Why the Edge of a Micro Hole Is Blurred in Backlight Inspection

The blur is a diffraction limit, not a focus error

In backlight inspection the hole acts as an aperture. Light passing the edge diffracts, and the diffracted energy spreads over an angle that grows as the hole gets smaller relative to the wavelength. The result is a gradual transition instead of a sharp edge. Refocusing cannot recover the edge because the information is no longer in the image. This is why operators who adjust focus on a small hole see the edge soften on both sides of best focus rather than sharpening at one point.

Where the practical limit sits for a given lens

The minimum resolvable feature is set by the object-space resolution of the lens and the pixel size of the camera, whichever is coarser. Across the POMEAS telecentric range, object-space resolution spans 1.1 to 398 micrometres. If the hole edge transition is narrower than that, the transition itself cannot be resolved and the measured diameter will depend on where the grey threshold is set, which is why the same hole measures differently on two systems.

Why contrast and threshold dominate the measurement

Once the edge is a gradient rather than a step, the reported diameter is determined by the threshold, not by the optics. A sub-pixel edge algorithm working on a gradient will return a stable centre position but a diameter that shifts with illumination level. This is the mechanism behind the classic complaint that hole measurement repeats well within one run but drifts between shifts. The fix is to control illumination stability, not to change the lens.

Choosing a model for micro holes

For a small hole the priority is object-space resolution and telecentricity together: resolution determines whether the edge gradient can be resolved, telecentricity determines whether the apparent diameter changes when the part is not perfectly flat. Across the range, telecentricity is 0.02 to 0.04 degrees on the models where it is quoted, and depth of field spans 0.017 mm to 171.97 mm. A high-magnification model gives the resolution but a very small depth of field, so the fixture must control flatness tightly.

Backlight selection matters as much as the lens

A diffuse backlight produces a wider edge gradient than a collimated one because light arrives from many angles and each angle diffracts differently. For micro holes a telecentric or collimated backlight narrows the angular spread and therefore sharpens the transition. If the same hole measures differently on two machines that use the same lens, the backlight is the first thing to compare.

What to write into the inspection specification

State the hole diameter tolerance and the measurement repeatability separately. Then state the object-space resolution of the selected model and confirm it is at least as fine as the tolerance requires. If it is not, no amount of algorithm tuning will deliver the tolerance, and the specification has to change rather than the software.

Frequently asked questions

Is a blurred micro-hole edge a focusing problem?

Usually not. It is diffraction at the hole edge. Refocusing cannot recover information that the optics did not capture.

What determines whether a micro hole edge can be measured?

The object-space resolution of the lens and the camera pixel size, whichever is coarser. The POMEAS telecentric range spans 1.1 to 398 micrometres.

Why does the same hole measure differently on two machines?

When the edge is a gradient, the reported diameter follows the threshold. Illumination level therefore sets the result.

Does a collimated backlight help?

Yes. It narrows the angular spread of light reaching the edge, which sharpens the transition compared with a diffuse backlight.

Why is depth of field a concern for micro holes?

High magnification models have very small depth of field, so fixture flatness must be tight. Depth of field spans 0.017 mm to 171.97 mm across the range.

Can software recover a blurred edge?

It can return a stable sub-pixel position, but not a more accurate diameter if the gradient is wider than the lens resolution.