I have a question regarding measuring natural illuminant spectra underwater. I see that in the sample data file there is a csv for “5m green water 400-700 nm”. Could you share the technique of how this was measured? I’m interested in doing something similar but would like to know if the illuminant spectra should be measured in the same viewing plane as the animal or if measuring downwelling irradiance is ok.
I’m not sure exactly how Cedric measured these irradiance spectra underwater, though I imagine using a cosine corrector and spectrometer.
The illuminant you measure should ideally match the illumination falling on the object of interest you’re measuring. On the sea floor things will normally be receiving light almost exclusively from above, so simply measuring the light in the upper nearly-half hemisphere (as with a cosine corrector) would make most sense. But if you’re dealing exclusively e.g. with pelagic conditions and sideways-facing surfaces, then measuring the light coming from the side would make most sense (i.e. turn the cosine corrector 90 degree to point towards the horizon). If the surface being measured is also not-at-all-diffuse then you might want to not use the cosine diffuser at all.
The grey standard you use should also then match the orientation of the measured illuminant.
Jolyon is spot on. One issue that we have been discovering is that, if your grey standard is not lambertian you might run into calibration issues when you have it illuminated sideways (i.e. in shallow water with lots of light coming from the top but the image taken sideways e.g. of an anemone or a wall). The measurement of the light environment is pretty straight forward, just as Jolyon said.