Photometer straylight is an indication of the optical conditions of the Light Scattering Chamber (LSC) at a given moment in time. Due to the transient nature of the observed straylight value there is no tolerance established.
Increases in the straylight occur through ordinary usage over the standard calibration cycle. Sudden and/or catastrophic increases in the straylight value, typically the result of component failure or operator error may render the unit inoperable.
Some examples of catastrophic failure include introduction of liquid to the scattering chamber, formation of condensate on the LSC optics, accumulation of solid particulate and failure of discrete components within the LSC. Photometers which remain operational within limits of ratio-metric setup instructions, even with elevated straylight values, are still capable of accurately measuring filter leakage values.
Note: Extreme increases in the observed straylight value are a good indicator that a photometers Internal Reference feature will no longer provide accurate readings.