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A Unique, Accurate LWIR Optics Measurement System


Stephen D. Fantone and Daniel G. Orband

Optikos Corporation, 107 Audubon Road, Bldg. 3, Wakefield, MA 01880

ABSTRACT

A compact low-cost LWIR test station has been developed that provides real time MTF testing of IR optical systems and EO imaging systems. The test station is intended to be operated by a technician and can be used to  measure the focal length, blur spot size, distortion, and other metrics of system performance.  The challenges and tradeoffs incorporated into this instrumentation will be presented.

The test station performs the measurement of an IR lens or optical system’s first order quantities (focal length, back focal length) including on and off-axis imaging performance (e.g., MTF, resolution, spot size) under actual test conditions to enable the simulation of their actual use.  Also described is the method of attaining the needed accuracies so that derived calculations like focal length (EFL = image shift/tan(theta)) can be performed to the requisite accuracy. The station incorporates a patented video capture technology and measures MTF and blur characteristics using newly available low- cost LWIR cameras.   This allows real time determination of the optical system performance enabling faster measurements, higher throughput and lower cost results than scanning systems.   Multiple spectral filters are also accommodated within the test stations which facilitate performance evaluation under various spectral conditions.


Infrared Imaging Systems: Design, Analysis, Modeling, and Testing XXII, edited by Gerald C. Holst, Keith A. Krapels, Proc. of SPIE Vol. 8014, 801413 • © 2011 SPIE • CCC code: 0277-786X/11/$18 • doi: 10.1117/12.884267


INTRODUCTION

The market for infrared imaging systems for the military, commercial and industrial customers is rapidly expanding. Lower cost imagers and the sourcing of optical fabrication in low cost labor markets has resulted in modestly priced thermal imaging systems which are finding widespread acceptance in both consumer, security, fire, law enforcement and military product.  This increased production has created the need for broad spectrum, easy to use, cost effective systems to accurately inspect LWIR (7-15 um) components that are used in these imaging systems.  Furthermore, optical components and systems are frequently sourced in facilities with minimal quality control processes, metrology personnel and equipment.

The challenge for a production facility or incoming inspection department is the required inspection equipment must have broad applicability and flexibility but still be able to be operated by a technician. The LensCheck™ IR from Optikos, see figure 1, is the next step in expanding our  cost effective test platform into the infrared.  You can now evaluate the polychromatic performance of LWIR optical systems over a broad range of test conditions including the entire 7-15um spectral range. With space at a premium in most manufacturing and testing facilities, the compact LensCheck’ s table top footprint is less than 4 square feet allowing it to be located on a standard laboratory table; a floating table is not required. It is portable and can be readily relocated and reconfigured for testing within a few minutes.

In contrast with interferometric testing at 10.6um, the LensCheck IR provides full polychromatic evaluation over the entire LWIR spectrum.  Also, MTF testing with the LensCheck and its associated OpTest™ VideoMTF® software provides real time inspection of the imaged formed in the wavelength and directly assesses the imaging performance of the optical system including any effects from stray light.  Measurement of blur spot size, image contrast, MTF, distortion, astigmatism, FWHM and many other image parameters are easily made and direct visual inspection by the operator of the video image allows the operator to apply the intuition gained from inspecting visual images to the assessment of the IR imaging performance. This is particularly useful when trying to diagnose the root cause of image degradation in a sample lens.

The Optikos LensCheck allows the operator to confirm the basic performance of an optical component or system and assess more complicated performance requirements. The light source and multi-position filter wheel enable lens and system testing using the spectral distribution encountered in actual use. It is not restricted to specific wavelengths and enables characterization of the various chromatic effects (e.g., longitudinal and lateral color, spherochromatism),  that simply are not available with monochromatic interferometric testing.

Frequently, catalog optical components have tolerances in focal length and centration accuracy that require evaluation of the first order and imaging properties of the lens prior to installation to a system.  It is prudent to confirm the performance of infrared components by measuring their first order properties ( effective focal length (EFL), back focal length (BFL), flange focal length (FFL), see Figure 2) and on-axis imaging performance (MTF, resolution, and/or spot size) under test conditions simulating their actual use. In order to space components properly to achieve desired magnification and total track between object and image plane, it is essential to know the precise focal length and principle point locations in a lens relative to mounting surfaces.  Thus, an ability to accurately measure parameters such as focal length, back focal length, and flange focal lengths greatly simplify and accelerate the assembly of these systems making the manufacturing process more predictable and deterministic. These quick checks assure that a component is performing to its nominal performance and may be safely incorporated into a larger optical assembly.

Often, a rapid inspection of the on-axis image blur noting asymmetry in the blur shape allows one to detect and reject a lens that has a centration error. If a more quantitative metric is desired the blur spot size (in x and y) and the measured MTF can provide precise measurement of astigmatism and this information can be used to evaluate the impact on overall system performance.


HOW ACCURATELY CAN WE MEASURE THESE LENS PARAMETERS?

