We are measuring vegetation (grassland canopy) reflectance with a handheld spectrometer, using the sun as the light source. When taking white reference measurement Is it important that the distance from the optical fiber to the white reference standard is the same as the distance from the optical fiber to the sample (grassland canopy)?
Eg. Our optical fiber has a 30 degrees field of view (FOV), which would mean a ground footprint of about 0.25 square meters when holding the optical fiber at about 1m above the grassland canopy. We can use a small diameter ( 50 mm) white reference standard held close to the optical fiber end (to cover the whole FOV) or we could get a large diameter white reference standard (wide enough to cover the 0.25 square meters FOV) that we hold at grassland canopy level- 1m below the optical fiber.
Dear Anamaria Roman ,
to my opinion there should be no significant differences.
But you may play around with your 50mm diameter white reference.
Around a distance of 90mm to your fiber-tip the FOV is covered by the white reference sample.
You may go stepwise ( e.g. in 80mm, 70mm, 60mm distances) to towards your fiber tip and see what the outcome of your spectrometer is.
I expect that at positions close to the fiber tip (e.g. < 30mm?) the output of your spectrometer will change*) due to multi-order light scatter between your white reference and the support (clamping means**)) of your fiber.
Please avoid partly shading of the sunlight when appoaching white reference and fiber end.
Please avoid at this stage of investigations any tilting of the white refererence.
In a second step you may check how tilting (at constant distance to the fiber) of the reference will affect the spectormeter output.
*) in spectrum shape and max intensity
**) you set up should be 'colored' in matt black in order to avoid any light reflection or light scatter
good luck
Just an additional remark with respect to a distance dependence of the optical signal picked up by the fiber tip.
a) for a point source the intensity picked up by the fiber tip is proportional to 1/r², with r being the distance between the source (here a point of the white reference standard) and the fiber tip.
b) the number of point sources at the FOV (Field of View) is proportional to r².
So in consequence the distance (r) dependencies of a) and b) are cancelled.
A main uncertainty remains; that is the light absoprtion in air with increasing distance. But for 1m distance it can be neglected in the visible regime.
The total spectrum/wavelengths has the same intensity dependence with the distance of your fiber tip. So, in a real aplication, it is quite difficult to analyze the total intensity reflectance values, but you can analyze spectrum changes (intra-).
Try to use a wavelenght, that you are prety sure is not related with a characteristic of your plant analyzed, as reference, allowing you to check the intensity fluctuations (due to sun-light variation intensity or experimental changes, etc..) Now, use other spectral region related with the plant characteristic, divided by the intensity of the wavelength reference. Therefore, the spectral analysis is not more sentitive to sun-light fluctuations.
Many thanks, Gerhard Martens and Diego Rativa for your answers, they were really helpful! We will test the stepwise distance testing that Gerhard Martens suggested to see if we get different results. It is more difficult to try to find a wavelenght where there is no response from vegetation as Diego Rativa suggests, since our spectrometer only covers the range from 400 to 1100 nm.
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