Measuring with a Microscope

Microscope Measuring

When we measure everyday items most of us will grab a ruler in order to make a measurement. When using a microscope, generally magnification is higher and a ruler will not work unless you are using a very low power stereo microscope. When using a compound high power microscope an eyepiece reticle is used to make measurements. Most measurements that are made with a compound microscope are between 0.2um to 25mm. It is hard to make measurements above 25mm becasue most eyepieces on a microscope do not have a field of view greater than 25mm.

When you look through the eyepiece of a microscope that has a reticle installed in it the ruler (or cross-line, grid, whatever is on the reticle) is super-imposed upon your image. This makes it easy to make quick and accurate measurements, as long as you have calibrated your microscope.

Measuring Definitions

Stage Micrometer -a microscope slide (generally 1" x 3") that has a ruler etched on it. It is either made of glass (for transmitted light) or metal (for reflected light). The stage micrometer is used to calibrate an eyepiece reticle when making measurements with a microscope.

Eyepiece Reticle (or reticule) -a small piece of glass with a ruler etched into it that fits into a microscope eyepiece. This ruler is used to make measurements of objects viewed through the microscope. The image from the eyepiece reticle is imposed upon the image when looking through the microscope. There are many variations of eyepiece reticles including lines, cross-lines, grids, counting reticles, etc. Before purchasing an eyepiece reticle you would want to make sure your microscope eyepiece accepts a reticle, and you would want to determine the diameter of the reticle that fits in your eyepiece.

Field of View - the diameter of the circle of view that you see when looking through the microscope. When calibrating an eyepiece reticle it is best to use a stage micrometer where most if not all of the ruler on the stage micrometer fits into the field of view. The higher your magnification, the smaller your field of view will be.

Detent - when turning the zoom knob on a stereo zoom microscope, detent is a click stop at each of the objective values. Detent is helpful if you need to make measurements. The detent allows the zoom knob to click into set objective values so you know that you are on a precise objective value. Trying to make measurements with a stereo zoom microscope that does not have detent will result in inaccurate measurements.

Why it is important to calibrate your microscope!

Even though you may be using two identical microscopes - let's say each with 10x eyepieces and a 40x objective, they can stil have slightly different magnification factors. Therefore, it is important to calibrate your eyepiece reticle with a stage micrometer before making measurements with your eyepiece reticle. This ensures that you will be making accurate measurements with your microscope.

How to: Microscope Eyepiece Reticle Calibration

With the eyepiece reticle installed and a stage micrometer under the microscope, focus so that both scales are cleary in focus and lined up at the "0" point. You will want to calibrate for each objective that you plan to use to make measurements.stage micrometer image

Notice on the scale that the stage micrometer and the eyepiece micrometer (reticle) line up almost exactly at 30 on the eyepiece micrometer, but seem to line up more closely at 60. The larger the number on the eyepiece micrometer, the more accurate your calibration will be.

The stage micrometer is 1mm long with 100 divisions, which means that each longer division on the stage micrometer = 0.01mm or 10um. If you count the lines on the stage micrometer where the eyepiece micrometer matched up with it at 60, it equals 400um. So we take 400um / 60 eyepiece reticle reading and we can determine that with that specific objective each division on the eyepiece reticle = 6.67um.