Stories & Features

Edge Blackened Optical Filters

Aug 22, 2023 by IDEX Health & Science

Edge Blackened Optical Filters - now available for high volume and smaller orders

What is Edge Blackening?

Edge Blackening is the application of a matte black paint to the edges of an optical filter to prevent stray light inside the filter from re-entering the light path outside the filter. Our existing and new Edge Blackening capabilities are now available for both high volume and smaller orders of catalog and custom filters.

New in 2023: Machine printed Edge Blackening capability (see image above, minimum order of 10)

Why Use Edge Blackening?

The ideal filter mount is a metal housing, which is available for standard sizes of round Semrock filters. The purpose of such housings is to reduce light bypassing the filters just outside the edges, or entering or exiting the filter via the edges. However, metal housings are not always practical or available; for example, no standard metal housings are available for rectangular Semrock filters

Edge Blackening with a suitable black paint is an acceptable alternative to metal filter housings and is available for most Semrock filters. The Semrock Edge Blackening process precisely covers the edges with a diffusely reflecting, light-absorbing paint to prevent stray light from exiting the filter through the edges. Using machine printed Edge Blackening, filter edges can be marked with text or colored dots for identification.

Additional Edge Blackening Benefits

  • Enable easy identification of filters
  • Allow confident assembly by technicians for more assured production
  • Reduce demand on customers’ incoming quality control
  • Mark filter edges with colored text or dots for identification

Frequently Asked Questions:

  • How well will Edge Blackening prevent stray light from escaping through the filter edges?
    The paint absorbs ~98% of the light incident on it from the inside of the filter over the range 350 – 750 nm, with the remainder reflected diffusely back into the filter interior.

  • What are the differences between manual and machine applied Edge Blackening paint?
    The machine paints only edges of rectangularly shaped filters without corner cuts. The manual process can be applied to edges of all filter shapes.

  • Will the Edge Black paint peel off?
    The Edge Blackened filters are mechanically robust, and the paint will not peel off. The painted edges can be handled by fingers or plastic-tipped forceps, and the filters should be mounted to avoid repetitive chafing of the painted edges.

  • Does Edge Blackening affect filter lifetime?
    Both manually and machine applied Edge Blackening have excellent longevity. Manually edge-blackened parts have been installed in customer systems for up to ten years and more without loss of Edge Blackening function.

  • How can I clean my filter without destroying the Edge Blackening?
    The Edge Blackening paint remains adherent to the glass surface for the life of the filter, without decomposing and / or falling off, provided that the paint on the filter edge is neither abraded by an external agent nor dissolved using a chemical agent such as acetone. Examples of suitable use include being stably secured in an optical assembly with no exposure to such solvents.

    No effect on paint adhesion to glass has been seen when rubbed with a cleaning swab soaked in an acceptable solvent, when tested by abrasion with an eraser, or when adherent cellophane tape is pulled off.

    Use isopropyl alcohol (IPA) or methanol to clean Edge Blackened filters, as these solvents don’t affect the paint, provided the cleaning is gentle, i.e., not aggressive or of overly long duration. Do not use acetone, as this will remove the paint.

    Watch our Optical Filter Cleaning Tutorial Here

  • Are there other environmental limits or contamination on heating?
    Edge Blackened filters can be used in the usual operating environments for optical filters; such filters have been tested between -20 °C and 85 °C and at 85% Relative Humidity without incident.

  • How many characters can be printed on a machine-painted filter edge?
    One character can be printed per mm of edge length, parallel to the filter surface.

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