Ocularlens magnification

An important consideration is the size and shape of the polarizer.  This depends on your application or product. Manufacturers mostly commonly offer square and round/circular polarizers. Here, circular polarizer refers to the physical shape of the polarizer and should not be confused with the circular state of polarization. Although more expensive, Optometrics can design a custom wire grid polarizer that meets your application needs.

For instance, glass substrates offer a spectral range of 420-700 nm, which is good for applications like digital projectors. In contrast, fused silica can be used over a much broader range from 250 nm – 4 microns. Naturally, fused silica substrates are 5-10 times more expensive than glass.IR applications such as FTIR spectrometers and IR imagers require wire grid polarizers to operate from 2.5 – 30 microns. In such cases, Optometrics’ CaF2, ZnSe, BaF2, Ge and KRS-5 (Thallium Bromide) polarizers offer the best performance, although the transmission of these substrates varies across the IR range. KRS-5 has a flat transmission profile from 2-30 microns but is more expensive as compared to the other substrates. Optometrics offers holographic coatings on all these substrates with a high transmission of 80-90%, which is ideal for high-performance and low-loss applications.

Scanningobjective lens

Wire grid polarizers, as the name suggests, are polarizing elements that use arrays of fine metal wires to selectively transmit p-polarized light and reflect s-polarized light. Due to this function, they are used as linear polarizers and polarizing beam splitters. Wire grid polarizers can be fabricated with different metals like aluminum or gold on a range of substrates. This provides great design flexibility to produce polarizers that can operate in different spectral regimes, such as UV, visible, and IR. Additionally, they have high tolerance to heat and are suited for high-temperature applications. Due to these advantages they are a popular choice of polarizer in several applications from digital projectors to imaging and displays.

While designing a product, dimensional tolerances need to be considered. This helps minimize any optical misalignments and defects. Most manufacturers will mention dimensional tolerances for the thickness of the polarizer, and the diameter (round) or length (square). Typically, tolerances of +/- 0.2 mm – 0.5 mm are quoted for readily available polarizers. If your application needs lower tolerance and more precision, you will need to get custom parts.

To operate the tutorial, use the Reference Focal Length and Objective Focal Length sliders to alter the specifications of the virtual infinity optical system. The objective magnification (M) is calculated by dividing the reference focal length (L) of the tube lens by the objective focal length (F). As the critical focal length parameters of the microscope are varied, this calculation is automatically performed and the result is continuously updated and displayed in the space to the right of the objective drawing in the tutorial window. For example, a reference focal length of 180 millimeters and an objective focal length of 18 millimeters yield a magnification of 10x. The objective working distance is also presented graphically and updated as the microscope focal lengths are adjusted.

Low powerobjective magnification

Infinity-corrected microscope optical systems are designed to enable the insertion of auxiliary devices, such as vertical illuminators and intermediate tubes, into the optical pathway between the objective and eyepieces without introducing spherical aberration, requiring focus corrections, or creating other image problems. In a finite optical system, light passing through the objective converges at the image plane to produce an image. The situation is quite different for infinity-corrected optical systems where the objective produces a flux of parallel light wavetrains imaged at infinity, which are brought into focus at the intermediate image plane by the tube lens. This tutorial explores how changes in tube lens and objective focal length affect the magnification power of the objective in infinity-corrected microscopes.

Like most polarizers, a wire grid polarizer is characterized by its Extinction Ratio (ER) and Polarization Efficiency (PE). If, T1 is the transmission for p-polarized and T2 is the transmission when the polarizer is rotated by 90 deg (or crossed), then ER= T2/T1 and PE= (T1-T2)/(T1+T2). Extinction performance is typically expressed as the inverse of the ratio or (1/ER):1. E.g. If ER=0.0001, then extinction performance is 10000:1. Refer to our previous article on Understanding Polarization and Wire Grid Polarizers.A high ER is desirable in order to achieve high performance. Manufacturers typically offer wire grid polarizers with an ER from 10:1 to 10000:1 over a wide spectral range, particularly in IR. If an extremely high ER is required, a combination of two or more polarizers can be used. For instance, Optometrics provides holographic wire grid polarizers with an ER of up to 300:1. Using two such polarizers in series can result in a ER of 90,000:1.

