FLUORESCENCE (Dec, 1944)
ITS RAINBOW COLORS MAY LIGHT TOMORROW’S CITIES
by Samuel G. Hibben
Director of Applied Lighting, Westinghouse Lamp Division.
AGE-OLD mysteries of flourescence and phosphoresence are being solved today because the demands of war and the foretaste of post-war electrical living have spurred scientific research and development, formerly dormant for several generations. A great incentive has been given to extend scientific studies of this subjectâ€”generally termed “luminescence”â€”through recent developments of the practical methods of producing the chief ingredient, “black light.” True, black light, which is another name for invisible ultraviolet radiations just out of range of the human eye, does exist in sunlight, but it is overcome by the much more powerful visible radiations.
Although the ability of many materials to shine in the dark has been known for hundreds of years, very little commercial use was made of this knowledge until we learned how to generate and isolate just a small portion of solar radiationâ€”just that little bit of it which produces fluorescence.
So one answer to the present popularity of black light studies is the availability of convenient and powerful black light sources such as the Mazda high intensity mercury vapor lamps. Within a decade there have been made available at least six sizes of these lamps, from 100 to 1,000 watts, and with them we can make almost any amount of black light that we desire. Furthermore, the modern Mazda fluorescent lamp can do a double transformation job. First it generates short-wave ultraviolet within the glass tube; then, depending upon the ingredients of the coating (or phosphor) adhering to the tubing, this ultraviolet may be transformed to either ordinary visible light, or quite considerably to the longer-wave ultraviolet that is the identical black light which we get from filtering the radiations from a mercury lamp. These specially coated tubular fluorescent lamps are known by the title, “360 BL.” If we are satisfied with a rather small amount of black light, then use may be made of the argon-filled 2-1/2 watt glow lampâ€”convenient because it will burn in any regular lighting socket.
Once the particular kinds of illuminants or radiators had been developed which we may classify roughly as producers of (a) longwave or “near” ultraviolet and (b) shortwave or “far” ultraviolet, then the discoveries of luminescent materials grew apace. Literally thousands of common objects and materials of everyday life were found to have hidden beauties, rare undiscovered colors, and light-giving abilities that we had not heretofore suspected. Nor is the survey complete. Several kinds of black light seem neededâ€” usually a matter of selections of purple glass filters or screening the light through chemical liquids. And many fluorescent materials ought to be twice as bright as they are! Crystal structure, moisture, and temperature are all factors, and we must learn more.
Black light and luminescence in itself and standing alone can be classified as a major scientific development, but perhaps its most interesting characteristic is the fact that it represents a section of knowledge that fills in a gap, just as a large block of stone fills a gap in a long wall. It brings us much closer to the completion of knowledge of the entire electro-magnetic spectrum. It is an important piece in the jigsaw puzzle, leading to a picture of what electricity really is.
Stretched across the scientific sky is a great rainbow of promiseâ€”a widely extending spectrum of which visible light is only a small part! There is so much of this great gamut of radiations that we may never seeâ€” directly! But there are methods, such as through luminescence, whereby man’s range of vision can be vastly extended. If we could imagine the electromagnetic spectrum as likened to a picket fence extending for perhaps a mile, then the particular kind of radiation to which the nerves of the retina in the human eye are “tuned” would represent just one or two palings in this fence. There have been gaps in this fence, or at least sections of it about which we have known very little, because we have had no human sense such as hearing or seeing or smelling with which to record and appraise the invisible radiations. The nerves of the skin take up the detection job in a partial way, after the nerves of the retina cease to respond. In this way we detect infrared radiation or radiant heat, but this sensitivity also is over a fairly narrow range, and not quantitative. Another human receiving method where the eye leaves off is the reddening or tanning of the skin under the impact of one kind of ultraviolet light. However, the eye is like a photocell or like the receiving tubes in your radio set, limited to the reception of just a few wave bands of broadcast energy and capable of tuning in on just one little part of this great family of radiations.
