360 degree inspection lens: inner wall, outer wall and bore imaging

Caps, vials, preforms, pins, valve bodies and threaded bores share a problem: the area to inspect wraps around the part. A normal lens looking down sees only the top; a camera from the side sees less than half the wall. The usual fixes are three or four cameras, or a part that rotates in front of one camera. A 360 degree inspection lens does it with one camera and one image, with no rotation. This explainer shows how the different types work, what the ring image means for resolution, and where these lenses stop being the right tool.

Three ways to see all the way round

Outer-wall lenses

A standard lens has its entrance pupil inside the lens: rays spread out from the lens towards the part, so the camera only sees surfaces facing it. An outer-wall (pericentric or hypercentric) lens moves the point where the rays converge out in front of the lens, beyond the part. The rays now arrive from outside the part's outline and reach its side walls. The camera sees the top face in the middle of the image and the full side wall around it as a ring.

These lenses suit small cylinders: caps, closures, bottle necks, pins, screws and preforms. For example:

Its data sheet gives a minimum object diameter of 4.5 mm and lists drug containers, plastic caps, preforms and threaded parts as typical uses.

Inner-wall and bore lenses

An inner-wall lens looks into a bore, tube or cup and images the inside wall as a ring around the bottom. Some designs carry their own LEDs, because lighting the inside of a bore from outside is difficult:

Others leave the illumination to you, which allows coloured, polarized or infrared light:

Typical tasks are burrs and chips inside drilled holes, thread presence in tapped holes, coating gaps inside tubes and contamination inside containers.

Mirror (multi-angle) lenses

A third design surrounds the part with mirrors. Each mirror shows one side view, and the camera sees the top of the part in the centre with several side views around it. Because each side view is a normal, undistorted image of one segment of the wall, measurements and text are easier to read than in a ring:

It uses 8 mirrors for parts under 15 mm in diameter.

What the ring image means for resolution

Outer-wall and inner-wall lenses map the wall onto a ring between an inner radius r1 and an outer radius r2 on the sensor. Software unwraps the ring into a flat strip with a polar-to-rectangular transform. The circumference of the ring is not the same at both edges:

A worked example

A 30 mm diameter cap is imaged with a 5 MP camera of 2,448 × 2,048 pixels. The ring runs from r1 = 400 pixels to r2 = 1,000 pixels, and it covers 15 mm of wall height.

1. The part circumference is π × 30 = 94 mm.
2. At the outer edge of the ring: 2 × π × 1,000 = 6,283 pixels, so 94 / 6,283 = 0.015 mm per pixel.
3. At the inner edge: 2 × π × 400 = 2,513 pixels, so 0.037 mm per pixel.
4. Along the wall: 15 / 600 = 0.025 mm per pixel.

The same scratch therefore covers 2.5 times more pixels at the outer edge than at the inner edge. Work out which end of the wall lands on the outer edge for your lens and part, and if the critical feature (a thread start, a tamper band, a sealing lip) can go there, put it there. Set defect-size limits per zone rather than one value for the whole ring.

Depth of field and aperture

The wall of a cylinder sits at different distances from a 360 degree lens along its height and around its curve, so these lenses need depth of field. Most run at small apertures, F8 to F16, which costs light. Two consequences:

• Lighting must be bright and even. Unevenness shows as bright and dark sectors around the ring, which look like defects after unwrapping.
• Small pixels gain little. At F8 the diffraction spot is about 11 µm across at 550 nm; at F16 about 21 µm. On a 2/3 inch camera with 3.45 µm pixels, stopping down to F16 makes the image noticeably soft.

Size limits you cannot design around

Every 360 degree lens is designed for a range of part diameters, wall heights and working distances. Outside that range, part of the wall falls outside the ring or out of focus, and no software fixes it. Data sheets give these limits directly, and they are often tighter than the lens size suggests. This multi-angle lens is a good example:

Its data sheet allows a maximum object diameter of 60 mm at an object height of 5 mm, but only 30 mm at heights of 10 or 20 mm. Taller parts need a smaller diameter, because the lens has to see over the top edge and down the side at the same time.

Inner-wall lenses have the equivalent limit in bore depth: the deeper the bore relative to its diameter, the steeper the viewing angle on the far end of the wall, and the less detail survives. Check the working distance range on the data sheet against the depth of the zone you need to see, as with this inner-surface system:

When a 360 degree lens is the wrong choice

For outer walls of small round parts at production speed, though, one camera with a 360 degree lens is hard to beat: no rotation, no synchronisation between cameras and a single image to process. Thread inspection on fasteners, for example, often uses one, as described in screw thread inspection camera selection.

Lighting the ring evenly

• Outer wall: a diffuse ring or dome light around the part, sized so that it lights the whole circumference equally. The dome lights suit shiny caps and metal parts; low-angle ring lights suit embossed text and thread profiles.
• Inner wall: built-in illumination where the lens offers it; otherwise a small coaxial or ring light that fits the bore opening.
• Check uniformity on a plain, defect-free part before setting any thresholds: unwrap it and look for brightness bands.

Next step

Browse the 360 degree inspection lenses by sensor format and working distance, then build the camera, lens and light together in the configurator. More lens topics are on the lenses topic page.