In the case of wire grid polarizers, there are two types of polarizers based on the fabrication approach that is used to achieve the metal grid:

One of the first parameters to consider when selecting wire grid polarizers is its spectral characteristics. Depending on the intended application, the operating spectral range needs to be determined. Some questions to ask here are:

In contrast to the ruled fabrication approach, holographic wire grid polarizers are fabricated using interference lithography. Lasers are used to generate fine interference patterns that are incident on a substrate coated with a photoresist. After a certain duration of exposure, the film is developed to create the grid pattern. Finally, a metal layer is evaporated on to the grid. This method offers a finer grid spacing that is suitable for short wavelength operation. It can be deposited on softer substrates like KRS-5, which have excellent broadband transmission. Polarizers fabricated using this method have lesser defects and higher uniformity. Holographic wire grid polarizers have several advantages over their ruled counterparts.Optometrics, offers holographic wire grid polarizers on CaF2 (Calcium Fluoride), ZnSe (Zinc Selenide), BaF2 (Barium Fluoride), KRS-5 and Ge (Germanium), covering a wavelength region from 2.5 to 30 microns.

Objective lensmicroscope function

In a finite optical system of fixed tube length, light passing through the objective is directed toward the intermediate image plane (located at the front focal plane of the eyepiece) and converges at that point, undergoing constructive and destructive interference to produce an image. The situation is quite different for infinity-corrected optical systems where the objective produces a flux of parallel light wavetrains imaged at infinity (often referred to as infinity space, and labeled in the tutorial window), which are brought into focus at the intermediate image plane by the tube lens. It should be noted that objectives designed for infinity-corrected microscopes are usually not interchangeable with those intended for a finite (160 or 170 millimeter) optical tube length microscope and vice versa. Infinity lenses suffer from enhanced spherical aberration when used on a finite microscope system due to lack of a tube lens. In some circumstances it is possible, however, to utilize finite objectives on infinity-corrected microscopes, but with some drawbacks. The numerical aperture of finite objectives is compromised when they are used with infinity systems, which leads to reduced resolution. Also, parfocality is lost between finite and infinity objectives when used in the same system. The working distance and magnification of finite objectives will also be decreased when they are used with a microscope having a tube lens.

Many applications require anti-reflection (AR) coatings, particularly in displays and imaging. If your application demands a reflectance of under 1%, then you need a wire grid polarizer with an AR coating. Typically, the spectral response of the coating is designed to match the substrate transmission. However, AR coatings cannot be deposited on all substrates and they only offer the best performance at a particular wavelength. For instance, the advantage of KRS-5 substrates is their high transmission over a wide spectral range. For such substrates, an AR coating is counterproductive as it interferes with the transmission range and narrows it down. Polarizers with a protective glass surface are more resistant to scratches, can be easily cleaned and therefore last longer. They are ideal in applications where the polarizer is subject to frequent cleaning or prone to damage.

The tube length in infinity-corrected microscopes is referred to as the reference focal length and ranges between 160 and 200 millimeters, depending upon the manufacturer. Correction for optical aberration in infinity systems is accomplished either through the tube lens or the objective(s). Residual lateral chromatic aberration in infinity objectives can be easily compensated by careful tube lens design, but some manufacturers choose to correct for spherical and chromatic aberrations in the objective lens itself. This is possible because of the development of proprietary new glass formulas that have extremely low dispersions. Other manufacturers utilize a combination of corrections in both the tube lens and objectives.

Wire grid polarizers based on glass or fused silica substrates can operate over a wide temperature range, typically from -40oC to 200oC. They can be designed to have low thermal expansion and high stability at high temperatures. In contrast, thin film polarizers on plastic substrates can only operate in the -20oC to 80oC range. If high-temperature performance is an important design consideration for you, then wire grid polarizers are a good choice. Additionally, for applications involving high power lasers, wire grid polarizers offer high damage thresholds as compared to other types of polarizers.

High powerobjective lens

Ian D. Johnson, Robert T. Sutter, Matthew J. Parry-Hill, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.

What isobjective lensin microscope

Ruled Wire Grid PolarizerWith ruled wire grid polarizers, a fine diamond ruler is used to carve fine lines on the substrate. Once the grid is generated, a thin metallic layer is evaporated on to the grid to create the polarizer. Although this method is relatively inexpensive, it can only be used on hard substrates and cannot achieve the grid spacing needed for short wavelength operation.Optometrics, offers ruled wire grid polarizers on CaF2 (Calcium Fluoride) and ZnSe (Zinc Selenide) covering a wavelength region from 2.5 to 20 microns.

