What Is The Function Of Eyepiece In Microscope?

Eyepiece and Ocular Lens

the eyepiece, or ocular lens, is the part of the microscope that magnifies the image produced by the microscope’s objective so that it can be seen by the human eye.

microscope objectives to further magnify the intermediate image so that specimen details can be observed.

oculars, or ocular lenses, are alternative names for eyepieces. to maintain consistency during this discussion, we will refer to all oculars and ocular lenses as eyepieces.

the ocular lens may refer to the eyepiece as a whole or specifically to the eye lens—the lens closest to the eye.

the ocular lens magnifies the image produced by the objective so that the microscope user can see it.

a compound microscope is a type of optical microscope that uses multiple lenses to magnify small objects. it consists of two sets of lenses: the objective lens, which is closer to the specimen and provides the initial magnification, and the eyepiece lens, which further magnifies the image for the viewer's eye.

light passes through the specimen and is magnified by the objective lens, then further magnified by the eyepiece lens, resulting in a highly magnified image visible to the observer.

• the objective lens is the primary magnifying element in optical instruments.
• positioned closer to the object being observed, it captures and magnifies the incoming light, bringing the specimen into focus.
• conversely, the ocular lens, also known as the eyepiece, is situated near the observer's eye.
• its primary function is to further magnify the image produced by the objective lens.
• the process begins with the objective lens capturing light from the specimen, forming an intermediate image.
• this image is then further magnified by the ocular lens, delivering a detailed and enlarged view to the observer.

Eyepiece Characteristics

to achieve the best results in microscopy, combine objectives with eyepieces that are appropriate for the correction and objective type.

the basic anatomy of a typical modern eyepiece is illustrated in figure 1 below. inscriptions on the side of the eyepiece describe its characteristics and functions.

the eyepieces illustrated in figure 1 are inscribed with uw, which is an abbreviation for the ultra-wide viewfield.

often eyepieces will also have an h designation, depending on the manufacturer, to indicate a high-eyepoint focal point that enables microscopists to wear glasses while viewing samples.

the eyepiece magnification of the eyepieces in figure 1 is 10x, as indicated on the housing.

the inscription a/24 indicates the field number is 24, which refers to the diameter (in millimeters) of the fixed diaphragm in the eyepiece.

these eyepieces also have a focus adjustment and a thumbscrew that allows their position to be fixed.

manufactures now often produce eyepieces with rubber eyecups that serve both to position the eyes the proper distance from the front lens and to block room light from reflecting off the lens surface and interfering with the view.

wf

for widefield

uwf

for ultra-widefield

sw and swf

for super widefield

he

for high eyepoint

cf

for eyepieces intended for use with cf corrected objectives

compensating eyepieces

are often inscribed with k, c, or comp, as well as the magnification.

flatfield objectives

eyepieces used with flatfield objectives are sometimes labeled plan-comp.

Lens and Diaphragm Arrangement

there are two major types of eyepieces that are grouped according to lens and diaphragm arrangement: negative eyepieces (or huygenian eyepieces) with an internal diaphragm and positive eyepieces (or ramsden eyepieces) that have a diaphragm below the lenses of the eyepiece.

negative eyepieces have two lenses:

• upper lens, which is closest to the observer's eye, is called the eye lens
• lower lens (beneath the diaphragm) is often termed the field lens
• in their simplest form, both the eye and field lenses are plano-convex, with convex sides facing the specimen.
• about midway between these lenses is a fixed circular opening or internal diaphragm.
• the size of the diaphragm defines the circular field of view that is observed when you look into the microscope.

the simplest negative eyepiece design, often termed the huygenian eyepiece, is found on most teaching and laboratory microscopes fitted with achromatic objectives.

although the huygenian eye and field lenses are not well corrected, their aberrations tend to cancel each other out.

more highly corrected negative eyepieces have two or three lens elements cemented together to make the eye lens.

if an unknown eyepiece has only the magnification inscribed on the housing, it is most likely a huygenian eyepiece and is best suited for use with achromatic objectives of 5x to 40x magnification.

the other main type of simple eyepiece is the positive eyepiece with a diaphragm below its lenses, commonly known as the ramsden eyepiece.

this eyepiece has an eye lens and field lens that are also plano-convex, but the field lens is mounted with the curved surface facing toward the eye lens.

the front focal plane of this eyepiece lies just below the field lens, at the level of the eyepiece diaphragm, making this eyepiece readily adaptable for mounting reticles.

to provide better correction, the two lenses of the ramsden eyepiece may be cemented together.

Modified and Compensating Eyepieces

a modified version of the ramsden eyepiece is known as the kellner eyepiece.

these improved eyepieces contain a doublet of eye-lens elements cemented together and feature a higher eyepoint than either the ramsden or huygenian eyepiece, as well as a much larger field of view.

a modified version of the simple huygenian eyepiece is illustrated in figure 3 on the right.

while these modified eyepieces perform better than their simple one-lens counterparts, they are still only useful with low-power achromat objectives.

chromatic difference of magnification in the intermediate image, especially when combined with high magnification achromatic objectives or fluorite or apochromatic objectives.

to fix this issue, manufacturers produce compensating eyepieces that introduce an equal but opposite chromatic error in the lens elements.

