Calibrating Calibrations

VX4In this era of incredible plug and play technology, a Video Measuring System or reticle disc for your microscope eyepiece feels like it should be enough to begin accurately measuring metallographic samples, but if you skip your calibration steps, your results could be wildly inaccurate.

Calibration begins with a certified stage micrometer. For all LECO microscopes, either metric or English units can be used. These guides are general directions. Your specific system may vary.

Video Measuring System Calibration

  1. First, focus the micrometer on the monitor and rotate the micrometer so the lines are parallel with the left and right edges of the guard frame of the video system.
  2. Enter calibration mode for your system. When you are asked for the distance you are using to calibrate, calculate it in this way:
    1. Count the number of complete stage micrometer divisions visible on the screen.
    2. Multiply the number by the distance per division as marked on the micrometer.
    3. Enter the calibration distance in the unit you will want for your results, i.e. if you want results in mm, enter 100 μm as 0.1.
  3. If the system provides two measuring lines: Adjust the left measuring line so the inside edge is placed on the left-hand edge of the leftmost micrometer line. Then, adjust the right measuring line and place it so the inside edge is placed on the left-hand edge of the rightmost micrometer line.
Met Tips Calibration Thick Horizontal
 

If the system does not provide measuring lines: Click the leftmost micrometer line and drag to the rightmost micrometer line to indicate the distance to be measured.

It is very important that the calibration distance you entered is correct. If you are using computer-generated measuring lines, it is also important that you are using the inside of the lines and that you use the same edge of the micrometer lines when performing your calibration.

After the horizontal calibration, the next step is to repeat the calibration vertically. Rotate the micrometer 90° and repeat steps 1 through 3 using the top and bottom of the guard frame. Again, it is important that you use the inside of the measuring lines and the same edge of the micrometer lines.

Each objective and magnification combination needs to be separately calibrated. Make sure you assign a clear and unique label to each calibration, as the system may not warn you if you select the wrong calibration for your objective and magnification.

Micrometer Reticle Disc

If you have a micrometer reticle disc, the eyepiece of your microscope will usually have a holder that can be removed by unscrewing it from the bottom of the eyepiece. Place the reticle into the holder so the numbers may be read normally. Do not touch the reticle or allow any dust to contaminate it, as this will be visible in the field of view. Screw the holder back onto the eyepiece.

  1. Place the stage micrometer on the microscope.
  2. Focus through the eyepiece containing the reticle disc and rotate the eyepiece until the scale on the disc is parallel with, but slightly above, the scale on the stage micrometer.
  3. Move the stage micrometer until one of the lines on the left is aligned with one of the lines on the reticle disc. Then, look to the right until you find another set of lines that are completely matched. (If you cannot find a second set of lines, estimate to a fraction of a division.)
  4. Divide the total distance between those two lines on the stage micrometer by the number of increments on the eyepiece disc between the two reticle micrometer lines. This gives you the value of each measurement on the reticle disc for that objective.

Met Tips Calibration ReticleIn the above example, there are 12 increments on the micrometer disc and 13 lines on the stage micrometer between two sets of matching lines. If each line on the stage micrometer is 0.001 inches, then: (0.001 * 12)/13 = 0.000923 inches. Therefore, each division on the micrometer reticle disc is 0.000923 inches for this objective. Repeat the calibration for every objective and create a table for your reference.

This type of micrometer reticle disc with a fixed scale should only be used for approximate measurements.

Once your calibrations are complete, you can measure with confidence!

Topics: Metallographic Science, Microscopes, Imaging