The tutorial initializes with the major optical train components (condenser, specimen, objective, tube lens, and eyepiece) of a virtual infinity-corrected microscope appearing in the window. A beam of semi-coherent light generated by the source passes through the condenser and is focused onto the specimen plane, subsequently being collected by the objective. The parallel flux of light rays exiting the objective are focused by the tube lens onto the intermediate image plane positioned at the fixed diaphragm of the eyepiece. The distance between the tube lens and the fixed eyepiece diaphragm is adjustable within a range of 160 and 200 millimeters using the Reference Focal Length (L) slider (equivalent to the tube length in older microscopes). In addition, the objective focal length can be varied from 2 to 40 millimeters by translating the Objective Focal Length (F) slider. As these sliders are translated, the individual components of the virtual microscope are readjusted to new positions.

Low powerobjective lens

As previously listed, the basic optical components of an infinity system are the objective, tube lens, and the eyepieces. The specimen is located at the front focal plane of the objective, which gathers light transmitted through or reflected from the central portion of the specimen and produces a parallel bundle of rays projected along the optical axis of the microscope toward the tube lens. A portion of the light reaching the objective emanates from the periphery of the specimen, and enters the optical system at oblique angles, advancing diagonally (but still in parallel bundles) toward the tube lens. All of the light gathered by the tube lens is then focused at the intermediate image plane, and subsequently enlarged by the eyepiece.

In contrast, a reflective or beam-splitting polarizer creates two states of orthogonal polarizations, one that is transmitted and the other is reflected. Polarizers based on Fresnel reflections at the Brewster angle, thin-film and wire grid polarizers fall under this category. Reflective polarizers can be commonly achieved in thin film configurations, where dielectric multilayer films are coated on a range of transparent substrates. The dielectric stack can be designed to create wavelength dependent polarization due to interference effects. This offers greater flexibility in achieving polarizers with high damage thresholds to laser irradiation. Wire grid polarizers also offer the same advantages as thin film polarizers, as metal grids can be deposited over large area substrates.

William K. Fester and Mortimer Abramowitz - Olympus America, Inc., Two Corporate Center Drive., Melville, New York, 11747.

Magnification objective lensuses

In general, the electric and magnetic fields in an electromagnetic wave oscillate in random planes perpendicular to the direction of propagation. A linear polarizer blocks all random oscillations and only permits one polarization state. Several applications are made possible through the manipulation of polarization of light. The most popular use of linear polarizers is in photography. Unwanted scattering and glare from flat or reflective surfaces are common problems faced by photographers. Linear polarizers offer a simple and straightforward solution to this problem. Light that is directly reflected from surfaces has a strong s-polarization component due to Fresnel reflection at the Brewster angle. A linear polarizer in the camera can be used to suppress the glare by rotating it such that s-polarization component is suppressed. This results in a much better color contrast in the images. Glare suppression is also used in sub-surface imaging particularly imaging under water surfaces.Linear polarization can be achieved in a transmission mode or a reflection mode. In a transmission mode, all unwanted polarization states are absorbed, and only one is transmitted. These types of polarizers are called absorptive polarizers. Certain crystals, polaroid filters, and nano-particle based polarizers come under this category. Polarizer sheets made from stretched plastics are commonly found in inexpensive 3D viewing glasses.

Since there are several choices of wire grid polarizers in the market, how do you go about buying right polarizer for your application? Here are some important parameters to consider when choosing a wire grid polarizer.

For square polarizers, typical sizes are 12.5 mm x 12.5 mm, 25 mm x 25 mm, and 50 x 50 mm. There is more flexibility in sizes with round polarizers. For instance, Optometrics’ holographic wire grid polarizers come in diameters of 25 mm, 29 mm, 35 mm, 38 mm and 50 mm.Finally, it is important to consider the type of mount. If you are building a custom application and designing your own mounts, then you should choose the unmounted option. Typically, when setting up experiments on an optical table, mounts are required to hold the polarizer in position. In such cases, ring or square mounted options are available.