Telecentric lens filters: where to place them and what they change

A telecentric lens is not a normal lens with a flat field: it is built around an aperture stop placed so that the principal rays leave the front (and, in a bi-telecentric design, enter the rear) parallel to the optical axis. That architecture is what makes measurements immune to part distance, and it is also what makes filtration awkward. The stop sits inside a sealed, factory-aligned barrel where you cannot reach it, so the question is never "which filter at the stop" but "which of the accessible positions costs the least". This guide walks through each placement, the numbers that tell you how much it hurts, and the setups that avoid the problem entirely.

Why the aperture stop is off limits

A simplified telecentric layout is two lens groups with the aperture stop at their common focal plane. The front group images the object through the stop, which is why chief rays are parallel in object space; the rear group relays that image to the sensor. Move, tilt or space the groups and the parallelism degrades by degrees that show up directly as magnification error across the field.

For a gauging application, a 0.1% magnification error on a 20 mm part is 20 µm — often your whole tolerance. This is why manufacturers ship telecentric lenses as one aligned unit and why no reputable product line offers a split front/rear system you can rebuild. If you need to experiment with optics at the stop plane, you are building a custom system, not modifying a measuring lens, and the result will not hold calibration. For measurement work, stay with sealed units and filter where the design tolerates it. If you are still deciding whether you need telecentricity at all, start with when you need a telecentric lens for gauging.

The three places a filter can actually go

On a practical machine-vision rig with a sealed telecentric lens, a filter has three possible homes:

1. Rear C-mount thread, between lens and camera. Most C-mount telecentric lenses have, or accept, a filter thread at the rear. This is the standard position for IR-cut and band-pass filters.
2. In front of the lens, over the barrel. Possible, but the front surface of a telecentric lens accepts a cone of parallel rays over a large diameter, so a small threaded filter vignettes the field badly.
3. Inside the illumination path. For lenses with a built-in coaxial light, the illumination branch can carry the wavelength selection for you, keeping the imaging path filter-free.

There is no fourth home. Anything that requires opening the barrel is out, for the reasons above. In practice the rear thread wins almost every time, and the rest of this article is about doing that without losing focus, magnification or your calibration.

What a rear filter does to focus and magnification

A plane-parallel glass plate inserted into a converging beam shifts the focal plane away from the lens. The shift is:

where t is the glass thickness and n the refractive index. For standard filter glass n ≈ 1.5, so:

A 3 mm thick filter therefore moves the focal plane outward by about 1 mm. The same plate also reduces magnification slightly, because the effective cone geometry changes; the magnification change is typically well under 1% for filters of a few millimetres, but "well under 1%" is not zero when you are measuring.

Worked example. Suppose you add a rear band-pass filter to a 1x telecentric lens on a 2/3" sensor. The filter is 3 mm thick, so focus shifts by about 1 mm. If you simply refocus by turning the focus barrel, you change the lens-to-sensor distance by 1 mm, and at 1x magnification that refocus itself perturbs magnification by roughly 1 mm / f, which for a short focal length is noticeable. The clean fix is not to refocus at all: compensate the 1 mm with a C-mount spacer so the sensor stays where it was, and verify magnification on a calibration target afterwards.

Rear filtering in practice

The rear position is the right one for the filters machine-vision systems actually use:

• IR-cut filters when a broadband light or ambient daylight leaks long wavelengths into a mono sensor.
• Band-pass filters matched to the illumination wavelength, to reject ambient light and raise contrast on reflective parts.
• Neutral-density filters to tame an over-bright strobe when the controller is already at minimum pulse width.

A matched pair of a narrow-band light and a band-pass filter at the rear of the lens is the standard way to get contrast on the shop floor. The LS-LXR-0225, for example, is an 850 nm band-pass filter in an M30.5 x 0.5 thread — check that the thread matches your lens or plan an adapter.

Rear filtering has one honest cost: it eats back-focus. Every millimetre of filter glass and every adapter adds spacing in the optical path, and telecentric lenses have fixed, often tight, working distances and flange distances. Before ordering, lay out the stack — lens, filter, adapters, camera mount — and confirm the total still fits the camera's flange distance. Extension tubes and adapter rings are listed in the lens accessories category; the accessories finder lets you filter by type and thread.

Why a front filter usually fails on telecentric lenses

On a normal lens a front filter works because the front element accepts rays over a modest aperture. The front of an object-side telecentric lens must accept a bundle of parallel rays as wide as the largest feature you measure — that is the whole point. Put a 40 mm threaded filter in front of a lens that needs a 60 mm entrance bundle and you vignette the corners of the field; the measurement quietly becomes position-dependent, which is the exact failure telecentricity exists to prevent.

Front filtration only makes sense when the filter is at least as large as the lens's entrance pupil and held parallel to it in a fixed mount. That is a fixture, not a threaded filter, and for most gauging rigs the rear position is simpler and cheaper.

Let the illumination path do the filtering

There is a way to get wavelength selectivity without touching the imaging path at all: use a telecentric lens with a built-in coaxial light. The illumination branch is collimated and shares the optical axis, so you pick the wavelength at the light — a red or IR LED, or a coaxial light source with its own band selection — and keep the rear thread free for nothing more than a protective window.

The LS-TCL-2194 is a 0.5x telecentric lens for a 1" sensor with built-in coaxial light and a 110 mm working distance, which leaves real room for fixtures under the lens. For flat, polished parts — gaskets, shims, stamped contacts — the coaxial route usually beats any filter on image quality, because the contrast comes from geometry (light returned along the axis versus light reflected away) rather than from wavelength rejection alone. The collimated backlight or diffuse backlight for edge gauging guide covers the related choice on the backlight side.

Checks before you order a filter

Work through this list and you will avoid the two most common outcomes: a filter that vignettes, and a stack that no longer reaches focus.

• Thread match. Filter thread size and the lens's rear thread must agree, or you need a documented adapter in the stack.
• Glass thickness. Get the filter's optical thickness and compute the focus shift, Δ ≈ t/3, before the filter is in the building.
• Spacing budget. Sum lens overhang, filter, adapters and camera flange distance. If the total exceeds the lens's design range, order spacers to rebalance rather than refocusing.
• Clear aperture. The filter's clear aperture must exceed the lens's rear exit cone diameter, not just the sensor diagonal, or you clip corners.
• Passband versus light. Band-pass centre and width must match the illuminant, not just the nominal colour.
• Calibration plan. Budget one re-calibration per filter change; a calibration target pays for itself the first time.

When filtering is the wrong answer

If you are adding a filter to fight glare on a shiny part, stop and check the geometry first. Glare on flat specular parts is usually an illumination problem that a coaxial light or a diffuse dome solves better than any filter, because filters reject wavelength, not direction. Similarly, if ambient light is your enemy, better shielding, a shorter exposure and a strobed narrow-band light often do more than a band-pass filter at a fraction of the alignment effort. And if the real question was whether a telecentric lens belongs in the build at all, the machine vision lens selection for reliable inspection guide puts telecentric and conventional options side by side. The broader lens selection finder and the lens configurator both let you filter telecentric models by sensor format, mount and coaxial-light option before you commit to accessories.

A short summary of the decision:

1. Never open the lens; the stop is inside for a reason.
2. Filter at the rear C-mount thread, sized to the exit cone.
3. Compensate thickness with spacers, not refocus, and re-calibrate.
4. For specular parts, prefer built-in coaxial light over filtration.
5. Verify magnification on a target after every change in the optical stack.

If a stack-up does not close, ask us through the contact page — the application support that comes with an order is the fastest way to check thread and spacing combinations against a specific lens series.