Consider a 50 mm focal length F/2 well corrected and centered system in the LWIR

Note that these capabilities scale with F/#, focal length, and wavelength so that highly corrected faster systems can be measured to higher accuracy and slower systems to lower accuracy. Typically, longitudinal aberrations in a highly corrected system may be measured to a fraction (< 1/5) of the longitudinal diffraction depth of focus allowing one to assemble a system with confidence that the predicted performance will be achieved.   Figure 3 depicts a full screen shot showing the real-time video image, line spread and MTF, and through-focus MTF measurements.

REQUISITE ACCURACY INTEGRAL TO THE INSTRUMENT DESIGN

In order to obtain results to these accuracies, the lens under test must be properly fixtured and aligned relative to the optical bench axes. Also, all slides used for this measurement are instrumented with glass scale encoders so that derived calculations such as focal length (EFL = image shift/tan(theta) can be performed to the requisite accuracy. Accurate measurement of MTF requires that stray light in the system be captured and therefore use of a camera with an extended bit depth and dynamic range is essential. To achieve high throughput, the Optikos LensCheck measures blur spot size and MTF using real-time video capture (US Patent 5,661,816) allowing real-time measurement of the lens or system performance

TESTING OVER EXTENDED FIELDS AND FINITE CONJUGATES

We have seen a number of highly unusual systems requiring characterization over very wide fields (beyond +/- 90 degrees) or very low F/#s.  The Optikos LensCheck family of instruments allow the user to test lenses over fields of view extending to +/- 105 degrees.  This has proven to be extremely useful to evaluation fisheye lenses, endoscopes with angled lines of sight and wide field angles, and other systems with deviated lines of sight.

In the visible, the high quality re-imaging objectives enable testing of lenses with numerical apertures as high as 0.8.  In specialized cases using an immersion objective as part of the image analyzer relay, it has been possible to test optical systems that are part of fluorescent imaging systems with numerical apertures approaching 1.5. For the LWIR, we have designed high performance reimaging lenses with a numerical aperture of 0.7 providing for measurement of exceptionally fast lenses with steep chief ray angles.


When the most precise measurement of chromatic properties of lenses are desired, a 60 mm clear aperture, 750 mm effective focal length reflective collimator is available.

While in many cases an optical system is used at infinite conjugates, numerous optical systems are intended to relay images and must be tested at their designed finite conjugates. For example, a short 4:1 finite conjugate relay system should be tested at the actual finite conjugates of use; in most cases, the design performance at infinite conjugate would be substantially degraded. Optikos has developed the ability to perform these tests and to characterize distortion and other field aberrations to extreme precision by incorporating high accuracy encoders into the image analyzer stages. Distortion accuracy of less than 0.1% can be obtained over a 100 mm field and usually limited by the distorted blur shape formed by the lens.  The finite conjugate configuration is depicted in Figure 5.

All of this is accomplished through the use of a USB based motion control system working closed-loop with encoders on multiple axes. The patented OpTest™ software assures a level of metrology accuracy and precision not previously available at this price point and unit size.

TESTING IN THE LONG-WAVE THERMAL INFRARED

The availability of low cost thermal cameras in bringing large scale manufacture and installation of thermal cameras in applications ranging from security, automotive, marine and to firefighting.  The LensCheck LWIR provides the ability to verify the performance of optical systems used in the LWIR.  Figures 6 and 7 depict the LensCheck LWIR with refractive and reflective collimators, respectively.  The image analyzer incorporates a high numerical aperture (0.7NA, 7.5X) relay objective that images the image formed by the system under test onto the LWIR camera. With 25um pixels, the combined camera and relay optics group has an effective Nyquist frequency of 150 lp/mm. Both slit and pinhole targets can be used and available cameras have been measured with NETDs with less than 35 mK at F/1.

The measurements that are available with the LensCheck LWIR including blur spot size, MTF, field curvature, distortion, and astigmatism. The ability to visualize the IR image allows one to diagnose any problems with the lens under test and further allows for rapid validation of sample lenses.

The Optikos LWIR bring a new level of capability to production facilities and allow technicians to make measurements previously made by development engineers. This enables manufacturers to qualify every incoming product quickly and reliably while greatly minimizing the costs and risks of sub-standard complete assemblies.

 

LensCheck, LensCheck VIS, LensCheck LWIR, and OpTest are trademarks of Optikos Corporation. VideoMTF is a registered trademark of Optikos Corporation. OpTest software is covered under US patent 5,661,816.

 

 

Figure 1 - LensCheck VIS Bench – 58 mm clear aperture collimator, rotary stage and image analyzer – intended for visible and near IR

Figure 2 - Several First Order Optical Parameters

Figure 3 - Typical mid-frequency MTF curve readily quantifies system astigmatism

Figure 4 – Screen shot depicts the real-time video image, line spread and MTF.

Figure 5 - Table top bench allows for finite conjugate testing

Figure 6 - LensCheck LWIR with Refractive Collimator

Figure 7 - LensCheck LWIR with Reflective Collimator