Table 6 — Luminous Efficiencies of Various Illuminants
| Illuminant and commercial description | Commercial rating | Lumens per watt | Efficiency (visible radiation × visual sensibility ÷ total radiation) |
|---|---|---|---|
| Carbon incandescent lamp oval anchored (treated) filament | 4 watts per mean horiz. c. | 2.6 | 0.0042 |
| Tungsten incandescent lamp, vacuum type | 1.25 watts per mean horiz. c. | 8.0 | .013 |
| Mazda, type c | 600 C. P. 20 amp., 0.5 w. p. c. Series type C. | 19.6 | .032 |
| Carbon arc (open) | 9.6 amp. clear globe | 11.8 | .019 |
| Open arc, yellow flame, inclined trim | 10 amp. D. C. | 44.7 | .072 |
| Quartz mercury arc | 174-197 volt, 4.2 amp. | 42.0 | .068 |
| Glass mercury arc | 40-70 volt, 3.5 amp. | 23.0 | .037 |
| Nernst lamp | 4.8 | .0077 | |
| Acetylene | 1 L per hr. consumption | .67 | .0011 |
| Petroleum lamp | .26 | .0004 | |
| Open flame gas burner | Bray 6 high pressure | .22 | .00036 |
| Incandescent gas lamp, low pressure | .350 lumens per B. T. U. per hr. | 1.2 | .0019 |
| Incandescent gas lamp, high pressure | .578 lumens per B. T. U. per hr. | 2.0 | .0032 |
| Firefly | 629.0 | .96 |
The firefly light by the above method of calculating efficiency is not 100 per cent. efficient because its maximum (λ = 0.567µ) does not correspond with the maximum sensibility of the eye (λ = 0.565µ), but taking into consideration also other effects of color, the firefly light would be a still more inefficient and trying one for artificial illumination, as all objects would appear a nearly uniform green hue. Indeed the distortion would be even greater than with the mercury arc, whose objectionable green hue is so well known. “We may say, therefore, that the firefly has carried the striving for efficiency too far to be acceptable to human use; it has produced the most efficient light known, as far as amount of light for expenditure of energy is concerned, but has produced it at the (inevitable) expense of range of color. The most efficient light for human use, taking into account both color and energy-light relationships, would be a light similar to the firefly light containing no radiation beyond the visible spectrum, but differing from it by being white.” (Ives, 1910.) Although the spectral energy curve for Cypridina light has not been worked out, it will be noted that the Cypridina spectrum is much longer than that of the firefly, more nearly approaching the spectrum of an incandescent solid giving white light. It approaches, but does not attain the ideal.
Although Muraoka (1896) and Singh and Maulik (1911) have described radiations coming from fireflies which would pass opaque objects and affect a photographic plate, and Dubois reports the same from bacteria, the existence of such radiation has been denied by Suchsland (1898), Schurig (1901) and Molisch (1904 book). The experiments of Molisch on luminous bacteria are of greatest interest, for they are very carefully controlled and show without a doubt that black paper or Zn, Al, or Cu sheet will allow no rays from these organisms to pass that will affect a photographic plate, even after several days’ exposure. The visible light of luminous bacteria will affect the plate after one second exposure. Moreover, Molisch has pointed out the errors of those who claim to have found penetrating radiation in luminous forms. It seems that certain kinds of cardboard, especially yellow varieties, or wood, will give off vapors that affect the photographic plate. The action is especially marked with damp cardboard at a temperature of 25°-35° C., and Dubois and Muraoka must have used such cardboard to cover their plates. A piece of old dry section of beech or oak trunk, placed on a photographic plate for 15 hours in a totally dark place, will register a beautiful picture of the annual rings of growth, medullary rays, junction of bark and wood, etc. Russell (1897) had previously found that many bodies, both metals and substances of organic origin (gums, wood, paper, etc.), placed in contact with photographic plates, would affect them, and concluded that vapors and not rays were the active agents. As a dry piece of wood has a very definite smell, there is something given off which can affect our nose and there is no reason why it should not change, by purely chemical action, the photographic plate. This action of wood on the plate is prevented by interposing a sheet of glass. Frankland (1898) has described similar vapors coming from colonies of Bacillus proteus vulgaris and B. coli communis which affect a photographic plate laid directly over the colonies in an open petri dish. There is no effect if the glass cover of the petri dish is between plate and bacteria. There is, then, no specific emission of X-rays or similar penetrating radiation from luminous tissues which will affect the photographic plate through opaque screens.
