In most cases, a microscope objective is mounted on the nosepiece of a microscope using a thread. Unfortunately, there are different thread sizes used by different manufacturers and for objectives of different kinds. In some cases, special adapters can be used for applying an objective to a microscope with different threads.

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.

The highest numerical apertures achievable with dry objectives, operated with air between the objective and the object, are approximately 0.95. Substantially higher values of e.g. 1.5 or even higher can be achieved with immersion objectives, where the gap between the object and the objective is filled with a liquid – water or some immersion oil with a higher refractive index, often somewhat above 1.5. Optimized immersion oils do not only have a high refractive index, but also a suitable viscosity and a low tendency for producing stains on the surfaces. They can be left on an objective over longer times without damaging it.

Objective lens microscopefunction

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.

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At least for high magnifications, the influence of a cover slip in terms of chromatic and spherical aberrations can be quite important. Therefore, objectives for use in fields like biology, where cover slips are often needed, are designed with integrated cover slip correction. The correction is often done for a standard slip thickness of 170 μm. A deviation of only 10 μm can already be quite problematic for an objective with a high NA of e.g. 0.95. Some objectives allow the adjustment of the corrected cover slip thickness.

Objectives for dark-field illumination are tentatively larger, providing extra space for the illumination light; therefore, they are typically used with larger threads.

For such applications, chromatic aberrations are often no issue, so that one does not exploit the chromatic correction of the objective. Also, a wide field of view would not be required. On the other hand, a microscope objective for visible light may well not have ideal properties e.g. for launching near infrared light into a fiber, and its power handling capability is limited (but usually not specified). Therefore, a microscope objective may not be the ideal solution for such an application. However, it may have to be used, e.g. if no other lenses are available for reaching the required small spot size.

Objective lensmagnification

Another application is launching light into a single-mode fiber or collimating light from such a fiber. Again, the objective should have an appropriate numerical aperture of the order of that of the fiber. For more details, see the article on fiber launch systems.

Unfortunately, perfect solutions are not available; therefore, one has to accept certain trade-offs, which lead to different optimized solutions for different applications. For example, optimum flat field properties are most important for measurement microscopes; one may then tolerate somewhat larger chromatic aberrations.

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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.

Shanghai Optics custom microscope objectives are designed with the assistance of CAD, Solidworks and Zemax software using top quality glass having highly specific refractive indices. This enables us to produce microscope objectives that are very low in dispersion and corrected for the most of the common optical artifacts such as coma, astigmatism, geometrical distortion, field curvature, spherical and chromatic aberration.

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.

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.

The design of a high quality microscope objective is a rather sophisticated task, for which substantial optics expertise and powerful optics design software are required. Such designs involve complicated trade-offs, which should be properly handled according to the importance of different aspects for a particular application.

Compoundobjective lens microscope definition

Note that some microscope designs count on the correction of some residual aberrations of the objective by the ocular lens.

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.

Microscope objectives are sometimes used for applications outside microscopy. For example, they can be used for tight focusing of laser beams, with spot sizes of a few micrometers or even below 1 μm. If the input beam is a collimated beam, an infinity-corrected objective will work best. The objective should have a numerical aperture which fits well to the beam divergence related to the required spot size. The input beam radius should also be chosen appropriately, i.e., calculated from the required spot size and the focal length. A difficulty may be to find out the focal length, as the objective barrel often only indicates the magnification, and the conversion to the focal length depends on the microscope design.

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There are also often color-coded rings indicating different magnification values, e.g. black for 1 ×, yellow for 4 ×, green for 10 ×, etc.

Note that it is essential not only to have a good transmittance over the full wavelength range, but also achromatic performance. In conventional light microscopes, this is needed to avoid colored image distortions. In confocal multi-photon fluorescence microscopes, it is important to have the same focus positions for infrared laser light as for the fluorescence light.

Objective lens microscope definitionand function

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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Types ofobjectivelenses

Modern microscopes mostly require infinity-corrected objectives, where the intermediate image of the objective alone lies at infinite distance. Here, one requires an additional tube lens in the microscope for generating the intermediate image at the diaphragm of the eyepiece.

Some microscopes allow the injection of illumination light through the objective to the sample. It is then important that there is no significant scattering of light in the objective.

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Older microscopes usually require finite-corrected objectives. Here, the object is supposed to be placed a little below the front focal plane of the objective, and the intermediate image occurs at a finite distance of e.g. 160 mm from the objective. Such an objective is designed for minimum image distortions in that configuration.

