Going smaller - a challenge we can meet
Introduction of 12µm optimized catalog lenses at CIOE 2018
Over the past 2 decades, the pixel pitch for uncooled thermal imaging detectors has been gradually reducing. The 50-micron pixel pitch detectors available in the 1990’s were replaced by 38-micron detectors and these in turn were replaced by 25-micron and 17-micron detectors. While the vast majority of today’s detectors feature 17-micron pixels, the next wave of detectors has already been launched and is gradually gaining traction in the market. The newest generation of detectors works with 12-micron pixels, and there are even a few detectors with 10-micron pixels. These new detectors present new challenges for lens designers and manufacturers and Umicore is launching a range of lenses compatible with the new detectors.
The first aspect considered in the design of the new lenses is increased image sharpness required by the smaller pixels. The Nyquist limit for a 12-micron pixel detector increases to 42 cy/mm. A second, but equally import impact of the increase in the Nyquist limit is to require a faster f/number to allow a larger diffraction limit. The faster f/number is therefore, an enabler to reach the image sharpness requirements. A complementary benefit of the faster f/number is increased energy collected resulting in greater sensitivity in the system. However, faster f/numbers and increased image sharpness are a significant design challenge. The easy answer would be to add optical elements to meet the changing requirements. However, at Umicore we pride ourselves on looking beyond the easy answer. Additional optical elements would mean additional size, weight and cost, so we are using advanced design techniques and knowledge of the optical materials to keep the number of elements low. Since the 12-micron pixel gives a smaller active detector area, we are optimising the size and weight of the lens elements to take full advantage of this.
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
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
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
Quantitative comparison of the chalcogenide glass GASIR with Germanium and Silicon for use in LWIR sensor lens design
In recent years, improvements in sensor fabrication technology have allowed infrared imaging devices to penetrate the commercial market. Applications in non-contact thermal sensing, domotics and security and surveillance are becoming affordable to a wide range of end users. As detector prices drop, the importance of the cost of the optical components increases. In a large part, this is determined by optical requirements such as resolution, focal length and f-number, which in practice translates into size and complexity of the lenses. Another key determinant of both design and price is the material being used. Germanium lenses have long been a favourite of the infrared lens designer due to the high refractive index, but suffer from drawbacks such as thermal drift and price fluctuations. Chalcogenide glasses have been developed to overcome these limitations, albeit at a lower refractive index. Where resolution is not as critical (i.e. for detectors with few pixels), silicon lenses are widely used. However, the fact that even sensing detectors are now available with many more pixels is driving change. In this work, we compare the performance and potential of three different LWIR lens materials, namely germanium, silicon, and GASIR. A quantitative analysis is made of several optical parameters. In addition, we demonstrate the potential... Read more
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... Read more