Microscopes give us a large image of a tiny object. Greater magnification can be achieved if the light from an object is made to pass through two lenses compared to a simple microscope with one lens. A compound microscope has two or more converging lenses, placed in line with one another, so that both bend and refract the light in turn. The result is to produce an image that is magnified more than either lens could magnify alone. Light illuminating the object first passes through a short focal length lens or lens group, called the objective, and then travels on some distance before being passed through a longer focal length lens or lens group, called the eyepiece. A lens group is often simply referred to singularly as a lens. Usually these two lenses are held in paraxial relationship to one another, so that the axis of one lens is arranged to be in exactly the same as orientation as the axis of the second lens. It is the nature of the lenses, their properties, their relationship, and the relationship of the objective lens to the object that determines how a highly magnified image is produced in the eye of the observer.
The first lens or objective, is usually a small lens with a very small focal length. A specimen or object is placed in the path of a light source with sufficient intensity to illuminate as desired. The objective lens is then lowered until the specimen is very close to, but not quite at the focal point of the lens. Light leaving the specimen and passing through the objective lens produces a real, inverted and magnified image behind the lens, in the microscope at a point generally referred to as the intermediate image plane. The second lens or eyepiece, has a longer focal length and is placed in the microscope so that the image produced by the objective lens falls closer to the eyepiece than one focal length (that is, inside the focal point of the lens). The image from the objective lens now becomes the object for the eyepiece lens. As this object is inside one focal length, the second lens bends the light in such a way as to produce a second image that is virtual, inverted and magnified. This is the final image seen by the eye of the observer.
Alternatively, common infinity space or infinity corrected design microscopes employ objective lenses with infinite conjugate properties such that the light leaving the objective is not focused, but is a flux of parallel rays which move and do not converge until after passing through a tube lens where the projected image is then located at the focal point of the eyepiece for magnification and observation. Compound microscopes such as described herein are commonly employed to view biological material.
Many microscopes, such as the compound microscope described above, are designed to provide images of certain quality to the human eye through an eyepiece. Connecting a Machine Vision Sensor, such as a Charge Coupled Device (CCD) sensor, to the microscope so that an image may be viewed on a monitor presents difficulties. This is because the image quality provided by the sensor and viewed by a human eye decreases, as compared to an image viewed by a human eye directly through an eyepiece. As a result, conventional optical systems for examining biological material often require the careful attention of a technician monitoring the process through an eyepiece. Real time images via a monitor are of poor quality.
While a scanning electron microscope can provide highly magnified images of biological material, there are several limitations associated with scanning electron microscope images. For example, it is difficult to impossible to provide real time images of a biological material. Scanning electron microscopy is typically a destructive technique, preventing further use of the imaged sample. A scanning electron microscope is a large apparatus requiring dedicated facilities, and is not portable.
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