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A digital microscope is a variation of a traditional optical microscope that uses optics and a charge-coupled device (CCD) camera to output a digital image to a monitor. A digital microscope differs from an optical microscope in that there is no provision to observe the sample directly through an eyepiece. Since the optical image is projected directly on the CCD camera, the entire system is designed for the monitor image. The optics for the human eye are omitted.
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History
The first digital microscope was made by a lens company in Tokyo, Japan in 1986, which is now known as Hirox Co Ltd. With the invention of the USB port, there came a flood of digital microscopes that connected directly to the computer. This invention has resulted in numerous companies creating some form of USB digital microscope that range in quality and magnification. They continue to fall in price, especially compared with conventional optical microscopes.
Optical and digital microscopes
A primary difference between an optical microscope and a digital microscope is the magnification. With an optical microscope the magnification is found by multiplying the lens magnification by the eyepiece magnification. Since the digital microscope does not have an eyepiece, the magnification cannot be found using this method. Instead the magnification for a digital microscope is found by how many times larger the sample is reproduced on the monitor. Therefore the magnification will depend on the size of the monitor. The average digital microscope system has a 15" monitor, would result in a average difference in magnification between an optical microscope and a digital microscope of about 60%. Thus the magnification number of an optical microscope is usually 60% larger than the magnification number of an digital microscope.
Since the digital microscope has the image projected directly on to the CCD camera, it is possible to have higher quality recorded images than with an optical microscope. With the optical microscope, the lenses are made for the optics of the eye. Attaching a CCD camera to an optical microscope will result in a image that has compromises made for the eyepiece. Although the monitor image and recorded image may be of higher quality compared with the digital microscope, the application for the microscope may dictate which microscope is preferred.
Resolution
With a typical 2 megapixel CCD, a 1600×1200 pixels image is generated. The resolution of the image depends on the field of view of the lens used with the camera. The approximate pixel resolution can be determined by dividing the horizontal field of view (FOV) by 1600.
Increased resolution can be accomplished by creating a sub-pixel image. The Pixel Shift Method uses an actuator to physically move the CCD in order to take multiple overlapping images. By combining the images within the microscope, sub-pixel resolution can be generated. This method provides sub-pixel information, simply averaging a standard image is also a proven method to provide sub-pixel information.
Limitations
Few USB models are supplied with stands capable of supporting the camera in a stable way, so it can be racked up and down, for example, but this is only true with USB microscopes. The higher end digital microscopes, that do not connect by USB, do not suffer this limitation.
See also
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