1. compensating eyepieces may be the positive or negative type.
2. compensating eyepieces must be used at all magnifications with fluorite, apochromatic, and all variations of plan objectives.
3. they can also be used to advantage with achromatic objectives of 40x and higher.
4. compensating eyepieces play a crucial role to help eliminate residual chromatic aberrations inherent in the design of highly corrected objectives.
5. it is preferable that the microscopist uses the compensating eyepieces designed by a particular manufacturer to accompany that manufacturer's higher-corrected objectives.

using an incorrect eyepiece with an apochromatic objective designed for a finite (160 or 170 mm) tube length application results in dramatically increased contrast with red fringes on the outer diameters and blue fringes on the inner diameters of the specimen details.

additional problems arise from a limited flatness of the viewfield in simple eyepieces, even those corrected with eye-lens doublets.

more advanced eyepiece designs resulted in the periplan eyepiece. this eyepiece contains seven lens elements cemented into a single doublet, a single triplet, and two individual lenses.

design improvements in periplan eyepieces lead to better correction for residual lateral chromatic aberration, increased flatness of field, and a general overall better performance when used with higher power objectives.

Viewfield and High Eyepoint

manufacturers now produce widefield eyepieces that increase the viewable area of the specimen by as much as 40 percent.

because eyepiece-objective correction techniques vary from manufacturer to manufacturer, it is important to use only the eyepieces recommended by a specific manufacturer for use with their objectives.

light rays emanating from the eyepiece intersect at the exit pupil or eyepoint, often referred to as the ramsden disk, where the pupil of the microscopists eye should be placed in order to see the entire field of view (usually 8–10 mm from the eye lens).

by increasing the magnification of the eyepiece, the eyepoint is drawn closer to the upper surface of the eye lens, making it much more difficult for the microscopist to use, especially if they are wearing eyeglasses.

• manufactures have designed high eyepoint eyepieces that feature eyepoint distances approaching 20–25 mm above the surface of the eye lens.
• these improved eyepieces have larger diameter eye lenses that contain more optical elements and usually feature improved flatness of field.
• these eyepieces are often designated with the inscription h somewhere on the eyepiece housing, either alone or in combination with other abbreviations.
• high-eyepoint eyepieces are especially useful for microscopists who wear eyeglasses to correct for near or far sightedness.
• they do not correct for several other visual defects, such as astigmatism.
• the large eye clearance reduces fatigue and makes viewing images through the microscope much more comfortable.

the diameter of the viewfield in an eyepiece is expressed as a field-of-view number or field number (fn).

information about the field number of an eyepiece can yield the real diameter of the object viewfield using the formula:

viewfield diameter = (fn) / (m(o) × m(t)

where fn is the field number in millimeters, m(o) is the objective magnification, and m(t) is the tube lens magnification factor (if any).

Measurement and Photomicrography

eyepieces can be adapted for measurement purposes by adding a small circular disk-shaped glass reticle (sometimes referred to as a graticule or reticule) at the plane of the field diaphragm of the eyepiece.

reticles usually have markings, such as a measuring rule or grid, etched onto the surface.

because the reticle lies in the same plane as the field diaphragm, it appears in sharp focus superimposed over the image of the specimen.

eyepieces using reticles must contain a focusing mechanism (usually a helical screw or slider) that allows the image of the reticle to be brought into focus.

the reticle in figure 5 (a)

is a common element of eyepieces intended to frame viewfields for photomicrography.

the reticle in figure 5 (b)

is a linear micrometer that can be used to measure image distances.

the crossed micrometer in 5 (c)

is used with polarizing microscopes to locate the alignment of samples with respect to the polarizer and analyzer.

the grid illustrated in figure 5 (d)

is used to partition a section of the viewfield for counting.

for highly accurate measurements, a filar micrometer is used. this micrometer replaces the conventional eyepiece and offers several improvements over conventional reticles.

in the filar micrometer, a reticle with a measuring scale and a very fine wire is brought into focus with the specimen.

the wire is mounted so that it can be slowly moved across the viewfield by the calibrated thumbscrew located on the side of the micrometer.

filar micrometers (and other simple reticles) must be calibrated with a stage micrometer for each objective with which it will be used.

some eyepieces have a movable pointer located within the eyepiece and positioned so that it appears as a silhouette in the image plane.

this pointer is useful to indicate certain features of a specimen, especially when a microscopist is teaching students about specific features.

manufacturers often produce specialized eyepieces, often termed photo eyepieces, that are designed to be used with photomicrography.

these eyepieces are usually negative (huygenian type) and are incapable of being used visually. for this reason, they are typically called projection lenses.

projection lenses must be carefully corrected so that they will produce flat-field images, a definite must for accurate photomicrography.