Purple, Pink or Magenta Video in Daylight: Diagnosing IR-CUT Faults
A colour fault is one of the few camera problems that tells you its own cause, if you know which symptom maps to which mechanism. Purple video in daylight is nearly always an infrared problem, and the pattern of when it appears points to the specific component at fault.
Two installers can report “the picture goes purple” and be describing two completely different faults: one a missing filter, the other a software timing issue that resolves itself in under a second. The difference matters because only one of them is a warranty claim.
Why a colour sensor produces purple when infrared appears
A CMOS colour sensor does not measure colour. Each pixel sits under a red, green or blue filter in a mosaic pattern, so it only measures brightness through one of those three windows. Colour is reconstructed afterwards by comparing neighbouring pixels — the process is literally called guessing, and it works because the filters pass light in a predictable way.
Infrared light breaks that predictability. Near-infrared wavelengths pass through the red, green and blue filters almost equally, so an infrared-rich scene drives all three channels towards the same high value at once. The reconstruction has nothing valid to work from. Attempting to guess anyway produces the familiar magenta and pink cast; refusing to guess produces a monochrome image, which is exactly what the camera does at night when infrared illumination is deliberate.
Daylight contains a great deal of infrared. That is why a colour camera needs a filter to exclude it during the day — and why a vehicle camera has to be able to remove that filter again at night, when the same infrared wavelengths are the only illumination available.
The IR-CUT filter, and the two ways it fails
The IR-CUT assembly is a small motorised carrier holding two filters. In daylight the infrared-blocking filter sits in front of the sensor; in the dark it is physically swung out of the way so the sensor can see the camera’s own infrared illumination. Both states are correct. Failure happens in between.
- Stuck open — the blocking filter is not in place when it should be. The camera sees infrared all day and the image is permanently purple or desaturated. Common causes: a mechanism that has jammed or lost power, a solenoid that will not release, or a filter that has physically broken free.
- Slow or unstable switching — the mechanism works but takes its time, and the automatic white balance computes a colour correction before the filter has finished moving. The result is a purple flash lasting a fraction of a second, appearing at tunnel entrances, under bridges and under tree cover.
Match the symptom to the cause
| What you see | Most likely cause | What to do |
|---|---|---|
| Purple or desaturated image all day, in every condition | IR-CUT stuck open, or the camera was built with a fixed full-spectrum lens and no filter at all | Check whether the mechanism clicks when the lens is covered; if there is no mechanism, the camera is the wrong specification for colour work |
| Purple flash for roughly half a second entering or leaving a tunnel or underpass | Automatic white balance computing before the filter has finished moving — a firmware timing issue | Ask for a white-balance freeze during filter transition, and a wider brightness hysteresis so the filter does not oscillate at the threshold |
| Black fabric, tyres and blue number plates turn magenta in bright sun; other colours look correct | Filter present but its infrared blocking depth is marginal — cheap filter glass, or a filter designed for a narrower lens angle | Reject the sample. This is a material quality problem and it will be worse in summer and at altitude |
| Thin purple or blue fringe on high-contrast edges only | Chromatic aberration in the lens, not infrared | A lens property. It is cosmetic unless it is severe; reduce by choosing a lens with better colour correction |
| Daytime colour is correct, but night vision is almost black even with the IR LEDs lit | The filter is stuck in the daylight position, so the camera’s own infrared illumination never reaches the sensor | Mechanical failure in the opposite direction to the usual one. Check the mechanism moves at all; replace if it does not |
The timing problem, in numbers
The tunnel flash is worth understanding because it is the fault customers most often mistake for a defect. The two events happen on completely different timescales:
- The filter is mechanical. A solenoid-and-carrier assembly takes on the order of 200–500 ms to move from one position to the other. This is not a manufacturable-part problem; it is what a moving part does.
- White balance is statistical. The image processor needs several frames — typically 100–300 ms of video — before it can compute a reliable colour temperature for the scene.
