Flash from radar not visible on 0803 video ?

[chris-s]

Hi all !

Today, I have been flashed on the highway.
Not me, but the bus just in front of me.

The speed-radar has flashed too late to see the front of the bus, I was surprised si I have looked at the 0803 video, but no flash is visible...
I've checked frame by frame... no flash !!!

How is it possible ???

With 30fps, I thought I can't miss a speed-radar flashing ?

Any explanation ?

For information, anti-flickering is set to 50hz.
Is it related to this ?

Here is the Youtube video :

The speed radar looks like this :
It is located to the right, before the bridge, near the blue road-sign.

 

[Nigel]

If the radar camera is going to read the plates of cars doing 200 kph then to avoid motion blur it needs to use shutter speeds much faster than 30 fps. In good light the 0803 will also use shutter speeds much faster than 30 fps, again to avoid motion blur so most of the time neither camera will be collecting light. They simply missed each other.

 

[chris-s]

Ok for shutter speed of the speed camera, but the flash light time itself can't be so short !?!?
Or is it a new generation of flash led ?

 

[Nigel]

Ok for shutter speed of the speed camera, but the flash light time itself can't be so short !?!?
Or is it a new generation of flash led ?

I think a normal Xenon flash tube takes around 0.2 milliseconds - (1/5000 second) although they can be faster. Synchronising it with the shutter opening has always been the difficult bit.

Some speed cameras use flashguns that are mainly infra-red so that people don't get blinded by them, it's possible that even though you saw the flash, the camera filtered it out. I think the reason will be timing though.

 

[Schorschi]

30 fps does not mean the shutter is open for 1/30 of a second for each frame, at least not during normal daylight hours.

For simplicity, let's say, each of the 30 frames per one second has a shutter speed of 1/300 second. Let's divide a second into 300 parts (or time slots) of 1/300 s length each. So, your first frame of the video is taken in the first of those 300 slots, the next nine slots the camera would not record anything. Come slot #10, the camera takes the next frame, the third frame would be taken in slot #30, and so forth until you reach slot #290 in which the 30th frame would be taken. Wait/pause another 9 slots and up to here you have recorded one second of video. So, in effect, the camera takes one frame for 1/300 s length, then pauses for 9/300 s before it takes the next frame.

A household flash has a duration of between 1/1000 to 1/10000 second. I don't know what flash duration speed cameras use, but would think it'd be in the 1/10000 s area, if not shorter.

Again, for simplicity, let's assume the flash duration of the speed camera at hand lasts 1/3000 s. So, the pause between two frames was 9/300 s. If we express that in 1/3000 s, we get 90/3000 s. So, the speed camera could fire its flash 90 times inside the pause between two frames, and you would never see it on your video. Now think about a single flash of 1/3000 s length. The chances of that occurring in the pause between two frames are much higher than it actually happening during the time a single frame is taken.

Of course, this is a simplified example, but with ballpark figures to give you an idea of the ratios between shutter speed, frames per second (fps) and flash duration.

 

[chris-s]

Thanks all !
EEverything is clear now