LED 'afterglow' time?

Thread Starter

Hypatia's Protege

Joined Mar 1, 2015
3,200
Kind friends:

It occurs to me that stroboscopic tachometery represents a highly satisfactory solution to determination of rotor bearing condition (Re: Coolidge RA types) --- Moreover, owing to enhanced switching stability and favorable (i.e. compact) geometry, LED technology would seem a better choice than traditional ‘flash-tube’ systems…

Hence my questions:

1) Do ‘ultra bright’ white LEDs exhibit significant ‘after glow’? If yes, what is the antcipatable range in characteristics (i.e. luminance vs time following interruption) –- failing precise data - how do such LEDs compare to Xe 'photoflash tubes' in this regard?

2) Are such LEDs 'tolerant' of rapid switching?

Many advance thanks for any and all assistance!:)

Best regards
HP
 

kubeek

Joined Sep 20, 2005
5,647
Frankly I have little idea about this, but I have a few things to consider. Does a forward biased led exhibit any capacitance? Also, do laser diodes have different parameters, since they are used at very high speeds quite often?

Also, you definitely don´t want to use white LEDs since they use a luminofor and I guess will be very slow.
 

BReeves

Joined Nov 24, 2012
410
luminofor
Just had to look this up as I had never heard of this before. Had better luck searching for "Lumiramic" which is a technology developed by Phillips that places a Phosphor layer over the LED to control the color temperature of the light. Very interesting and can understand why you would suspect the afterglow time would be longer than an LED without this layer.
 

kubeek

Joined Sep 20, 2005
5,647
Oops sorry didn´t realize this is not used in english. But if you search for luminophore, you get much better results ;)
 

Thread Starter

Hypatia's Protege

Joined Mar 1, 2015
3,200
Also, you definitely don´t want to use white LEDs since they use a luminofor and I guess will be very slow.
That's just the sort of issue I was 'afraid of':( --- Clearly 'low intensity' LEDs have rather low 'persistence' (as evinced by 'antique' calculators, and their ilk) wherein the strobing of the display may be 'detected' via a photo transistor --- I might have guessed such performance from high output devices was too much to hope for:rolleyes:

do laser diodes have different parameters, since they are used at very high speeds quite often?
That's an interesting point!:) -- In my experience diode pumped YAGs tend to 'start' rather slowly - but then I expect there's a difference between application specific instrumentation and 'hacked' pointers:oops::D

Many thanks four responses!!!:):):)
 

OBW0549

Joined Mar 2, 2015
3,075
...owing to enhanced switching stability and favorable (i.e. compact) geometry, LED technology would seem a better choice than traditional ‘flash-tube’ systems…
Not necessarily. Flash tube systems dump an enormous amount of energy into the Xe plasma in a very short amount of time. LEDs can be strobed, of course, but I don't know that they can achieve the extremely high ratios of peak current to average current that a flash tube can withstand without being destroyed.

1) Do ‘ultra bright’ white LEDs exhibit significant ‘after glow’? If yes, what is the antcipatable range in characteristics (i.e. luminance vs time following interruption) –- failing precise data - how do such LEDs compare to Xe 'photoflash tubes' in this regard?
I don't have any data on this, but my understanding is that these parts exhibit significant afterglow, lasting on the order of many microseconds up to several milliseconds. IIRC, I've read they have a complex afterglow "tail" since the white color is obtained by a mix of phosphors, each having its own decay characteristics. (Caveat: this is all secondhand info, unverified and cited from my somewhat dim memory.)

2) Are such LEDs 'tolerant' of rapid switching?
Any LED, like all diodes, is going to be tolerant of rapid switching; what they will NOT tolerate is any significant reverse bias (>4V or so) for even the briefest of durations. With high-power LEDs driven by short pulses of many amperes, I imagine that paying close attention to lead inductance is necessary and it's probably a good idea to parallel the LED with a co-located Schottky diode to clamp any reverse "kickback."

Hope this helps a bit...
 

GopherT

Joined Nov 23, 2012
8,012
@Hypatia's Protege

I've tested with photodiodes (photo-transistors are too slow). LEDs are extremely fast. Even with phosphors, white LEDs are very fast. "Phosphors" is a misnomer because most of the color-shifting coatings are actually fluorescent (nearly immediate quantum-mechanically "allowed" triplet to singlet transitions) that occur on the low nano-second timescale. Phosphorescence is a "forbidden" decay of an excited molecule and can occur in the high nano-seconds to minutes time frame (can be described as a half-life like nuclear decay).

