Function ofstage in microscope

Microscope Objectives or Objective lenses are in many ways the heart of the microscope, and are typically mounted on a rotating nosepiece or turret to enable easy selection. Many microscopes will be equipped with a scanning objective (4x), a low power objective (10x), a high power objective (40x), and perhaps even an oil immersion objective lens.

A simple magnifier (magnifying glass), works when the object to be examined is situated within focal length of the magnifier lens, enabling larger virtual image is produced. This type of magnifier is very limited in both resolution and magnification. A compound microscope, on the other hand, uses a relay lens system instead of the single lens, and since each lens component can contribute magnifying power, the result is greatly increased capability.

Each microscope objective is itself a complex assembly of lenses, and besides contributing to the magnification, it is the objective lens which determines the resolution power of the microscope. An objective lens can also provide optical aberration corrections.  A reflective objective, for instance, includes two mirrors within the assembly. These mirrors can focus laser light as well as provide chromatic corrections.

where θ is the maximum 1/2 acceptance ray angle of the objective, and n is the index of refraction of the immersion medium. Figure 2 shows the ray angle θ of an infinity-corrected objective.

Typesof objectivelenses

Figure 2: An unpolarized wave. The direction of polarization changes randomly along the wave. Sunlight and most other light sources are unpolarized but become partially polarized on reflection. Image source: astronomy.nmsu.edu

Important specifications are marked on the barrel of the objective, so students or researchers can easily identify the properties of an objective and determine the optical performance and working conditions for proper use. Figure 1 shows a diagram of an objective lens. A detailed discussion of the objection specifications is provided below.

Objective lensmagnification

Figure 6: Polarized sunglasses block horizontally polarized light (red) but transmit vertically polarized light (blue). Image source: microscopyu.com

Since indirect backlight illumination is generally more effective than direct illumination, most microscopes do not include an internal light source.  Instead, they rely on daylight or on background illumination such as a lightbulb. In brightfield illumination, also known as Koehler illumination, two convex lenses saturate the specimen with external light admitted from behind. These two lenses, the collector lens and condenser lens, work together to provide a bright, even, and constant light throughout the system: on the image plane as well as on the object plane.  This system of illumination is used in many compound microscopes, including student microscopes and those found in many research labs.

The optical aberration correction determines the optical performance of an objective lens and plays a central role in the image quality and measurement accuracy of imaging or microscopy systems. According to the degrees of the aberration corrections, objective lenses are generally classified into five basic types: Achromat, Plan Achromat, Plan Fluorite (Plan Semi-Apochromat), Plan Apochromat, and Super Apochromat.

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The ocular lens, located at the top of a standard microscope and close to the sensor (receiving eye) receives the real image from the ocular lens, magnifies the image received and relays a virtual image to the sensor. While most eyepieces magnify 10x, there are some which provide no magnification and others which magnify as much as 30x. The magnification power of the microscope can be calculated by multiplying the magnification power of the eyepiece, or ocular lens, by the magnification power of the objective lens. For example, an objective lens with a magnification of 10x used in combination with a standard eyepiece (magnification 10x) would project an image of the specimen magnified 100x.

What is objective lensin microscope

The Fresnel equations quantitatively show how unpolarized light becomes partially polarized after reflection from a dielectric surface--such as water or glass. The equations are mentioned here just so the reader can see the horizontally polarized component of a wave differs in its reflection coefficient from the vertically polarized component. Unequal reflection coefficients lead to unpolarized light becoming partially polarized. Interestingly, the Fresnel equations predict the existence of an angle for which the glare is completely polarized, not just partially polarized. In other words, glare coming from this angle (known as the Brewster angle) can be completely blocked by an ideal polarizing filter. The effect is dramatic. (see Figure 4).

Objectives are complex multi-element lenses. For any given application, careful consideration of the optical parameters and specifications is necessary. In many cases, custom-designed objective assemblies provide the best-fit solution for meeting all the requirements of a specialized application.  Custom parameters may include antireflection coatings, chromatic focus shift, working distance, image quality (MTF and spot size), lens mount, glass window thickness, and field of view, among others.

