Telecentric Lens for PCB and FPC Inspection: What Perspective Error Actually Costs

A PCB is never flat. Solder paste, component bodies, connector shells and board warp all put features at different heights, and a conventional lens reports a feature's height difference as a size difference. On a board where the tolerance is a fraction of the component height, that error is larger than the tolerance. A telecentric lens is specified for this case because its magnification does not change with object distance.

The size of the error is calculable, and it is worth calculating before deciding whether telecentric optics are necessary.

Sizing the perspective error

For a conventional (entocentric) lens, magnification varies with object distance:

ΔM / M ≈ Δz / WD

where Δz is the height difference between two features and WD is the working distance. Converted into a measurement error on a feature of size L:

error ≈ L × Δz / WD

Worked case: WD = 110 mm, a component standing 1 mm above the board, measuring a 10 mm feature.

error ≈ 10 mm × 1 mm / 110 mm = 0.091 mm = 91 µm

Against a ±50 µm tolerance, the perspective term alone exceeds it. Increase the component height to 2 mm and the term doubles to about 182 µm.

The same expression rearranged gives the working distance needed to hold the perspective error under a chosen budget:

WD ≥ L × Δz / allowed error

For L = 10 mm, Δz = 1 mm and a 10 µm budget, WD ≥ 1,000 mm — which is why pushing the working distance is not a practical fix, and why telecentric optics are used instead.

What telecentricity removes

In a telecentric lens the aperture stop sits at the focal plane, so chief rays run parallel to the optical axis. Magnification becomes independent of object position within the depth of field. The Δz term above drops out of the error budget, leaving resolution, distortion and illumination as the remaining terms.

Three variants are specified differently:

- Object-side telecentric — chief rays parallel on the object side; removes the magnification-vs-height error. This is the configuration used for dimensional measurement of parts with height variation. - Image-side telecentric — chief rays parallel on the image side; keeps ray angles constant at the sensor, which matters for sensors with microlens arrays. - Bi-telecentric (双远心) — parallel on both sides; the most stable against both object height and sensor position, at higher cost and bulk.

Selecting on PCB and FPC: the four numbers

1. Magnification from field of view. The field has to cover the largest panel region inspected in one image:

magnification = sensor width / required field width

An 8.8 mm (2/3-inch) sensor covering a 44 mm board region needs 8.8 / 44 = 0.2×. POMEAS offers 0.2× in several variants — PMS-LTCM02-110 (16.78 µm resolution) and LTCM02-180-G / PMS-LTCM02-180S-G (1-inch, 180 mm working distance, 12 mm depth of field).

2. Resolution against the smallest feature. The lens resolution figure is the optical floor. POMEAS telecentric lenses are specified with measured resolution values:

For fine-pitch FPC work, the higher magnifications carry the finer resolution figures: 2.0× at 2.2 µm (PMS-LTCM2-65-S) and 8.0× at 1.1 µm (PMS-LGX8-95).

3. Working distance for the mechanics. Long working distances are needed when the lens must clear a handler, a conveyor edge, or lighting mounted above the board. POMEAS long-WD telecentric options include PMS-LTCM0061-398 (398 mm), PMS-LTCM0088-398-G (398 mm), PMS-LTFM0106-468 (468 mm, F-mount bi-telecentric), PMS-LTFM017-416 (416 mm), LTCM02-180-G (180 mm), PMS-LMC03-185 (185 mm), and PMS-LTC08-90 (90.5 mm).

4. Depth of field against the height range present. Telecentricity removes magnification error, not defocus. Features still have to sit inside the depth of field. Where the height range is wide, a large-depth-of-field design is specified: PMS-LTCM02-180S-G is specified with 12 mm depth of field at 1-inch sensor format, and PMS-LMC04-150 with 3.03 mm.

Bi-telecentric and F-mount options

For large-area panels and line-scan integration, POMEAS specifies F-mount bi-telecentric lenses: PMS-LTFM0106-468 (0.106×, 468 mm), PMS-LTFM017-416 (0.17×, 416 mm), PMS-LTFM05-179 (0.5×, 179 mm), PMS-LTFM1-127 (1×, 127 mm), and the dual-magnification PMS-LTDC00880176-120 (0.088× / 0.176×, 120 mm).

Coaxial illumination variants exist for reflective surfaces: PMS-LTC2-110C (2.0×) and PMS-LTC1-110C (1×, 13.42 µm) carry coaxial illumination, which is the usual answer for specular pads and component leads.

Illumination is half the measurement

A telecentric lens used with a diffuse dome will not produce the edge definition its resolution figure implies. For轮廓 (profile) work on PCBs and small parts, a telecentric parallel backlight is the matching choice: POMEAS VP-GLTD Series covers 30–265 mm and VP-GLTDL Series 35–265 mm, both telecentric parallel LED backlights.

The pairing matters because the edge the algorithm finds is created by the illumination, not only by the lens. Telecentric optics plus a collimated backlight is the standard configuration for silhouette gauging of pins, leads, and connector geometry.

What telecentricity does not fix

Telecentricity removes the magnification-versus-height term. It does not remove defocus outside the depth of field, it does not fix a part that is tilted rather than translated, and it does not compensate for poor edge contrast. A telecentric lens on a board with 3 mm of warp still needs enough depth of field to keep the whole height range in focus, or a height sensor to follow the surface.

Measurement accuracy remains distinct from optical resolution. Resolution is a property of the lens; accuracy additionally depends on pixel sampling, contrast, illumination stability, datum definition and the algorithm.

FAQ

How much error does perspective cause on a PCB?

Approximately L × Δz / WD. At 110 mm working distance, a 1 mm tall component measuring a 10 mm feature introduces about 91 µm of apparent size error.

Is a telecentric lens always necessary?

Not when the part is flat and all features sit at the same height. It is necessary when features sit at different heights and the tolerance is smaller than the perspective term.

What is the difference between telecentric and bi-telecentric?

Object-side telecentricity fixes magnification versus object height; bi-telecentricity additionally stabilises ray angles at the sensor. Bi-telecentric costs more and is physically larger.

Does telecentricity increase depth of field?

No. Depth of field is a separate specification. A telecentric lens keeps magnification constant within its depth of field, it does not extend it.

Which magnification for fine-pitch FPC?

Higher magnification carries finer resolution: 2.0× at 2.2 µm and 8.0× at 1.1 µm in the POMEAS line. Select the magnification from the field of view first, then check the resolution figure.

Why does my telecentric measurement still drift?

Common causes outside the lens: defocus from warp beyond the depth of field, tilted parts, unstable backlight intensity, or a datum defined on a feature that itself varies.

Next Step

Contact POMEAS with your requirements.