I don't know your definition of rapid switching or high power is exactly but 100k Hz is easily done and measured. In most cases, the LED will not be your biggest problem at high speeds.

LEDs have a datasheet just like every other component and the On lag, off lag (or some company-specific analogous measurement is usually given. High-nanosecond range is common for white, less for red and blue/UV. Many high brightness GaN green LEDs are actually made with phosphors so a bit slower.
 
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hp1729

Joined Nov 23, 2015
2,304
On a quick search I couldn't find such things mentioned on an LED data sheet. It makes sense that they must be fast since they are used in fiber optics which can reach pretty high speeds. I also would expect the "phosphor" type white LEDs to be slower than other types.
Can we devise a fixture to measure such a characteristic? A high speed counter that starts when power to the LED is removed and stops when a phototransistor senses decay of the light. A scope would probably be easier.
Phototransistors have a rise and fall time in the microseconds, at least the one I checked on did. This would be more than suitable for an RPM sensor for a motor under 10,000 RPM if you just wanted to monitor the RPM instead of using a strobe light.
 

GopherT

Joined Nov 23, 2012
8,012
That looks like a 20 nanosecond tail...50MHz on the half cycle. Fiberoptic service for Internet use suggests faster methods are employed. Or am I being fooled by TV commercials for 30 megabyte (30MHz x 8 bits) performance?
Listen carefully, they advertise 30Mbps - lower case "b", means Megabit per second, not byte. Scammers!
 

Thread Starter

Hypatia's Protege

Joined Mar 1, 2015
3,200
Thanks for your informed responses! -- Sounds encouraging!:)
Time for me to actually (gasp!) bench test it -- (an ounce of 'empiricism' and all that...;))

Again, many sincere thanks for your knowledge, insight and suggestions!:):):)

Best regards
HP:)
 

Thread Starter

Hypatia's Protege

Joined Mar 1, 2015
3,200
Is that the same meaning as, "One valid test result is worth a dozen theories."?
I wouldn't go that far:D --- 'Twas merely an acknowledgment of my proclivity for the 'abstract' -- Now in possession of some knowledgeable, affirming, feedback -- 'tis time my mitts were 'sullied' with some real-world (i.e. laboratory) investigation:eek::D

Best regards
HP:)
 

RichardO

Joined May 4, 2013
2,268
@Hypatia's Protege

I've tested with photodiodes (photo-transistors are too slow). LEDs are extremely fast. Even with phosphors, white LEDs are very fast. "Phosphors" is a misnomer because most of the color-shifting coatings are actually fluorescent (nearly immediate quantum-mechanically "allowed" triplet to singlet transitions) that occur on the low nano-second timescale. Phosphorescence is a "forbidden" decay of an excited molecule and can occur in the high nano-seconds to minutes time frame (can be described as a half-life like nuclear decay).
I hear what you are saying but my eyes see an afterglow from white LED's. :confused: I have been meaning to verify this observation for quite some time usxing a photodiode...
 

GopherT

Joined Nov 23, 2012
8,012
I hear what you are saying but my eyes see an afterglow from white LED's. :confused: I have been meaning to verify this observation for quite some time usxing a photodiode...
The point source is very bright. It is more a question of object permanence in your eyes than of the phosphor on the LED.
 

GopherT

Joined Nov 23, 2012
8,012
I wouldn't go that far:D --- 'Twas merely an acknowledgment of my proclivity for the 'abstract' -- Now in possession of some knowledgeable, affirming, feedback -- 'tis time my mitts were 'sullied' with some real-world (i.e. laboratory) investigation:eek::D

Best regards
HP:)
If you don't have a photodiode (faster than photo transistor), make your own by simply using a old-school green LED - cathode connected to ground and anode connected to a 1M resistor and other end of resistor connected to ground.

Then connected the resistor cathode node to an amplifier. Always make sure the absorbing LED is longer wavelength than your transmitter LED - but not too much. An IR LED will not detect flashing from a blue or green LED. As mentioned above, lots of newer high-brightness GaN LEDs are emitting blue or UV and red-shifted with phosphors. That means, you cannot use certain green LEDs to detect blue light.



Good luck.
 
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