Figure 5: Molecules in a polarizing filter are long in one direction and short in the perpendicular direction. Electrons can freely oscillate along the length of the molecule, absorbing or reflecting the light energy, while they are unable to oscillate very far along the short direction. The "E-field" arrows in the Figure show the direction of polarization. The small spheres labeled "e-" represent electrons. Note in the Figure how the horizontally polarized wave (top) emerges) from the its electron interaction with its amplitude reduced, while the vertically oscillating wave (bottom) comes through with undiminished amplitude. Image Source: voer.edu.vn

At Shanghai Optics, we design and manufacture custom objectives and imaging systems to support our customers’ needs in many industries, including medical, biomedical, machine version, scientific research, and metrology, etc. Taking the client’s budget and precision requirements into consideration, our experienced engineering team ensure that each design can be manufactured at a reasonable cost and the optical performance is being met based on fabrication, assembly, and alignment tolerance analysis.

To understand how polarized sunglasses block horizontal polarization, it's important to know how the electrons in the molecules of the sunglass filter behave. Electrons are set to oscillation by the incoming light wave and therefore some of the wave energy of the light gets transferred to the electrons to be dissipated or reflected by the electrons. Polarizing filters used in sunglasses contain molecules which make it is easy for the electrons to oscillate in the horizontal direction (long direction) thus dissipating more horizontally polarized light energy. On the other hand the long molecules make it hard for electrons to oscillate in the vertical direction (short direction)—thus diminishing the electrons' dissipation of vertically polarized light waves. (see Figure 5).

The ocular lens, or eyepiece, is also an optical assembly rather than a single lens, but it is typically more simple than the objective. Often it is composed of two lenses: a field lens and an eye lens. The design of the ocular lens determines the field of view of the microscope, as well as contributing to the total magnification of the system.

Objective lenses can be classified based on the objective construction, field of use, microscopy method, performance (optical aberration corrections), and magnification. Many microscope objective manufacturers offer a wide range of objective designs, which provide various degrees of optical aberration corrections for supporting different needs. Mirrors or reflective elements are used in objective lenses for the applications that requires chromatic aberration over board spectral ranges. Most traditional microscopy systems use refractive objectives such as achromatic objectives (the cheaper objectives) for laboratory microscope applications and plan apochromats (expensive objectives) for biological and science research microscope applications.

Figure 4: Glare reflecting at Brewster angle from a window. The light producing the glare is highly polarized allowing the polarizing filter (on the right window) to virtually remove all of it. For fresh water, the Brewster angle is about 53 degrees, so peak performance of polarized sunglasses on a calm lake happen when the sun is at an angle of about 37 degrees from the horizon (90-53= 37). Image source: boundless.com/physics

Magnification is one important parameter. Magnification is usually denoted by an X next to a numeric value.  Objectives are available in a range of magnifications from 2X to 200X.

A microscope objective is an important component of a microscopy or imaging system for a range of science research, biological, industrial, and general lab applications.. An objective lens determines the basic performance of an optical microscope or imaging systems and is designed for various performance needs and applications. It is located closest to the object and is an important component in imaging an object onto the human eye or an image sensor.

Function ofcondenser in microscope

Figure 1: "Polarization angle" is the angle of the transverse direction of oscillation of a wave. Here, horizontal (0 degrees) and vertical (90 degrees) are shown. Angle in between can be thought of as a mixture of the two directions with different mixture ratios. The grey rectangle with a vertical slot represents a polarized filter designed to transmit the vertical oscillations of the string and block the horizontal ones. Image source: cnx.org

Field of View is the area of the object that can be imaged by a microscopy system. The size of the field of view is determined by the objective magnification or focal length of the tube lens for an infinite-corrected objective. In a camera system, the field of view of the objective is related to the sensor size.

High powerobjectivemicroscopefunction

Ari Siletz is president of CCDMETRIX. His company specializes in automated vision system inspection and metrology. With a background in both optical and software engineering, Ari has been developing instruments for the the ophthalmic and optical coating industries since the 1980s. Writing is one of Ari's serious hobbies. He is a published author whose short stories have appeared in numerous literary anthologies. He lives in Sebastopol, California.

