NIR cutoff filter for true color imaging sensors

Ralf Biertümpfel 1  and Steffen Reichel 1
  • 1 SCHOTT AG, Advanced Optics Hattenbergstr. 10, 55122 Mainz, Germany
Ralf Biertümpfel
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  • Dr. Ralf Biertümpfel studied Mechanical and Aerospace Engineering at the Technical University of Darmstadt (Germany) and Cornell University (USA). After his diploma and his Master of Engineering in 1996, he worked on a research project in the field of heat transfer and thermodynamics and finally earned his doctorate at the Technical University of Darmstadt in 2001. Since 2001, he works at SCHOTT AG in the fields of hot forming of glass and precision optics. At present, he is the application manager for optical filters.
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and Steffen Reichel
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  • Steffen Reichel studied Electrical Engineering at the University of Kaiserslautern and spend 7 months at the Michigan State University, Michigan, USA. After his diploma degree in 1996, he worked as a researcher at the University of Kaiserslautern working on his doctorate (Dr.-Ing.) in the field of optical communications (fiber optics, erbium-doped fiber amplifiers). In 1999, he joined Lucent Technologies working on fiber optical communications, erbium-doped, and Raman amplifiers. Since 2001, he works for SCHOTT on several fields of optics from imaging optics, fiber optics, waveguide optics, laser optics, illumination optics, and optical filters. He is a Senior Member of the IEEE and Member of the SPIE, and since 2013, he is a professor (honorary) for ‘Optics & Photonics’ at the University of Applied Science, Darmstadt, Germany. In addition, he is co-author in three books, has about 60 publications, and has 20 granted or applied patents.
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The function of a near-infrared (NIR) cutoff filter for imaging sensors is being described. The main purpose of the NIR cut filter is to obtain correct color recognition; therefore, the NIR filter is made of an absorbing filter glass and an interference coating. The absorbing filter glass is needed to minimize multiple reflections inside the lens system, which are the cause for ghost images. An additional interference coating enhances the function of the filter. Coating and filter glass are strongly dependent on each other. This requires high reproducibility and low tolerances of the filter glass and interference coating. In addition, features like inner quality – especially striae – and stability of the refractive index are important. A NIR cut filter may be designed as a flat plate or as a lens. Our analysis provides an estimation about striae level and variation of transmittance and their effect on image quality and color recognition. Furthermore, the use of an absorption filter glass as a lens (shrinking down the overall size) is discussed in terms of the influence on transmission and striae.

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Advanced Optical Technologies is a strictly peer-reviewed scientific journal. The major aim of Advanced Optical Technologies is to publish recent progress in the fields of optical design, optical engineering, and optical manufacturing. Advanced Optical Technologies has a main focus on applied research and addresses scientists as well as experts in industrial research and development.