind the investigator of relevant phenomena that they need to keep in mind. As new and/or improved methods are developed they will be reflected in this document.
Background
The Cary 500 is typical of high performance laboratory spectrometers in that it has a dual-beam design. The instrument alternately measures the spectral radiance from a reference material and from the sample material of interest (or the unknown). The instrument uses a common light source, common monochrometers, and common detector for both beams. A chopper wheel controls which beam the detector is viewing during the measurement cycle. The chopper wheel further facilitates the measurement of the detector dark current during each measurement cycle. The spectral transmission or reflectance of a material in the most basic form is determined by the ratio of the two measured beams, Eq. 1.
(1)
Unfortunately, the influence of an integrating sphere, reference standards, optics, etc. in a spectrometer must be accounted for. As a result, Eq. 1 will not sufficiently describe the measurement. To better understand the operation of a dual beam spectrometer, let us first consider the components in each beam. The case of reflectance measurements with an integrating sphere will be described first since that is the more complicated configuration. A brief discussion will follow describing the simpler transmission measurement.
Reflectance Measurements
The reference beam as sensed by the detector is:
(2)
where:
is the reference beam spectral radiance measured by the detector
is the spectral irradiance incident on the reference target
is the spectral reflectance of the reference target
is the spectral efficiency of the integrating sphere for the reference beam
is the optical spectral efficiency for the reference beam
The kO terms in the above and following equations account for the differences in the optical paths the two beams follow in the instrument. These two paths (and associated optical elements) can contribute to an imbalance between the two beams that must be accounted for.
For the sample beam we have:
(3)
where:
is the sample beam spectral radiance measured by the detector
Es()is the spectral irradiance incident on the sample
s()is the spectral reflectance of the sample
kS,s()is the spectral efficiency of the integrating sphere for the sample beam
kO,s()is the optical spectral efficiency of the sample beam
It would be convenient if Es() = Er(), kS,r() = kS,s(), and = kO,s() in Eq. 2 and 3. A simple ratio of these equations would reduce easily reduce to Eq. 1. While instrument designers strive for such perfection, it is seldom realized and cannot be expected to be stable over the life of the instrument. For now, lets assume these terms are not equal. To account for these inequalities, a baseline scan across the wavelength range of interest is necessary to characterize the response of the instrument. For this document, the methodology for 100% Baseline and 0% Baseline is discussed.
100% Baseline
For the 100% Baseline [B100()], a white reference target is placed at both the sample and reference ports. While it is desirable for these two reference targets to be of equal reflectance, it is not always possible, nor is it absolutely necessary. As will be shown, it is necessary that a NIST-traceable white reference be placed in the sample port while acquiring the baseline. From now on, let s() in Eq. 3 be the spectral reflectance of the NIST-traceable white reference during the baseline scan. The instrument will scan these targets and compute the ratio of the sample vs. reference beam radiance values. This ratio will form the 100% Baseline response of the spectrometer.
(4)
After the 100% Baseline scan is performed, the target material of interest (the unknown) is placed in the sample port of the integrating sphere for measurement. The sample beam signal is now made up of:
where:
Lu()is the unknown sample spectral radiance measured by the detector
u()is the spectral reflectance of the unknown sample
kS,u()is the spectral efficiency of the sphere with the unknown material in the
sample port
The resulting ratio of the sample beam with the reference beam is:
D() will contain within it the baseline response B100() of the instrument. To remove B100(), D() is divided by B100(), resulting in a relative reflectance rel().
Reducing this equation results in Eq. 5.
(5)
For unknown materials with scattering properties similar to those of the white reference in the sample port during the baseline scan, kS,u() = kS,s().
The above equation then further reduces to:
Rearranged into a practical form:
u() = rel()s()(6)
This model illustrates the need to have a NIST-traceable standard in the sample port of the spectrometer during the baseline scan. All subsequent reflectance measurements of unknown samples are easily corrected for the spectral properties of the NIST-traceable standard by multiplying by the absolute spectral reflectance of this standard, Eq. 6.
0% Baseline
While a 100% Baseline is simple and commonly used, it is not sufficient to fully characterize a reflectance spectrometer. At very low spectral reflectance values, some stray light from the sample beam may reflect off the integrating sphere, and be measured by the detector. Furthermore, any variations in the detector response at these extremely low light levels will also contribute to measurement error. The error is removed by following up the 100% Baseline scan with a 0% Baseline scan. A 0% Baseline [B0()] characterizes the instrument with a 0% reflectance target or light trap in the sample port. The Cary documentation recommends using the black inside of the sample cover as the light trap. However, experience has shown that a low level of reflectance (1 – 2%) results from the closed sample cover. Until a better light trap is available, it has been determined that the best practice is to completely darken the room, open the sample cover, and leave the sample port empty. Black velvet fabric hanging across the room (~ 8 feet) from the Cary will greatly reduce any stray reflections.
