Well, it depends on your experimental design. First, are you calculating these figures for a one off estimate? If not, you would be well served by using remote sensing. It allows you to monitor growth and productivity without spending excessive amounts of time in the field. Your calculations may also be far more accurate using remote sensing- kinda depends on your sampling.

The down side is, you need to know what you are doing otherwise including it is a complete waste of time. And it's not so easy to learn. There are alot of details which can influence your study one way or the other. So you need someone who has experience with that sort of data collection and understands the fundamental mechanics of the data.

I like the equipment from ASDI (http://www.asdi.com/). I think they have rentals.

Goodluck

1: Advantages and Disadvantages

Similar response to above answer; it depends on what you are trying to do.

The methods and complexity in sampling and spectral analyses also broadly depend on what type (and structure, e.g., height) of vegetation are you sampling; forest, grasses, etc. And, what you trying to estimate, the canopy reflectance or leaf and background reflectance? And, what you hope to use the reflectance spectra for; calibrating aircraft or satellite RS data, linking the biomass to the site-specific reflectance in order to extend to aircraft-satellite mapping, as inputs into models, e.g., radiative transfer, and etc. And as in the above response, field spectral methods and data analyses can be quite complex and time-consuming, so the benefit depends on your objective and the advantages of adding spectral data to reach your objective.

2: Different models of Field Portable Spectrophotometers with prices

It depends as well. If the visible-near infrared (or most of it) will do, Ocean Optics makes quality, high spectral resolution hand radiometers that are reasonably priced.

Thanks for your suggestions and comments. Firstly I am working on historic changes in the forested areas (Close to Sparse & Broadleaved to Needleleaved). Based on strata, will define the number of samples and distribution.

I am looking biomass and carbon stock from plot to landscape level. In this context I can get/collect the spectral library and range of broadband and narrow band vegetation indices. As per my knowledge we don't have spectral signatures of trees, specially when we are talking about the tropical to sub-tropical and alpine ranges in the Asia.

So it sounds like you are doing a historical estimate? In that case I suggest working entirely with a Geographic Information system (GIS). You can build your landscape up from the photos and make a quantitative model within the GIS that reflects real surfaces, light, drainage, soils etc.

Later you can develop another model of current stocks based on your field estimates and use all the spatial data and probably overlay a nice quality image.

Goodluck

Field spectroscopy can be very helpful but it also very much depends on the vegetation in your plots (e.g. trees are really a challenge for field spectroscopy!). Also it takes quite some time to collect the spectra and the optimal window for collection is rather short (depending on your latitude and season), 300 sample plots sound ambitious and you might need more than one spectrometer. We used ASD for our research projects. If you have any questions regarding field spectroscopy the NERC field spectroscopy facility in Edinburgh is a wonderful place to find answers (ttp://fsf.nerc.ac.uk/)

The ASD is an excellent instrument but the the Ocean Optics suite (USB4000 or USB2000) is much cheaper. I have used both the ASD FieldSpec and the OOUSB4000 and OOUSB2000. The ASD FieldSpec generally produces less noise and is more useful for examining bands in the mid-IR; however, if you are interested in only the VIS-NIR region, OO are a much better way to go. You can use a pair of OO instruments to reduce acquisition time. It is also much more reliable than the ASD in cloudy conditions.

Forests are very tricky. I also agree that 300 sample plots is very ambitious. How are you taking these measurements? Do you have some type of boom to get you above the canopy? Many researchers use hemispherical photos because they can be collected underneath the canopy. You might be better off taking leaf level reflectance measurements and scaling up to satellite imagery. Depends on your budget and time requirements.

It isn't forests, but you can get an idea of the methodology using the OO from the papers of my mentor here: http://calmit.unl.edu/people/agitelson2/

I would be careful with the OO systems. From what I know, they have no cooling system so in the field they tend to overheat and can lead to serious spectral response variability. You can probably minimize this with really good in-field calibration procedures but it's a good issue to keep in mind.

As Anthony suggested, having a way to measure downwelling irradiance allows sampling during cloudy conditions. If you are looking for a system measuring only in the VIS-NIR, the dual channel Unispec spectrometer form PP systems may be a good option. It's incredibly stable, portable and having the two channels (upward and downward) in one system is really nice. It's a little more expensive than the Ocean Optics systems but much less than the ASD.

Good luck!!!

Thanks a lot for your comments and guidance. Now I concluded that "I can't use spectrophotometer for my study" due to following reasons;

1: Finance: Instruments are very expensive, minimum 5k $

2: Project period: The duration of my project is only one year and need to cover 300 plots so multiple spectrophotometer require.

3: Technical issues: I can get only spectral signatures of leafs (standing on ground) not overall canopy, so can't link directly with the passive RS data.

I would enquire about a UAV-based lidar survey with some VIS/NIR imagery. For biomass and vegetation monitoring, this could be really useful and time effective. The imagery may be expensive, but you should see if you could negotiate a research/pilot study deal with them and share the results for adveritisng purposes. You may be able to negotiate a reduction in cost that way.

