You set an old phone on the windowsill, point it at the gate, and check the footage the next morning. The daytime part looks fine. The night part shows your own living room: the ceiling light, the TV, the shelf, and maybe the phone itself, floating over a faint, dark street.
Nothing is wrong with the phone. This is plain physics, and it happens to every camera that records through glass from a lit room. The good news is that the fixes are free, and most of them take a minute.
This guide covers three things people run into when they put a phone camera behind a window: night reflections, the "double image" from double glazing, and the mosquito mesh or window grille that sits in front of so many windows in homes and shops.
Why does my phone security camera only show a reflection at night?
Because at night, the inside of your room is much brighter than the outside, and window glass is a weak mirror.
Ordinary uncoated glass reflects about 4% of the light that hits it straight on, at each surface. That number comes from the Fresnel equations: with glass at a refractive index of about 1.5, the reflectance is ((1.5 − 1) / (1.5 + 1))² = 0.04. A single pane has two surfaces, front and back, so it reflects roughly 8% in total (Wikipedia: Fresnel equations).
In daylight, 8% does not matter. The street is thousands of times brighter than your room, so the weak mirror image of the room is lost under it.
At night the balance flips. Here are some typical light levels from the Wikipedia table on lux:
| Scene | Illuminance |
|---|---|
| Full moon on a clear night | 0.05–0.3 lux |
| Dark limit of civil twilight | 3.4 lux |
| Public areas with dark surroundings | 20–50 lux |
| Family living room lights | 50 lux |
Now put those together in a simple model. I am treating the room and the street as if their surfaces reflect light equally well, which real scenes don't, so read these as rough ratios, not measurements:
| Outside scene | Glass | Room reflection vs. outside scene |
|---|---|---|
| Moonlit yard (0.3 lux) | Single pane | reflection ≈ 14× brighter |
| Moonlit yard (0.3 lux) | Double glazing | reflection ≈ 30× brighter |
| Dusk (3.4 lux) | Single pane | about equal (1.25×) |
| Lit street (20 lux) | Single pane | scene ≈ 5× brighter |
(For double glazing I used four surfaces: 1 − 0.96⁴ ≈ 15% reflected. The script that made these numbers is one line of arithmetic per row; the room is 50 lux in every case.)
That table explains almost every complaint. If the outside is lit by a streetlight, you usually see the street with a faint ghost of the room on top. If the outside is lit only by the moon, the room wins by a wide margin and the camera records a picture of your ceiling light.
The camera's auto-exposure makes it worse, not better. It sets brightness for the whole frame, and the brightest thing in the frame is now the reflection, so it exposes for that. (If faces or objects come out black for a different reason, a bright background, see the companion piece: Why Do Faces Come Out Black on My Phone Security Camera?)
The one fix that matters most: lens flat against the glass
A reflection reaches the lens only if light from the room hits the glass in front of the lens and bounces back into it. If the lens is touching the glass, or nearly touching it, the patch of glass in front of the lens is covered by the phone itself. Room light cannot reach that patch, so there is nothing to bounce.
In practice:
- Put the camera lens right against the glass. A few millimetres is fine; a few centimetres is not. Every centimetre of gap lets room light in from the sides.
- Block the sides with something dark. A ring cut from black foam, a folded black cloth, or a strip of dark tape around the lens. Photographers use rubber lens hoods pressed against windows for exactly this reason. Your version can cost nothing.
- Keep the phone square to the glass. A small tilt is fine. A steep angle is not: glass reflects more as the angle gets shallower, which is why a window looks like a mirror when you look along it.
If you only do one thing from this article, do this one. It removes the problem rather than reducing it.
Turn off the light behind the camera
If the lens can't sit flat on the glass (a deep window ledge, a curtain rail in the way), the next best fix is to take away the thing being reflected.
- Switch off or dim the light directly behind the phone. Ceiling lights and tube lights are the worst offenders, because they are the brightest objects in the room.
- Move lamps to the side. A light that is out of the camera's reflected view does not appear in the footage, even if it is on.
- Watch for small, bright sources. A TV, a router's LEDs, a charger light, or a lit phone screen all show up as bright spots in a dark night frame.
That last one is worth a moment. A phone recording with its screen on is itself a light source, sitting a few centimetres from the glass, aimed straight at it. Recording with the screen off removes that source entirely. That is one of the reasons Background Camera RemoteStream records with the screen off: no glowing screen on the windowsill, no screen reflection in the footage, and less heat and battery drain on top.
Double glazing: why you see a ghost of every streetlight
Double-glazed windows have two panes and four surfaces. Putting the lens on the inner pane stops room reflections, but there is a second effect you can't fix the same way: bright outdoor lights bounce between the two panes and show up as a faint second copy, slightly offset from the real one.
You'll notice it as a "ghost" streetlight or a double headlight. For a security camera, it is mostly cosmetic. It does not hide a person walking past. What helps:
- Aim so the brightest streetlight is near the edge of the frame or out of it. Ghosts are offset in a consistent direction, so a light at the edge often has its ghost pushed out of the frame.
