Six Inches: What UL 325 Asks of Your Photo Eyes, and How Often We Find It Ignored

Published August 20, 2026

There is a number that governs the safety of your garage door and almost nobody who owns one knows it. The number is six inches. That is the maximum height above the floor at which the infrared beam between your two photo eyes is permitted to sit, and it comes from UL 325, the safety standard every residential garage door opener sold in the United States has been built to since 1993.

We started measuring that height on every job a while ago. It was meant to be a quick check, the sort of thing you note and move on from. It turned out to be wrong so often that it changed what this business mostly does.

What The Sensors Are Actually For

Two small housings sit near the bottom of your door tracks, one each side. One transmits an infrared beam, the other receives it. While the receiver can see the transmitter, the opener is willing to close the door. Interrupt the beam and the door stops and reverses.

This is called non-contact entrapment protection, and the word non-contact is the important part. The door is not supposed to touch the obstruction at all. It is supposed to see it and change its mind.

There is a second, separate protection, which is the force limit. If the door does make contact with something and meets resistance, it should reverse. That one is contact entrapment protection, and it is the backstop. UL 325 wants both, working independently, because each one covers a case the other misses.

Why Six Inches And Not Twelve

The number is not arbitrary and it is not a rounding of something else. Six inches is close to the height of a small child lying on the floor, a dog asleep across the threshold, or a foot stuck out ahead of a bicycle. It defines the slice of the doorway where a body is most likely to be when a door comes down on it.

Now consider a beam at knee height, which is where we find a great many of them. That sensor is not broken. It works perfectly. Walk through the doorway while the door is closing and it reverses instantly, every time. It will pass any test a homeowner would think to run, because every test a homeowner runs involves a hand or a leg at a comfortable height.

What it will not see is a child on the concrete. A door closing on a beam set at twenty inches will pass right through the space where somebody is lying down and only notice when the panel reaches them, at which point the only thing left protecting them is the force setting, which is the protection that operates after contact rather than before it.

That is the whole argument for measuring the height. A sensor that works but watches the wrong volume of air feels like a working safety system while providing very little of the protection it is credited with.

The Three Ways Height Goes Wrong

Somebody knocked it and put it back. A car door, a ladder, a bike handlebar. The bracket comes loose and gets refitted wherever the screw holes line up, which tends to be higher, because reaching down to five inches with a screwdriver is more awkward than working at knee level.

The installer raised it deliberately. Garage walls fill up. Kayaks, surfboards, paving slabs, stacked storage totes, all of it ends up along the wall where the beam runs. A beam at five inches gets interrupted by that clutter, the door will not close, and the fastest answer is to move the beam above the obstruction rather than move the obstruction.

Nobody ever set it. On a rushed opener install the sensor brackets go on at whatever height was convenient and no tape measure comes out at all. This is the most common one on doors fitted by people who fit doors occasionally rather than for a living.

The Ones That Were Defeated On Purpose

The second half of what we find is worse and more interesting. Photo eyes that were disabled deliberately.

Electrical tape stuck across a lens. Both housings unbolted from the tracks and zip tied to a shelf bracket a foot apart, facing each other, so the beam is permanently made and the opener believes the doorway is clear at all times. Wire nuts at the opener head joining the sensor terminals into a closed loop, which is the one that unsettles us most, because the eyes are still on the wall, still bolted up, still with a little light showing, and doing absolutely nothing at all.

Here is the thing worth understanding about every one of those. Not one was done out of carelessness or contempt for safety. Every single one started with a door that kept reversing for no reason the owner could see. It would go halfway down, bounce back up, and refuse to close. They tried again. It happened again. They had a car to park and a job to get to.

So they solved the annoyance. And solving the annoyance meant removing the protection, because from where they were standing the sensor was the problem.

A Nuisance Reversal Is A Symptom

This is the point we would most like to get across. A door that reverses at random is not a door with an oversensitive safety feature. It is a door telling you something is wrong, in the only language it has.

