Ricardo E. Trelles wrote: > Does anyone know exactly what this Reflectance drift test means, is it > serious to fail it, > what to do when it happens? Hi, unfortunately I'm not sure exactly what it does. If you could describe what the instrument is put through, it might help (ie. what the sequence and timing of the various reflection measurements is - something you can get an idea about by propping the instrument slightly clear of the reference tile with a piece of paper). I do know that the Rev A I have suffers from thermal drift - the lamp is quite sensitive to how hot it is, so a reflective reading directly after a previous reading has a noticeable difference to a reading made after the lamp has fully cooled down. I speculate that this is a lamp aging effect. Judicious use of spot read using it's delta against reference capability can give you an idea whether this is the case with your instrument. My newer and less used Rev D does not exhibit this effect to any noticeable degree. Graeme Gill.