KERN OKM 173 Metallurgical Microscope, reflected light, trinocular, infinite-corrected optics

Geometric Optics of Metallurgical Reflected-Light Microscopy

In materials science fractography, metallurgical quality control and the examination of opaque (light-impermeable) semiconductor structures, conventional transmitted-light microscopes fail due to the total physical absorption of photons within the solid. The KERN OKM 173 is designed as a high-precision metallurgical epi-illumination microscope. In this complex optical setup, the objective lens also functions as a condenser. The integrated 30-watt halogen light source emits a continuous, broadband spectrum of photons. This light is precisely directed into the optical axis via a partially transparent beam splitter in the microscope stand, focused onto the sample through the objective lens and reflected by the metallic solid-state surface. The reflected light passes through the objective lens once more and produces, in the trinocular tube, an image of the microscopic grain structures, voids or microcracks that is exceptionally high-contrast and free of artefacts.

Physical superiority of the infinity-corrected optical path

The metrological heart of the OKM 173 is its infinity-corrected optical system (Infinity Optics). In contrast to finite optical systems with a fixed tube length (e.g. 160 mm), with infinity optics the light rays emerge from the objective as parallel beams. Only a specially calculated tube lens in the microscope head focuses these parallel photons into a real intermediate image. This physical space occupied by the parallel rays between the objective and the tube lens is of essential importance: it allows additional optical components – such as polarising filters, analysers or DIC prisms – to be introduced without causing a shift in the optical focal length, spherical aberrations (aberrations) or microscopic ghost images.

Polarisation optics and anisotropy analysis

Particularly with reflective metallic surfaces or polished micrographs, unpolarised halogen light often leads to severe flare, which masks the finest phase boundaries in the metal lattice. The OKM 173 is therefore equipped as standard with a high-performance polarisation and analyser unit. The polariser forces the transverse electromagnetic light waves into a defined plane of oscillation. Reflection off the anisotropic crystal structure of the metal sample specifically alters the polarisation state of the light. The downstream analyser physically blocks out interfering specular reflections and makes optical stress-induced birefringence, grain boundaries and heterogeneous alloy phases visible with maximum microscopic contrast.

Planachromatic optics and digital image conversion

To reproduce the absolutely flat polished surfaces typical of metallurgy without geometric errors, the system is equipped with planachromatic objectives. These highly complex lens systems correct chromatic aberration for two spectral colours and physically flatten the image field so that both the centre and the extreme peripheral areas of the solid-state sample are simultaneously in absolutely sharp focus. The trinocular Siedentopf tube also enables simultaneous binocular visual inspection by the user and the loss-free coupling of the photon flux to a digital microscope camera (C-mount). This is essential for comprehensive digital documentation and software-based metrology in certified laboratories.

Technical details

• Optical system: Infinity-corrected (Infinity Optics)
• Microscope type: Metallurgical epi-illumination microscope
• Tubus design: Trinocular (Siedentopf), 30° inclination, 360° rotatable
• Illumination type: 30 W halogen epi-illumination (continuously dimmable)
• Optical path: Partially transparent mirror for coaxial illumination
• Objectives: Planachromatic LWD (Long Working Distance) optics
• Standard magnifications: 5x, 10x, 20x, 50x (plan objectives)
• Eyepieces: HWF 10x / Ø 18 mm (with dioptre adjustment on both tubes)
• Optical contrast enhancement: Includes polariser and analyser unit
• Mechanical cross-table: Large surface area (approx. 200 x 150 mm) for solid workpieces
• Focusing: Coaxial coarse and fine adjustment with mechanical stop
• Camera output: C-mount adapter for digital image capture
• Power supply: Integrated mains power supply (230 V)

Scope of delivery

• 1 x KERN OKM 173 metallurgical microscope (stand and tube)
• 4 x Planachromatic LWD objectives (5x, 10x, 20x, 50x)
• 2 x HWF 10x wide-field eyepieces
• 1 x polarisation and analyser unit
• 1 x 30-watt halogen replacement bulb
• 1 x C-mount camera adapter
• 1 x dust cover and mains cable
• 1 x Detailed operating instructions

Individual requirements & accessories

The trinocular optical path of the OKM 173 is mechanically and optically designed to accommodate a high-resolution digital microscope camera (e.g. KERN ODC series with CMOS sensor). To carry out metrological length or angle measurements on metal structures via the software, such a camera, in conjunction with an object micrometre for calibration, is essential. For analyses requiring extremely high magnification, the system can also be upgraded with a plan-achromatic 100x LWD objective. Please contact our procurement service at any time regarding the optoelectronic configuration and integration into your quality management system.