Note: While tempting to merely block the incoming sample beam to the sphere (as suggested in some of the Cary documentation), this will not properly characterize the 0% response. Such a practice will only replicate the sample beam dark current measurement, which is made during each measurement cycle. Using a light trap as directed will fully account for stray light reflecting off the sphere.
When determining B0(), there is another equation involved with the reflectance measurement.
L0() = Es()0()kS,0()kO,s()
where:
L0()is the light trap spectral radiance measured by the detector
0()is the spectral reflectance of the light trap
kS,0()is the spectral efficiency of the integrating sphere with a sample port light
trap
The 0% Baseline spectra will be:
(7)
The instrument will perform the following computation when an unknown sample is scanned when the 0% Baseline method is used.
(8)
Substituting in the appropriate expressions into Eq. 8 and rearranging yields:
(9)
As above, we will assume the scattering properties of the unknown and the white reference sample are approximately equal as with the 100% Baseline case. If there is no stray light in the sample beam, 0() will be assumed to be 0 since no light will be reflected from the light trap. Eq. 9 will now reduce to:
u() = rel()s()(10)
However, if there is stray light and detector/electronic variations, the error will appear in the Bo() term, which is subtracted from both the sample and reference beams during the measurement of the unknown. The stray light and detector/electronic error can be very subtle; however, this subtle correction is particularly important when measuring low reflectance materials.
Transmission Measurements
Transmission measurements typically are made of liquids in cuvettes or solid materials that have transmissive properties. If the sample has scattering properties and the total transmissive properties are of interest, the transmission measurements need to be made with an integrating sphere. The sample would be placed at the sample beam entrance port, and the resulting measurement will be the transmittance of the sample. Given the need for the integrating sphere, much of the phenomenology and methodology described for reflectance measurements will apply.
If the sample is non-scattering, or only the non-scattered transmissive properties are of interest, the integrating sphere is removed from the Cary 500, and the instrument is used in the transmission configuration.
The standard practice for transmission measurements of liquids requires the placement of a blank solution in the reference beam, and the solution of interest in the sample beam. The blank solution is often the solvent used to dissolve the material of interest. It may be water, methanol, benzene, etc. In a remote sensing application, it is likely that the sample will be the combination of the material of interest and the “solvent”. Examples include sea water and algae in tidal pond water. In these situations, an empty cuvette would be placed in the reference beam, and a cuvette filled with the combination liquid in the sample beam. Since the cuvette is not part of the sample, having a cuvette in both beams will ratio out the effects of the cuvette during the transmission measurement. For solid transmissive materials, (such as glass, Plexiglas, window covering films, tree leaves), there is no need to put anything in the reference beam.
The mathematics describing transmittance measurements are similar to those used for reflectance measurements. Starting with Eq. 7, 8 and 9 for a 0% Baseline Correction, we make the following adjustments:
- Since the sample beam will be attenuated by the sample, u() is added to Eq. 3.
-s() is substituted for u() and 0() since the Spectralon standard is replaced
in the sample port during transmittance measurements.
- kS,s is substituted for kS,0 in Eq. 7 since the Spectralon standard is replaced in
the sample port during transmittance meaurements.
A 0% Baseline Correction would be performed by placing an opaque material in front of the sample beam entrance port to the integrating sphere requiring the addition of a t0 term to Eq. 7.
Eq. 7 now appears as:
Incorporating all these modifications into Eq. 9 results in Eq. 11.
(11)
Since = 0 for an opaque material, the B0() terms in Eq. 11 be 0.
Eq. 10 now easily reduces to:
(12)
For transmission measurements with the integrating sphere removed, the terms are removed since reflectance standards are not involved with these measurements. Since a 0% Baseline Correction is performed with an opaque material in the sample beam, Eq. 11 is simplified to Eq. 12 in the same fashion as the transmittance case.
Since the Bo() = 0 in the transmission and transmittance case, the need for a 0% Baseline Correction is removed.
General Considerations
Spectral measurements between 0.35 and 2.50 m in the laboratory are relatively simple; however, there are considerations that need to be taken into account that will insure the measurements are of high quality. Even though this document is focused on the Cary 500 Spectrophotmeter, the follow considerations are common to any similar instrument.
