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I am designing high-speed flash for my microscopy project and as I know from my practical experience when you stop applying voltage on the LED it needs some time to fully stop illuminating, because of the phosphorous cover. So, my question now is, how it will be possible to get rid of that effect because in my application is key to maintain short pulses of light for a good quality picture. The duration of the pulse should be around 100ns to pulses long some minutes.

K H
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Nejc Klanjscek
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    "*... in my application is key to maintain short pulses of light for a good quality picture.*" This should only be a requirement if there is motion involved. Is there? Do you need white light? Red, green and blue LEDs don't use phosphors. Could you use an RGB LED instead of a white? – Transistor Mar 30 '21 at 16:40
  • Yes, I will be measuring the speed of blood cells that are moving and need a better picture. Besides that, it would be best to use white light LED that needs to be powerful enough. I looked for RGB LEDs but as I read on the internet that wouldn't solve my problem because there is some kind of phenomenon there too. Correct me if I am mistaken. – Nejc Klanjscek Mar 30 '21 at 16:48
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    Actual phosphor persistence might not be too long. I recall numbers in tens of ns for the usual rare-earth phosphors though there was some attempt to use long persistence phosphors to reduce flicker I don't think that's the norm. But you can always use RGB LEDs if you want. Are you sure your "practical experience" was not limited by the driver circuit? – Spehro Pefhany Mar 30 '21 at 16:52
  • Probably some use phosphors for certain colours but most RGBs should not need to. Why white light? Do you want true colour rendition or high contrast (which would suggest one wavelength that is passed at different intensities by your target and background). – Transistor Mar 30 '21 at 16:52
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    Are you trying to build a flow cytometer? Typically you'd use a diode laser for that as you don't need broad spectrum if you're just trying to profile cells... – Synchrondyne Mar 30 '21 at 16:53
  • @Transistor White light is used because I would be watching cells with my eyes and is more pleasing to watch them under withe light than blue for example. If I have white light my microscope has some filters to view it in different light shades. – Nejc Klanjscek Mar 30 '21 at 17:01
  • @SpehroPefhany yes the problem is in datasheets I could not get those parameters. In the past, I designed some driver circuits for lasers that reached pulses width around 20ns powering the lasers with a current of 5 amperes. – Nejc Klanjscek Mar 30 '21 at 17:04
  • Can you actually get a usable snr using 100ns pulses from an LED? Using high power (multiple watts) LEDs in a microscope, it's usually challenging to get usable images even at microseconds. For this reason lasers are often a much better choice. – user1850479 Mar 31 '21 at 13:21

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You need to switch to LEDs without phosphors. The original red, green and blue LEDs don't need phosphors, and should have no trouble flashing at that speed. Some of the more modern ones use phosphors to deliver fancy colors, suck as pink.

You should be able to get reasonable illumination using a combination of red, green and blue. You will have to tweak the currents through each LED color to get something that looks right.

Simon B
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