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Stereo microscope basics

There are two main types of light microscope, compound and stereo. Generally stereo microscopes are used for low magnification applications, (tyically upto around 100x). There are two optical paths transfering the image from the eypieces al the way to the to the final objective lens and even all the way to the sample, for some very low cost units. This gives a perception of 3D when veiwing the subject, this is because you are viewing the sample from two slightly different angles, unlike an upright which views with a single optical path vertically to the sample. The trick with most optical inspection tasks, is only to use just enough magnification as is required, allowing you to view the detail required. This will retain depth of focus and larger fields of view. Higher magnifiction is often used, to try and overcome poor illumination or resolution.

Two main types of stereo microscope exist and in brief they are the greenough optical design, which have converging angled optical paths. The only real advantage is that they are relativley inexpensive to manufacture.

The second type have parallel optical paths from the eyepieces to the final objective lens. The optical paths then converge between the final objective and the subject. These tend to be more expensive, have wider magnifcation ranges and more importantly, allow accessories to be fitted between the eyepieces and objective (camera adapters etc). If you can stretch to one, go for a parallel optical stereo system!

The following list explains the basic stereo and dissecting microscope parts and functions of each part: Stereo Head: Two eyepieces - looking through them is something like looking through a pair of binoculars, with simliar controls for eyestrength correction and interpupilary distance settings. Eyepiece: The part of the microscope that you look through. Eyepieces in stereo microscopes, as in most compound microscopes, typically have a lens with a 10x magnification level. Both eyepieces have the same magnification on stereo head microscopes. Eyepiece diopter setting: Compensates for focusing differences between your eyes, it is very important this is set correctly, in order to prevent eye strain. Objective: The second lens of the microscope. The objective lens together with the lens of the eyepiece makes up the microscope's magnification. Stereo microscopes can have a fixed single objective, a rotating multiple lens turret (such as the one pictured)or ideally a zoom. All of these allow you to change the magnification level for different applications. Stereo head microscopes actually have two separate objectives so that each eye is looking through an eyepiece lens as well as an objective lens. Focus knob: Moves the head of the microscope up and down to bring the object sharply into view.

Most stereo microscopes have only one focus knob. Lighting: Top lighting shines down and reflects off opaque or solid specimens; bottom lighting shines up through transparent objects. The microscope pictured below has top and bottom lighting, though not all microscopes will have both. Rack and Pinion Focusing: Most stereo and dissecting microscopes have standard "rack and pinion" focusing. Turn a knob to slide the head of the microscope up and down (closer or farther from the specimen). Stage clips: For holding microscope slides or other thin objects in place on the stage. Stage plate: Where the specimen is placed for viewing; located directly under the objective lens. Some stereo microscopes have reversible black and white stage plates to provide appropriate contrast with the object being viewed.

Generally speaking, in conclusion, if you require less than 100x magnifaction and your sample is 3 dimentional, or in particular you wish to manipulate, dissect or re work it - go for a stereo. However if you want to measure on the subject, work at higher magnification, or view down deep holes, a compound or monocular optical system may be required. If in doubt, call us for a chat!


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