Brightfield/Darkfield Objective (BD or BF/DF)
Brightfield/Darkfield objectives are meant for viewing objects in reflected light, with no glass or coverslip between the objective and the object being viewed. For examples, microscopes used to inspect electronic components or metallurgical samples might carry a BD objective. The chief characteristic feature of the brightfield/darkfield objective is a hollow collar surrounding the barrel which houses the optical elements. Light from exterior sources can be directed into this collar, where mirrors or prisms direct it down toward the viewing stage in a hollow cone of light. In essence, the BD objective is both an optical element, and part of the instrument’s illumination system.
The presence of the collar makes BD objectives significantly larger in diameter than other types of objectives, requiring either a nosepiece specifically designed to accept them, or a special adapter to allow them to be used in a standard nosepiece.
Long Working Distance Objective (LWD)
In the traditional microscope design, the objective lens is positioned very close to the object being viewed – often only a millimetre or less away. The distance between the objective lens and the specimen under observation is known as the working distance. In some types of microscopy, such as performing dissections under a microscope, or inspecting or repairing electronic circuit boards, it is necessary that the object being observed or worked upon be at a much greater distance from the objective lens. In other words, the working distance must be much larger. Objectives designed for such applications are known as long working distance objectives. Such objectives usually carry the designation LWD, though some manufacturers also use designations such as L, LL, LD, ELWD (extra-long working distance), SLWD (super-long working distance) or ULWD (ultra-long working distance). Many of today’s objectives have the working distance (in millimetres) inscribed on the lens barrel.
Due to the limitations of the optics involved, long working distance objectives ordinarily provide relatively low magnification, with magnification usually decreasing as the objective’s working distance is increased.
Metallurgical Objective (M)
A metallurgical objective is one meant to be used with a metallurgical microscope. The designation “metallurgical” can be a bit misleading, since microscopes of this general type are used for any sort of viewing where there is no coverslip between the object being viewed and the objective lens. Examples include inspection and repair of electronic circuit boards, and observation of insects, animals or plants (in their complete form, rather than just thin tissue samples on a glass slide).
The illumination used in such viewing is normally also different from that used in biological microscope: in a metallurgical microscope, the light is reflected off the solid, opaque object’s surface (called episcopic illumination), rather than passing through a thin, translucent sample (diascopic illumination). Objectives made for such viewing have special design features that optimise them for this type of viewing.
Phase Contrast Objective (Ph)
Phase contrast objectives are specifically designed for use with a phase contrast illumination system. Such a system makes use of the tendency of different frequencies of light to behave differently when they pass through semi-transparent specimens. The technique reveals features that would be invisible using normal illumination and optics. One advantage of phase contrast is that cells and tissues do not have to be killed and stained before viewing, so that details of their structure will be visible.
To help distinguish phase contrast objectives from other types, many manufacturers use green lettering on the lens’s barrel to indicate its various specifications.