Protective filter window for a vision lens: what it changes

A scratched front element writes off an expensive lens; a scratched window unscrews and gets replaced. That trade is why flat protective windows have become standard fittings on vision lenses that live near washdown, dust, swarf or operator traffic. The window is not optically invisible, though: a flat plate shifts best focus by a third of its thickness, and an uncoated one gives up light at every surface. Below you get the shift formula, the transmission arithmetic, the substrate rules per wavelength band, and the five-minute refocus check that proves the fit before the line runs.

What a protective window is, and when it earns its place

A protective window is a flat, parallel-faced plate in front of the lens's front element, either threaded into the filter ring or mounted as a panel in the housing wall. Substrates are optical glass or acrylic, normally with an anti-reflection coating for transmission and a hard anti-abrasion coat for wiping. Filters, adapters and plates sit together in the lens accessories category.

Fit one when the alternative is damage: splash and steam from washdown, conductive dust, fibres in the air, or a telecentric or long-working-distance lens whose front group is effectively irreplaceable. If a spectral filter already sits in the front thread, it is doing the protective job; an 850 nm band-pass filter with an M30.5 × 0.5 thread shields the element while it narrows the band to the illumination. The spectral side of that choice is covered in machine vision optical filters; this page handles the plain protective case.

The focus shift: a third of the thickness

A plane-parallel plate adds optical path wherever the beam converges, and the paraxial result is one short formula:

Optical glass at n = 1.52 gives a multiplier of 0.34, acrylic at n = 1.49 gives 0.33, germanium at n = 4.0 gives 0.75. For glass and acrylic, remember it as a third of the thickness: the scene effectively sits a third of a window-thickness closer, so a 3 mm glass window shifts best focus by 1.0 mm.

The shift is a pure axial offset. It changes no magnification, adds no measurable on-axis aberration, and disappears into the depth of field at long working distances. Refocus once after fitting and the image is optically what it was without the plate.

Worked example: a 3 mm window at 200 mm working distance

Camera: a 2/3"-type sensor with 3.45 µm pixels, 2448 × 2048, so 2448 × 3.45 µm = 8.4 mm wide. Lens: 35 mm. Working distance: 200 mm. Field of view = 200 × 8.4 / 35 = 48 mm, so magnification m = 8.4 / 48 = 0.175. Thin-lens depth of field:

At F2.8: 2 × 2.8 × 0.00345 mm × (1.175 / 0.031) = 0.0193 mm × 38 ≈ 0.7 mm. At F8: 2 × 8 × 0.00345 mm × 38 ≈ 2.1 mm.

The 3 mm window shifts focus by 1.0 mm. At F2.8 that exceeds the whole 0.7 mm depth of field, so the image is visibly soft until the barrel turns. At F8 the shift hides inside the 2.1 mm margin but consumes half of it. Either way the fix is one refocus at the working distance, and the full margin comes back.

The example lens class is a 35 mm, 2/3", 6 MP, C-mount machine vision lens:

Transmission: what the AR coating buys you

Each air-glass surface reflects Fresnel's share, R = ((n − 1)/(n + 1))². At n = 1.5 that is (0.5 / 2.5)² = 4% per surface, so two uncoated surfaces pass (1 − 0.04)² ≈ 92% and you lose about 8% of the light. Acrylic at n = 1.49 reflects 3.9% per surface, effectively the same. A single-layer magnesium-fluoride coat cuts that to roughly 1% per surface; a broadband multilayer coat runs below 0.5%, putting window transmission above 99%.

Order both faces AR-coated. One uncoated face hands back the 4% you engineered away, and the reflected light second-bounces between window and lens as veiling glare, one of the causes traced in machine vision stray light.

Choosing the substrate: glass, acrylic or the filter you already have

Glass is the default for wipe-down lines; 1-3 mm covers most close-range work. Acrylic wins when you need a cut-to-shape housing panel, accepts laser cutting cleanly and weighs less, but give it a hard coat and keep solvents off it. Fused silica is the only flat choice when the band starts below 400 nm. Germanium passes a thermal image and nothing visible, and at n = 4.0 its focus shift is 75% of thickness — a 2 mm germanium plate moves focus 1.5 mm, so refocus is mandatory, not optional.

