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 reflection of about 98%.

The reflection 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% reflection will be by definition 98%. From now on it will measure all samples lighter than they are.
Therefore it is necessary to calibrate a spectrophotometer frequently 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 wipe the mist away. 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