Short answer: yes. A 940 nm proximity or parallax sensor sees through a few millimetres of clear acrylic or glass nearly as well as through air, because those panes pass about 90% of near-IR on each crossing. You pay about 10% of your detection range, and you manage one small reflection off the pane's inner face. Pick the pane from its transmission curve at your sensor's wavelength and kill crosstalk with a matte divider. Recalibrate with the pane fitted, and seal the pane into the enclosure's rating. The numbers below are for light at 850-940 nm.
What near-IR passes through
Your sensor almost certainly emits at 850 nm or 940 nm, the two wavelengths used by proximity detectors and parallax rangers. At those wavelengths most clear glazing is nearly invisible. Typical transmission for a 3 mm [1/8 in] pane at normal incidence, from the curves glazing makers publish:
| Pane, 3 mm [1/8 in] | At 850-940 nm | Pick it when |
|---|---|---|
| Clear acrylic (PMMA) | about 90% | you want scratch and UV resistance outdoors |
| Clear polycarbonate | about 85-90% | impact matters more than optics |
| Tempered glass | about 90% | washdown runs hot and the pane must stay clear |
| Tinted or mirrored polycarbonate | a few percent to about half | the maker's curve at your wavelength confirms it |
| Black IR-transmissive polycarbonate | blocks visible, passes much near-IR | the sensor must sit behind a dark lens |
Thickness barely matters for the clear grades, because nearly all the loss happens at the two surfaces. Going from 3 to 6 mm [1/4 in] clear acrylic costs only a percent or two more. The black IR-transmissive grade is what remote-control windows use, and a patent on light-tight infrared-transmissive polycarbonate describes the idea; the number to order against is that sheet's curve at your wavelength. For the mechanical trade-offs between the two clear plastics, polycarbonate versus acrylic for machine windows weighs impact against scratch resistance.
The optical case has a boundary, though. Sound and radio follow different rules: ultrasonic sensing through an enclosure wall couples the transducer to the metal, and a radio signal through a metal enclosure needs a plastic window or an external antenna.
The 4% you lose at every surface
Every air-to-plastic boundary reflects a fixed share of the light, given by ((n2 - n1) / (n2 + n1)) squared, with n the refractive index. Acrylic and polycarbonate both sit near n = 1.5, so ((1.5 - 1.0) / (1.5 + 1.0)) squared = 0.2 squared = 0.04. That is 4% per surface, about 8% for the two faces of a pane, plus 1-2% absorbed in the plastic. The 90% figure in the table is this arithmetic, nothing exotic: a clear pane is a dimmer, not a wall. The same 4% also bounces inward, and that half of the story is the one that bites.
Crosstalk, the reflection that bites
Your emitter throws light forward; the pane's inner face throws 4% of it sideways, straight into a detector mounted millimetres away. At close quarters that bounce can outshine the return from a target a metre out, so the sensor trips with nothing in front of it. Four fixes, used alone or together:
- Run a matte black divider between emitter and detector, from the board to the pane, touching it.
- Tilt the sensor 5-10 degrees so the specular bounce misses the detector.
- With a triangulation ranger, let the pane sit inside the dead zone: those read nothing closer than their minimum range, commonly 4-20 cm [1.6-8 in] depending on the model, so a pane a few millimetres away is invisible to them.
- With an amplitude proximity pair, which has no dead zone, the divider carries the whole job.
For standoffs, aiming and board layout, mounting an infrared sensor behind an enclosure window covers the mechanics.
Light budget for the sensor behind the pane
The inputs, each from the table above or a datasheet:
- Pane: the clear acrylic from the table, about 90% transmission per crossing at the sensor's wavelength.
- Sensor: a compact proximity ranger rated to 100 cm [39 in] on a white card in open air.
- Job: detect that same white card at 100 cm through the pane.
The rule in words: the beam crosses the pane twice, out and back. The return from a diffuse target falls with the square of distance, so usable range scales with the per-pass transmission.
The arithmetic:
- Two crossings: 0.90 x 0.90 = 0.81, so the detector receives 81% of its open-air signal and 19% is spent at the pane.
- Range: 100 cm x the square root of 0.81 = 100 cm x 0.90 = 90 cm [35 in].
