Micrometer Calibration

Eyepiece Micrometer

Eyepiece micrometers enable the size of a specimen to be measured.

Microscope Reticle

The eyepiece micrometer also known as a reticle is a glass disc that goes inside eyepiece of the microscope. They are generally placed in the the lower part of the eyepiece by unscrewing the lower portion. Then the micrometer is inserted in the lip or by a retaining ring. Varies by type and manufacturer.

Custom sizes are available to accommodate varying size eyepieces. Eyepieces can be purchased with micrometers. The micrometer/reticle above is marked with a cross fine scale from 0 to 100 and other measurements and type of micrometers are available.

Stage / Slide Micrometer

The stage or slide micrometer is used to calibrate the eyepiece micrometer. A stage micrometer consists of a microscope slide with a fine engraved precise scale. Certified stage micrometers are available at significantly higher prices.


Eyepiece Stage Micrometer

The stage micrometer is placed on the platform/stage of the microscope. Then view through the eyepiece and focus and align with the scale of the eyepiece reticle. The left side should be line up at the 0 with the stage micrometer so they are overlapping best possible.. In the picture above the eyepiece reticle is 10mm long divided into 100 divisions with major markings every 1 mm. (10, 20, 30, 30, etc.). The stage micrometer is 1mm long divided into 100 units, each unit is .01mm...

Then a reading is taken from the scales. These readings are then used to calculate the calibration factor for each objective. Objectives with the same magnification can vary by a few percent from the same manufacturer.

When you look through the eyepiece the markings remain constant but the size of the specimen magnified will increase along with the magnification. Thus, switching to a higher power objective lens, the represented value between marks will change proportionately. For example, if each mark represents 0.1mm with a 1X objective lens, then with a 4X objective lens, each mark will roughly represent 1/4 of 0.1mm or .025mm, which equals 25 micrometers (there are 1000 um in one millimeter).

As an example, for a reticle that is 10mm long with 100 divisions (as shown above), each division was found to represent the following distances:

  • 1x objective (true reading) 100um (0.1mm)
  • 4x objective, 25um
  • 10x objective, 10um
  • 40x objective, 2.5um
  • 100x objective, 1um

The correct way to calibrate a reticle with a stage micrometer is to line up the left edges of each. For instance if 22 lines from left to right on the reticle line up with 55 micrometers (actual measurement) on the stage micrometer. Then 22 markings is to 55um as one line is to X micrometers. The calculation and "X" can be solved by dividing 55 by 22. Each line on the reticle then equals 2.5um. Recording keeping is important so you do not have to recalculate again.

Additional information:

Using a unit converter is quite helpful.

For additional information view the following video: