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 Post subject: Make a note
PostPosted: Tue Dec 04, 2007 6:15 pm 
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From the latest addition of "The Australian Gemmologist."

A line from an abstract of an article concerning the use of white LEDs in conjunction with a dichroscope, "this combination will produce anomalous dichroic or pleochroic colours from many coloured gemstones."

Clearly, white LEDs should not be used in conjunction with any gem instruments for diagnostic purposes unless you are fully aware of their effect on the instrument and readings.

Until improvements are made to even out the color temperature of white LEDs I would recommend against their usage for diagnostic tests.

They still look nice on a Christmas tree though.


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PostPosted: Wed Dec 05, 2007 1:58 am 
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One other opine of mine. Alot of these scopes are now showing LED illuminators. I have said in ten years we will all be using them and we probably will. But right now they are simply little flourescent lights. None of them specify a color rendition index. I will stick with quartz halogen for the moment. All "white" Leds have a big hump in the blue region. You want white light not blue light.


I have been saying that for a while. :smt003


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PostPosted: Wed Dec 05, 2007 10:51 am 
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Gene, I knew you were aware of this and have posted about it. So many people come through these forums at different times they may have missed your posts or others on this subject.

There are so many of these white LED type maglites on the market now, that the unsuspecting may erroneously be using them with their gem equipment and and getting improper results.

It's just one of those subjects I thought deserved reiteration. 8)


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 Post subject: Re: Make a note
PostPosted: Thu Dec 06, 2007 10:10 am 
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JB wrote:
Until improvements are made to even out the color temperature of white LEDs I would recommend against their usage for diagnostic tests.


Hi JB,
Do you think they refers to illumination devices too? there are some manufacturers who sells microscopes with darkfield illumination provide by LED illuminators (Comdiam, for example and even Ted Themelis).
i've made some custom made darkfield gemological stands with filtered white led ringlights and they works great: NO heating, long life, less space needed due to the absence of the steel mirror.
There are many led manufacturer selling white led with color temperature suitable for gemological instruments (warm light).
ciao
alberto


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PostPosted: Thu Dec 06, 2007 11:07 am 
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The Led manufacturing world is one where there is lots of market to be gained, zillions of dollars of research going on and generally a LOT of activity.

I sold a microscope to an electronics engineer who works close to that part of the world and he showed me some really amazing things. He says if a piece is six months old its way behind the times.

Volpi and Schott , venerable manufacturers of fiber optic lightsources are both making LED rings. But these are primarily for machine vision.
The advantages Alberto enumerates are all quite correct. And people DO criticize their Gemolites both for not having intense enough illumination AND at the same time for getting too hot on extended use. These are valid criticisms.

But even the warm white Leds that Alberto mentions should not be used for lighting a dichroscope. They can be used for photography with a digital camera if you do a white balance to correct for the excessive blue that is in ALL white Leds. And they are nice for cool bright extended viewing sessions. I am not sure about color grading.

Remember that white LEDs are like little bitty fluorescent lights. There is a blue LED in there exciting a little tiny square of a phosphor. Since its a little tiny square they can afford to use better phosphors than you usually find in a conventional fluorescent lamp. So if you look at that light in the hand spectroscope it doesn't have all those bright lights from the mercury excitation , nor peaky phosphor emissions. (at least the ones I have looked at) But what it does have is leakage of the blue excitation light from the blue LED that is driving the white emittiing phosphor. You can't really see it well in the spectroscope but if you look at a spectrogram its right there. And again, until they start specifying Color Rendition Index and having them rate in the mid to high nineties I would be careful about using them for color judgements.

There is an outfit right here in St. Louis that is making an LED ringlight to put on your microscope. It has an extra knob which when you turn it shuts off all but one quadrant of the ring and allows you to move the lit quadrant one led at a time (out of like 80) to make the light come from where you want to come from. Cool idea.


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PostPosted: Thu Dec 06, 2007 11:11 am 
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MK Digital Direct is selling what they call "color correct" led grading lights and boxes. I can't vouch for how accurate they are.

Here.

Jason

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PostPosted: Thu Dec 06, 2007 11:26 am 
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If you look at their in counter lighting at $175/foot you will see that they require not one but two cooling fans. Probably a nice product. Those super bright LEDs produce much less heat than a quartz halogen, but not NO heat.

I don't know what they mean by color correct. In the retail environment
people assume that crafty jewelers will juice up the lighting anyway. Which they probably should.

Take a look on ebay and see whether the offerings of "ledwholesalers" look the same. But MKs prices aren't too bad. People have been saying nice things about their products.


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PostPosted: Thu Dec 06, 2007 11:39 am 
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I bought this light box from them. At the time I bought it they were offering a 1 foot led strip mounted inside as a freebie so I got that too. It's not a 'color correct' one and is definitely blue but it really enhances the sparkle effect of my photos. I've been very pleased with the results.

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PostPosted: Thu Dec 06, 2007 11:52 am 
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G4Lab wrote:
Those super bright LEDs produce much less heat than a quartz halogen, but not NO heat.


i've played with some hi-power leds made by Luxeon (KII). Very nice and powerful comes in 1, 3 and 5W and are quite HOT....

alberto


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PostPosted: Thu Dec 06, 2007 11:53 am 
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Ravirus,
What Gene said. :)

For years the incandescent gem torchlight and daylight equivalent fluorescent along with the standard mag lites have been used by gemologists for grading and some instrumentation work. Even taught in the classroom.

The question of using a white LED light for the use with the spectroscope came up a good while ago when ROM questioned a suspected anomalous reading he was getting while using that light type. I checked a white light LED with the handheld spectroscope and found that there was indeed some very faint line and absorption activity from the LED, in line with what ROM suspected.

