Place the reference at the specimen plane
The stage scale replaces the specimen during calibration. Keep the objective, adapter, camera mode and optical path the same as for measurement. Focus the scale at the working plane and measure its image using the same software coordinate system. An eyepiece reticle remains inside the viewing optics; the stage micrometer supplies the physical reference for its divisions.
Calculate length per image unit
Divide the known reference span by its measured pixel span to obtain length per pixel. For an eyepiece, divide the reference span by the number of eyepiece divisions. Use a measured reference interval when supplied and identify whether coordinates are based on mark centres or specified edges.
| Reference span | Image reading | Conversion |
|---|---|---|
| 0.5 mm | 1000 pixels | 0.5 µm/pixel |
| 100 µm | 40 eyepiece divisions | 2.5 µm/division |
Choose a span that reduces endpoint sensitivity
A longer observable span reduces the relative effect of locating each endpoint, provided it stays within the usable field and suitable reference region. Repeat the reading with consistent line-centre or edge conventions. For an XY scale, establish X and Y conversions separately. To evaluate changing scale across the field, measure several locations with a suitable grid or translated reference.
Separate nominal geometry from measured values
A drawing defines nominal spacing; an inspection record reports measured intervals under stated conditions. Measurement uncertainty describes the uncertainty associated with those reported values. Traceability requires a documented calibration chain with uncertainties. A report title or serial number alone does not establish that chain. Identify the intervals relevant to the application, measurement conditions and the records required with the part.
| Entry | Meaning |
|---|---|
| Nominal interval | Design value for the identified marks. |
| Measured interval and error | Reported length and its difference from the nominal value for the same endpoints. |
| Uncertainty | Value, coverage convention and method associated with the measurement result. |
| Conditions and reference | Measurement setup, temperature where relevant, reference and report identification. |
Verify the conversion after configuration changes
Changing the objective, zoom, camera adapter, binning or saved-image resizing can change the length represented by one pixel or division. Cropping alone can retain the scale if sampling is unchanged, but a saved camera mode may combine cropping with resampling. Verify the actual output. Lens distortion and non-telecentric distance changes can make scale position-dependent.
Keep a usable calibration record
Record the reference and interval, optical configuration, saved image dimensions, endpoint convention, field location, repeated readings and resulting scale. Recheck after relevant configuration changes, damage, contamination or a failed comparison with the retained baseline. Set the routine review interval from use and stability rather than assuming one period suits every instrument.