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Types ofobjectivelenses

The curvature of field and distortion refer to the location of image points with respect to one another. The former three aberrations mentioned so far can be corrected by making corrections in the design of the lens, but these two aberrations could remain. In curvature of the field, the image of a plane object perpendicular to the optical axis will lie on a paraboloidal surface known as the Petzval Surface. Distortion, on the other hand, refers to the deformation of an image. There are two kinds of deformation, namely barrel distortion and pincushion distortion.

What are the 3objectivelenses on amicroscope

An optical microscope is used with multiple objectives attached to a part called revolving nosepiece. Commonly, multiple combined objectives with a different magnification are attached to this revolving nosepiece so as to smoothly change magnification from low to high only by revolving the nosepiece. Consequently, a common combination lineup is comprised from among objectives of low magnification (5x, 10x), intermediate magnification (20x, 50x), and high magnification (100x). To obtain a high resolving power particularly at high magnification among these objectives, an immersion objective for observation with a dedicated liquid with a high refractive index such as immersion oil or water charged between the lens end and a specimen is available. Ultra low magnification (1.25x, 2.5x) and ultra high magnification (150x) objectives are also available for the special use.

High powerobjective microscopefunction

The name “Coma” is derived from the fact that a point image is blurred into a comet shape when rays from an off-axis object point are imaged by different zones of the lens. In spherical aberration, the images of an object point that fall on a plane perpendicular to the optical axis are circular in the shape of varying sizes and superimposed about a common centre. In a coma, the images of an off-axis object point are circular, of varying size, but displaced with respect to each other.

A variety of microscopy methods have been developed for optical microscopes according to intended purposes. The dedicated objective lenses to each microscopy method have been developed and are classified according to such a method. For example, "reflected darkfield objective (a circular-zone light path is applied to the periphery of an inner lens)", "Differential Interference Contrast (DIC) objective (the combination of optical properties with a DIC( Nomarski）prism is optimized by reducing lens distortions)", "fluorescence objective (the transmittance in the near-ultraviolet region is improved)", "polarization objective (lens distortions are drastically reduced)", and "phase difference objective (a phase plate is built in) are available.

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When the light of a single wavelength is considered, there are five monochromatic aberrations to be considered and they are:

Axial chromatic aberration correction is divided into three levels of achromat, semiapochromat (fluorite), and apochromat according to the degree of correction. The objective lineup is divided into the popular class to high class with a gradual difference in price. An objective lens for which axial chromatic aberration correction for two colors of C ray (red: 656,3nm) and F ray (blue: 486.1nm) has been made is known as Achromat or achromatic objective. In the case of Achromat, a ray except for the above two colors (generally violet g-ray: 435.8nm) comes into focus on a plane away from the focal plane. This g ray is called a secondary spectrum. An objective lens for which chromatic aberration up to this secondary spectrum has satisfactorily been corrected is known as Apochromat or apochromatic objective. In other words, Apochromat is an objective for which the axial chromatic aberration of three colors (C, F, and g rays) has been corrected. The following figure shows the difference in chromatic aberration correction between Achromat and Apochromat by using the wavefront aberration. This figure proves that Apochromat is corrected for chromatic aberration in wider wavelength range than Achromat is.

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Objective lenses are roughly classified basically according to the intended purpose, microscopy method, magnification, and performance (aberration correction). Classification according to the concept of aberration correction among those items is a characteristic way of classification of microscope objectives.

Ocular lensmicroscope

Astigmatism is the result of the failure of a single zone of the lens to focus the image of an off-axis point at a single point. In the figure, we see two planes perpendicular to each other passing through the optical axis. These planes are known as the meridian plane and the sagittal plane, the meridian plane being the one containing the off-axis object point. Skew rays, rays not in the meridian plane are focused farther away from the lens than those lying in the plane. In either case, the rays do not meet in a point focus but as lines perpendicular to each other. Intermediate between these two positions, the images are elliptical in shape.

The image shows two images, one resulting from a central cone of rays and the other from a cone passing through the rim.

