MICROMETRY (A Type of microscope )

 MICROMETRY

Micrometry is the measurement of microscopic objects using a calibrated eyepiece micrometer. The unit of measurement is the micron (u orum). The eyepiece micrometer scale requires calibration from a measured scale engraved on a micrometer glass slide (graticule).

The eyepiece micrometer is calibrated for each microscope objective at which the measurement is required. For example, for parasitology, the scale should be calibrated to 40X objective while for haematology, it should be 100X objective.

Method of Calibrating an Eyepiece Micrometer

Example

Distance measured

= 0.4 mm Number of divisions

= 25 1 division measures 0.4/25 = 0.016 mm To convert mm toum 0.016 x 1000

= 16 microns. To measure an object

1. Focus the eyepiece micrometer scale and the object sharply.

2. Position the object along the scale and count the divisions covered.

3. Refer to the previously prepared table for the objective in use to obtain the measurement for the number of divisions counted. This will give the size of the objectin microns.

For example, an object covering 2 divisions on the scale will measure 16 x 2 = 32 microns. Table shows the relationship between units used for the measurement of microorganisms.

Relationship between units used for the measurement of microorganism,
Unit of lenghth	Millimeter(mm)	Micrometer( UM)	Nanometer
Milimeter (mm)	1	1000(103)	1000.000 (106)
Micrometer(um)	0.0091 (103)	1	1000 (103)
Nanometer (nm)	0.000001 (106)	0.001(103)	1
Angstrom	0.0000001 (107)	0.0001(10-4)	0.1(10-1)

Method of Calibrating an Eyepiece Micrometer

1. Unscrew the upper lens of a 10X huygenian eyepiece and insert the eyepiece micrometer disc, with engraved side facing down.

2. Place the stage graticule slide on the stage; rotate the required objective into position and bring the scale into focus. Both the eyepiece scale and the calibration scale should be focused clearly.

3. Adjust the field until O line of the eyepiece scale aligns exactly with the O line of the calibration scale.

4. Look along the scales and note where a division of the eyepiece scale exactly aligns with a division of the calibration scale.

5. Measure the distance between the 0 point and where the alignment occurs on the graticule. The measurement of the calibration scale are 0.1 mm to 2.0 mm.Each small subdivision measures 0.01 mm.

6. Count the number of divisions of the eyepiece scale between the 0 point and where the alignment occurs.

7. Calculate the measurement for 1 division of the eyepiece scale in microns (um). Figure shows calibration of eyepiece micrometer.