- The two overlap badly. While the filter is halfway across, the sensor is receiving a mix of unfiltered infrared and the ever-changing light of a tunnel mouth. Whatever number the white balance computes from those frames is wrong, and it is applied to the next second of video.
So a camera can be electrically perfect and still flash purple for up to about a second at every tunnel. The fixes are firmware, not hardware: freeze the white balance when the filter command is issued and resume when the filter has settled, and require the scene to stay below the threshold for a few seconds before switching at all, so that passing under a single tree does not trigger a switch. Cheap cameras deal with the problem by removing colour instead — dropping saturation to zero the moment light falls below a threshold, which trades a purple flash for a grey picture. That is a legitimate engineering decision, and one worth knowing about before you buy, because it also means the camera will not show you a signal lamp’s colour after dusk.
Acceptance tests that actually catch it
- Midday sun test. In bright sunlight, point the camera at a black cotton shirt and a blue registration plate. Cotton reflects infrared strongly; a marginal filter will show the shirt as purple and turn a blue plate towards violet. Both must stay correct.
- Tunnel test. Drive into and out of an underpass at normal speed and watch the recording afterwards at normal speed and frame by frame. Count how many colour excursions last longer than a quarter of a second.
- Cover test. Cover the lens with your hand and uncover it repeatedly. This forces the filter to cycle. It should click audibly and change state, and it should do so every time.
- Cold start test. If the vehicle operates below freezing, leave a sample in a cold chamber or a freezer overnight and repeat the cover test. Solenoid IR-CUT mechanisms are the components most likely to stick at low temperature.
- Sun-angle test. In open ground at low sun angles, look for a wash of magenta across the frame. That is a filter blocking-depth failure that only appears when the infrared load is at its highest.
Specifying it so that the problem does not arrive
- State the requirement as colour, not as a part. “Must display colour within X seconds of entering daylight, and must show no infrared contamination on black fabric at midday” is a testable requirement. “Must have IR-CUT” is not.
- Ask about the filter, not just the presence of one. A filter designed for a 60° lens may not perform correctly in a 150° housing, because light arrives at the glass at a much steeper angle. Wide-angle cameras need filters specified for wide-angle use.
- Be explicit about night colour behaviour. If operators rely on seeing a blue signal lamp at dusk, a camera that drops to monochrome below a brightness threshold is unsuitable however good its daytime colour is.
- If the camera is for detection rather than viewing, remember that infrared contamination can defeat automated plate recognition entirely: a blue plate read as violet is failed by the recognition algorithm even though the human eye can still read it.
Colour faults are cheap to test for and expensive to discover in service, because by then the cameras are on vehicles in another country and the conversation is about replacements rather than specifications. Half a day of testing on a first sample is the cheapest insurance in this category.
Frequently asked questions
Why does my camera show purple only when driving into a tunnel?
Because the mechanical filter is still moving while the colour calculation has already started. The mechanism needs roughly 200–500 ms; the white balance needs another 100–300 ms of frames. During that overlap the processor settles on a wrong colour temperature and applies it. It is normal behaviour on low-cost designs and can be reduced with firmware — white-balance freeze during transition and a wider brightness threshold before switching.
Why do black clothes look purple but everything else is fine?
Black cotton reflects near-infrared strongly even though it absorbs visible light. If the image processor sees a strong infrared return where it expected black, it produces magenta — the visible failure of an infrared-blocking filter that is not cutting deeply enough. Some other materials, such as leather, reflect far less, which is why the fault appears to be selective.
My camera is black and white during the day. Is that a setting?
Almost always a stuck IR-CUT mechanism rather than a setting. If the filter is unable to move back into position, the sensor keeps seeing infrared, and most firmware then drops colour entirely rather than showing a purple picture. Listen for the mechanism when you cover the lens; if it never actuates, the assembly has failed.
Does a 150° camera need a different IR-CUT filter?
Yes. Light arriving at a steep angle passes through an interference filter at a different effective angle, which shifts the wavelengths it blocks. A filter specified for a narrow-angle lens can leak infrared at the edges of a wide-angle frame. Wide-angle housings need filters designed for wide-angle incidence.
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