In optics, aberration is a property of optical systems such as lenses that results in light being spread out over some region of space rather than being focused to a point. An image-forming optical system with aberration will produce an image that is not sharp. Aberration can be caused due to a variety of reasons such as lens size, material, thickness and position of the object. In this article, let us learn more about aberration and its type.

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Objectivelensmicroscopelabeled

When the light is not monochromatic (not of a single wavelength), a sixth aberration is found in lenses but not in mirrors and is named chromatic aberration.

Photography or image pickup with a video camera has been common in microscopy and thus a clear, sharp image over the entire field of view is increasingly required. Consequently, Plan objective lenses corrected satisfactorily for field curvature aberration are being used as the mainstream. To correct for field curvature aberration, optical design is performed so that Petzval sum becomes 0. However, this aberration correction is more difficult especially for higher-magnification objectives. (This correction is difficult to be compatible with other aberration corrections) An objective lens in which such correction is made features in general powerful concave optical components in the front-end lens group and powerful concave ones in the back-end group.

In the optical design of microscope objectives, commonly the larger is an N.A. and the higher is a magnification, the more difficult to correct the axial chromatic aberration of a secondary spectrum. In addition to axis chromatic aberration, various aberrations and sine condition must be sufficiently corrected and therefore the correction of the secondary spectrum is far more difficult to be implemented. As the result, a higher-magnification apochromatic objective requires more pieces of lenses for aberration correction. Some objectives consist of more than 15 pieces of lenses. To correct the secondary spectrum satisfactorily, it is effective to use "anomalous dispersion glass" with less chromatic dispersion up to the secondary spectrum for the powerful convex lens among constituting lenses. The typical material of this anomalous dispersion glass is fluorite (CaF2) and has been adopted for apochromatic objectives since a long time ago, irrespective of imperfection in workability. Recently, optical glass with a property very close to the anomalous dispersion of fluorite has been developed and is being used as the mainstream in place of fluorite.

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The image depicts a case of spherical aberration. The most spherical aberration-free image is found at the circle of least confusion.

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Objectivelensmicroscopefunction

The purposes of optical microscopes are broadly classified into two; "biological-use" and "industrial-use". Using this classification method, objective lenses are classified into "biological-use" objectives and "industrial-use" objectives. A common specimen in a biological use is fixed in place on the slide glass, sealing it with the cover glass from top. Since a biological-use objective lens is used for observation through this cover glass, optical design is performed in consideration of the cover glass thickness (commonly 0.17mm). Meanwhile, in an industrial use a specimen such as a metallography specimen, semiconductor wafer, and an electronic component is usually observed with nothing covered on it. An industrial-use objective lens is optically designed so as to be optimal for observation without any cover glass between the lens end and a specimen.

Objectivelensmicroscopenames

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Meanwhile, an objective lens for which the degree of chromatic aberration correction to the secondary spectrum (g ray) is set to medium between Achromat and Apochromat is known as Semiapochromat (or Flulorite).

In spherical aberration, rays of light from a point on the optical axis of a spherical lens do not all meet at the same image point. Rays passing closer to the center are focussed farther away than the rays passing through a circular zone near its rim. A circular cross-section is formed whenever a plane held perpendicular to the optical axis is made to intersect a cone. The area of the cross-section varies with the distance along the optical axis. The smallest size is known as the circle of least confusion. The most spherical aberration-free image is found at this distance.

The failure of a lens to focus all colours in the same plane is known as chromatic aberration. The refractive index for red is least at the red end of the spectrum, hence the focal length of a lens in the air will be greater for red and green than it would be for blue and violet. Chromatic aberration affects magnification along the optical axis and the axis perpendicular to it. The former is known as longitudinal chromatic aberration and the latter is known as lateral chromatic aberration.

Objectivelensmicroscopemagnification

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An objective lens is the most important optical unit that determines the basic performance/function of an optical microscope To provide an optical performance/function optimal for various needs and applications (i.e. the most important performance/function for an optical microscope), a wide variety of objective lenses are available according to the purpose.

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In an ideal lens, light from any given point of an object would pass through the lens and come together at a single point in the image plane. Contrary to ideal lenses, real lenses do not focus light to a single point. These deviations from the idealized lens performance are known as an aberration of the lens.