Were we to compare the electro-magnetic or radiant energy spectrum with a somewhat similar gamut of sound vibrations or pulsations, we could appreciate how the ear may record sounds over a tremendously wide range, whereas the eye has a very narrow band of reception, or sensitivity. For example, the keyboard of an ordinary piano comprises about seven octaves, from the deep bass notes of about 32 beats per second up to the highly pitched tones (at the right hand end) of about 4,096 beats per second. An octave is any section where the vibrations double in number. For instance, one octave would be from 32 to 64 beats per second or from 2,048 to 4,096. There has been no “soundless sound” nor any unperceivable region of sound that is comparable to the invisible light or black light because the human ear is tuned to such a wide scale that it can actually record about ten octaves of vibration extending from approximately 16 up to 18,000 beats per second. The ear is capable of hearing the loudest air raid siren as far as 75 miles distant and it can record the footsteps of a fly on a piece of paper. The ear sensitivity is such that when we listen to ordinary speech, we are capable of recording 10,000 million times the intensity of the smallest audible sound. The very loudest sound that we can endure would be perhaps one million million times the intensity that the ear measures when listening to faint sounds. In scientific language we determine the range of hearing by measuring the amount of energy produced by sound waves beating against the ear drum, just as we might record the energy of waves at the seashore beating against the rocks of the coast. We say that the minimum sound perceivable represents an energy of 10-16 watts per square centimeter; the maximum sound at the edge of pain, about 10-3 watts per square centimeter.
Comparing the wide sensitivity range of the ear with the very narrow range of the eye idicates the importance of black light and luminescence in extending our range of visibility. If we cannot stretch the receptive faculty of visionâ€”why, then we must transform invisible radiations or emissions into such wave-lengths as can be seen. We must recall that the great spectrum of radiations or vibrations really in its entirety is composed of more than 80 octaves. One octave alone of this vast range is used for direct seeing purposes.
When we determine whether a certain kind of radiant energy is visible or invisible we are merely speaking about the simple matter of wave-lengths. We know, for instance, that in radio broadcasting, we deal with wavelengths measuring from perhaps a few centimeters up to several miles. When we talk about the 60-cycle frequencies of alternating current electricity, we are speaking of wave-lengths each measuring approximately 5,468,000 yards. When we think about radiant heat, we are speaking of waves that measure roughly 0.1 millimeters to about .001 millimeters. This, in terms of frequency, is in excess of three million megacycles. When we undertake to measure the wave-lengths of visible lightâ€”very much shorter than heat wavesâ€”we need a new yardstick. We adopt the linear unit called the Angstrom, which is one ten-millionth of a millimeter long. The wave length of deep red visible light would then be about 8,000 Angstroms (or .0008 millimeters) and the wave length of the visible violet light on the other side of the rainbow would be about 4,000 Angstromsâ€”this spread representing just about one octave of wave-lengths. It is within this octaveâ€”and only within these wave-lengthsâ€”that the human eye functions.
It is hard to conceive of the spectacular fact that we know of more than 80 octaves of electro-magnetic vibrations and yet can directly perceive with the eye only one octave. Below the sensitivity range of the eye, or in the long-wave range, we classify the vibrations about as follows:
Heat (infra-red) …………………………17 octaves
Micro Waves ……………………………… 8
Ultra-short Waves………………………. 5
Short Waves (radar, etc.)……………. 4
Commercial Broadcasting (radio) 1.5
Long Wave Radio………………………… 5
Extending above the range of eye sensitivity, through the ultraviolet and beyond, we find this classification:
Near and Far Ultraviolet……………… 6 octaves
Commercial X-ray ……………………..12
Diagnostic X-ray ………………………… 3
Gamma Rays (from radium) ………. 8
Secondary Cosmic Rays ………………15
Black light, according to the commonly accepted commercial understanding, has a wavelength in the region of 3,600 Angstroms. We term it “black” simply because it, or rather its source, is almost or wholly invisible to the eye. Strangely enough, however, impinging energy of this peculiar wave-length is capable of setting up a mad dance of some of the Electrons or outermost particles comprising the atoms of certain substances. If we hammer an iron bar, it becomes hotâ€”even red hotâ€”because the hammer blows are transferring energy from the muscles to the molecules of iron and because the movement of these molecules or atoms develop a radiant energy that we call heat.
