LED Fluorescence (cont'd)

INFRARED (IR) AND ULTRAVIOLET (UV) FLUORESCENCE

The previous photos show various materials emitting visible light in response to excitation by LEDs (UV and visible). Many specimens do emit (fluoresce) invisible "light" in the IR as well. Chlorophyll, for example, emits strongly in the far red and near IR (685 nm and beyond) when excited in the near UV or blue. The dye Indocyanine Green has an absorption peak around 800 nm and an emission peak around 835 nm. That dye has been used for angiography employing video microscopy. CCD video cameras are very sensitive to IR out to approximately 1100 nm, if they do not have an IR blocking filter. This characteristic makes these relatively inexpensive cameras very useful for certain IR studies. Note should be taken that LEDs usually emit some radiation in the IR that needs to be blocked. This will prevent reflection of the radiation that could pass as IR fluorescence. Incidentally, some researchers have referred to infrared fluorescence as infrared luminescence.

Unlike the terms x-ray fluorescence or UV fluorescence, infrared fluorescence usually denotes the emission, rather than the exciting, wavelengths. Below are several photographs of various minerals taken using visible light LEDs emitting in the blue-green to excite IR fluorescence, along with a video camera equipped with an IR-transmission filter to record the fluorescence. An IR-cutting filter (to filter out any extraneous infrared) was placed on the LED lamp.

A sample of greenockite, a rare mineral, from Llallagua, Potosi, Bolivia

A cadmium sulfide encrustation on the rock matrix is visible. The blue-green LED infrared-fluorescence image was inserted into the red channel of a white-light photo of the specimen.

A fragment of synthetic ruby (Al₂O₃:Cr) producing very strong IR fluorescence

Cruder but very effective specimens for fluorescence studies can be easily made by melting a mixture of aluminum oxide and chromium trioxide in a ceramic crucible in a laboratory carbon-arc setup.

Kyanite (Al₂SiO₅) exhibiting strong IR fluorescence

Many specimens of kyanite from different locales exhibit IR fluorescence. This sample practically jumped out at me as I was screening specimens from my mineral collection. Unfortunately, the point of origin for this rock was not given. Very, very few minerals exhibit IR fluorescence well. One way of quickly screening specimens is to illuminate the rock with a green laser pointer (532 nm) and look for a bright spot on the monitor delineating good luminescent areas. If the rock doesn't pass this simple test, the chance for finding any fluorescence under ultrabright LEDs is practically nil. Another sample of kyanite from Imperial County, California, exhibited practically no emission in the infrared.

Below is a comparison between the use of LEDs in UV fluorescence versus a standard mercury-lamp wand. In brief, LEDs do not currently emit light much below ~378 nm. As such, this prevents their use for examining minerals that glow under short-wave only (e.g., scheelite). The next pair of photos shows a sample of franklinite (so highly prized by collectors specializing in fluorescent minerals) illuminated by a short-wave lamp (on the left) and a UV LED (on the right). Clearly, LEDs have a secure place in the lab, but they cannot completely replace mercury lamps. At least, not yet!

Franklinite, willemite and calcite under UV

Seen here are two views of franklinite (with associated calcite and willemite) from Franklin, New Jersey. The sample in the left-hand image is being viewed under a standard short-wave mercury lamp with appropriate filtration. On the right is the same sample but viewed under a 378 nm wavelength UV LED. Under long wave, only the green willemite (zinc silicate) glows in the visible range. Under short wave, the calcite (calcium carbonate) glows red, contrasting nicely with the green willemite. The black crystals are non-fluorescent franklinite (ZnFe₂O₄).

Some minerals which fluoresce under ultraviolet illumination also fluoresce in the infrared. The following is a specimen of pectolite (sodium calcium silicate) that exhibits this dual fluorescence in certain areas.

Pectolite in UV and IR

The left-hand image is a photo of pectolite viewed under a UV LED emitting at 378 nm. The observed fluorescence is in the visible spectrum. On the right is an identical view of that specimen with blue-green LEDs stimulating IR fluorescence. Not many specimens exhibit both UV and IR fluorescence. This is to be expected given the rarity of fluorescence in UV or IR in general.

LEDs are already quite suitable for special illumination applications in many areas of science as a whole and microscopy in particular. On the next few pages, some of the best and brightest LEDs currently available are demonstrated being used on a new microscope I designed for the study of microorganisms.