- Clean both inner surfaces you can reach. Smudges and dust spread a bright light into a haze that covers far more of the picture than a ghost does.
- Check for condensation. A damp morning can fog the inner pane on the room side. Fog turns every outdoor light into a big glowing blob.
Mosquito mesh and window grilles
In many homes and shops, the window has a mosquito mesh, a metal grille, or both. These cause different problems depending on how far they are from the lens.
Mesh pressed against the lens. Any object much closer than the lens's minimum focus distance is rendered as a heavy blur. So a mesh touching the lens does not appear as a sharp pattern. Instead it blocks some of the light and lowers contrast, like looking through a dirty glass. At night it also scatters streetlight into a soft glow. A phone camera's opening is only a few millimetres across (for example, a lens with a 4.7 mm focal length at f/1.8 has an entrance opening of about 4.7 / 1.8 ≈ 2.6 mm), so a mesh strand crossing it blocks a meaningful share of the light.
Mesh or grille further away. If the grille is, say, 20–40 cm in front of the phone, it may be far enough away that the camera can focus on it. In low light, autofocus has less to work with and may settle on the grille bars instead of the street, leaving the gate soft all night.
What works:
- Find a gap. If the grille has openings bigger than the phone's lens, put the lens through a gap or right in the middle of one, so the bars are out of the view.
- If you can't avoid the mesh, make it touch. A mesh pressed flat against the lens is better than a mesh 5 cm away. Up close, it becomes a light blur; a short distance away, it becomes a visible pattern over everything.
- Open the mesh panel if it slides. Many mesh frames slide or hinge. A small opening in one corner, with the phone inside it, solves the problem entirely, and mosquitoes will not care about a gap the size of a phone.
- Test focus at night, not in the daytime. Autofocus that looks fine at noon can behave differently at 11 pm.
For developers: on Android's Camera2 API, LENS_INFO_MINIMUM_FOCUS_DISTANCE tells you the closest distance a lens can focus, in diopters, and is 0 for a fixed-focus lens. Setting CONTROL_AF_MODE_OFF and choosing a LENS_FOCUS_DISTANCE lets an app hold focus at a set distance so it can't drift onto a grille in low light. (There's more on how focus, exposure and white balance behave over long recordings in Camera2 API: Handling Orientation, Focus, and Exposure in Background.)
Offline security camera through a window: the night test
Here is a five-minute test that tells you whether your setup works, without guessing. It needs no internet, no account, and no cloud.
- Set up the phone in the exact spot, lens against the glass, at the time you'll actually use it: after dark, with the room lights as they normally are.
- Start recording with the screen off.
- Look at the live picture from another device on the same Wi-Fi. Background Camera RemoteStream has a built-in web server, so you can open the live view in a browser on a second phone or laptop on your home network (the Play listing puts this browser view in the Pro tier). Without it, record for five minutes and play the clip back instead. The camera's video stays on the camera phone; nothing is uploaded anywhere. This also lets you check the frame without walking up to the window and lighting it with your own phone screen.
- Walk past the window outside while someone watches the live view, or review the recording afterwards.
- Change one thing at a time: lights off behind the phone, a dark cloth around the lens, lens moved to a grille gap. Re-check after each change.
If you want an off-site copy as well, the app can also stream to YouTube Live (also listed under Pro), but that needs a working internet connection. Local recording and the LAN live view work without one.
What the glass cannot fix
Being honest about the limits saves wasted evenings:
- Glass blocks some light. Each surface reflects about 4%, so a single pane passes about 92% and double glazing about 85%. On a moonlit night, that loss sits on top of a scene that is already very dark. For what phones can and can't see at night in general, read Can an Old Android Phone See in the Dark?
- Tinted or coated glass is different. Some windows have coatings or films that reflect more than plain glass. The lens-on-glass fix still works best, but expect a darker picture.
- Most phones have no infrared light of their own. A dedicated CCTV camera with infrared LEDs behind glass has the opposite problem: its own IR bounces straight back off the window. A phone without an IR illuminator doesn't have that issue, but it also can't light up a dark yard. A small outdoor light on a sensor helps more than any setting.
Quick checklist
- Lens touching the glass (or within a few millimetres)
- Dark foam, cloth or tape around the lens
- Light directly behind the phone off or moved
- Screen off while recording
- Lens in a grille gap, or mesh pressed flat against it
- Glass clean on both sides you can reach
- Night test done, checked live or from a playback
If you're setting up an old phone as a camera for the first time, start with Turn an old Android phone into a screen-off security camera (no cloud, LAN-only), then come back here for the window.
Background Camera RemoteStream turns an Android phone into a screen-off camera: it records with the screen off and keeps recordings on the phone, with no cloud account needed. Its Pro tier adds a live browser view over your own Wi-Fi and YouTube Live streaming; check the Play listing for what is free and what is Pro.
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