Types of Special Objectives for Metallographic Microscopes
① Correction Collar Objective An adjusting collar is fitted in the middle of the objective lens. Rotating the collar can adjust the distance between the lens groups inside the objective, so as to compensate for the cover glass thickness deviation from the standard value. The scale on the adjusting collar ranges from 0.11 to 0.23, and these figures are also marked on the objective housing. It indicates that this objective can correct errors caused by cover glass thickness ranging from 0.11 mm to 0.23 mm.
② Iris Diaphragm Objective An iris diaphragm is installed in the upper part inside the objective barrel, with a rotatable adjusting ring on the outer side. Rotating the ring can change the aperture size of the diaphragm. This type of objective is generally oil immersion objective. When observing specimens in a dark field, the background of the field of view tends to darken, resulting in degraded image quality. Adjusting the diaphragm aperture darkens the background further, brightens the specimen, and improves inspection performance.
③ Phase Contrast Objective This objective is dedicated to phase contrast microscopy. It features a phase plate mounted at the rear focal plane of the objective.
④ No-Cover (Ncover) Objective Some specimens cannot be covered with a cover glass. A no-cover objective must be used for observation; otherwise, the image quality will deteriorate significantly, especially at high magnification. Such objectives are marked with NC on the housing, and "0" is engraved at the position where the 0.17 mm marking is located on ordinary objectives.
⑤ Long Working Distance Objective This objective has a longer focal length than standard objectives. It is designed for the observation of liquid materials (high-temperature metallography, liquid crystals, tissue cultures, suspensions and other materials).
