VIS-NIR FAQ
1. What is the difference between a spectrometer and a spectrophotometer?
A spectrometer is an instrument that measures the quantity of light over a spectrum of wavelengths. The spectrometer is the "measurement engine"
of the spectrophotometer. A spectrophotometer is a complete system for making spectral measurements. The system includes a light source, means for
collecting light that has interacted with the material under test, and a spectrometer to measure the collected light.
2. What wavelengths are you using?
The system uses two spectrometers to cover the 350-2200
nm range. One spectrometer measures the visible and near-infrared from 350-1000 nanometers, and the second spectrometer measures in the NIR from
1100-2200.
3. Why not use mid-infrared?
Soil samples have to be dry when using mid-IR, due to excessive optical absorbance of moist soil. The weaker absorbance in the near-infrared
region (overtones of the fundamental molecular vibrations), allows measurement of field-moist samples—a requirement for on-the-go soil mapping.
4. How much soil VIS-NIR research has been conducted?
Over the past several years, several studies of soil VIS-NIR measurements have been reported in scientific literature. These have been with fixed
base spectrometers, either in a lab or field setting. These have laid the groundwork for on-the-go VIS-NIR sensing by studying the relationships
between VIS-NIR and various soil properties. A partial list is presented below.
- Sudduth, K.A., Hummel, J.W., 1993. Soil organic matter, CEC, and moisture sensing with a portable NIR spectrophotometer. Transactions of the
ASAE, vol 36, pp. 1571-1582.
- Chang, C.W., Laird, D.A., Mausbach, M.J., Hurburgh, Jr., C.R., 2001. Near-infrared reflectance spectroscopy – principal components regression
analysis of soil properties. Soil Science Society of America Journal 65, 480-490.
- Reeves, J.B., McCarty, G.W., Meisinger, J.J., 1999. Near infrared reflectance spectroscopy for the analysis of agricultural soils. Journal of
Near Infrared Spectroscopy 9 (1), 25-34.
Veris Technologies Research: with prototypes from 2002-2007, more than 50 fields were mapped in 4 US states with over 500 lab samples for
validation. Since shipping the first commercial unit in late 2006, field studies have continued for internal research and in conjunction with public
sector researchers. In 2007 32 fields in 7 states were mapped , with another 400 samples for validation. Results from Veris Technologies Research
are presented in journals listed below:
- Christy, C.D., P. Drummond, and D.A. Laird. 2003. An on-the-go Spectral Reflectance Sensor for Soil. ASAE Paper 031044. Presented at the
2003 ASAE annual meeting July 27-30, 2003. Las Vegas, NV.
- Christy, C.D. 2008. Real-Time Measurement of Soil Attributes Using On-the-go Near Infrared Reflectance Spectroscopy. Computers and Electronics in
Agriculture. 61 (1): 10-19.
- Lund, E.D., G. Kweon, C. R. Maxton, P.E. Drummond. 2008. Soil Carbon and Nitrogen Mapping: How These Relate To New Markets And Public Policy.
Proceedings of 9th International Conference on Precision Agriculture. Paper 251
Results from public sector research using Veris VIS-NIR sensors is beginning to be published, and considering the number of projects currently
underway, many more papers will soon be published. Here is one of the first:
- Huang, X., S. Senthilkumar, A. Kravchenko, K. Thelen, and J. Qi. 2007. Total carbon mapping in glacial till soils using near-infrared
spectroscopy, Landsat imagery and topographical information. Geoderma Volume 141, Issues 1-2, Pages 34-42
5. What soil tests does VIS-NIR relate to? Could VIS-NIR someday replace wet chemistry lab measurements?
Because of the vibrations that occur when molecules
containing O-H, C-H, NH bonds are illuminated, VIS-NIR soil measurements correlate well with water, carbon, and nitrogen. Relationships to other
properties such as calcium, magnesium, pH, CEC, clay content have been reported in the literature. Veris Technologies research results confirm this,
however results are not as consistent as they are for C and N. Extensive research is still required in soil VIS-NIR, and as spectral libraries grow,
improved calibrations to soil properties may result.
6. What agricultural application can make the best use of soil VIS-NIR data?
Primarily soil carbon mapping, for the rigorous measurement, monitoring, and verification that would be needed under a regulated carbon trading
program. (Partial development funding for Veris VIS-NIR equipment was provided by USDA and DOE SBIR awards.)
7. Can this measure carbon with the accuracy needed for a carbon-trading program?
While the details of carbon baseline measurement requirements have not yet been established, research results show that Veris Technologies VIS-NIR
is able to measure field soil carbon cost-effectively.
9th Intl Prec Ag conf paper [PDF] 972 KB
Carbon White Paper [PDF] 828 KB
8. What is the status of carbon trading?
Currently, voluntary markets for carbon such as Chicago Climate Exchange (CCX) are providing carbon sellers and purchasers with valuable
experience in carbon trading. The measuring, monitoring, and verification requirements for these initial programs are low, as is the price per ton of
soil C. It is anticipated that legislation may become law in 2009 or 2010 that would mandate significant reductions in carbon emissions, and generate
a more lucrative market for ag soil carbon. Commonly called ‘cap-and-trade’, this law would also mandate stricter carbon measurement, monitoring, and
verification.
