23 May 2013
"As textile testing professionals we are all aware of the requirement to change the glassware in our light fastness testing instruments, but just how important is it? There are several lamp types and light filters specified in various testing standards. The most commonly used standards for members of this group will be ISO 105-B02 and AATCC TM16. Both of these have tolerances for the measured irradiance at the samples surface. Maintaining this irradiance level relies upon a combination of the lamp and filters.
There are a range of acceptable light filters depending on the equipment type and standard, but generally they perform the same function. This is to control the wavelength of the electromagnetic spectrum that can transmit through them and reach the test specimens. They are used to produce a desired ‘cut on’ at the UV end of the spectrum for consistent fading and to provide and desired ‘cut off’ at the IR end of the spectrum to control the temperature of the test, with all wavelengths in-between allowed to pass as freely as possible.
Many filters over time are susceptible to a phenomenon called ‘solarisation’, and this means that over many hours of exposure to the lamp the transmission properties of the filters change and they start to absorb the UV that at the start of their life was allowed to pass through. If the filters are not replaced as required, then firstly the lamp will have to work increasingly hard to maintain the specified irradiance. This will dramatically reduce the life of the lamp. Secondly, after an extended period the filters will act like UV filters and the specified irradiance can no longer be achieved, at this point the testing is out of specification to the standard and all results are unreliable.
It is also very important that the lamp is replaced as required too. If we talk about Xenon Arc Lamp technology, this lamp emits a powerful, broad spectrum light and it is the UV component of this lamp that is responsible for the fading of the samples and the Blue Wool references, and is closely controlled as part of the testing standards. The number of hours of use and number of strikes (powering up the lamp) diminish the output of this lamp and after many hours, the lamp can no longer produce the specified irradiance.
You can see how important changing the glassware in your light fastness instruments becomes. You should always change your glassware at the required intervals stated in the testing standard or if this is information is not available then you should follow the manufacturer’s instructions on replacement intervals."
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