Using material advances in chalcogenide glasses to improve imaging lenses in the 8-14 µm waveband
Changes in the position of best focus over temperature are a major source of contrast degradation in the long-wave infrared. The prime sources of this focus shift are the difference between thermal expansion coefficients of lens material and housing material, and the change in refractive index over temperature ∂n/∂T. These parameters, combined with the limited depth of focus when using lenses for uncooled detectors, can rapidly degrade performance with changing temperature. First-order paraxial calculations to model these changes are discussed, with a demonstration of its application to single-element imaging systems. The model is then expanded to include two-element systems where both elements are made of the same optical material, or the more general case where different materials are combined. It is shown how a chalcogenide glasses... Read more
Evaluate depth of field limits of fixed focus lens arrangements in thermal infrared
The desire to implement lens arrangements without a focusing mechanism demands a deeper quantification of the Depth of Field problem. A new approach avoids the classic “accepted image blur circle”.
Very often, basic functions like recognition or detection of specified targets are tested for a certain lens. But what is the impact of... Read more
Molded wafer level optics for long-wave infrared applications
The initial introduction of LWIR thermal imaging to the public was with Night Vision systems in their cars. Today the consumer market continues to expand with the launch of a number of consumer focused, smart phone add-ons. This has brought a step change in system costs, with the possibility to turn your mobile phone into a thermal imager for under $300.
For lenses, the quantity manufactured, quality and costs will require... Read more
Depth of Field in Modern Thermal Imaging
Modern thermal imaging lenses for uncooled detectors are high aperture systems. Very often, their aperture based f-number is faster than 1.2. The impact of this on the depth of field is dramatic, especially for narrow field lenses. The users would like to know how the image quality changes with and without refocusing for objects at different distances from the camera core.
The Depth of Field approach presented here is based on... Read more
Designing Athermal Lenses For Long-Wave Infrared Systems
Due to advances in materials and manufacturing technologies, the vast majority of lenses used on long-wave infrared (LWIR) cameras are simple: One- or two- element designs with an emphasis on low-cost manufacturing. One effect of this is that most of these cameras are supplied without any means for the user to focus them. This means that the lens must stay in focus over a broad range of temperatures. Since the standard temperature range for use outdoors is -40 °C (really cold) to +85 °C (much too hot!), this can provide some interesting design challenges.
Most materials... Read more
Methodology for lens transmission measurement in the 8-13 micron waveband: Integrating sphere versus camera-based
Transmission is a key parameter in describing an IR-lens, but is also often the subject of controversy. One reason is the misinterpretation of “transmission” in infrared camera practice. If the camera lens is replaced by an alternative one the signal will be affected by two parameters: proportional to the square of the effective aperture based F-number and linearly to the transmission. The measure to collect energy is defined as the Energy Throughput ETP, and the signal level of the IR-camera is proportional to ETP.
Most published lens transmission values are based on... Read more
Two-lens designs for modern uncooled and cooled IR imaging devices
In recent years, uncooled, or thermal, detectors with a 17µm pixel pitch have become well-established for use in various applications. Examples of applications include thermal imaging in cars for driver’s vision enhancement. This has allowed the civilian infrared market to steadily mature. The main cost driver for the manufacture of these lens designs is the number of lenses used.
The development of thermal detectors... Read more
What is Perfect Optical Performance?
They say that beauty is in the eye of the beholder, however, perfection is often more difficult to define. In lens systems the diffraction limit is often used to describe the limits of optical resolution. In visible optics this is rarely relevant but in the 8-14 micron waveband, this is a very real limitation on what can be done in any optical system and can, conveniently, define perfection.
The diffraction limit is... Read more