The decision: hold your stand-off at 90 cm or less, or order the next range class up, typically 150 cm [59 in], and bank the margin. A parallax ranger keeps its distance reading true through the pane; the 81% costs sensitivity margin, not accuracy. Use range-scales-with-transmission as your planning rule: at 80% per pass, 0.80 x 0.80 = 0.64 and range falls to 80%, which is where threshold recalibration becomes part of commissioning.
Keeping the rating at the pane
A window is a hole until it is glazed, and the box keeps its type only with the pane sealed into the design. Two builds hold a hose-down rating:
- Bonded glazing, the pane bedded in glazing adhesive in a machined seat, the standard build for custom enclosures with a window.
- A gasketed pane in a recess; sealing and supporting a long window works the gasket spacing and pane support.
For outdoor life, silicone and EPDM are the two gasket materials that hold up to UV and weather, where most common gasket compounds break down. Silicone spans -60 to 200 C [-76 to 392 F]; EPDM spans -50 to 150 C [-58 to 302 F] and takes hot water and steam best. The finished enclosure is rated NEMA 4X / IP66 by the manufacturer; it is not UL listed. NEMA does not test or endorse enclosures. For what a panel keeps after every hole is cut, what a rating survives after you cut the panel works it device by device. The fundamentals of pane openings sit in the cutouts and windows guide.
What to specify
Six lines on the drawing or quote request settle this job:
- Pane: thin clear acrylic or tempered glass for either emitter wavelength; black IR-transmissive polycarbonate to hide the sensor; transmission stated at your wavelength.
- Divider: matte black, emitter to detector, touching the pane, or a 5-10 degree sensor tilt.
- Gasket: silicone for -60 to 200 C service, EPDM for steam and hot-water washdown.
- Recalibration: record trip thresholds with the pane fitted, at 30, 60 and 90 cm [12, 24, 35 in].
- Anti-fog: a fogged pane reads as a blocked beam at the sensor's wavelength; condensation near a cold window carries the fixes.
- Layout: place the pane opening in the cutout designer, so the quote carries the exact hole and pane seat.
Built to your print, with material, size, pane seat and every cutout defined by you, the enclosure is delivered in 10 business days. Counted from your order; for a custom build like this, from your approval of the drawing.
Catalog dimensions to compare
Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Will a 940 nm sensor work behind polycarbonate instead of acrylic?
Yes. Clear polycarbonate of the same thickness passes about 85-90% at the sensor's wavelength, against about 90% for acrylic, so the two-pass budget is 0.85 x 0.85 = 0.72 to 0.81 and you keep 85-90% of range. Choose polycarbonate for impact, acrylic for scratch and UV resistance; both glaze into a rated window the same way.
Can I hide the sensor behind a black lens?
Yes, with the right black. Black IR-transmissive polycarbonate, the material remote-control windows use and the subject of a patent on light-tight infrared-transmissive polycarbonate, blocks visible light while passing much of 850-940 nm. Ordinary tinted or mirrored lenses can cut that band to a few percent. Bench test before ordering: hold a TV remote against the material and watch through a phone camera; if you see the LED flash, your sensor sees through it too.
Will a PIR motion sensor work behind a glass or acrylic window?
No. A PIR sensor reads thermal infrared at 8-14 um, and both glass and acrylic block that band. A sealed PIR needs a high-density polyethylene pane, which passes 8-14 um; fit it as a bonded or gasketed opening and plan on somewhat shorter range than in open air.
Why does the sensor now trip with nothing in front of the window?
The pane's inner face is a weak mirror. It reflects about 4% of the emitter's light straight into the detector, which at close quarters can outshine a distant target's return. Fit a matte black divider between emitter and detector that touches the pane, or tilt the sensor 5-10 degrees so the bounce misses the detector, then recalibrate the trip threshold.
Which gasket should seal an outdoor window pane?
Silicone or EPDM. Those two hold up to UV and weather where most common gasket compounds break down, and their ranges bracket the jobs: silicone from -60 to 200 C for wide temperature swings, EPDM from -50 to 150 C with the best resistance to hot water and steam washdown. Nitrile belongs around oil, at -30 to 120 C.
