Martin Weberg wrote: > >> 1) There is something different about the instrument calibration for UV vs. >> no UV filter. Measuring the calibration tile may >> reveal whether this is the case. >> > > The readings are different. See attached. > Martin, I assume, after switching between UV-filtered and non-filtered, you always calibrate the instrument against the calibration tile again, before taking some readings, do you? Well, if we'd assume that the filtered light does not contain any energy below 400nm or 420nm, then basically we would have to discard the readings for these wavelengths completely, as they would be the result of a zero by zero division (no light multiplied by the reflectance of the sample (+ noise), divided by no light multiplied by the reflectance of the reference tile (+ noise)). I could also imagine that the instrument possibly applies some kind of regularization in order to avoid completely unfeasible readings (dominated by noise only) at the UV wavelengths. Regards, Gerhard