Figure 3: Viewing the same scene with and without a polarizing filter. The reflected sunlight in the left image is partially polarized. The right image is taken with a filter that blocks horizontally polarized light. Image source: photography.ca/blog

Sunlight can become partially polarized by the scattering of air molecules or by reflecting off something like a lake. This means that after scattering or reflecting the sun's waves oscillation angles, they are no longer random in all directions but have a preferred direction on average. In the case of a horizontal surface—like a lake or a road—the preferred direction is horizontal. This horizontally vibrating reflected sunlight is the nuisance we see as glare, and this is why polarized lenses are so useful to beach goers and motorists: they block glare. The polarized filters on these lenses preferentially block the horizontal component of light oscillation while transmitting the vertical component. The result is a darker image but with better contrast (see Figure 3).

A microscope is a special optical device designed to magnify the image of an object. Depending on the type of microscope, it may project the image either onto a human eye or onto a recording or video device. As an example, consider the photographs of cells that can be found in a science textbook. These photographs have all been taken by a specialized microscope, and may be called micrographs.

For keeping the objective at the proper position, there are mounting threads on almost all objectives. Commonly used mounting threads include RMS, M25 x 0.75, M26X 0.706, M32 x 0.75.

The parfocal length is the distance between the objective mounting plane and the specimen / object. This is another specification that can often vary by manufacturer.

Since the objective is closest to the specimen being examined, it will relay a real image to the ocular lens. While doing so, it contributes a base magnification of anywhere from 4x (for a scanning objective lens, typically used to provide an overview of a sample) to 100x (for oil immersion objectives).

Two major lens components—the objective lens and the ocular lens, or eyepiece—work together to project the image of the specimen onto a sensor. This may be the human eye or a digital sensor, depending on the microscope setup.

Many objectives are designed to be used with a cover glass. Using an incorrect coverslip thickness can greatly reduce the optical performance of a microscopy system.

High powerobjective lens

What is the function of objective lensin microscope

In practice it's difficult to get the long molecules all lined up in one direction, but somewhat lined up molecules are still effective in making a polarizer. One way to accomplish this is to put long-chain molecules on a piece of transparent stretchable material then heat and pull the stretchable material. The long molecule chains, originally in random orientations, will more or less line up in the direction of the pulling.

To see how polarized sunglasses work, it is convenient to think of light as a wave moving along a string. Just like a wave on a string, a light wave wiggles transversally to its direction of motion. And just like a wave on a string, the plane in which the string oscillates can have different orientations. For example, the oscillation can vibrate up and down, side to side or in any combination of the two directions. If the string is made to oscillate at, say, a 45 degree angle, this can be mathematically decomposed into a combination of horizontal and vertical oscillation components in equal amounts. A string oscillating at, say, 75 degrees has more vertical component than horizontal component. The angle of transverse oscillation is called the "polarization angle" (see Figure 1). By the way, when we say that a certain light source is "unpolarized" we mean that it emits waves at random angles so that on average there is no preferred direction of polarization. Direct Sunlight, light bulbs and candles are examples of unpolarized light sources. Almost no natural source of light is polarized at the source; polarization happens after some sort of interaction with matter. Therefore, for almost all sources of light, it is as though we wiggled the string in one transverse direction to generate a wavelet along the string then unpredictably changed to another transverse direction, and so on (see Figure 2). The waves come out swinging in hodge-podge polarization angles, but all travel along the direction of the string.

Most objectives are designed to image specimens with air as the medium between the objective and the cover glass. However, for achieving higher working numerical apertures, some objectives are designed to image the specimen through another medium such as special oil with a refractive index of 1.51.

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While the simplest of microscopes is simply a magnifying glass with a single lens, compound microscopes used today are highly complex devices with a carefully designed series of lenses, filters, polarizers, beamsplitters, sensors, and perhaps even illumination sources. The exact combination of optical components used will depend on the application of the microscope; the wavelength of light with which it is intended to be used, and the resolution and magnification required in the final image.

Figure 6 below illustrates a simplified summary of everything we just discussed about polarization: glare reducing sunglasses!