Substitute for the hammer blows the wave beats of black light striking the molecules and atoms of some object with a rapidity of several million blows per second and it is little wonder that under such an impact, things begin to happen to the particles of that material. Actually, in luminescence, they do not become hot, but they do become displaced and distorted from the normal arrangement and when, either during the impacts or afterwards they tend to resume a condition of normalcy, then if visible light is emittedâ€””cold light” in popular terminologyâ€”we say that the material is fluorescent or phosphorescent. If the visible light emission continues only so long as the impacts or exposure to black light continues, we consider this to be fluorescence. If, however, there is an afterglow or a persistence of light emission, or a slow return to normalcy, we generally speak of this as phosphorescence.
Governed by the material, the afterglow of phosphorescence may be visible up to twelve hours or more. It lessens (we say “decays”) rapidly at first, then quite slowly. After two or three hours the brightness is approximately the same as full moonlight on a white surface. When plotted on a logarithmic scale, the decay curve is a straight line. If the phosphorescent material is warm, the decay is more rapid. At the low temperature of liquid air, the phosphorescent action is “frozen” and the emission of light is suspended until the material is allowed to approach normal temperature.
Black light develops fluorescence and phosphorescence in any substance that possesses these characteristics of luminescence. Among some of the common materials showing such characteristics may be included:
(1) Petroleum oils, jellies and paraffins
(2) Coal-tar derivatives, anthracene, etc.
(3) Quinine salts and many medical compounds
(4) Calcium, such as in the fingernails, eyeballs and teeth
(5) Many plant extracts and dyes, including chlorophy
(6) Many minerals or ores, chiefly of zinc, tungsten, etc.
(7) Inorganic compounds, chiefly silicates, tungstates and sulfides
The phosphorescent compounds such as used in luminous plastics, coated tapes, and the pigments of paints are usually zinc sulfide, cadmium sulfide, or sulfides of calcium, strontium, etc. The so-called phosphors with which the commercial fluorescent lamps are coated are usually those compounds which can only be excited by shortwave ultraviolet, generally around 2,200 to 3,200 Angstrom wave-lengths. The secret of the ability of these phosphors to convert invisible ultraviolet radiations into the longer wave-lengths that are visible, lies primarily in the crystal structure of the particles. Strangely enough, the ability to transform wave-lengths depends upon certain very small quantities of impurities; and the control of these two factors represents one of the finest tasks of modern science.
PHOSPHORS USED IN FLUORESCENT LAMPS Phosphor Exciting Range Emitted Range
General Color (Angstroms) (Angstroms)
Blue 2,200â€”3,000 3,800â€”7,000
Blue-white 2,200â€”3,200 3,800â€”7,200
Green 2,200â€”2,960 4,500â€”6,200
Zinc beryllium silicate
Yellow-white 2,200â€”3,000 4,500â€”7,200
Yellow-pink 2,200â€”3,200 4,300â€”7,200
Pink 2,200â€”3,600 4,000â€”7,200
360 BL phosphor
Blue ultra 2,200â€”3,200 3,200â€”4.000
When we put black light to use it has a marvelous versatility. For the medical man it discloses characteristics of the skin to aid the knowledge of disease and it easily designates the healthy condition of teeth, or is used to measure the speed of blood circulation or even to determine whether an unconscious person really is living or dead. Fluorescent substances make otherwise invisible flaws and cracks visible, hence they are used in inspection processes. Black light determines vitamin content and other characteristics in foods, and discloses spoilage. Fluorescence of mineral ores aides the prospector or the minerologist just as the jeweler can sort diamonds which look the same in daylight, but fluoresce as differently as black from white. Phosphorescent markers guide the invasion troops at night-time on a strange beach and the phosphorescent cape protects the traffic policeman at a dark intersection.
