I’m working on a project where I need to track frames, and I was thinking about using an LED alternating on and off every other frame. When I tried to do this, the LED was not fast enough, and the camera would always see it on. Does anybody know of any IR LED’s that have a fast enough reaction time for 30 FPS, or of any other way I can make the camera image change every frame that i can track.
Thanks.
If you wanted the LED to be on one frame, and off the next, wouldn’t you need to synchronize the camera and whatever is controlling the LED in some sort?
It seems highly unlikely that the LED was not fast enough.
In fact there’s a trick I helped someone with where you walk into a pitch black room that stays that way but the camera sees you because it’s switching on and off an LED IR light source in sync with the camera captures.
That same art display has the opposite trick. You walk into a blindingly lit room and the camera makes it look like it’s night time. Not through a lense. By quickly turning off the LED lighting and snapping a frame in sync. Your eyes do not adjust quickly enough to match the very high quality camera used.
Couple of things:
Your camera probably has a white balance system built into it. Especially if it is a CMOS camera like a webcam.
How do you synchronize this because if the camera is digital there is almost always a delay that may vary for the capture and transmission of a single frame?
When you say track a change per frame can you use several LEDs?
Light them up in sequence fast enough and you should see a change every frame. Plus with several LEDs you can trick the white balance. I assume any preditable change is enough. So put several LED in a ring around the outside of the camera’s field of view. Rotate around the circle lighting a single LED. You might miss a transistion or so but it will take time for the lit LED to rotate the full circle increasing the odds of you making this work. Or make a bar of LEDs and do the same. Scanning a single lit LED down the bar then starting over.
Ultimately this will likely trick a CMOS camera’s white balance because with several LED you can effectively reach a nominal amount of light produced over the frame and achieved by different lit LEDs. The LEDs can change but the white balance will tend to stay more fixed which makes the camera’s behavior more predicatable. In case you are, say, recognizing colors.
I can try using multiple LED’s. Thats a good idea. And I was thinking as long as i made the switch happen more then the framerate, it would usually change by the time the next frame came around. One or 2 missed would not be a problem, but I think the multiple LED’s would be better.
The film and video industry use time code to do that. Here are a few links you might like. Your camera and code must be synchronized.
-Hugh
Just make sure if you try that trick to recognize that you don’t need to blast a bright light at your camera lense. This trick will likely best be done by adjusting down the LED light intensity and potentially using wide viewing angle LED. LEDs tend to be directional light sources so a little experimentation is required. Another possible solution is using those cheap rectangular LEDs which are generally diffused anyway.
You can turn down the LED intensity either with current limiting resistors (or potentiometers), PWM or even sanding the LED gently with some 220 grit sand paper. Obviously if you intend to use PWM you’ll have to deal with the timing issues that is prone to create. If you use resistors be aware that sometimes batches of LED will have different intensities with nearly the same current limiting resistor. So you might be better off with potentiometers so you can adjust the LEDs just the way you want. I would seriously consider testing the sand paper though it could really be that simple.
If you wanna get really clever adjusting the LED intensity for each individual LED can also be done with a multichannel D/A converter or the specialized version a ‘digital potentiometer’.
We actually found a blinking LED thing that somebody got from a rave that worked perfectly. It has 3 different colored LEDs, and works perfectly. Thanks for the help.