The ASDI is a must if reflectance are needed up to 2500 nm, but it's pretty expensive. If the visible spectrum is enough, I would recommend Ocean optics USB2000+ which cost around 4000 USD with all needed accessories. If you also need reliable measurements in the NIR (around 800-1000 nm), a cooled CCD spectrometer may be needed. I had a great deal of trouble with Ocean Optics proprietary software, which wasn't suited to automated measurements. And every additional software is pretty expensive.

Lately, I tried ASEQ spectrometers (http://www.aseq-instruments.com/) which are pretty cheap (1200 USB with peltier cooling) and reliable. Moreover, their use is quite practical since labview modules as well as VC++ libraries are provided for free.

The University of Nebraska has developed their own software for operating multiple instruments including the OO simultaneously. It may seem like steep cost initially but it is a one time cost and it integrates several other instruments (e.g. Trimble GPS, digital cameras, backscattering instruments, etc.).

http://nutechmarketplace.com/magento/nutech/licenseAgreements/CDAP-2_LicenseAgreement.pdf

Fieldspec4 is the recent model in spectrometers. it is time consuming and cost of the instruments is very high. but it is the best way to validate the spectral signature curves which was got through from satellite image

Beware of Ocean Optics. I used the JAZZ extensively and it is very unreliable. Besides, as soon as the outside temperature goes over 25°C, measuring becomes almost impossible because of the lack of internal cooling (measurements saturate even at the shortest integration time).

If you want an affordable but bulletproof and reliable instrument, try out the GER1500 (or other spectroradiometers) from SVC: http://www.spectravista.com/1500.html

Geert, it is unfortunate you had problems with the OO system, but I have found them to be very reliable. We do cool them using 80 mm CPU fans in our all-terrain vehicle, but in our backpack system we do not have a cooling mechanism. We have never had any problems with saturation unless they were poorly calibrated. I suggest using a dual-headed system. The CDAP software we developed in-house does a great job at collecting data fast and efficiently. Do you cover your instruments? We do not let them get exposed to direct sunlight. Since we have gone wireless, only the fibers really need to be exposed.

Hi Anthony,

Well, the JAZZ was always shielded from the sun (but did not have any cooling). However, also the lab measurements proved to be unreliable. Not that they would saturate, but the reported reflectance was often more than 3-4% off even after a few seconds of operation. I communicated this to the OO people, but they did not believe me. Until I drove there with my instrument and I showed them a simple test I do: calibrate the radiospectrometer using the Spectralon, wait 20 seconds and measure the Spectralon again. Result: 95 % reflectance, 105% reflectance etc. instead of the expected 100 % reflectance. It is a simple test I always do with spectrometers and the JAZZ fails every time. They now have put another grating in it, which improved the performance in the UV-blue section, but it I still insufficient accurate and consistent in the Red-NIR region. Maybe it is only a JAZZ-specific problem, but I never go for OO again (also, the customer support was poorly from my point of view). Anyhow if you have good results with it, then I applaud that (and certainly the dual-head is a nice solution). As said, maybe the other OO spectrometers are better, but I stick with my very reliable GER right now. Cheers.

I am having the same issue, and trying to get a field spectroradiometer and decided on the OO. But after going through the comments I am thinking again. Could anyone please suggest me a good one, I do not have a big budget though and I am trying to go for a cheaper and a simple one. 

The saturation issue may not be caused by the spectrometer. There may be too much incoming light. If you are using a diffuser or lense directly mounted on the spectrometer, you should try using an optical fiber to limit the amount of light which pass through the spectro slit.

If you are already using a fiber, you should try another one with smaller core diameter.

Most of the time, one should avoid working with small integration times since it is much more sensible to changes in incoming light, thus hard to get a stable measurement.

Thanks for this thread. We have just purchased an OO Jazz spectrometer and I hope they have improved over the last 4 years!

Hi everyone, I found your discussion as I am curious about the problems that overheating in spectral measurements causes. What is exactly happening when my spectrometer overheats? Why are the measurements unrealiable? Does it have something to do with voltage/signal conversion? What are you describing if you say that the measurement is saturated? I really try to understand the technical aspects here. Thanks for any thoughts you are spending on this. Cheers René

Thanks Dimitriy!

Thanks. I had the chance to read a bit about the topic. Thanks to all of your support I know a little bit more now. Thanks heaps for your effort to explain everything in a simple fashion Gustavo Alckmin

Instruments such as Spectroradiometers are available for collecting spectra from 0.3500 to 2500 nanometer. Good spectral signature can be generated (except for sharp dips in middle infrared bands) and can be correlated with satellite data for geospatial modelling of biomass and carbon assessment. Ceptometer/Plant Canopy Analyser are used for PAR/ipAR/APAR/Gap Fraction Analyses and LAI (leaf area index) and can be correlated with satellite data for biomass and carbon assessment.