Centration Measurement, Alignment and Assembly of Infrared Lenses

Transcription

1 Centration Measurement, Alignment and Assembly of Infrared Lenses Contents Centration Measurement of Infrared Lenses 2 Special Considerations for Centration Measurement of Infrared Lenses 2 Infrared Spectral Ranges 2 Differences to VIS from the Operator Side 3 Differences to VIS from a Technical Perspective 3 Measurement Examples for Infrared Lenses 4 Centration Testing of Highly Sensitive Single Lenses 5 Centration Measurement of Infrared Single Lenses with Plane Reference Flange 6 Centration Testing of Aspherical Lens Surfaces 7 Characterization of Complex Lens Systems 9 1/9

2 Centration Measurement of Infrared Lenses This whitepaper focusses on centration measurement of infrared lenses: it describes the special considerations for the centration measurement of infrared lenses and provides a detailed description of typical applications for infrared lens measurement. The majority of centration measurements can be carried out with a more cost-effective visual measurement head. An infrared measurement head is only required for the measurement of optical systems or for measurements in transmission. As the whitepaper focuses on infrared lens applications the general description of centration measurement is not part of this whitepaper. Please feel free to visit our website to get more information about centration measurement and low-coherence interferometry. Special Considerations for Centration Measurement of Infrared Lenses For testing single lenses and completed assemblies that are only transparent in the infrared range, TRIOPTICS provides measurement heads specifically designed for the infrared wavelength ranges that are equipped with focal-plane array infrared image sensors for ease of use. Typical applications are e.g. testing of lenses and assemblies for thermal imaging, military applications or industrial process control made from lens materials like e.g. Ge, Si, ZnSe, ZnS or CaF2. Infrared Spectral Ranges The infrared wavelength range is divided in three distinct ranges for imaging applications. These bands exhibit high transmission of light through air and are separated by strongly absorbing bands in the spectrum. The three imaging infrared ranges are: short wave infrared (SWIR) from µm, medium wave infrared (MWIR) from 3-5 µm and finally long wave infrared (LWIR) from 8-12 µm wavelength. Due to limitations in detector technology, so far no image sensor can cover all three wavelength ranges, however a LWIR system can also cover most MWIR lenses and vice versa. 2/9

3 For an overview about compatible materials and the wavelength regions covered by the * Comparison of transparent regions of typical infrared lens materials and the regions covered by available OptiCentric measurement heads. *Depending on doping level and dopant type available measurement heads please refer to adjactent drawing below. Silicon is a special case as the transparent region depends on the doping level and dopant type, so a LWIR head might be a suitable depending on application. Differences to VIS from the Operator Side In contrast to VIS systems, the light emitted from the focused autocollimator head cannot be seen by the naked eye which is however no problem in practice for aligning the sample. Apart from that, the operation of the infrared systems is not different to the VIS systems, so a user can be quickly trained for a new wavelength range. Differences to VIS from a Technical Perspective From a technical side, apart from using suitable optics and illumination sources in the measurement heads, the most important difference between visual and infrared range is that in the infrared range every object, including the sample, emits light in this wavelength region, so the instrument needs to compensate for the thermal background before taking a measurement. This is done automatically by the software and requires no operator intervention. Also, the contrast between background and illuminated areas is lower than in 3/9

4 the VIS, so specialized image processing algorithms are used to reach the required high resolution. OptiCentric in the standard reflection mode relies on back-reflection from the lens surface, so the light intensity of the reflected reticule image strongly depends on the type of coating used. Typically, all infrared imaging lenses are AR-coated, however there is a wide variation in efficiency which the instruments compensates by adjusting illumination power and shutter times where available. In general, the typical accuracy of the centration error measurement in IR range is approximately 1 µm, which is due to the longer wavelength and larger pixel size of the cameras used in the autocollimators in the IR range. Measurement Examples for Infrared Lenses In order to understand the measurement and assembly of different infrared lenses, some typical measurement, alignment and assembly tasks and their solutions are presented below. Centration Testing of Single Lenses Centration testing of non-vistransparent or transparent single Measurement of an infrared lens on lens rotation device lenses with OptiCentric is the described measurement task. The required instrument is equipped with a lens rotation device and a visual measurement head. The lens under test is placed onto the ring chuck support of the lens rotation device. This fixes the center of symmetry of the bottom surface in space. When the lens rotates with its edge against the V-block, the lens rotation axis is given by the geometrical center of the circumference and the center of curvature of the bottom surface. In this way the rotation axis corresponds directly to the reference axis for the determination of the wedge error. So the remaining parameter to be measured is the center of curvature position of the top surface, which can easily be tested by OptiCentric in reflection mode. Advantages of the Single lens centration testing in reflection mode with an OptiCentric System with the lens rotation device allows the evaluation of infrared lenses with a cost-effective instrument without the need for expensive infrared devices. OptiCentric with lens rotation device with vacuum chuck 4/9

5 Centration Testing of Highly Sensitive Single Lenses The easiest way to protect highly sensitive lenses like calcium fluoride lenses against scratches is to reduce handling during the production and testing process. The OptiCentric Dual instrument in combination with an air bearing and a non-contact distance sensor fulfills this requirement, instead of using the lens rotation device with V-block. An OptiCentric Instrument with cost-effective visual measuring heads is used regardless of the material properties of the lens under test. The lens is carefully placed in the lens holder. During the measurement the dual autocollimators measure the center of curvature positions of the top and bottom surfaces; the distance sensor measures the radial run-out of the lens edge. In the analysis, the software determines the reference axis from the center of the circumference and the bottom surface. The measured offset of the top surface to this axis gives the lens centration error. For lenses that are Measurement the edge of a highly sensitive lens with a distance sensor transparent in the VIS range the centration error can alternatively be measured in transmission. Therefore, the top autocollimator measures the run-out of the focus spot when the lens under test is illuminated with collimated light by the bottom (auto-) collimator. A minimum of handling protects the sensitive surfaces of the lens. Both surfaces are measured in one step with cost-effective visual measurement heads. OptiCentric 100 Dual or OptiCentric 300 Dual with distance sensor 5/9

