Microscopes - Basic Types of Objective Lenses
(1) Based on the microscope lens tube's length (in millimeters): 190 lens tube for reflected light without a cover glass; 160 lens tube for transmitted light with a 0.17mm thick or thicker cover glass; and transmitted light and reflection The lens barrel, which has an infinite length, is used to focus light.
(2) In accordance with the features of the immersion method: immersion type, dry type, and non-immersion type. (oil immersion, water immersion, glycerin immersion and other immersion methods).
(3) Transmissive, reflecting, and catadioptric, depending on the optical tool.
(4) Low magnification (NA0.2 and 10X), medium magnification (NA0.65 and 40X), and high magnification (NA>0.65 and >40X), as determined by numerical aperture and magnification.
(5) Achromatic objective lenses, semi-apochromatic objective lenses, apochromatic objective lenses, flat field achromatic lenses, flat field apochromatic lenses, and monochromatic objective lenses are typically separated according to the condition of correcting aberration.
a. Achromatic objective
The name "Ach" is frequently found on the housing of this type of microscope objective, which is the most popular. It maintains the Qi Ming condition and corrects the chromatic aberration (red and blue dichroism), spherical aberration (yellow-green light), and sinusoidal aberration of the point on the axis. Off-axis spots' astigmatism does not exceed the permitted level (which is -4 for luminosity), and the secondary spectrum is not adjusted.
The two lenses that make up low magnification, achromatic objectives with a numerical aperture of 0.1 to 0.15 are typically adhered together. Two sets of doublet lenses make up achromatic objectives with numerical apertures up to 0.2. Plano-convex lenses are added when the numerical aperture reaches 0.3, determining the focal length of the objective lens while other lenses correct for the aberration caused by its spherical and plane surfaces. By sinking, the high magnification objective lens's plane aberration can be removed. Immersion-type high-magnification achromatic objectives typically have four parts: a front lens, a crescent lens, and two double-gel lens groups.
b. Apochromatic objective
The structure of this type of objective lens is complicated, and the lens is made of special glass or fluorite and other materials. The outer shell of the objective lens is marked with the word "Apo". It realizes the sinusoidal condition for the two color lights, and requires strict correction of the positional chromatic aberration (red, blue two-color), spherical aberration (red, blue two-color) and sinusoidal difference of the point on the axis, and requires correction of the secondary spectrum (recalibration) positional chromatic aberration of green light). The chromatic aberration of its magnification cannot be completely corrected, and generally must be compensated with eyepieces.
Due to the perfect correction of various aberrations, it has a larger numerical aperture than the achromatic objective lens of response magnification, which not only has high resolution and excellent image quality, but also has a higher effective magnification. As a result, apochromat objectives are high performing and suitable for advanced research microscopy and photomicrography.
c. Semi apochromatic objective
Semi-apochromatic objective lens is also called fluorspar objective lens, and the shell of the objective lens is marked with "FL". In terms of structure, the number of lenses is more than that of the achromatic objective lens and less than that of the apochromatic objective lens. In terms of image quality, it is far better than the achromatic objective lens and close to the apochromatic objective lens.
d. Plan objective
The plan objective lens is to add a half-moon-shaped thick lens to the lens system of the objective lens to correct the defect of field curvature and improve the imaging quality of the edge of the field of view. The field of view of the plan objective lens is flat, which is more suitable for microscopy and photomicrography. For flat-field achromatic objectives, the chromatic aberration of magnification is not large, so it is not necessary to use special eyepieces to compensate. For flat-field apochromatic objectives, eyepieces must be used to compensate for its chromatic aberration of magnification.
e. Monochromatic objective lens
These objectives consist of a set of singlet lenses made of quartz, fluorite, or lithium fluoride. It can only be used in individual areas of the ultraviolet spectral region (the width does not exceed 20mm), and the monochromatic objective lens cannot be used in the visible spectral region. These objectives are made into reflective and catadioptric systems. The main disadvantage is that a considerable portion of the beam is blocked in the center (25% of the entrance pupil area). In the new catadioptric system, due to the glued structure of the semi-transparent mirror and the objective lens, this shortcoming is greatly alleviated, so that the shading of the mirror frame can be cancelled. And the residual aberrations of the two coaxial mirrors are mutually compensated, and the numerical aperture is increased by the lens group. If the calibration of the system is satisfactory, when the aperture reaches NA=1.4, the central occlusion may not exceed 4% of the entrance pupil area.
f. Special objective lens
The so-called "special objective lens" is specially designed and manufactured to achieve certain specific observation effects on the basis of the above-mentioned objective lens. There are mainly the following types:
(a) Correction collar objective
There is a ring-mounted adjustment ring in the middle of the objective lens. When the adjustment ring is turned, the distance between the lens groups in the objective lens can be adjusted, thereby correcting the coverage difference caused by the non-standard thickness of the cover glass. The scale on the adjustment ring can be from 0.11--.023, and this number is also marked on the shell of the objective lens, indicating that the error between the thickness of the cover glass from 0.11-0.23mm can be corrected.
(b) Iris diaphragm objective
The upper part of the objective lens barrel is equipped with an iridescent diaphragm, and the adjusting ring can also be rotated outside. When rotating, the size of the diaphragm aperture can be adjusted. The objective lens of this structure is a high-grade oil immersion objective lens. During the microscope inspection of the field of view, the illumination light often enters the objective lens due to some reasons, so that the background of the field of view is not dark enough, resulting in a decline in the quality of the microscope inspection. At this time, adjust the size of the aperture to make the background black, make the object under inspection brighter, and enhance the effect of microscope inspection.
(c) Phase contrast objective
This kind of objective lens is a special objective lens for phase contrast microscopy, and its characteristic is that a phase plate is installed at the back focal plane of the objective lens.
(d) No cover objective
Some objects to be inspected, such as smeared films, cannot be covered with a cover glass. In this way, a coverless objective lens should be used in the microscope inspection, otherwise the image quality will be significantly reduced, especially in high-magnification microscope inspection. The shell of this kind of objective lens is often marked with NC, and there is no word 0.17 on the position of the thickness of the cover glass, but "0" is marked.
(e) Long working distance objective lens
This objective lens is a special objective lens for inverted microscopes, and it is designed to meet the microscope inspection of tissue culture, suspension and other materials.
