Invalid measure is typically caused by either a hardware failure like a burned out lamp or the user lets the instrument up before the measurement has completed. To check for a burned out lamp, press the read switch (located on the bottom side of the instrument just above the black optics cap) while holding the instrument a few inches above a table. During the measurement, three individual white spots, each representing a lamp, should be visible on the table. If three individual spots are not visible, the instrument will need factory service. If three individual spots are visible and the instrument is still giving 'Invalid Measurement', the batteries may need to be charged. Plug the A/C adapter into the instrument and let it charge for one hour before trying to calibrate again.
It’s not uncommon for the 500’s to display 'Optics Change' from time to time. There are two basic reasons for this; one and the most obvious, is when you change the aperture size or add the polarization kit. The second, which is most likely the reason, is when the system detects a signal change (amount of light returning back to the instrument) between now and the last time it did a full (white and black) calibration. The main cause for a signal change is dust getting into the optics. Carefully blow out the optics with clean, dry, compressed air and try calibrating again. In some cases the signal reference just needs to be updated due to time.
The frequency in which the instrument displays 'Optics Change' is completely dependent on the environmental condition the instruments are used in. The more dust flying around means the instrument will display 'Optics Change' sooner and more frequently. If the instrument is displaying 'Optics Change' on a daily bases there is a problem and it may need service. It is more common to see this message about once every other week, to once a month.