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Techniques for CollectorsLight Source

1st Feb 2009 04:55 UTCBarry Stevens

What's the best light source for a microscope (to get the truest colours)?

1st Feb 2009 16:45 UTCDonald Peck

Hi Barry, I don't know what the best light source is, but I have had good luck using a halogen lamp with a fiber-optic system. If you are taking digital photos, color can be corrected in almost any photo editor.

1st Feb 2009 22:41 UTCAlysson Rowan Expert

I have found that the "Ice White" LEDs give a good colour rendition for very little effort. The important thing is to have lots of them to give a good, even illumination of sufficient intensity.

2nd Feb 2009 21:09 UTCHenry Barwood

I have no idea who may make them anymore, but the 20W halogen desk lamps that had the bulb center mounted (as opposed to the much more common side mount) were easily converted into a powerful microscope light. If you removed the face plate and put a spot bulb in place of the one that came with the lamp, it made an intense light source. Two of them provided an even, but directional, illumination that was great for photography. They were actually brighter than a 250W bifurcated fiber optic light source. I have a dozen or so of the lights stored away for future use (I've had two of them go dead so far, but the other four are still humming along!). They originally cost under $10 in the stores.

Henry

4th Feb 2009 01:08 UTCAndrew Brodeur

I recently purchased a 20w halogen desk lamp a K-mart that i am having good luck with. It takes a small automotive style bulb and puts out an impressive amount of light. The part that i like best is it has about a 3" head on it so you can get it in close to the specimen without interfearing with the microscope. I doesn't produce the extreme heat that other larger halogens do either and it was only about $11.00......have fun with it, Andy

4th Feb 2009 03:20 UTCTony Peterson Expert

Halogen is the only way to go (I'd like to try xenon but I've been unable to find bulbs to fit standard fixtures here in Canada). Because they provide a continuous black-body spectrum, easy digital manipulation allows you to shift the colors to be very similar to those you would observe with sunlight. If you are using a variable voltage power source, settle on a single value and stick to it so your bulb temperature, and your digital correction, are always the same (and the hotter the better). I use 50 W bulbs but there should be no intrinsic difference with 20 W.

Fluorescent lights are much less desirable because their spectra are discontinuous (visible light is produced by UV light striking fluorescent chemicals lining the tube). And while "white" LEDs might produce good results for certain colors, they are to be generally avoided because they trick the human eye/brain into "seeing" white by producing only two colors: blue, and yellow (the latter by fluorescence). The yellow falls halfway between red and green and stimulates those color receptors equally: red+green+blue = 'white'. But in reality there is no red or green light to be reflected (nor blue-green, violet, etc. etc.) and hence some mineral colors cannot be accurately photographed.

To see a vivid demonstration of the inadequacy of fluorescent sources, check out my comparison pair of photos of cinnabar:

http://www.mindat.org/photo-52086.html

Tony

4th Feb 2009 15:54 UTCDonald Peck

Tony, is the dolomite matrix for the cinnabar actually pink? If it is white as in the fluorescent photo, an nice technique, assuming that the specimen and the camera were not moved, would be to digitally combine the cinnabar from the halogen specimen and the dolomite from the fluorescent shot.

4th Feb 2009 16:22 UTCTony Peterson Expert

The dolomite is quite white; the problem is that it's a closeup photo and red light is leaking out of the cinnabar and illuminating the dolomite in the halogen photo - I've noticed this problem in other cases. Well, it could also partly be that I took this photo long ago and was less skilled at and attentive to getting accurate colors at that time (I even considered removing the posted image).

The dolomite in the fluorescent image actually looks a little green to me.

But I think I'm already close to the edge regarding editing of my images, as I routinely use Combine Z and digitally clean up dust, fibers, CZ-produced halo artifacts, etc. when I can. I would never combine portions of images generated with different light sources. (:P)

tdp

4th Feb 2009 17:20 UTCKelly Nash 🌟 Expert

Curious if anyone knows if the continuous spectrum of a halogen light in a gooseneck sytem is affected by the fiber optic tubes (i.e. are any wavelengths maybe filtered out).

4th Feb 2009 18:30 UTCTony Peterson Expert

I've wondered the same - and would bet the farm that some blue light would be scattered out; the light that emerges would be slightly reddened. The price on those things is so high I would hope they'd use top quality fibers!!

4th Feb 2009 22:30 UTCAlysson Rowan Expert

Fibre has been a personal bugbear of mine for a number of years, so here's my take on the subject.

As to materials:-

Standard fibre-optics filter out some of the infra-red, virtually all the ultra-violet, and give the light the slightest green tint (only significant if you're doing photometric work, and easily corrected)

Boro-flex fibre (borosilicate) allows some UV through, has a very slight yellow colouration and is probably the best budget product.

Quartz-fibre (which is usually with borosilicate jacket) lets through 90% of the ultraviolet, and makes little measurable difference to the colour.

Polymer fibre imposes an increasingly yellow hue on the light (just before the fibre becomes very brittle, brown and largely opaque)

Other glasses are used.

Now, construction:-

The size of the fibre is important, since at certain diameters and refractive-index gradients the fibre will act like an interference filter. Read the specification and expect to lose a few specific wavelengths (mostly in the far-blue/violet)

Graduated RI has the highest bandpass (the light is bent, not reflected suddenly), and tends to not lose many wavelengths.

Evaporated (mirror-coated) fibre is unreliable, and imposes a distinctly blue cast on the light when new - which becomes decidedly sepia as the metal film oxidises.

Coherent bundles (transmit an image) are expensive, but generally have lower light losses because the fibres don't bend sharply over each other. Standard light-pipe bundles are semi-coherent, which is a good compromise for illumination.

Support etc :-

A jacket that prevents excessively tight bends is obviously better than one that provides no rigidity.

There was a phase when light-pipes had lubricants in the fibre bundle - this loses a lot of light and the lube turns brown with age, so not a good idea.

Ferrules that crimp the fibre bundle tend to cause colour aberrations (as well as losing a good percentage of the light), so impregnated (resin-bound) ferrules are preferable - these also tend to be well polished (a good thing)

Finally -

Optiflex, (amongst other trade names) is a very fat, low flex light pipe which uses hinges mirrors instead of fibre and obviously causes no colour shift.

There are other, specialist light pipes out there, but they all have their own issues.

I hope this helps.

4th Feb 2009 22:48 UTCAlysson Rowan Expert

Tony Peterson Wrote:

-------------------------------------------------------

> Halogen is the only way to go (I'd like to try

> xenon but I've been unable to find bulbs to fit

> standard fixtures here in Canada).

Xenon lamps require specialist high-voltage, medium current power supplies (usually 2400V and 60V, or there abouts). They produce an excellent black-body spectrum - with the Xenon lines imposed on top (and including lots of ultra-violet radiation as well as heat)

The fused-quartz tubes used to have a nasty habit of exploding unexpectedly, but are still decidedly fragile (they run at very high pressure, after all), so a metal shield is required. The point-source of light (the arc) is incredibly intense, so eye-protection is an absolute must (even ignoring the ultraviolet output).

The xenon illuminator on my microscope had to be converted to halogen a good few years ago - the life expectancy of the tube was a few tens of hours at best, and at a very high unit cost (current price is over £150 compared with £4 for the equivalent 150W halogen)

Good luck with your search, I look forward to hearing about your results.

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