The calibration lamp costs more than $ 1000. It's a lot for my research budget.
If you just want a wavelength calibration, that's quite easy ; you can use the Fraunhofer lines to find the corresponding wavelength. You can fit a polynome using measured and known lines wavelength.
But using the sun as a reference for radiometric calibration is not possible. It's possible to use a PAR sensor and to re-scale a standard solar spectrum so that its integral over the 400-700 nm range is equal to the measured PAR and use the result as a reference, but that wouldn't be an accurate calibration.
If you're looking for a radiometric calibration, I would advise you to look for cheaper calibration sources. Stellar-net is selling a 750 USD calibrated light source.
Hi German,
You can calibrate your Licor with the sun AND a teflon reference panel with very high (calibrated) reflectances in the range of 450 to 2500 nm. These reference panels are known under their commercial name Spectralon and are sold by Labsphere. See also the site:
http://www.pro-lite.uk.com/File/spectralon_material.php
To calibrate your Licor for field measurements for example you sequentially take a spectrum of the spectralon followed by a field measurement. An important condition is that the irradiation hzs to be the same for the two measurements. On a cloudless day and with a short time interval between the Spectralon and field measurement, this condition is is met. From the specralon calibration curve which is provided by Labsphere, you can read the reflectance corresponding with you Licor signal and this for the whole 450 to 2500 nm range.
Then you apply a rule of three, to calculate the reflectance of the target of your field measurement. Be sure of the IFOV of the Licor, to use the correct distances for the Spectralon measurement and the target you want to measure in the field.
So that's the way to use the sun as a calibration source German. Buy a Spectralon reference panel (and a tripod to make life easier in the field) and off you go.
Good luck,
Frank
PS.: Evidently this is a reflectance calibration and not a radiometric one.But you question leaves the option open for reflectance measurements. For radiometric calibration, you will have to look for sponsors to increase your budget.
Hi German,
A NIST calibrated Mylar sheet with ten narrow, sharp spectral absorption features in the shortwave infrared spectral region 1.1 to 2.5µm is a very good tool for wavelength calibration. For radiometry, a mercury-argon spectral line lamp is a good choice. This is a common source lamp in most of the teaching labs offering undergrad courses in physics/chemistry. Physics department in your university must be having one.
Good Luck,
D.R.
If your instrument as the necessary attachment to measure direct (rather than global) irradiance, you can use the Langley-plot technique to calibrate it wavelength by wavelength. This calibration is more precise than with a lamp _if_ you do that at a high-elevation site with very clear and stable atmosphere, such as Mauna Loa, Izana, Pic du Midi, or Jungfraujoch.
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