Dimming led's ?

And by "yellow coating" he means phosphors.

What a joke. But that's how Phil is. He knows a few things, as Dirty Harry says, a man has to know his limitations. Phil doesn't know his. Often the only thing he actually knows is profanity.

Glad to be a source of amusement.

No, that would last micro/milliseconds - this afterglow lasts for seconds.

The light was white, that was what the original topic was about...

Phosphors in other words.

Enjoy your day, you do give us all a laugh too from time to time.

John ;-#)#

One way to find out if the afterglow is capacitor discharge or decaying phosphorescence would be to keep the LED lamp unpowered and instead illuminate the yellow stuff with an external intense light source?

piglet

I'd guess that the LEDs are being fed a PWM signal, with a variable duty cycle. The PWM wouldn't have to be super-fast to give a visually flicker-free appearance. For example, if you're clocking out on/off signals to each LED at a 1 kHz rate, and having the on-time be anywhere from 0 to 8 out of 8, you'd have a flicker rate of 125 Hz which is about the same as that of a fluorescent light.

If you can clock out at 10 kHz or faster you could have more duty-cycle variations and still keep the updates too fast for the eye to see (unless, possibly, you flick your vision across the display left-to-right and "strobe" the flashes out onto your retina).

It isn't, it's just the smoothing cap voltage decay. When I was hazarding my 'guess' I'd overlooked that effect with many LED GLS lamps. If the phosphor afterglow TC is just a few hundred nanoseconds, that kicks my attempted explanation into the long grass.

The only RGB type LED lamps I see on general sale are those fancy remote controlled mood colour types. Since the red and green LEDs come nowhere near the efficacy of the blue, you lose some efficiency over the 'yellow' phosphor coated blue LED GLS types despite avoiding the losses incurred in the fluorescence conversion of the blue wavelength emission into red and green wavelengths.

Getting back to the OP's question, it looks like an entirely electronic technique similar to that used in modern day DSOs to emulate the phosphor decay effect of a CRT display.

Oh, there's a rating system, all right. That's because it's possible to make some terrible color-rendering illumination with fluorescent materials, so every photographer distrusts fluorescent (gas-in-glass or LED) sources.

An artistic rendering of a subject under LED lighting is a common exercise in Photoshop.

A UV laser pointer makes both fluorescent lights and "white LED" lamps glow brightly, and CRT phosphors, too. None of these is notably color-accurate matched to daylight.

The same is true for fine art. It's amazing how artworks will look entirely different in a gallery, in daylight, in your home, etc. "Garage shop" galleries often do their artists injustice by failing to invest in lighting that best shows the works.

It's been "painful" getting suitable working light (for my other half), indoors. We've even considered reshuffling room usage to exploit the magical "north light"...

OK - my point was more about a way to settle the argument about whether the queried afterglow was due to electrolytic cap discharge versus phosphor decay. When the external excitation is removed does the phosphor fade away slowly or darken instantly?

piglet

Yes, to both. I've got compact fluorescents that are dimly visible for a quarter hour after lights-off, and some glow-in-the-dark tape that takes a few seconds (can write one's name in it, almost), and there's some relatively new materials (strontium aluminate) with hours of emission. There's a long tail (probably due to impurities) on lots of the short-time emitters.

Ruby phosphorescence times out at about a millisecond, which is useful for light-chopped detection: just chop the excitation at 1 kHz, and phase-sensitive-detect with a delay that exactly extinguishes the signal due to the exciting source.