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Author Topic: How fast can LEDs flash; where can I get a fast one?  (Read 2341 times)

Offline Atomic-S

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What kind of flash rates (10% to 90% full brightness or better) are achievable with commercially available LEDs, and what should I search for if I seek a combination of rapid flash rate and high output?


 

Offline evan_au

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Re: How fast can LEDs flash; where can I get a fast one?
« Reply #1 on: 25/07/2014 11:45:29 »
Telecommunications lasers can turn on and off 10 billion times per second (or faster, if you can afford the price). But these typically work at infra-red wavelengths, and have connectors designed for optical fibers.

Normal visible-light LEDs have a larger junction area, and a higher capacitance. Output typically drops if you try to drive them much faster than 1 MHz, but current work on Li-Fi (optical Local Area Networks) is likely to produce LEDs which can flash even faster; the current standard works up to 96MHz.

If you want really fast on/off time, leave the LED permanently illuminated, and use an Mach-Zehnder interferometer to turn the light on and off.
« Last Edit: 25/07/2014 22:18:06 by evan_au »
 

Offline evan_au

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Re: How fast can LEDs flash; where can I get a fast one?
« Reply #2 on: 25/07/2014 22:36:54 »
"White" LEDs used for room illumination usually consist of a blue LED and a yellow phosphor - they look yellow if you look through the lens when they are turned off.

The blue light causes the phosphor to glow - but the glow continues for a short time after the blue LED has turned off. If you turn the blue LED on and off too fast, the phosphor just emits a steady yellow glow.

So it is probably best to use a monochromatic LED with no phosphor; LEDs are commonly available in red, green and blue (plus infra-red and some near-ultraviolet). Generally the longer wavelengths (red and infra-red) are more efficient.

Another limitation to how fast you can turn the light on and off is how big a room you are using - if light reaches the detector over many paths, such as direct line of sight plus reflected off the walls, the reflections can be turning off when the direct line is turning on; this causes problems at 100 MHz and above.

Another limitation is interference from room lights; old-fashioned fluorescent lamps flash on and off at 100/120Hz, while the newer compact fluorescent lamps typically flash on and off at many kHz. Due to thermal inertia, incandescent lamps don't flash very much.

You can detect the flash more easily if you filter out irrelevant light - use an optical filter on the detector which allows through the colour of the transmitter LED, but blocks other colours.

Infra-RED LEDs and detectors as used for TV remote controls may be a good place to start with mass-produced components. You could even order a cheap remote control on eBay and take it apart...
 

Offline Atomic-S

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Re: How fast can LEDs flash; where can I get a fast one?
« Reply #3 on: 27/07/2014 07:11:37 »
Interesting and useful information, some of which I had no idea about (structure of white LEDs). Anyway, this seems very doable.
 

Offline syhprum

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Re: How fast can LEDs flash; where can I get a fast one?
« Reply #4 on: 01/08/2014 08:35:15 »
If you want a really high flashing rate you could use a laser or any other source of polarised light and a Kerr cell.
These were used in pre war projection TVs and the early HELL film recorders with which I used to work.
« Last Edit: 01/08/2014 08:38:02 by syhprum »
 

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Re: How fast can LEDs flash; where can I get a fast one?
« Reply #4 on: 01/08/2014 08:35:15 »

 

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