Micrometers and Reticles for the Microscope

In microscopy, micrometers (also refered to as reticles) are used to measure and or count specimens or portions of specimens. A straight line reticle is used to measure the length of a specimen under observation with microscope. There are two types of micrometers for straight line measurements. The most common type, and preferred type is an eyepiece reticle. The less common type is a stage micrometer. To overstate the obvious, the eyepiece reticle is installed in the eyepiece, where as the stage micrometer lays on the stage and is manipulated by hand.

The eyepiece reticle is made from glass and has minute etchings used for measurements. The most common straightline reticle is 10mm in 100 divisions ( No. 0 thru 10 ) which is common in clincal laboratories.

Eyepiece micrometers can also be used to count portions of specimens within specimens. Take for instance the Howard Counting Reticle used in food processing (typically tomato paste and fruit juices). The pattern on the Howard Counting Reticle is a 13.82 diameter circle encompassing an evenly distributed network of squares in a grid pattern. As the specimen is viewed under the 10x objective, a count of mold fibers takes place for each square in the grid. If the number exceeds recommended amounts as set by the USDA, a secondary check using the 20x objective is used to confirm the existence of mold in the process.

Other counting reticles and micrometers such as the Miller Disc or Walton Beckett micrometer are available from most microscope dealers.

* How to determine the size of the eyepiece micrometer you need.

1)If you are sure that the eyepieces that you have on your microscope are the original ones, you can look at your owner's manual which will give you the size of the micrometer/reticle shelf. This number is typically between 18mm and 29mm, but not always.

2)If you do not have the instruction booklet or the original eyepieces have been changed out, you will need to take an accurate measurement.

3)High quality micrometers are custom cut, so your measurement should be exact. In order to measure, you need to remove one of the eyepieces and flip it upside down. (taking special care not to get the eyepiece lens dirty). I use a caliper to measure the inside diameter of the eyepiece. This is where the micrometer enters the eyepiece. You also need to determine if the bottom of the eyepiece srews off at this point. If the bottom of the eyepiece screws off, it usually means that they reticle will sit on a reticle shelf and be held in place by the plastic part of the eyepiece when it is screwed back in.


If the bottom of the eyepiece does not screw off, it will be necessary to order a retaining clip with your micrometer in order to lock it into place. Clips are relatively cheap costing less than $3.00 in most cases.

So, for a microscope like a Nikon Eclipse 200, the reticle/micrometer size is 27mm which is the inside diameter of the bottom of the invoice. Do not be confused by the field number printed on the side of the eyepiece. This is the field number, not the measurement of the inside diameter of the eyepiece. You will NOT find the micrometer size printed on the eyepiece

There is a company that allows you to send your existing eyepiece to them for sizing and installation of micrometers. This is nice becuase they make sure the reticle is clean, right side up, and seated properly when installed. Installing a micrometer sounds simple, but it can be a really excercise in patience, when trying to keep it clean and get the numbers facing right side up.

Reticles