Shutter and exposure time: why license plates smear at night
How is smear produced?
Each frame of the camera requires an exposure time. If the distance the subject moves during this period If there is more than one pixel on the screen, the image will be "blurred". The higher the vehicle speed and the longer the exposure time, the more serious the smear will be.
When there is insufficient light at night, the camera will automatically extend the exposure time in order to obtain sufficient brightness. Therefore, the clear number plate during the day turns into a bright band at night - this is a common "night capture rate decrease" of the bayonet.
How to set the upper and lower shutter limits
| Scene | Recommended shutter speed limit | Explanation |
|---|---|---|
| Pedestrian passage | 1/50 s | The walking speed is slow and can be relaxed for brightness |
| Park entrance and exit (within 20 km/h) | 1/250 s～1/500 s | Taking into account license plate and body characteristics |
| Urban road intersection (60 km/h) | 1/1000 s or more | Must be equipped with fill light |
| Highway or expressway | 1/2000 s or above | Requires special bayonet unit and strong fill light |
The trade-off between noise and noise
Shortening the shutter speed will inevitably reduce the amount of light entering, and the picture will become darker and the noise will increase. The correct approach is not to rely on gain to harden brightness. Instead, it is equipped with supplementary light: a strobe light or a steady-on LED fill-in light is used for the bayonet, and a low-brightness steady light can be used at the entrance and exit. The brightness and angle of the fill light and the reflective characteristics of the number plate jointly determine the final effect and must be debugged on site. You cannot just make selections on the parameter table.
Troubleshooting points
- Confirm whether the shutter limit is set to "auto". In automatic mode, the night time will be extended to 1/25 s or even longer.
- Check whether wide dynamic range or strong noise reduction has been turned on by mistake, both of which will indirectly affect the shutter strategy.
- The angle between the fill light and the camera should avoid overexposure caused by shining directly on the lens. It is recommended that the angle between the light and the lens be at least 30°.