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A consensus of opinion throughout the optical filter industry has developed over the last decade; in which the hard coating of bandpass filters offers a higher performance and greater durability when compared to original lamination of bandpass filters. This thought process has led to the belief that there is no longer a need for traditional bandpass optical filters. While this may be true in some cases, not all laminated filters are created equal. In this paper we will illustrate the advantages and disadvantages of both processes, while dispelling some of the many myths about traditional laminated filters. Andover Corporation provides both types of optical coatings and we hope this information will help you decide which option is best for you.
Rather than leaving our customers with only one typically more expensive choice for their filter needs, Andover Corporation chose not to limit its technologies in this way. Andover developed new techniques to improve the strength and durability of the lamination. These techniques include laser ablation to create a sealing perimeter and the use of proprietary epoxies and techniques to assemble the laminated surface into a high quality optical filter. Andover believes that all of the coating technologies and methods have a place, depending on your application and specifications. Some comparisons to keep in mind are:
  • Sharp Edged Pass-band Profiles
  • High Transmission Levels
  • Lifetime
  • Cost
Myth: Traditional coating technologies cannot yield sharp edged pass-band profiles. 
Andover Corporation has fully automated optical systems that deliver square shaped 3-5 cavity bandpass filters consistently and always to specification.  
Myth: Laminated glass filters cannot provide high-transmission levels. 
The filters in the graphs below are Cary 5000 spectrophotometric measurements showing actual transmittance of filters Andover has delivered to customers with full CSV data at no extra charge.


                              Myths3 Myths4 

Myth: Traditional laminated glass filters do not last .  
This is also a misleading statement.  Andover Corporation has optical filters in space that have been operating effectively for decades. Andover Corporation uses epoxy purification techniques, detailed curing methods, construction and scribing procedures to extend the life of traditional filters. This is important for equipment manufacturers to know.  
The question is; do I purchase a $25 to $90 optical bandpass filter that will last 10 years, or a $300 to $500 filter that will last 30 to 40 years?  A typical manufactured piece of equipment (for OEM) has a life cycle of 5-10 years.  In 5-10 years a company will release another product version that is more effective than the first. It is a rare case that an optical filter needs to survive 30 years. However, there are some cases where this is true, Andover’s traditional filters can last decades in space, and as long as 10 years here on earth.  
Hard coating first surface thin films are a significant advancement in coating technology and has many benefits. However, it also has disadvantages other than just price. A large disadvantage of first surface hard coatings, created by Magnetron Sputtering or Ion Beam Sputtering, is the build-up of stress. This tremendous stress will distort the optical surface of a filter, creating a distorted transmitted wavefront with reduced image quality.  If the hard coating film is of considerable length (and they usually are) then the optic can be visibly bent like a potato chip if the substrate is not thick enough.  
A second disadvantage is limited blocking range, because it requires so many layers and long coating runs (sometimes a full week) to block the NIR.  If your application requires it, keep in mind, that traditional laminated filters can block the entire out-of-band spectrum regardless of whether it is in the ultraviolet, infrared or visible spectrum.
In conclusion, when choosing the appropriate specifications and requirements for an optical bandpass filter, be sure to consider the true facts about what is really needed. Andover Corporation’s design and quotation team can help refine specifications to get you the correct bandpass filter for your needs. In this way, Andover can help limit any unnecessary costs that may be driving your project. Let us prove it to you.

Laminated vs- Hard-coating

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