Instrument Parameters
1.Modern laboratory spectrometers are capable of amazing spectral resolutions and signal to noise performance. Great care must be taken when selecting the measurement parameters. It is important that realistic measurement parameters be selected that will give the highest probability of producing high quality data. Generally, if wider spectral bandwidths and/or longer integration times are used, the noise level will be decreased. However, these two parameters must be balanced with the requirements of the data use and the need for sample throughput.
Reflectance Measurements
- For reflectance measurements, an integrating sphere needs to be installed into the spectrometer. Upon installation, it is crucial the alignment of the integrating sphere and associated optics be checked and adjusted according to the manufacturers instructions.
- Reflectance measurements rely on a NIST-traceable standard in the sample port to return absolute reflectance measurements. A Spectralon standard is suitable for this purpose, and is available with a NIST-traceable spectral characterization. The accompanying Traceability Protocol document describes how to maintain the calibration of these standards.
- The scattering properties of the unknown sample should be similar to those of the Spectralon standard. If they are not, error will be introduced to the measurement due to differences in sphere efficiency.
Transmittance Measurements
- For transmittance measurements, all the scattered light transmitted through the sample will be collected by the integrating sphere and measured by the detector. There may be a different optical layout for the optical elements of the integrating sphere attachment. Therefore, it is necessary that the manufacturer’s instructions be followed in setting up the sphere attachment.
Transmission Measurements
- Transmission measurements are probably the easiest measurements with a dual beam spectrometer. However, care must be taken to insure the two beams are balanced and properly baselined. For liquids, it is important the cuvettes in the sample and reference beams are a matched set. Any differences in the spectral transmission of the cuvettes will manifest itself as error in the final spectral measurement. If a solution is used to dissolve or suspend the unknown material, it is important that the solution also be present in the reference cuvette. Otherwise, the resulting spectra will be that of both the material and solution combined, which may not be the desired result.
Specific Considerations for the Cary 500
The Cary 500 Spectrophotometer is one of the premier instruments of its type. The wavelength range is from 0.175 to 3.30 m in transmission mode. With a Spectralon lined integrating sphere, the wavelength range is .25 to 2.50 m. It uses a dual-beam optical system, and a dual-grating monochromator for increased spectral resolution. Like many similar instruments, a photomultiplier tube (PMT) detector is used for the UV/VIS and NIR wavelengths. For the transmission measurements in the SWIR wavelengths, the Cary 500 uses a thermoelectrically cooled lead sulfide (PbS) detector. Reflective and transmittance measurements at these wavelengths use an uncooled PbS detector. To accommodate these different detectors and the broad wavelength range, different gratings are used in the UV/VIS/NIR and the SWIR.
As good as the Cary 500 may be, there are specific considerations that must be taken into account when using the instrument.
- The instrument must be allowed to warm up before use. Sufficient warm-up time will allow the internal components and the thermoelectrically cooled PbS detector to achieve thermal equilibrium. This will result in less drift from the electronics, sources, and optics. Varian suggests a 1 - 2 hour warm up period. Tests on the RIT instrument showed a linear signal drift over a two hour warm up period of less than 0.10% reflectance. We conclude that a shorter warm up period of approximately 30 minutes is sufficient.
- Dust on the optical elements or in the integrating sphere will degrade the instrument performance. To reduce dust accumulation, it is helpful to have a constant purge of filtered and dry air. The RIT instrument is continuously purged with filtered air from a Balston dry air system that produces air with a dew point of approximately –100oF. Purging with dry air has the added benefit of reducing the interference of the water absorption bands in the SWIR region. The repeated opening and closing of the sample compartment will continue to allow dust to enter this part of the instrument. However, the purge will maintain a positive air pressure and minimize further dust accumulation.
- As robust as the Cary 500 is, it is still sensitive to vibration. Therefore it is important that the instrument be positioned on a firm and stable bench.
- The electronics of the Cary 500 are among the most sensitive for an instrument of its type. While much design effort went into making the electronics resistant to electrical noise and variable line voltage, it is prudent to attach a high quality power conditioner to the instrument. Besides filtering out electrical line noise and smoothing out line voltage fluctuations, a power conditioner will also protect the instrument and attached computer from any surges that may appear on the power line. Such inexpensive protection may have significant cost savings in the future.
Making Measurements
Spectrophotometers are common instruments in analytical laboratories. As a result, the measurement methods have become some what standardized. Spectral measurements of remotely sensed materials are not a common application of a spectrophotometer such as the Cary 500. However, the measurement protocols have elements borrowed from the other common laboratory measurements.