Along with the article cited above concerning the dichroscope and LEDs as well as the fact that the color rendering index of these off the shelf mag-lite type white LED aren't usually included on the packaging, makes them poor choices.

As far as the darkfield lighting for microscopes goes, I think the LEDs may have less impact diagnostically as the microscope as used by gemologists is primarily used for inclusion studies and surface characteristics than it is for color rendering.

I will feel more comfortable with LEDs when the big Gem Labs have done their studies and deemed them by name and brand suitable for gemological applications.


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PostPosted: Thu Dec 06, 2007 12:15 pm 
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I bought a pair of Luxeon powered flashlights at Sam's a while back and with fresh batteries they are like a pair of little arc lamps. Bright intense and bluish. If you run it a while and then open it the led is very hot.

I did say in ten years we will all be using them. But don't dump your quartz halogen illuminators yet.

If I may speak for my pal Marty Haske, HE likes Solux quartz halogen lamps which have a built in daylight correction filter. They are only available in MR16 lamps (MR= Miniature Reflector, 16=16/8 or two inch diameter)

They are threatening to start using White LEDs in automobile headlights.
Its a very interesting area to watch.


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PostPosted: Sat Dec 08, 2007 5:49 am 
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Quote:
8a. LED lights and color
Posted by: "xxxxxxxxx@xxxx.com" xxxxxxxxx@xxxx.com
Fri Dec 7, 2007 8:19 am (PST)
Earlier there was a message cautioning using led's to judge stones. The
message gave the presence of ultra violet light as the reason. The ultra violet
light did not seem right so I checked. The point about not using it to
judge color is right on. LED's emit light a single frequency (color). That is
one reason they are efficient. Many stones absorb light at specific
frequencies. If these two don't match then the perceived color can be very far off.
Just think of the difference between color change garnets and Alexandrite
under incandescent and fluorescent lights and make it much larger. But the
point about the difference being due to ultra violet light is only partially
correct.

One of the problems with LED's was due to the fact that they produce just
one color. Shorter wavelengths (blue colors) contain more energy. This meant
that the earlier LED's were red and green because there were easier to
produce. Producing a blue LED was a major advance when it was first done. When
they advanced the technology to produce a blue LED, they then made white light
by using one red, one green, and blue LED. This was expensive because 3
LED's were required for a white light, so they then coated a blue LED with a
yellow phosphor because yellow activates the red and green sensors in the eye and
requires only one LED. Because some blue gets through, this gives the
appearance of white. The phosphors also produce light over a band of colors
which also helps.

They now have LED's in the ultra violet region and can coat these with a
phosphor just like they do with fluorescent lights top produce a white color.
This is the path to cheaper and better, but it is relatively new and not
necessarily cheaper yet.. For us that means that the appearance of a stone under
an LED light is not only different from the appearance under other light
sources, but varies with the specific LED. Using LED's to determine color can be
very misleading.

xxxxxx

This just appeared on the usfgfaceters list. The poster is a longtime member but I have xxed his name out for confidentiality.It wasn't me.

This one posing a good devil's advocate question followed immediately.

Quote:
Xxxxxx, does LED's characteristics affect looking for
inclusions, cracks, etc? Xxxxx from Mass


Its a technology that is going to roll in on us like digital cameras versus
film or CDs versus vinyl LPs. (MP3s vs CDs??)The color part requires our awareness just as it does for Compact Fluorescent lamps.

The fluorescent white LEDs that I have looked at in the hand spectroscope ,had much smoother wavelength responses than any tube fluorescent I have ever looked at. In addition to the absence of the mercury light peaks, most of the phosphors used in tubes and CFLs have their own line structure for efficiency. The LEDs, I have read, use a square of doped YAG. There is a big hump in the blue, but its not liney. I haven't put one infront of a spectrometer because I have been busy. Brian got any slave stoonts?? :D


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PostPosted: Sat Dec 08, 2007 1:10 pm 
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Here's a website that presents spectra for LEDs, including white LEDs: http://www.superbrightleds.com/leds.htm. As the website says, "Click dot for specs" and then scroll down to the bottom of the page to see a spectrum.

I've got some white LEDs, including some special "warm white" LEDs direct from Nichia. If I get a chance, I'll run spectra. But I can tell you from experience that you can't identify that blue peak with a spectroscope.

Green, violet, and yellow atomic mercury lines in fluorescent light are easy to pick out because they are so narrow. But the blue line in white LEDs is just too broad. It peaks in a wavelength region where our eyes don't have good response, and then slowly decreases into the region where our eyes have maximum response. So the overall effect is a smooth blending into the rest of the rainbow you see.


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PostPosted: Sat Dec 08, 2007 4:03 pm 
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Quote:
Xxxxxx, does LED's characteristics affect looking for
inclusions, cracks, etc? Xxxxx from Mass

That's a good question to ask concerning uncorrected LEDs and darkfield lighting with microscopes.

We know there are some diagnostic features relating to color with some inclusions. The flash effect in fracture filled diamonds, possibly a flash effect in some glass filled gemstones and there may be other ones you may think of.

Whether the LED light has any effect on these characteristics or not, I don't know. I would want to know if they did however.


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PostPosted: Mon Dec 10, 2007 2:04 am 
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G4Lab wrote:
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They now have LED's in the ultra violet region and can coat these with a
phosphor just like they do with fluorescent lights top produce a white color.



So therein lies the hope for a "white" LED that can reproduce daylight. It isn't too difficult to come up with phosphors that reproduce a daylight spectrum, and also it isn't too difficult to use a long-pass filter or to apply a coating to stop the UV that leaks through. But I haven't seen ultraviolet LEDs in the places I commonly troll. I'd like to have a couple pure UV LEDs for a student experiment I run.


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