written by: Robyn Broyles•edited by: Leigh A. Zaykoski•updated: 8/27/2008
The digital microscope combines the principles behind light microscopes and digital cameras. Handheld versions called USB microscopes, as well as 3D digital microscopes, offer great flexibility.
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Optical microscopes have been around for about four hundred years. This oldest form of microscope uses lenses made of transparent material to magnify objects solely by bending light. In the past few years, new technology has led to the development of new types of light microscopes. Relying on the same technology that is behind digital photography, these new devices are called digital microscopes.
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A standard digital microscope works in exactly the same way as a traditional optical microscope, except that the image falls on a sensor instead of passing through an ocular directly to the viewer's eye. A digital microscope connects to a computer through a USB cable and comes with specialized software. In some cases, a separately purchased microscope camera must be used in conjunction with the microscope.
To operate a digital microscope, the user must first install the software on a computer and connect the computer to the microscope via a USB cable (a few models come with their own viewers instead). Slides are prepared in the same way as with an optical microscope, but instead of being viewed through a viewfinder, the image appears on the computer monitor. Living organisms will appear to move in real time on the screen. The images can be saved as still images or as video.
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Not all digital microscopes are built with a traditional optical microscope's design. Some are handheld devices that are often called USB microscopes. A USB microscope does not require slide preparation; it can be used on anything -- for example, documents, coins and stamps, household objects, and living things (from insects to human body parts). These pocket-sized devices can be purchased for under USD $130. The wide range of objects on which they can be used, combined with their size and inexpensive price tag, make them flexible, powerful tools for many applications.
To operate a USB microscope, the user connects the device to a computer via a USB cable, then gently touches the lens directly onto the object to be magnified. The lens can also be adjusted for distance viewing, making it possible to view wet or moving objects.
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3D digital microscopes
In a regular optical microscope, a view from an unusual angle, such as from the side, would require mirrors to bend light up to the eyepieces. A 3D digital microscope takes advantage of the fact that the image does not need to fall on human eyes; instead, the image sensor is free to be located anywhere, so light does not need to be reflected in order to achieve non-standard viewing angles. With a 3D digital microscope, the user can adjust the arm to view the target from any angle. Typically, these microscopes have a much greater depth of field, as much as twenty times greater than that of a slide microscope, allowing the user to focus on objects that would not be viewable with a slide microscope.
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Magnification and optics
Digital microscopes have lenses and optics, but for much of their magnification power, they do not rely on an optical lens at all. Instead, the nature of the camera sensor makes additional magnification possible.
Many digital microscopes use a charged-couple device (CCD sensor), which is composed of a square array of light-sensitive pixels. Each pixel detects photons as they fall upon it and stores the information as electrons. The process is controlled by a clock signal, so that the sensor is "read" at measured intervals.
Other digital microscopes, particularly handheld USB microscopes, use CMOS sensors. Also known as active-pixel sensors (APS), CMOS sensors are integrated circuits that are less expensive and consume less power, but they currently produce lower quality images than CCD sensors.
Whether CCD or CMOS, the way the sensor works can result in fine resolution independent of optical magnification. When the CCD's analog information is converted to a digital image, the microscope software can magnify the image. Digital microscopes thus use both optics (magnifying lenses) and CCD/CMOS magnification.