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Microscope Eyepiece Reticle Basics

A guide to using and selecting the correct microscope eyepiece reticles covering the reticle's diameter, stage size, eyepiece and objective magnification.

The terms ocular and eyepiece mean the same thing.

The terms reticle, reticule and graticule mean the same thing

It is not normal to have a calibration certificate for an eyepiece reticle. The reason for this is that the reticle is installed in the eyepiece and all of the optics on the microscope can effect the size measured by the reticle. It is normal practice to use a stage micrometer to calibrate the complete microscope.

Size conversions and equivalents.

1mm = 1000µm1inch = 1000thou1mm = 39.4thou
0.1mm = 100µm0.1inch = 100thou1thou = 0.0254mm (25.4µm)
0.01mm = 10µm0.01inch = 10thou
0.001mm = 1µm

It is far better to increase the objective magnification than use a reticle with finer details. A recent example had a customer that wanted to have a reticle with a horizontal scale to measure spores of 2-5µm in diameter. They said they normally used a 10x objective. We answered as follows;

'In this case, our NE1, which has a 10mm scale subdivided into 0.1mm divisions will measure 0-1mm when used with a 10x objective lens. Each division will represent 0.01mm (10um). This will not be good enough to measure spores in the 2um - 5um region. If you use a reticle with a smaller scale in the eyepiece it is very difficult to read so the answer is for the customer to use the 40x objective. Now the 10mm scale will measure 0-0.25mm and each division will represent 2.5um. If the customer is able to use the 100x objective then the measurements can be even more accurate as each division will represent 1um'.

The diameter of the reticle is important to ensure it fits the eyepiece you have.

When we select an eyepiece reticle we are only concerned with the objective magnification. The mathematics of microscope magnification is very simple - all you do is divide the size of the feature on the reticle by the objective magnification to get the size that it will actually measure at the stage.

Eg. a 10mm length scale when used with a 10x objective will measure 1mm at the stage. The same 10mm length reticle when used with a 40x objective will measure 0.25mm (250µm at the stage).

If you want to look at another way, if you have a 10µm at the stage and are using a 40x objective lens this will be magnified to 400µm (0.4mm) at the reticle image plan. The next section gives more sizing information.The eyepiece merely magnifies the image on the reticle.

Table Relating Stage Size and Objective Magnification with Size at Reticle Image Plane.

The two columns on the left hand side show the size of the specimen or feature at the stage. Along the top are typical objective magnifications. So, if you want to measure something that is about 50µm using a 40x objective lens this will actually be 2mm at the reticle image plane. A reticle with a 10mm scale in 0.1mm divisions (Our NE1) or one with a 5mm scale in 0.05mm divisions (Our NE5) may be suitable.

Measurement at Stage (m)Measurement at Stage (mm)1X1.25X2X5X10X20X40X100X
10 000m10mm10mm12.5mm20mm50mm100mm200mm400mm1000mm

Colour of reticle lines does not matter in a microscope eyepiece. All lines, whatever their colour, block the light from the specimen/stage so the pattern will always appear dark against a brighter background.

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