Optical Filter Visual Inspection Criteria

Learning Center, Quality Assurance

As experts in the design and manufacture of thin film optical filters, Iridian employs rigorous quality assurance processes.  Among these is our comprehensive visual inspection process, which includes the following steps:

Critical Area

We inspect the clear aperture (typically central region of a filter) through the light beam is transmitted or reflected.

Edge/Corner (Chips)

After a coring or dicing process, we inspect for both chips in the substrate and in filter coating.  If a chip of either type extends into the clear aperture of the filter, then this results in a filter failing visual inspection.

Sleeks, Scratches and Digs

Sleeks are scratches on the substrate underneath the coating. A sleek appears as a bright white line when observed under bright field illumination. Sleeks can be long (across the entire filter) or short and they can be straight or have various orientations.

A filter containing a sleek or scratch results in a visual inspection failure if, within the clear aperture area, the width is greater than a specified scratch value.  In the case of a filter with more than one sleek or scratch in the clear aperture area, the sum of all sleeks in the clear aperture area is used.


We also inspect for defects not found in other categories, such as pits or areas with insufficient coating.  A filter with a defect in the clear aperture area fails visual inspection if its width is greater than a specified dig value.  In the case of a filter with more than one defect present in the clear aperture area, the sum of all clear aperture area defects must be less than the specified dig value in order to pass visual inspection.


A protrusion is any excess material protruding from the side of the filter.  Any visible protrusion results in the filter failing visual inspection.

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