Two bands where the usual window is wrong

UV. Below 400 nm, assume a standard AR coating blocks more than it passes: coatings tuned for the visible band are designed to cancel reflection around 400-700 nm and cannot be trusted outside it, and plain soda-lime glass absorbs heavily below roughly 370 nm. For a UV lens such as the LS-SWL-0291, a 25 mm, 240-1000 nm, 2/3", F2.0, C-mount design, fit fused silica or run windowless behind a fixed guard. UV illumination is invisible and hard on eyes and skin, so keep the station enclosed or interlocked. What changes outside the visible band generally is covered in SWIR lens and UV lens basics.

LWIR. Oxide glass transmits essentially nothing beyond about 2.5 µm, so a glass or acrylic window on a thermal lens is a black cap.

For a thermal lens such as the LS-SWL-0006, a 17 mm, F1.2 LWIR thermal lens for a 1/1.2" sensor, protection comes from the housing front or from a germanium window. Budget the germanium shift at three quarters of the thickness and refocus after fitting.

Two band-critical lenses where window choice decides the image:

Tilt, wedge and mounting

Keep the window square to the optical axis. A tilted plate in a converging beam adds astigmatism that grows with aperture and tilt angle, which makes it a close-range problem: hold tilt under about a degree at magnifications of 0.1-0.3; at long working distances a few degrees is harmless.

Wedge, the angle between the two faces, behaves like a thin prism and bends the image by (n − 1) × wedge. Five arcminutes of wedge on glass deviates the beam by 2.6 arcminutes, a constant 0.15 mm image shift at 200 mm working distance — an offset to calibrate out, not a distortion. Order custom panels with faces parallel to a few arcminutes and mount the window rigidly so vibration cannot re-aim it. On telecentric assemblies, the placement logic is the same as for telecentric lens filters, and magnification stays fixed regardless.

Fit, refocus and verify

1. Measure the thickness t and note the substrate; compute Δ = t × (n − 1) / n before touching anything.
2. Confirm the lens focus travel covers Δ at your working distance. A lens already parked at its close-focus limit needs a fix first, as diagnosed in lens focuses only at close distance.
3. Fit the window square to the axis, coated faces as specified, tightened evenly so the plate is never bent.
4. Refocus at the working distance on a live part.
5. Check the corners. Soft corners mean tilt or a wedged mount, not the window's flatness.
6. Confirm the focused band on a ruler target and lock the focus and aperture rings; the sequence in lens locking screws covers locking order and clearance.
7. Clean the window exactly as you would a front element — the method in how to clean a machine vision camera window safely applies unchanged — and replace it when scratches accumulate. It is the sacrificial part by design.

For step 2, the LS-LXR-0087 focusing ring adds 24-45 mm of adjustable length when the barrel has none left:

For step 6, the LS-LXR-0029 target pairs 0.5 mm dots with a 76 × 76 mm grid and reads the focused band directly:

The decisions, and what to order

The one-line decisions:

• Visible or NIR lens in a harsh environment: 2-3 mm AR-coated glass, square to the axis, one refocus after fitting.
• Cut-to-shape housing panel: hard-coated acrylic, solvent-free cleaning routine, shorter scratch life accepted.
• UV lens below 400 nm: fused-silica window or none at all; never a standard coated plate.
• Thermal LWIR lens: germanium or zinc-selenide only, with 0.75 × t focus shift budgeted; otherwise housing protection.
• Filter thread already in use: the working filter is the window; keep a spare.

Thickness above 3 mm needs a specific reason, because the shift scales with it and cleaning scratches show sooner on heavily used thick plates. Pick fittings from the accessories shop finder and the configurator; the machine vision lenses page filters by mount and band when the window decision feeds back into the lens choice. More focal-length and mounting material sits on the lenses topic page, and you can contact the applications team with your working distance and band to size the whole stack together.