On offer is high quality 60X POL microscope objective (6001)
Probably from a Prior (UK) compound polarising microscope. This is a brand new objective from unused stock.
This objective has a standard RMS thread.
Markings on the steel barrel are:
(POL= made with strain free glass suitable for use with transmission polarising microscopes (also works well with biological upright microscopes))
60 = 60X magnification /0.85 indicates the numerical aperture of the objective
infinity = optical system 'tube length' (very common in current microscopes) / 0.17 = coverglass corrected for 0.17mm thickness of glass
Information about microscope objectives:
There are three basic types of microscope objectives:
Compound microscope objectives
This is the most common type where the microscope typically has a turret of objectives. Within this type of objective are a host of different sub-types and magnifications, probably there over 4,000 different types of compound microscope objectives. Examples include objectives for biological microscopes, metallurgical microscopes, phase contrast, fluorescence & polarising. Then there are different grades of optics (in increasing quality): eg achromatic, semi-plan achromatic, plan achromatic, plan apochromatic etc. There are also different optical tube systems (reference focal length) eg infinity or 160mm. Finally there are different threads, the most common being RMS thread --- see below for more details.
Stereo/Dissecting Zoom microscope objectives
These are larger, flatter, wider objectives with low magnification powers typically, 0.3X,0.5X,0.7X, 0.75X, 1X, 1.5X, 2X.
Monozoom system objectives
These are specialised objectives that only fit monozoom microscopes, they are typcally very small and flat (thin) and have similar low magnification powers to stereo/dissecting zoom microscope objectives above.
Information about Compound Microscope Objectives:
The most common thread is the RMS (Royal Microscopical Society) standard microscope objective thread 0.8in (20.32mm) in diameter (outside diameter of thread) based on a Whitworth thread pitch of 36 threads per inch.
Meaning of the markings on the microscope objective barrel
Plan (objective quality grade)
100X oil/1.25 (magnification, whether immersion oil is needed to use it/numerical aperture)
160/0.17 (Optical tube length & compatible optical system in the microscope / coverslip correction indicating the thickness of the coverslip (in mm) that the optics are corrected for)
Numerical Aperture (NA)
The most important, factor in determining the resolution of an objective is the angular aperture, which has a practical upper limit of about 72 degrees (with a sine value of 0.95). When combined with refractive index, the product:
is known as the numerical aperture (abbreviated NA), and provides a convenient indicator of the resolution for any particular objective. Numerical aperture is generally the most important design criteria (other than optical correction) to consider when selecting a microscope objective. Values range from 0.1 for very low magnification objectives (1x to 4x) to as much as 1.6 for high-performance objectives utilizing specialized immersion oils. As numerical aperture values increase for a series of objectives of the same magnification, we generally observe a greater light-gathering ability and increase in resolution.
Objective Quality Grade
The least expensive (and most common) objectives, employed on a majority of laboratory microscopes, are the achromatic objectives. These objectives are corrected for axial chromatic aberration in two wavelengths (blue and red; about 486 and 656 nanometers, respectively), which are brought into a single common focal point. Furthermore, achromatic objectives are corrected for spherical aberration in the color green The limited correction of achromatic objectives can lead to substantial artifacts when specimens are examined and imaged with color microscopy and photomicrography. If focus is chosen in the green region of the spectrum, images will have a reddish-magenta halo (often termed residual color). Achromatic objectives yield their best results with light passed through a green filter (often an interference filter) and using black and white film when these objectives are employed for photomicrography. The lack of correction for flatness of field (or field curvature) further hampers achromat objectives. In the past few years, most manufacturers have begun providing flat-field corrections for achromat objectives and have given these corrected objectives the name of plan achromats.
The next higher level of correction and cost is found in objectives called fluorites or semi-apochromats (illustrated by center objective in Figure 2), named for the mineral fluorite, which was originally used in their construction. Figure 2 depicts the three major classes of objectives: The achromats with the least amount of correction, as discussed above; the fluorites (or semi-apochromats) that have additional spherical corrections; and, the apochromats that are the most highly corrected objectives available. Although similar in construction to fluorite objectives, the lenses have different curvatures and thickness, and are arranged in a configuration that is unique to apochromat objectives.
Objective Correction for Optical Aberration
(information courtesy of www.gxoptical.com and Olympus)