Fluorescence makes possible the generation of nearly cold light, at an efficiency double that of incandescent methods. After five years of commercial production, some forty million fluorescent (tubular) lamps are used each year to light American factories and offices.
Black light is therefore filling in a gap by its ability to make invisible things visible. It is a magic wand with which the lighting scientist touches a drab or dark material and makes it magically spring to life with lovely color or with a visible radiation that permits the eye to do its appointed task. Black light is white magic!
… How to take BLACK LIGHT COLOR PICTURES
by Louis Hochman
MY ASSIGNMENT was to take color pictures of fluorescent objects. I had two light sources to contend with: the visible, colored light of the fluorescent object, and black light the invisible near ultra-violet rays of the lamps activating the materials. This latter light source, although practically invisible to the eye, proved far from invisible to the camera, and in my first kodachrome tests caused a deep, violet-blue haze over the transparency. This interfering light source was prevented from entering the camera lens by a Wratten 2A filter.
Next my calculations showed that the fluorescent light source had a much higher Kelvin temperature than ordinary daylightâ€”therefore, I used daylight type kodachrome. The matter of exposure was another story. Exposure meters proved useless in getting any readings. My exposures were, literally and figuratively shots in the dark.
I used three black-light lamps: one large 250W floodlight and two small 100W spotlights, all of the mercury-arc type, with ultra-violet No. 587 transmitting filters over them, transmitting the near ultra-violet radiations in the 3650 Angstrom Unit band, and cutting off the visible light These lamps were placed at three and four foot distances from the subject and the average exposures ranged between five and 15 seconds at f 4.5.
In one case, where the model was lying comfortably on a fluorescent rug, I took a chance on her ability to hold still and stopped my lens down to f 16 for greater depth of field. At this opening the exposure given was 40 seconds, and the model didn’t budge an eyelash until the picture was taken.
FLUORESCEIN SAVES LIVES
THE principle of fluorescence has gone to work in the medical field, too, with astonishing results. Over a period of years, Drs. Kurt Lange and Lynn J. Boyd, along with several others, have worked out a remarkably accurate method of diagnosis in several maladies, using an injectable solution called Fluorescein.
The technique is essentially simple. Once the fluorescent fluid is injected into the patient’s vein, it circulates through the blood stream. If the room is now darkened and a mercury-vapor ultrd-violet lamp beamed on the patient, the progress of the Fluorescein through his body can be observed. A strong green-golden color is emitted from every section of the body which the fluid has reached.
For example. Dr. Lange wishes to study a patient’s rate of blood circulation, which is sometimes vital in medical diagnosis. (Reason: cardiac failure and circulatory difficulties such as hardening of the arteries usually slow up the blood’s progress through the body; hyperthyroidism, on the other hand, is invariably asso-ciated with a shortened circulation time.) Before the Fluorescein method was perfected, the techniques of clocking the blood’s flow were elaborate and not too accurate. Today, all Dr. Lange has to do is make the injection in the patient’s arm, then wait until his lips suddenly acquire the bright green-golden color. The normal circulation time in adults, for passage from the arm to the lips, averages 17.1 seconds. If the time is significantly above or below the norm, specific maladies may be indicated.
Fluorescein is also used with telling effect in the operating room. Suppose a patient is operated upon for strangulated hernia, a condition in which a section of the bowel has been damaged to the point where its circulation has been cut off and its tissues have turned black. Often in the past, surgeons had no recourse but to cut out the injured loop, making the operation much more hazardous. But today Fluorescein may be injected and the circulation through the intestine studied. In many cases, thanks to this visual aid, doctors find it possible now to restore the blood supply to such injured parts and thus avoid serious resections.
If circulatory rate is normal, girl’s lips should begin glowing with this brilliant golden-greenish color in IS to 20 seconds. Device called a dermo-fluorometer measures the degree of change in her skin color.