Why do Lachat wavelength filters go bad?

Document ID

Document ID TE6800

Version

Version 4.0

Status

Status Published

Published Date

Published Date 03/13/2018
Question
Why do Lachat wavelength filters go bad?
Summary

Explanation of filter deterioration

Answer

To explain how filters deteriorate it helps to understand how they are manufactured.

Interference Filter Manufacture

 

The filters are made from sheets of glass which are then coated with thin films. Each film coated glass sheet functions to block a portion of the spectrum. The correct glass sheets are then chosen to give a certain wavelength interval (10 nm) centered at the specified wavelength. The sheets (3 or 4) are then laminated like a sandwich using epoxy adhesive. The individual filters are then cut out, like a cookie cutter, and mounted with epoxy adhesive in a ring.

 

Deterioration of the filters

 

The epoxy absorbs moisture from the environment and may forms "rings" or "halos". These look like foggy spots on the filter. This should generally not occur in the first year. Though it may occur more quickly in environments that are high in humidity or when exposed to chemicals.

 

After the epoxy has become saturated with moisture, the film coatings will begin to absorb moisture and will crack. The technical term for this is "crazing". These cracks leave light leaks and may or may not be serious depending on which of the 3 or 4 films is cracking. This cracking is difficult to observe with the naked eye, but can be seen under 10X magnification.

 

The initial deterioration (absorption of moisture) has no effect on any of these. Further deterioration, crazing, can cause light leaks (not blockage) in between cracks. The performance of a given chemistry may or may not suffer but over time peaks will get smaller even if the chemistry/color developed has not changed. 


Do the Lachat wavelength filters go bad?

Is there any way to prolong the life of the Lachat wavelength filters?
 

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