Infrared - IR - filters are normally combinations of thin film deposition interference filters. Out of band blocking can be achieved by the proper choice of substrate, reflectance filter stacks or by adding absorption glass.

The thin film infrared filters are comprised of stacks of thin layers (of different optical materials) in controlled thicknesses. Light is reflected at each film interface and by the correct choice of parameters can be made to interfere with that from other interfaces, constructively for certain wavelengths giving high reflectance, and destructively for others giving high transmittance. Long wave pass, short wave pass and bandpass filters can be designed and manufactured in this way.

IR Filter Applications

The applications for IR filters are diverse. EOC supplies a broad range of mid infrared filters and coatings for a variety of detector applications including  Pyroelectric, Thermopile, Lead Selenide (PbSe), Lead Salt (PbS) and MerCadTeluride (MCT) Detectors:

• Environmental monitoring equipment
• Emission monitoring for power plants and factories
• Flame detection for petrochemical plants and oil rigs
• Home and industrial security
• Automotive exhaust analysis
• Spectroscopy equipment
• Ozone layer monitoring equipment
• Blood analyzer equipment (breathalyzers)
• IR cameras
• Aircraft avionics
• Missile and space applications

To determine your infrared filter requirement consider the kind and intensity of the source, detector characteristics, and optical path geometry. EOC will assist you in selecting a suitable standard IR filter or in developing the specification for a custom-made infrared filter.

Mid IR Filter and Coating Types

Narrow Bandpass Filters (NBP)
Narrow bandpass IR filters are designed to isolate a narrow region of the infrared spectrum. This is accomplished using a complex process of constructive and destructive interference.

Wide Bandpass Filters (WBP)
Wide bandpass filters are designed to isolate a very wide spectral band. They are classified by having bandwidths (measured at half peak transmittance levels) greater than 13%.

Long Wave Pass Filters (LWP)
Long Wave Pass filters (also referred to as edge filters) are constructed from stacks of thin layers. They are distinguished by a sharp transition from a zone of rejection to a zone of transmission.

Short Wave Pass Filters (SWP)
Short wave pass filters are distinguished by a sharp transition from a zone of transmission to a zone of rejection.

Neutral Density Filters
The majority of neutral density filters offered are manufactured with a metallic coating which attenuates incident light by both reflection and absorption.

AR (Anti-Reflection) Coatings
Capabilities in anti-reflection coatings range from single layers to advanced high efficiency multi-layer coatings. These coatings can be applied to windows, lenses or customer supplied substrates.

IR Filter Technical Notes:

• Glossary of Filter Terms (PDF)
• Filter Technology
• Temperature Shift
• Angular Shift
• Military Specifications
• Custom Infrared Filters
• NDIR Filter Requirements
• Northumbria Optical Coatings History