LED illumination

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Graham Stabler
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Post by Graham Stabler »

These LEDs are not being strobed they are being used for my object movie rig. But still I am probably not as bothered as many about the colour. I have an led torch that is quite blue, these don't show that to the naked eye.

They are only 1W LEDS 45 Lumen, not as bright as the K2 leds I use for my video stuff, they were chosen mainly for convenience. They are described as cold white which I think means they have more of a peak in the blue that the warm white which is more uniform however these specifically are an improved version which is supposed to have better colour uniformity, I don't know if that is between parts or across the spectrum.

Graham

AndrewC
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Post by AndrewC »

If they are on continuously you should probably be heatsinking them. I have mine mounted on some little Northbridge finned heatsinks I found cheap somewhere but then I am sinking about 1000ma thru them

Cyclops
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Post by Cyclops »

I wonder how those wind up LED torches measure up. I have one,altho the winder snapped off,but I've noticed the light is bluish which tells me they may look white on a photograph. (I would normally say they would look white on film but well,i haven't used film in ages-old habits die hard huh!)
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g4lab
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Post by g4lab »

Rare earth doped scintillators have been used in the electron detectors of SEMs for decades. The composition trade secrets as I am sure the LEDs are.

Graham you COULD strobe them if you want, I think they are quicker than most camera flashes at least. And if you do heat sink them you can give them bigger pulses of current synchronized with your camera gate if you have a gate-able camera. You knew that.

The second composition of LEDs referred to reminds me about compact fluorescent lamps. If you take a CD and look at the reflection of a compact fluorescent lamp you will see a series of very distinct images of the lamp in green and blue and red. These are the sharp emission lines of these sources.
There is almost no continuum radiation. Our eyes don't like this which is why these things give lighting a ghastly quality IMO. I performed the same test a long while ago on an LED and was very surprised at how black body like the light appeared. In the reflection of a CD with my eye as a detector Very scientific test. Nonetheless if they start, or are already using, using the CFL phosphor sets then the photographer will want to be even more careful.

Graham Stabler
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Post by Graham Stabler »

Andrew, being only 1W, mounted on an aluminium substrate and then on another piece of aluminium I don't forsee a problem. For the K2 Leds I used cast pin heatsinks intended for BGA surface mount electronic devices.

They won't produce enough light for my strobe work even with higher peak currents is my impression.

Graham

rjlittlefield
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Post by rjlittlefield »

g4lab wrote:There is almost no continuum radiation. Our eyes don't like this which is why these things give lighting a ghastly quality IMO.
It's a fine point, but the problem here is not your eyes. It's that the narrow spikes in the illumination often line up badly with whatever bumps or spikes there are in the reflection spectra of the stuff you're looking at. Swapping in film or digital sensors will capture effects that are bad too, though probably a little different from what your eye would see.
... if they start, or are already using, using the CFL phosphor sets then the photographer will want to be even more careful.
Agree completely.
In the reflection of a CD with my eye as a detector
That's a stroke of genius. Dang good diffraction gratings at pennies per square inch. Thanks for mentioning -- that one had slipped past me!

--Rik

g4lab
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Post by g4lab »

There is almost no continuum radiation. Our eyes don't like this which is why these things give lighting a ghastly quality IMO.
I'm talking about how awful things look if illuminated solely by CFLs They are truly nasty. Some people get a headache from them and I suspect the headache arises out of retinal irritation in a fashion similar to the headache that limelight used to give actors,, headaches which gave rise to "the green room". supposedly.

Also people who do inspection under monochromatic light such as low pressure sodium and visible line mercury supposedly get similar severe headaches if they work too long under monochrome conditions.

Perhaps only one of the bleachable pigments gets depleted and the unusual imbalance causes the headache. I don't really know.

That's a stroke of genius. Dang good diffraction gratings at pennies per square inch. Thanks for mentioning -- that one had slipped past me!
Thank you. I actually noticed it myself ages ago. But others have expanded on it and you can google "CD and cereal box spectroscope"
but if you go to Ioannis Galidakis home page and link to spectroscopy he says they aren't actually too good. But you can have alot of fun with them.
When I checked the link below I discovered that he made an improvement which is at the bottom of the link. His website is well worth exploring.
Here

rjlittlefield
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Post by rjlittlefield »

g4lab wrote:I'm talking about how awful things look if illuminated solely by CFLs They are truly nasty. Some people get a headache from them.
...
Also people who do inspection under monochromatic light such as low pressure sodium and visible line mercury supposedly get similar severe headaches if they work too long under monochrome conditions.

Perhaps only one of the bleachable pigments gets depleted and the unusual imbalance causes the headache. I don't really know.
Hhmm... Headaches from monochrome I can understand. I'm having trouble with the part about CFL's, though, because their phosphors are specifically balanced to stimulate human visual cones in the same proportions as natural light. That balance is accomplished by packing a lot of energy into a few narrow bands, but it's completely unclear to me how an eye could tell the difference.

Actually, it's clear to me that my eyes cannot. I can easily spot metamerism under CFL illumination, say by garish rendition of a salad. If I spent a lot of time looking at salads, it would definitely be a problem -- or maybe a solution, since garish salads look fresher. But I have several reading areas that are lit entirely by CFL's and they seem to be fine -- to me.

On the other hand... As I write this, I am reminded that some people have abnormal color vision caused by slight variations in the chemistry of the receptors. It occurs to me that for some of those people, CFL's could conceivably be balanced far away from natural lighting, and therefore prone to cause the "monochrome" effect. But on the other other hand, I'd be inclined to think that those people would identify the problem as "ghastly color" and not "gives me headaches".

This difficulty is new to me and I haven't tried to track down any reliable references. Quick scan finds mostly anecdotes, and a recent article in Scientific American suggests that the problem, assuming it's real, may be due to factors other than the color of the light. Do you happen to know of any good studies to read?

--Rik

g4lab
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Post by g4lab »

To clarify, I myself do not get headaches from CFL lighting. I do find them awful to live and work under but use them to save energy. Their light is almost totally line structure which is very abnormal and unnatural. But there were (anecdotal) reports from others on the gemology forum who claimed to have headaches from them. Even the headaches from monochrome light might be anecdotal at least according to web searches.

I have spent a little time under low pressure sodium lighting and the reduced chromatic aberration makes things look sharper. I did not get a headache but I also did not want to stay in that environment. And I get the same sensation from CFLs. I have normal vision at this particular moment. Checked including color vision by an ophthomalogist about three weeks ago.

It appears that the better the light quality from fluorescents the less efficient and the more expensive to buy they are Like LEDs I don't think I will be using them anytime soon.

I don't have any scientific references to offer other than the ones earlier in the thread.

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