6 Centration Measurement of Infrared Single Lenses with Plane Reference Flange Lenses with a plane reference flange are often used in infrared optical systems. The preferred instrument to measure these lenses is the OptiCentric Dual, with two visual measurement heads and lens rotation device with vacuum chuck. The lens is placed with the support flanges onto the ring chuck and rotated against the V-block. The upper autocollimator focuses in the center of curvature of the top surface. The lower autocollimator focuses in the center of curvature of the lower surface. Thus, the measurement is accomplished in reflection. In this case the reference axis Lens with plane reference flange is given by the center of symmetry of the circumference and the normal of the support flange. The result of this measurement gives information about the centration errors of both surfaces with respect to the reference. For VIS-transparent lenses the centration can be measured in transmission as well. Both surfaces are measured in one step with cost-effective visual measurement heads and lens rotation device with vacuum chuck. OptiCentric Dual with vacuum chuck 6/9

7 Centration Testing of Aspherical Lens Surfaces The use of aspheric surfaces in the design of optical systems makes it possible to achieve better spot size performance, or alternatively to achieve similar performance while using fewer elements in the system. Therefore, especially for infrared optics, aspherical surfaces are commonly applied. Aspherical infrared lenses are preferably measured with an OptiCentric Dual instrument equipped with AspheroCheck module. This configuration achieves the best results and allows for simple handling. The lens is placed in the lens holder with the aspherical surface to the top. During the measurement the lens rotates and the three parameters required for centration testing are measured: Center of curvature position of the spherical bottom surface Paraxial center of curvature position of the top aspherical surface Eccentricity of the outer edge of the aspherical surface under rotation These three measurements are taken at the same time during the rotation of the lens: The lower measurement head focuses in the center of curvature of the spherical bottom surface and measures its centration. The centration of the paraxial area of the upper Measuring an aspherical lens surface is measured with the help of the upper measuring head in reflection mode as well. Third, the AspheroCheck module measures the eccentricity of the aspherical surface. The results of these measurements of aspherical infrared lenses give information about: Orientation of the asphere with respect to the primary reference axis of the measurement system (corresponding to the axis of rotation) Orientation of the top asphere with respect to the axis of bottom sphere and center of circumference In case of a double-sided asphere: relative orientation of the two aspherical surfaces OptiCentric 100 Dual or OptiCentric 300 Dual with AspheroCheck module 7/9

8 Assembly of Infrared Optical Systems The precise assembly of lenses in a barrel is a decisive step during the production of objective lenses. An OptiCentric instrument with an air bearing and distance sensor greatly improves the accuracy of the production process and the performance of the final product. Measurement & Assembly: At first the barrel axis is aligned to the axis of the rotary air bearing using a distance sensor (e.g. a lever gage). The first lens is placed into the barrel. The autocollimator measures the center of curvature position of the top surface with respect to the reference axis. Then the lens can be realigned in magnitude and direction according to the measured values. Depending on the mechanical design of the sample the lens is fixed in position by using retaining rings or glue, for example. This procedure is repeated iteratively for all further lens elements until the optical system is completed. If a VIS&IR measuring head is used, the assembly can be done in the VIS range for highest precision and the final inspection is done I n the IR range. An optimized mechanical alignment of optical infrared systems is achieved using an OptiCentric instrument. Thanks to the step-by-step measurement and alignment procedure of freely accessible lens elements, a cost-effective visual measurement head can be used regardless of the material properties of the lens. If the assembly and final inspection of the lens is required on one workplace an instrument with VIS&IR measuring head is required. OptiCentric 3D 100 or OptiCentric 3D 300 with distance sensor, OptiCentric 3D system with VIS&IR measuring head OptiCentric MAX 300 with VIS&IR measuring head 8/9

9 Characterization of Complex Lens Systems In the quality inspection of lens systems, testing solely the final optical performance is often not sufficient as it does not reveal the causes of potential substandard performance. Instead, a full opto-mechanical characterization of the samples is required to identify potential issues in the assembly process. An OptiCentric 3D instrument equipped with an infrared measurement head or VIS&IR measuring head, rotary air bearing, tilt and translation table, and MultiLens and OptiSurf low-coherence interferometer modules fulfills these requirements. After the objective lens to be tested has been placed onto the sample table, the center of curvature positions of all single lens surfaces are measured following the MultiLens concept. Therefore, the autocollimator iteratively focuses into the center of curvature positions (or their image positions according to the MultiLens calculation) of all sample surfaces and measures their relative position with respect to the reference axis. In the subsequent analysis, the relative orientation of the single element s or group s axis is evaluated with respect to each other or a mechanical barrel axis. As the precise coincidence of sample and measurement axis is a particular requirement for the measurement of the center thickness and air gaps, data from centration testing is used for the precise alignment of the sample axis with the tilt and translation table. Then, the axial lens surface positions are measured. OptiCentric with IR measuring head OptiCentric 3D 100 with infrared measurement head is the only instrument which accomplishes the complete opto-mechanical characterization of infrared lens systems and delivers detailed information about the assembly of the instrument. OptiCentric 3D with IR, OptiCentric 3D 300 Infrared or OptiCentric 3D Dual If the objective lens only consists of lenses that are transparent in the visible range, standard VIS measurement heads can be used. 9/9