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Looking for list of minerals and color temperatures

Posted by Peter K. Szarka  
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Peter K. Szarka January 02, 2018 05:18PM
I've searched with little success for a list of common minerals and what temperature lighting should be used to best show their true colors.

Minerals of interest are:

Zeolites
Azurites
Malachites
Calcites
Fluorites
Smithsonites, Hemimorphites
Wulfenites, Mimetites
Metallic Oxides and Sulphides

Does anyone know if a comprehensive list can be found somewhere? Especially helpful would be references to the lighting used to optimize specific locality pieces, e.g Rogerley fluorites,
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Ronald J. Pellar January 02, 2018 06:10PM
The best lighting for specimen illumination is full spectrum lights, e.g., Tungsten, tungsten-halogen, daylight, etc.. Certain fluorescent lighting referred to as "daylight" fluorescents with a Color Rendering Index (CRI) of 92% or better are pretty good. Electronic flash is excellent for photography. Color Temperature can be very deceiving and should not be used as a measure of color rendering quality! Color Temperature is a simple means to designate the relative amounts of red and blue light.. Two complementary laser's with appropriate wavelengths can be adjusted to create a white light of any color temperature desired with essentially no capability to illuminate any specimen to look like it should under a full spectrum illuminant. Some newer LED's are getting better but are still not good enough, particularly in the green part of their spectrum. However, LEDs in the future should be better and bear watching.
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Georg Graf January 02, 2018 07:35PM
Hi Peter,

to illuminate mineral spezimens for photography, I use the light of an old slide-projector. It is very "white" and gives a very steady light intensity.

Kind regards, Georg
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Steve Hardinger January 02, 2018 11:10PM
You won't find a color reference list because there is some variation in colors in the same mineral from the same locality. So perhaps color ranges would be a better target?
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László Horváth January 03, 2018 05:39PM
Peter,
Cameras (at least the DSLRs) can be set to a wide range of color temperatures. You can take a trial shots to see how your existing light source works with the color temperature setting of your camera and a particular mineral specimen and adjust it until you are happy with the results. And, the results are instant.

Back in the days of film with essentially daylight and tungsten balanced films available, it was a lot more complicated and the processing would take days or weeks (Kodakchrome, Agfachrome) before you would see the results.
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Ronald J. Pellar January 03, 2018 06:13PM
Adjusting the camera to the "right" color temperature is easily accomplished with a proper gray card, i.e., a gray card made especially for white balance adjustment where the gray card has a flat spectral response. However, there will be some minerals that will have a slightly differenct color appearance to the camera than to the human eye, even with a correct white balance using a good gray card. This is due to the spectral response of the camera sensor not exactly matching the human eye spectral response. This effect is often referred to as camera metamerism.

If you are trying to get an exact appearance match, it can be a achieved with a lot of effort and care to match the monitor color temperature to the color temperature of the light source illuminating the specimen, as well as very neutral backgrounds, eye adaptation, etc. Only in very critical publishing requirements and at high cost will this be done.

The field of color perception and reproduction is very complicated and for most of us that just want good pictures, we must decide for ourselves what is "good enough". In most cases a few simple steps will yield "good enough" results. These steps are:

1) Use a broad spectrum light source, e.g., tungsten, tungsten-halogen, electronic flash, daylight, etc.
2) Use a spectrally flat gray card to set camera white balance.
3) Set camera output to Camera RAW or TIFF or, if the camera allows it, DNG. NO JPEG, i.e., no lossy compression!!!
4) Set camera bit depth to 12 or 14 bits to retain dynamic range.

Some cameras allow for fine tuning color temperature or white balance, but it easy to lose your way with this adjustment and is best left to the experts. A good spectrally neutral gray card can give very good results. A google search on camera gray cards will give many results to choose from at very reasonable prices.
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Peter K. Szarka January 07, 2018 03:24AM
Thank you all for the answers. I hadn't expected the photography angle. My question should have phrased differently. But nonetheless, I'll bookmark this thread so I can make use of the info for the camera.

My question should have been, what temp LED lights are used in a display cabinet for the groups of minerals mentioned. I'm trying to determine my needs for cabinet lighting based on the common standard of cool (5K), neutral (4K) and warm lighting (3K).
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Doug Daniels January 07, 2018 05:36AM
Peter
Thanks for the clarification. Unfortunately, with LEDs it's likely still not a simple matter. White LEDs, as I understand them, are made using red, green, and blue LEDs, in various proportions to create the different "colors" (more blue = cooler, more red = warmer). Each color LED has a specific wavelength, not a range. They are nowhere near the more full spectrum lights such as incandescent, etc. You may have to just get one of each of the three you mention, and see how they work with your particular specimens. You may find you need some of each type bulb, maybe with certain parts of your case lit with one type bulb, another part with another bulb, and so on, with each specimen placed under the "best" bulb. Or get really complicated and have all three lumped together to mimic a single bulb (probably not a good idea). Granted, I haven't started fooling with the LED bulbs yet, mainly because of the design - the light is emitted from the top half of the bulb, but apparently you get little shining downwards (thinking of typical table lamps).



Edited 1 time(s). Last edit at 01/07/2018 05:40AM by Doug Daniels.
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Jolyon & Katya Ralph January 07, 2018 07:59AM
> White LEDs, as I understand them, are made using red, green, and blue LEDs, in various proportions to create the different "colors"

No actually they're not made this way at all. the 'white' from an LED computer screen is generated this way, but not with 'white' LEDs in lighting.

