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Magnification Calibration

Magnification can be calibrated several ways in FTA systems. The subject is extensively discussed in the manual. The following is a short-form to guide you if you already understand the basics.

Magnification is expressed in terms of image size that covers one pixel in the camera. A typical value might be 10 or 20 microns per pixel (or, 10000 or 20000 namometers per pixel). This is not the physical size of the CCD pixel itself; instead it is physical size of the pixel divided by the optical magnification of the microscope.

There are two ways to calibrate the system from known objects:

  • Object with known linear dimension: Stainless steel dispense needle diameter is one convenient standard. Sapphire balls are another. The first step is to capture an image of the object and measure it with one of the available tools on the Movie form. Then go to the Calibration tab and enter the Actual size (what you know the object to be, say the diameter of the needle) and the size Measured with the current magnification (what the image analysis said the size was). Then click Apply. Notice how the number in the Calibration by Direct Setting field changes. This is the current magnification in nanometers per pixel.

    Standard needle diameter values are listed in the Reference section of the manual and are also shown on the Contact East website (see the Kahnetics section): www.contacteast.com. Inside and outside diameters are shown, albeit in inches (multiply by 25.4 to convert to millimeters).

  • Liquid with known surface tension: Measure the surface tension of a known, trusted liquid. Use its Actual and Measured values on the Calibration tab. Click Apply. This is the easiest way to calibrate because surface tension is very sensitive to magnification. Therefore, when surface tension is correct, magnification must be correct.