The causes are ordinary and nearly all of them are an hour of work:

• Low sun on the receiver. Facing west in Huntington Beach, late afternoon sun coming straight down the driveway can wash out the receiver so it cannot pick its own transmitter out of the glare. It happens at the same time of day and only for part of the year, which is exactly the pattern that makes people think the sensors are haunted. A hood or a repositioned housing fixes it.
• A pinched or corroded wire. Sensor wire is low voltage bell wire stapled along the jamb. Near the water the staples rust and bite through the insulation, and the circuit drops out intermittently. Dry day it works, damp morning it does not.
• A bracket bent just enough. The alignment margin between the two housings is not huge. A bracket knocked a few degrees out leaves the receiver only just seeing the transmitter, so any vibration in the track breaks contact.
• A binding track or a heavy door. If the door is fighting its way down, the opener reads the resistance as an obstruction and reverses on force. The photo eyes are innocent. People blame them anyway, because reversing is what the eyes are known for.
• Salt in the terminal screws. Coastal garages tarnish every connection inside the sensor housings. A tarnished terminal will pass on a dry afternoon and fail in the marine layer.

Every one of those is findable. None of them is a reason to bypass the sensor, and every one of them will still be there after you have taped over the lens.

The Test With The Board

The photo eyes are only half the standard. The other half is the contact reversal force, and there is a proper way to measure it that costs nothing.

Take a piece of 2x4 and lay it flat on the floor, directly under the path of the door, where the bottom edge lands. Flat, not on edge, so the door meets it at roughly an inch and a half. Close the door with the opener and watch what happens.

The door should touch the wood and reverse promptly. That is a pass.

If it lands on the board and keeps pushing, or if it stops and sits there straining, the down force is set too high. If the door were meeting a person rather than a piece of timber, it would keep applying that same force.

Do this monthly. It takes under a minute. Do the photo eye check on the same day: start the door closing and pass an object through the beam, not your arm, and confirm it goes back up. Two tests, one minute, twelve times a year.

Why Force Gets Turned Up In The First Place

Almost every over-forced door got that way for an understandable reason. The door started binding, or the springs weakened and it got heavy, so it began stopping partway. Somebody turned the down force up and the symptom went away.

The order is what is wrong there. A properly balanced door needs very little force to close. Force should be the last adjustment, made after the mechanical fault is fixed, not the first one made instead of fixing it.

So the sequence that actually works is: check the balance by pulling the release cord and lifting the door by hand to halfway, where it should stay roughly put. Then check the track and rollers for anything making the door fight. Only then set the force, to the minimum that closes the door reliably, and confirm it with the board.

The Battery, And Why California Asked For One

Separate from UL 325, there is a state rule. Since 1 July 2019, residential garage door openers sold or installed in California have been required to include a battery backup. That came from SB-969.

The reason is worth knowing, because it explains why the rule looks like a convenience feature and is not one. During wildfire evacuations, households could not get their cars out of attached garages. The power was cut, the opener was dead, and either nobody knew about the emergency release cord or the door was too heavy to lift by hand once disconnected. That is a genuine problem with a double door, or for anyone who is not strong enough to raise one.

A backup battery keeps the opener running through an outage so the door goes up the way it always does. If your opener was installed after that date and has no battery, it did not meet the requirement. Worth checking, and worth checking that the battery still holds a charge rather than simply beeping at you once a month.

What To Do This Evening

Go and look at your sensors. Put a tape measure on the beam if you have one, or use your hand: six inches is roughly the width of a spread palm. If the beam is above your ankle bone, it is too high.

Then check that both lenses are clear, that neither has tape on it, and that both housings are actually bolted to the tracks rather than sitting on something. Lay a 2x4 flat under the door and close it.

If any of that comes out wrong, or if your door has been reversing at random and you have been living with it, call us on (714) 814-3638. We measure the height, trace the wiring, fix the fault that is causing the nuisance reversals, and set the force against a balanced door rather than around an unbalanced one. It is not expensive work. It is just work that nobody thinks to ask for.