Hi Craig If you haven't already I would take a look at the wealth of literature in remote sensing for this question. Water stress indices in remote sensing sometimes use around 900nm while other indices looking at the ratio of carotenoids to chlorophyll use around 800 nm. Obviously the scale of observation is different but I would imagine you could relate SPAD at 805 & 850 to an existing remote sensing index. I believe the SPAD values of photosynQ output all use abs at 940 as well? is that correct? Is SPAD calculation for all = LOG((sample at 940 / calibrated 940) / (sample at X_nm / calibrated X_nm))*100 If so, a remote sensing index similarly structured might be useful to understanding... Hope this helps Jon

Hi Jon, thanks for the response. Relating the SPAD measurements of the devices to the vast remote sensing literature will be useful. I have looked at the water stress indices but not at the carotenoid to chlorophyll ratio indices. Both are certainly worth considering. The SPAD values of the multispeQ normalize using the absorbance at 940 nm to account for variations in leaf thickness. The SPAD 850 and 880 readings may be related to light scattering. It might be interesting to infiltrate leaves with water.