Being a student, you must have had a chance of having at least some interaction with maybe a simple or a complex type of microscope, once in your life. The microscope that you may have dealt with is mostly associated with your occupational tasks. Talking about the microscopes, the Ocular lens is one crucial part of any microscope’s design to which a human eye is initially exposed for magnifying and observing any specimen. This ocular lens, as it is often termed, is none other than the Eye-piece of the Microscope.
Table of Contents
- Purpose of Ocular Lens in Microscope
- Variations in the Ocular Lens
- Some Facts of the Essential features of an Ocular Lens
- Field Stop
- Field Number
- Negative Eyepieces
- Positive Eyepieces
- Different Types of Ocular Lenses used in Microscopes
- Huygens Lens
- Kellner Eyepiece
- Compensating Eyepieces
- Periplan Eyepiece
- Super-field Lens
Purpose of Ocular Lens in Microscope
The basic need of an eyepiece in a microscope is to enhance the magnification capacity of the microscope, by working along with the objective lens and multiplying the magnifying power. The optical power of the eyepiece ranges between 7x and 15x. The eye-piece tube isn’t as simple as it looks. The ocular lens tube is worth exploring.
Variations in the Ocular Lens
The eye-piece of one kind of microscope is different from the other kind. It enables us to magnify objects from as low as 10x, accelerating this power to an amazing 2000x. It has an adjustable tube to achieve the required focus. Eye-pieces are mounted with two to three ocular lenses. Each of these lenses may itself comprise multiple lens elements, fused with each other.
Later in the discussion, we will learn about different ocular lenses structurally and functionally. When a microscope is designed, it is of utmost importance to choose the right combination of ocular and objective lenses. The image aberrations produced by the objective lens are counter-checked, altered, and corrected by the Ocular Lens.
Some Facts of the Essential features of an Ocular Lens
Field Stop
An ocular lens is incorporated with a superb mechanism of field stop which inhibits all random light reflections from the surroundings to enter the visual field and pathway of the image itself. It also eliminates any sort of aberrations that can distort or deviate an efficient formation of the specimen’s image, for example, the aberrations formed by even an achromatic objective lens.
Field Number
Apart from the magnification power, the Field Number inscribed on the surface of the Ocular lens also indicates the performance of this lens. This number reveals the capacity of a particular eyepiece to cover the visual field of the specimen being observed.
Unlike the objective lens, the higher the magnifying power of the eye-piece, the lesser the visual field area covered by it. Eye-pieces can also be grouped on the basis of arrangements of the lenses and diaphragm within them
Negative Eyepieces
These eye-pieces are designed with their circular diaphragm at almost the same distance away and between the two ocular lenses within their structure. The basic negative eye-pieces have both of these lenses of plano-convex shape. The convex sides of these lenses direct towards the specimen.
The lens near the eye is termed the eye lens, and the one near the specimen is labeled as the field lens. The circular diaphragm delivers a circular field of vision when looked through the microscope. The diameter of this diaphragm also varies.
An example of a basic negative eyepiece is the Huygenian ocular lens.
Positive Eyepieces
A well-known type of this eye-piece is Ramsden Eye-piece. Both the eye and the field lens are also plano-convex in this eyepiece, but instead, their convex sides face each other. Here, the diaphragm is not sandwiched between these two lenses. In spite, it is situated below both of them. If these lenses are bonded closer together, they produce a higher image clarity with fewer chances of errors.
Different Types of Ocular Lenses used in Microscopes
Huygens Lens
This one is mentioned above as the best example of a negative eye-piece. This eye-piece comprises two plano-convex lenses. Its field stop is the curved diaphragm is situated in the lens tube. These are best fitted in microscopes with achromatic objective lenses (equalize refractive indexes of colors of different wavelengths).
They are most appropriately used in schools and colleges and other laboratories widely. The interesting part of these lenses is that although the eye and field lens is not free of visual errors, they eliminate each other’s visual deviations and tend to produce desired images. They work at their best with low magnifying objective lenses of power 5x to 40x.
Kellner Eyepiece
This is an advanced form of Ramsden ocular lens. Both its eye and field lenses constitute double lens elements, therefore, providing a finer deviation-free image than its basic Ramsden version. It delivers a wider visual field for observing all suitable objects for which it is used. However, this special edition also works best with low-power achromatic objective lenses.
Compensating Eyepieces
This one is your ultimate solution to correct the residing chromatic aberrations of even the highly corrected achromatic objective lenses. It works well with all types of objective lenses including fluorite and apochromatic, of any magnification power.
Periplan Eyepiece
This is integrated with the most advanced eye-piece building technology. This is structured to have seven basic lens units, with three to form a triplet, two fuses to a doublet, and two remaining as single lenses. It also works on the correction of chromatic aberrations of the lateral parts of an image’s visual field, and also offers a less curved field of vision. It also works best with high-power objective lenses.
Super-field Lens
This is very much required where wider visual field observation is required. Super-field eye-piece is integrated into dissecting microscopes.
I hope this will give you a satisfactory understanding of the purpose of all types of Eye-pieces that form a part of the microscopes.