There are a number of crystals available for use
as lenses in infrared viewing panes, the most common being
Calcium Fluoride (CaF2). This has been used for many years
in different applications as it has the additional benefit
of being transparent and therefore is also used for visual
inspections as well as infrared.
We are able to use this material at present
as crystals are at present exempt from impact test (if over
1.4 mm) and the impact and flammability requirement is only
required on polyermeric materials. This standard is for
switchgear up to 1.5 kV. This is an area that requires immediate
rectification as crystal can not take any form of impact
and we are therefore producing IR Viewing Panes using crystals
with no structural integrity.
Above 1.5 kV IR Viewing Panes manufacturers
can certify to IEEE requirements for Visual Viewing Panes
(IEEE standard C37.20.2, specifically the specification
required for section A.3.6.) the standard which specifies
the minimum MANDATORY test requirement for a viewing pane
(see IEEE datasheet). This is the standard that the VPFR
and VP12 infrared viewing panes have been built to.
Another
disturbing issue is that far too often companies purchase
crystal infrared viewing panes as they want to complete
the visual inspection of the switchgear and LOTO operations.
The crystal can not meet the minimum requirement for impact
under the mandatory requirement of IEEE and as such these
companies may indeed have inadvertently de-rated their switchgear,
infrared viewing panes are not exempt the tests that visual
viewing panes require, especially when used in this manner.
The fact that Calcium Fluoride cannot resist
impact should be enough to discount it from use in industrial
infrared viewing panes, however there are other disturbing
issues regarding is that it is hydroscopic (absorbs water)
and is affected by high frequency vibration levels. The
image to the left is of a Calcium Fluoride crystal in an
air-conditioned cabinet that was damaged by moisture, after
2 years the window had zero infrared or visual transmission.
We can coat crystals to reduce the ingress of water but
this only puts off the inevitable, it is for these reasons
that IRISS do not recommend the use of crystals in infrared
viewing panes.
Another
disturbing issue is that far too often companies purchase
crystal infrared viewing panes as they want to complete
the visual inspection of the switchgear and LOTO operations.
The crystal can not meet the minimum requirement for impact
under the mandatory requirement of IEEE and as such these
companies may indeed have inadvertently de-rated their switchgear,
infrared viewing panes are not exempt the tests that visual
viewing panes require, especially when used in this manner.