Those are made by wrapping a UV LED in a highly fluorescent coating so that the UV from the LED makes the coating fluoresce in a broad white light.
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Alfredo Petrov January 07, 2018 08:23AM
Anyone have a spectrum of such white LED lights, so we can see which portions of the spectrum are weak or missing? That would tell us better than the "color temperature" which minerals will look poorly and which will look better. I suspect this may well vary between manufacturers.
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Jolyon & Katya Ralph January 07, 2018 08:44AM
This is what the CRI is all about. Higher CRI = a more accurate & complete color rendition.

https://en.wikipedia.org/wiki/Color_rendering_index
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Jolyon & Katya Ralph January 07, 2018 08:47AM
For minerals you want to buy lighting ideally with 90+ CRI. Color temperature (the original question!) is a matter of personal taste, but I generally prefer lights around 4000K for my specimens.
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Alfredo Petrov January 07, 2018 09:30AM
The CRI looks useful for judging relative quality of light sources, but saying a light has a CRI of 90 doesn't tell us which part of the spectrum is deficient, and that will influence which minerals look bad under the light. Green minerals which get their colour from iron (ludlamite and boracite, for example) look great under incandescent bulbs but washed out under lots of artificial lights, which then obviously must be weak in some portion of the green, and has long been a source of frustration for me, since I have a lot of those types of green.
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Jolyon & Katya Ralph January 07, 2018 09:53AM
This is why a spectrometer is really what's needed :)
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Alysson Rowan January 07, 2018 10:12AM
Jolyon & Katya Ralph Wrote:
-------------------------------------------------------
> This is why a spectrometer is really what's needed
> :)

OR, you could just use a decent (dichroic) halogen spotlamp and play around with colour filters.
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Jolyon & Katya Ralph January 07, 2018 10:54AM
Alysson Rowan Wrote:

> OR, you could just use a decent (dichroic) halogen spotlamp and play around with colour filters.


That's not much help for cabinet lighting which was the original request.
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Ronald J. Pellar January 07, 2018 06:14PM
Jolyon,

The most common and cheapest white LED's are fluorescent phosphors on top of blue LEDs. The newer, more expensive, LEDs with highter CRIs are using the phosphors on UV LEDs. They both have deficiencies in the green part of spectrum, but the UV LEDs are better in the green but may be worse in the blue. There are white LEDs that are composed of separate Red, Green, and Blue LEDs and usually have adjustable color temperatures. These are a bit more expensive but are somewhat better spectrally than the phosphor using LEDs, depending upon how broad the emission lines are for the RGB LEDs.

LEDs will get better in the future with either broader line emission for RGB LEDs or better phosphors and shorter wave UV LEDs.

As I recall, there was a very extensive discussion on light sources in the "Mineral Photography" thread a while back that had a lot more information than has been discussed here.



Edited 1 time(s). Last edit at 01/07/2018 06:45PM by Ronald J. Pellar.
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Kevin Conroy January 07, 2018 08:46PM
You may want to consider getting a variety of lights, as the same lighting won't do well for all of your minerals of interest. For instance, blue fluorite from Southern Illinois looks great under the right fluorescent lighting, but those lights will make cuprite look dark. If you have an entire shelf of cuprite go with incandescent lighting for that shelf. Perhaps put together a varied sample flat of minerals, and take it to a store that has an array of lights on display to see what best fits your needs.
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Peter K. Szarka January 08, 2018 01:32AM
An interesting array of answers. Several allude to the issue I face. Especially rendering greens. For example, if one was to choose Congo minerals, would a mix of Chrysocolla, Azurite, Malachite and Dioptase all in the same cabinet work? All cool lights or will the two colours need different temps? Will I need 2 cases? Do I need to specialize the Dioptases and tailor the lighting around them?

@Ronald - speaking about the grey card makes me think the background for the cabinets should be grey, rather than black or white. Is there an argument to be made for this?
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Ronald J. Pellar January 08, 2018 06:53PM
I have always liked a black background in my displays, including my competitive displays. Black will add no color bias to the light reflected off of it. Gray and white backgrounds can be worse if they are not spectrally neutral and will no longer be gray or white under some lighting, this is referred to as metameric colors. White in particular can be problematic since many white cloth, paper, etc. have fluorescent whiteners added to them so different lights can cause them to shift from neutral. Since the eye will be somewhat adapted to the background color this can bias the visual appearance in an unwanted color direction for particular mineral.

in particular, dioptase, is very sensitive to deficiencies in the green part of the spectrum of the lighting used. Color Temperature is a relative measure of the amount of blue compared to red and is useless for the green sensitive dioptase. The best light for dioptase is tungsten-halogen lighting as it has a complete spectrum.

The choice of lighting is the same for displaying minerals as it is for photographing minerals and trying to get pleasing results. That is why I referred to discussions in the "Mineral Photograpy" forum for more detailed discussions. Although photography of minerals is further affected by considerations of camera metamerism, monitor considerations, etc., but the principles of lighting are the same.
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Peter K. Szarka January 08, 2018 11:56PM
@Ronald

Your explanation is highly appreciated.

Initially I decided use a black background but then I saw a few cabinets last Feb. with grey that looked nice. I think your reasoning helps solidify the choice for black. And I see I have a few challenges ahead. I'll look through the photo thread for more of the info. Thank you.

And thank you to everyone for their suggestions. The 4K avenue looks promising.
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