Blog Deep Dive into Optics: Objective Lenses
An objective lens is an important component of a microscopy or imaging system for a range of scientific research, biological, industrial and general laboratory applications. An objective lens is located closest to the object and plays a central role in imaging an object to the human eye or an image sensor to discern object detail.
Types of lenses
Depending on optical performance or degree of aberration correction, objective lenses are generally classified into five basic types:
- Achromatic objectives: the cheapest lenses with chromatic aberration corrected for red and blue light, and spherical aberration corrected for green light.
- Plan achromatic objectives: A type of microscope objective that has a flat field in addition to chromatic aberration corrected for two wavelengths.
- Plan Fluorite Objectives (Plan Semi-Apochromat): versatile lenses that provide flat-field images and better chromatic aberration, and spherical aberration is corrected for 2 wavelengths.
- Plan apochromatic objectives: Compared to Plan Fluorite objectives, Plan Apochromatic objectives have improved transmission between 400 nm and 100 nm, and superior corrections for curvature of field, chromatic aberration and spherical aberration.
- Super Apochromatic Objectives: provide diffraction-limited performance over the entire field of view, from visible to near-infrared.
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Degree of correction or resolution of aberrations: Lenses with a high degree of aberration correction provide a high resolution image across the field of view.
Magnification: the ability of a microscope to produce a larger image. The high magnification objective gives a very detailed image of the sample.
Numerical aperture: The objective with high NA collects more light emitted/scattered from the sample and has the ability to resolve fine details of the sample at a fixed distance.
Conjugate distance: lenses are corrected for a particular throw distance. In an infinity-corrected optical system, a beam of light emitted from a specimen passes through the objective lens which does not form an image and enters as a parallel beam at infinity into the lens of the tube that forms a real image.
Glass slide: Usually, objectives are corrected for a particular slide. The standard thickness of the lens covers is 0.17 millimeters.
Immersive media: it is important to use the correct media (such as water, oil, air/dry) specified by the lens.
Working distance: The distance from the front end of a microscope objective to the workpiece surface at which the sharpest focus is obtained. To obtain a good image with the specified magnification, it is very important to place the objective in the right place.
Parfocal length: the distance between the shoulder of the objective and the specimen plane.
Working wavelength(s): Lenses are corrected for a particular wavelength or wavelengths. Shorter wavelengths give higher resolution.
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