Robust Light Filters Support Powerful Imaging Devices
- Sunday, 01 November 2009
Originating Technology/NASA Contribution
Consider the anatomy of a rainbow: From the inner arch, violet shifts to blue, then green, yellow, and red. Contained in the rainbow is the spectrum of light that our eyes take in and translate into images of the world around us. But the human eye only registers a minute percentage of the electromagnetic spectrum, which theoretically stretches into infinity beyond the wavelengths (between 0.38 and 0.76 micrometers) of visible light. Past violet, the wavelengths of ultraviolet, X-rays, and gamma rays cramp closer and closer together. Going in the other direction, past red, stretch the expanding wavelengths of infrared, terahertz, microwaves, and radio waves. It is within these invisible ranges that many of the secrets of our universe remain.
Much of the universe is not observable using visible light. Clouds of cosmic gas and dust obscure the gaze of traditional telescopes, and the ongoing expansion of the universe following the Big Bang further complicates observation of distant galaxies. As the galaxies race away from our position, the visible light they emit is redshifted, or stretched, transforming it into infrared (IR) light—thermal, or heat radiation—which is able to pass unimpeded through cosmic dust clouds. Telescopes that detect IR light are therefore capable of unveiling vastly greater views of our universe and even peering into its very beginnings.
NASA has been at the forefront of IR astronomy since 1974, when its Kuiper Airborne Observatory (KAO), an IR telescope mounted on a transport jet, made astonishing observations on a series of flights that lasted until 1995. (The Earth’s atmosphere absorbs much of the IR radiation from space, making high-altitude telescopes far more effective than ground-based versions. Space-based telescopes are more sensitive yet, since they can by cooled to very low temperatures, limiting interference from their own heat radiation.) The Agency has developed many IR observatories, from the 1979 construction of the Infrared Telescope Facility 14,000 feet up the slope of Mauna Kea in Hawaii to the 1983 launch of the Infrared Astronomical Satellite, the first IR satellite telescope, to the Cosmic Background Explorer (COBE), to the near-infrared camera multiobject spectrometer (NICMOS) operating onboard the Hubble Space Telescope since 1997. In 2003, the Spitzer Space Telescope combined sensitive modern infrared detectors with a cooled telescope to produce a remarkably powerful observatory. NASA plans to come full circle in its IR observatory efforts in 2010 with the first operational flight of KAO’s next-generation cousin, the Stratospheric Observatory for Infrared Astronomy (SOFIA), a large, airplane-mounted IR telescope. Four years later, the Agency expects to launch the infrared James Webb Space Telescope, which will allow us to see deeper into the universe than ever before.
Lake Shore Cryotronics Inc., of Westerville, Ohio, entered into 2002 Phase I and Phase II Small Business Innovation Research (SBIR) contracts with NASA’s Jet Propulsion Laboratory (JPL) and similar contracts in 2004 with Langley Research Center. The NASA centers wanted robust and high performance optical filters for IR astronomy to select the wavelengths of light that reach the detector. Since warm objects emit IR radiation, steps must be taken to reduce the amount of local thermal noise that interferes with the desired signal, typically by cooling the detector and its filters to a near-absolute-zero temperature. Existing IR filters, composed of multiple layers of different materials with different mechanical properties, tend to delaminate (separate from one another) at such extreme temperatures. They are also extremely expensive and often comprised of mechanically and chemically fragile materials. Lake Shore, a leading supplier of cryogenic temperature sensors and instrumentation, magnetic test equipment, metrology systems, and probe stations for the characterization of magnetic and transport properties of material, was intrigued by the problem and used the SBIR funding to investigate porous silicon technology, and subsequently, metal-mesh technology, as possible elements of a new filter technology for the IR. The company invested significant internal funds in the project; Lake Shore prides itself in a research and development budget that is typically double the national average for instrumentation companies.