9. How deep does the shank measure?
The depth is adjustable from 1” to 4” (2.54-10 cm).
These data are used to create a map of the field carbon, at a depth just below the soil surface. This map is helpful for identifying overall carbon
variability, and for selecting locations for additional, deeper investigations.
10. How deep does the VIS-NIR probe measure?
45" (115 cm)
11. How does the core sampler on the probe unit compare to other hydraulic core samplers?
With the VIS-NIR-EC-Force probe in place, the Veris probe is the most advanced agricultural probe in the world. It also has many features that also make it the premier soil coring probe:
- can be set up for tractor 3-point or truck-bed skid-mount
- user-friendly--electric start, flat fold, fast retraction
- rack-and-pinion side-shift design provides close-coupled, lateral movement
- rotation and hammer options
- can be used with or without liner
- full line of cutting shoe options for various soil types
- rapid anchoring option for hard soils
12. Can I get a probe with just the EC and force sensor? Is the system upgradeable to VIS-NIR-EC-Force sensor?
Yes, the system is available with only EC and Force sensing, and can be upgraded to include the VIS-NIR. With EC and Force, the probe is the replacement product for the Veris Profiler 3000.
13. Can I use the hammer option when doing VIS-NIR probing?
No
14. Can the spectrometers handle harsh field environments?
The spectrometers are enclosed in a dust-free and moisture-resistant case, which is shock-mounted to the implement. The case is cooled, and humidity and temperature are monitored, with automatic shutdown of system if humidity or temperature parameters are exceeded.
15. What software comes with the system?
The Veris VIS-NIR spectrophotometer is controlled by a PC-based (Windows XP) operating system, which covers instrument control, data-recording, and data standardization functions. An auxiliary instrument, which controls the movements of the shank shutter during the dark-reference routine, also collects the auxiliary data. After data has been collected, the software module suggests locations of soil samples that will best calibrate to various soil properties, and guides the operator to those locations. See FAQ ‘How do you calibrate VIS-NIR data to soil properties?’ below for more information on this function.
16. What changes are required to move the spectrometers from Shank to Probe? Is there different software?
Moving the spectrometer and auxiliary cases from one unit to the other takes about 5 minutes. The Veris Spectrophotometer software runs both systems.
17. How many hours of training are required? Can a person operate it without training?
Ideally, each customer would come to our factory to pick up his unit—and we welcome those who do. This gives us a chance to do make sure the customer’s laptop PC is up and running the system, and we provide 2-4 hours of training. Yet less than 20% of our customers are able to arrange that, and are still able to operate successfully. The manual is detailed and thorough, and we provide excellent phone and email support.
18. How do you calibrate the system?
Each Veris VIS-NIR production system is considered a slave instrument and calibrated from a master instrument, which is kept at Veris Technologies, Inc. This master instrument is calibrated to known Avian reflectance standards. Keeping the master instrument calibrated to these standards allows the slave (production) VIS-NIR units to be calibrated by a system traceable to these known standards. This allows the data collected from any Veris Spectrophotometer to be compiled into soil VIS-NIR libraries—a method of leveraging local VIS-NIR measurements. A system check transform using external references compensates for any instrument variation due to wear. This ensures that over time the instrument will give the same readings as it did when it was first built.
19. How do you process spectral data? How do you calibrate VIS-NIR data to soil properties?
Step 1. Collect soil samples using Veris software (included). In order to create a calibration to use the VIS-NIR spectra for quantitative predictions, a set of soils samples are needed that are representative of the spectral data space. To determine meaningful sample locations, the spectra are compressed using principal component analysis (PCA) and then grouped into 5-15 (user-selectable) clusters according to spectral properties. Subsequently, one location from each cluster is selected for soil sampling. The scores used for this compression are those corresponding to the eigenvectors of the covariance matrix having the largest eigenvalues. The first few principal components typically account for 95% or more of the variation in the spectra. Clustering is accomplished using a fuzzy c-means algorithm. For each cluster, the sampling location is chosen so that it is close to the center of the cluster in spectral data space and geographically surrounded by the most points from the same cluster.
Use 3rd party chemometrics programs such as PLS Toolbox from Eigenvector Research, Inc., or Unscrambler® from CAMO Software Inc. to perform Partial Least Squares (PLS) regression analysis using spectral data and lab-analyzed results. Validate results with cross-validation techniques—leave-one-out or more rigorous leave-one-field out.
20. Can I run in growing crops?
Yes, soil-engaging components can be adjusted to fit into 30” (76 cm) rows.
21. Can I use in the lab as a benchtop spectrometer?
Yes, this is a good way to start using the system, and to process soils in situations where field operation isn’t feasible.