- Type of microscope objective lenses, such as Achromatic and Plan Apochromatic, are vital for imaging and address specific aberration correction needs.
- Corrections for cover glass thickness and working wavelengths are vital for optimal performance.
- Balancing magnification and resolution, alongside factors like working distance, ensures detailed observations.
Critical Role of Objective Lenses in Microscopy
In microscopes, objective lenses play a crucial role as the most complex and important component. These lenses, designed as multi-element lenses located closest to the specimen, receive light emitted by the specimen. Their primary function is to produce a real image, which is then transmitted to the eyepiece or computers. Widely employed in scientific research, biology, industry, and laboratory work, these lenses serve essential functions in various fields.
Types of Objective Lenses
A diverse array of objective lenses, classified as types of objective lenses, is available for selection based on design and quality. The classification is generally determined by intended purpose, microscopy method, performance, magnification, and aberration correction. In this article, we will introduce the five types of objective lenses classified by aberration correction.
- Achromatic Objectives: These objective lenses, recognized as achromatic lenses, are not only widely used but also the most affordable. If no markings suggest otherwise, one can assume their achromatic nature. These lenses proficiently correct chromatic aberration in red and blue light, ensuring the convergence of these wavelengths at a single focal point, while also addressing spherical aberration in green light.
- Plan Achromatic Objectives (Achroplan): In a non-corrected objective, you can find sharp focus around the edges of the field of view or the center of the field of view. This type of microscope lens offers correction for chromatic aberration in two wavelengths and a correction of field curvature.
- Plan Fluorite Objectives (Plan Semi-Apochromats): These lenses were originally manufactured using the mineral fluorite, but now are mainly made of synthetic materials. These versatile lenses provide improved chromatic aberration correction and a flat field, with spherical aberration corrected for two wavelengths.
- Plan Apochromatic Objectives (Plan Apo): These lenses outperform Plan Fluorite Objectives by achieving superior transmission in the 400nm to 100nm range. They actively correct chromatic aberration for three colors (red, green, and blue), ensuring the convergence of these wavelengths at a single focal point. Furthermore, they actively correct spherical aberration for two or three wavelengths.
- Super Apochromatic Objectives: These lenses deliver outstanding performance over the entire visible to near-infrared field of view with axial color aberration.
Objective Lenses Key Specifications in Microscopy
- Aberration Correction or Resolution: Spherical and chromatic aberrations limit the resolution of conventional microscopes. Lenses with a high degree of aberration correction result in high-resolution images over the entire field of view.
- Magnification: Magnification refers to a microscope’s capacity to produce larger images, and high-magnification objectives offer detailed specimen images. Observers often confuse magnification with resolution, which defines an imaging system’s ability to reveal object detail. High magnification without adequate resolution may render small microbes visible but prevents distinguishing between microbes or sub-cellular parts. Avantier has dedicated years to design and manufacturing, ensuring simultaneous fulfillment of both magnification and resolution requirements.
- Numerical Aperture (NA): NA is the measure of its capability to gather light and to resolve fine specimen details at a fixed object. A lens with a high NA collects more light and can resolve finer specimen details at a fixed distance.
- Conjugate Distance: Objectives are corrected for a specific projection distance. In finite conjugate optical design, light from a non-infinite source is focused down to a point. In infinity-corrected optical systems, light emitted from the specimen passes through the objective lens and enters the tube lens as an infinity parallel beam, forming a real image.
- Cover Glass: Objectives are usually corrected for a specific cover glass thickness, with 0.17 millimeters being the standard.
- Immersion Media: The main purpose of using different types of immersion media is to minimize the refractive index between the objective and the sample. It is crucial to use the correct media, such as water, oil, or air/dry, as specified by the objective.
- Working Distance: The distance between the front end of the microscope objective and the surface of the specimen at which the sharpest focus is achieved. Proper positioning is important to obtain a good image at the specified magnification.
- Parfocal Length: The distance from the shoulder of the objective to the sample plane.
- Working Wavelength(s): Objectives are corrected for specific wavelengths, with shorter wavelengths yielding higher resolution.
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