The absolute repeatability of a spectrophotometer depends also on the quality of the calibration. During the calibration the white standard shows the instrument a defined color, in our case a white with a reflectance of about 98%.
The reflectance of a dirty white standard will go down to example 90%. When this dirty white standard is used, the instrument will “learn” that this 90% reflectance will be by definition 98%. From now on it will measure all samples lighter than they are.Therefore it is necessary to frequently calibrate a spectrophotometer with a clean standard.
We recommend cleaning the standard with a super-microfiber cloth. If the standard is really dirty, alcohol or soap can be used to get a basic cleaning. The finish should be made by breathing against the glass and wiping the mist off. When the surface of the internal white standard is shiny (image of a light source is viewed sharp) the standard is clean.
We can also recommend a fluid called “Optical Wonder Reinigungsflüssigkeit” and “Optical Wonder Cloth” which can be ordered at www.baader-planetarium.de