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Another practically important factor is the working distance, i.e., the distance between the objective and the object. Small working distances are generally required for objectives with high NA, but also can to some extent be optimized as a design goal (possibly somewhat compromising the NA or the correction). For objectives with oil immersion, a relatively small working distance is actually good, since otherwise one would require more of the immersion fluid, and that would be more difficult to hold in place.

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There are also reflective objectives, containing curved mirrors and no lenses. They are naturally achromatic and may be advantageous for operation in extreme wavelength domains. Also, they can exhibit lower losses of optical power.

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Objective lensfunction

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Stagemicroscopefunction

The objective lens is the most important part of a microscope and plays a central role in imaging an object onto the human eye or an image sensor for discerning the object’s detail. Microscope objectives are ideal for a range of science research, industrial, and general lab applications.

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The higher the magnification, the higher is also the required numerical aperture because this is the factor which ultimately limits the achievable image resolution. There are different ways of calculating the image resolution and are slightly different circumstances, but they lead to similar resolution values, which are roughly <$\lambda / (2 NA)$>, where <$\lambda$> is the optical wavelength (about 400 to 700 nm) and NA is the numerical aperture. For example, an NA of 1 allows for an image resolution of roughly 250 nm for green light. For low magnification, an NA of 0.1 may be fully sufficient.

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Most microscopes objectives are based on refractive optics, containing several lenses. For example, a simple low-NA objective may contain a meniscus lens and an achromat. A high-NA objective typically contains a more complicated combination of various types of lenses of hemispherical, meniscus, achromatic doublet and triplet type.

Microscopeparts

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.

Note that oil immersion may not work properly e.g. when observing a biological sample in an aqueous solution and the oil is only between the cover slip and the objective. One may have to use special water immersion objectives for such cases.

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).

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Microscopes often contain multiple objectives on a rotatable nosepiece, for example a scanning lens with only 4 × magnification, an intermediate one (the small objective lens) with 10 × and a high-resolution large objective with 40 × or 100 × magnification. The eye piece may contribute another factor 5 or 10 in magnification, for example.

Optical microscopes usually work based on imaging with visible light, i.e., in the wavelength region from 400 nm to 700 nm. Therefore, most microscope objectives are optimized for that wavelength range, with most emphasis on the region from 480 nm to 640 nm. However, there are objectives with an enhanced range of e.g. 400 nm to 950 nm, and others which work further in the infrared. For example, that is required for laser microscopes where infrared laser beams need to be transmitted.

Particularly for objectives with high numerical aperture, a high image quality can be achieved only with substantial efforts for correcting various kinds of optical aberrations such as spherical, astigmatism, coma, field curvature, image distortion and chromatic aberrations. For example, plan-apochromatic objectives, having particularly sophisticated designs, provide optimum flat field correction combined with good achromatic properties.

Chromatic aberrations essentially result from the wavelength dependence of focal length. They lead to colored image distortions. For conventional microscopy, they can be quite relevant, in contrast to other types of optical microscopy, e.g. certain types of laser microscopy. Best suppression of chromatic aberrations is achieved with apochromatic objectives.

Finite-corrected objectives are always designed for a certain tube length, e.g. according to DIN or JIS standard (which differ by 10 mm in tube length). Using an objective of the wrong standard may significantly deteriorate the obtained image quality.

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Edmund Optics offers a wide variety of microscopy components including microscope objectives, inverted and stereo microscopes, or optical filters that are ideal for use in microscopy setups. Microscope objectives are available in a range of magnifications and include infinity corrected, finite conjugate, and reflective objectives in industry leading brands such as Mitutoyo or Olympus. Microscope objectives are ideal for a range of research, industrial, life science, or general lab applications. Microscopy filters are ideal for isolating specific wavelengths in fluorescence imaging applications.

Note that a large magnification alone is not helpful if it only makes images larger without increasing the level of detail; see below the section on the numerical aperture.

The microscope objective is a key component for reaching high performance of a microscope. It is the part which is placed next to the observed object, usually in a fairly small distance of a few millimeters. Usually, the microscope objective produces an intermediate image in the microscope, which is then further magnified with an eyepiece (ocular lens). Particularly in cases with high magnification, most of the magnification is provided by the objective.

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Although a microscope objective is sometimes called the objective lens, it usually contains multiple lenses. The higher the numerical aperture and the higher the required image quality, the more sophisticated designs are needed. High-end microscope objectives may also involve aspheric lenses.

The focal length of a microscope objective is typically between 2 mm and 40 mm. However, that parameter is often considered as less important, since magnification and numerical aperture are sufficient for quantifying the essential performance in a microscope.