The objective lens is the most important optical unit that determines the basic performance and function of an optical microscope . Therefore, in order to meet various needs and applications, we have developed an objective lens with the best optical performance and function (which is also the most important performance and function for optical microscopes), and has introduced a variety of objective products for different purposes. . Basically, the objective lens is classified according to the purpose, the observation method, the magnification, the performance (aberration correction), and the like. Among them, classification according to aberration correction is a classification method unique to a microscope objective lens. Objective lens classification: Classified by use 1. Magnification: The magnification of the objective lens refers to the ability index of magnifying the physical multiple on the line length of the objective lens. There are two representations, one is directly on the scale, such as the objective lens 8 ×, 10 ×, 45 ×, etc.; the other is a scale of the focal length f of the objective lens on the objective lens, the shorter the focal length, the magnification more high. The former objective magnification formula is M = L / f, L is the length of the optical barrel, L value is very accurate in design, but in practical applications, due to poor measurement, the length of the mechanical barrel is commonly used. The mechanical barrel length refers to the linear distance from the microscope eyepiece interface. An objective lens are each marked with the number of mechanical tube length. 2. Lens barrel length: The barrel length refers to the distance from the bottom surface of the objective lens to the top surface of the eyepiece. Since the objective lens based on the image aberration is corrected to a predetermined position, and therefore must be used in the objective lens barrel mechanically predetermined length, generally microscopic mostly mechanical tube length 160mm, 170mm, 190mm. When the metallographic microscope is photographed, the projection distance of the image varies greatly due to the difference in magnification. Therefore, the aberration of the excellent objective lens is corrected for the length of the arbitrary barrel, that is, the object mirror difference is corrected in the infinite length range. Formoterol Intermediate,Formoterol Api,Formoterol Cas 1049695-95-5,Formoterol Fumarate Intermediate Shandong Bolode Bio-technology Co., Ltd. , https://www.bldpharma.com
The use of optical microscopes is broadly divided into two categories: "biological use" and "industrial use". The objective lens can also be classified into a "biological" objective lens and an "industrial" objective lens according to these two uses. In biological applications, biological specimens are typically placed on a glass slide and covered with a cover glass from above. Since the biological objective lens needs to observe the sample through the cover glass, an optical system design considering the thickness of the cover glass (generally 0.17 mm) is employed. In industrial applications, specimens such as metal mineral chips, semiconductor wafers, and electronic components are generally observed without being covered. Therefore, the industrial objective lens adopts the optimal optical system design in which the front end of the objective lens and the specimen are not covered by the cover glass.
Classified according to observation methods
Various observation methods have been developed in accordance with the use of optical microscopes, and specialized objective lenses corresponding to these observation methods have also been developed. The objective lens can be divided according to the observation method. For example, "objective lens for reflecting dark field (circular illumination path around the inner lens)", "object for differential interference (reducing internal distortion of the lens, optimized for combination with optical characteristics of differential interference prism)", "object for fluorescence" (Improved transmittance in the near-ultraviolet field), "Objective for polarized light (too much reduced internal distortion of the lens)" and "Object for phase difference (built-in phase plate)".
Classified by magnification
An optical microscope is a plurality of objective lenses mounted on a device called an objective lens converter. In this way, by rotating the objective lens converter, the low magnification can be switched to a high magnification, and the magnification conversion can be easily performed. Therefore, it is common to install a set of objective lenses of different magnifications on the objective lens converter. For this reason, the product lineup of the objective lens is composed of a low magnification (5×, 10×), a medium magnification (20×, 50×), and a high magnification (100×) objective lens. Among them, especially in high-magnification products, in order to obtain high-definition imaging, we have introduced a liquid immersion objective lens which is filled with a special liquid having a high refractive index such as synthetic oil or water between the front end of the objective lens and the specimen. In addition, ultra-low magnification (1.25×, 2.5×) and ultra-high magnification (150×) objective lenses for special applications have also been introduced.
Aberration correction and classification of objective lenses
According to the degree of correction of the chromatic aberration correction classification (grade) according to the axial chromatic aberration (longitudinal chromatic aberration), it can be divided into three levels of achromatic, semi-achromatic (Fluorite) and apochromatic. Product lineups are also sorted from normal to high, with different prices.
3. Numerical aperture: The numerical aperture characterizes the concentrating ability of the objective lens, which is one of the important properties of the objective lens, and is usually expressed by "NA". The size of numerical aperture of the objective lens determines the resolving power of the objective lens (identification) and the effective magnification. According to the theoretical derivation: NA = nsin θ
There are two ways to increase the numerical aperture of an objective:
(1) Increasing the diameter of the lens or reducing the focal length of the objective lens, that is, designing a short focal length objective lens to increase the aperture half angle θ. However, this method will lead to increased aberrations and manufacturing difficulties, and is generally not used. In fact, the maximum value of sin θ can only reach 0.95.
(2) Increase the refractive index n between the objective lens and the observation object. The dry objective lens is air-based and has a refractive index n=1, which is generally used for low magnification objectives. The oil-based objective lens is often used as a medium for the high magnification objective lens with conifer oil (n=1.515, NA=1.4) and α-deuterated bromine (n=1.658, NA=1.60) . The numerical aperture of the oil objective lens can reach 1.30~1.40 at this time, and the magnification can reach 100~140 times. However, the dry objective lens cannot use oil as a medium.