A similar conclusion is reached if we attack the problem in another way. X-rays and radium rays (Becquerel rays) cause fluorescence of ZnS, barium platinocyanide, willemite (Zn2SiO4), and calcium tungstate. Coblentz (1912) showed that the firefly will cause no fluorescence of a barium platinocyanide screen and I have been unable to detect fluorescence of zinc sulphide, barium platinocyanide, zinc silicate (willemite) or calcium tungstate shielded from Cypridina light by black paper, although the light of this organism is quite bright enough to cause phosphorescence of zinc sulphide without the black paper. The samples of the above four substances all showed fluorescence in presence of radium rays, but only the ZnS phosphoresces after exposure to light rays, although the willemite was phosphorescent after exposure to the ultra-violet.
While photometry at low intensities is a difficult procedure at best, if the light varies in intensity or is a flash, accurate measurements become well-nigh impossible. The figures given for intensity of animal luminescence must, therefore, be accepted with a realization of the difficulties of measurement. By candle is meant the international candle, unless otherwise specified, equal to 1.11 Hefner candles (H. K.) 0.1 pentane lamp and 0.104 carcel units. It is a measure of intensity.
Amount of light, or light flux, measured in lumens, is that emitted in a unit solid angle (area/r2) by a point source of one candle-power. One candle-power emits 4π lumens. The latest figure for the mechanical equivalent of light at λ =.566 is.0015 watt (Hyde, Forsyth and Cady, 1919), i.e., 1 lumen =.0015 watt. One watt is 107 ergs (one joule) per second.
The illumination (of a surface) is that given by one candle at one metre, the candle metre (C.M.) or lux. The surface then receives one lumen per square metre. A metre kerze (M.K.) is the illumination given by one Hefner candle at one metre distance.
The brightness of a surface is measured in lamberts or millilamberts. A lambert is “the brightness of a perfectly diffusing surface radiating or reflecting one lumen per square cm.” A millilambert is 1/1000 lambert. For further definitions the reader is referred to the reports of the committee on nomenclature of the Illuminating Engineering Society.
Dubois (1886) states that one of the prothoracic organs of Pyrophorus noctilucus has a light intensity of 1/150 Phœnix candle of eight to the pound (probably about equivalent to 1/150 candle) and that 37 or 38 beetles (each using all three light organs) would produce light equivalent to one Phœnix candle. Langley (1890) found that to the eye the prothoracic organ of Pyrophorus noctilucus gave one-eighth as much light as an equal area of a candle and the actual candle-power of the insect was 1/1600 candle. It may be remarked in passing how widely divergent these observations are.
For the flash of the firefly (Photinus pyralis) Coblentz (1912) found variation from 1/50 to 1/400 candle, the predominating values being around 1/400 candle. A continuous steady glow is sometimes obtained from this insect and it proved to be of the order of 1/50,000 candle.
Steady sources of light can be more easily measured and we have two records of the light intensity from luminous organisms with continuous light. One of these is a fish, Photoblepharon palpebratus, with a large luminous organ under the eye, of flattened oval shape, 11 × 5 mm., which glows continuously without change of intensity. The organ can be darkened by a screen similar to an eyelid which pulls up over it. Steche (1909) reports the intensity to be.0024 M.K.3
Luminous bacteria probably glow with less intensity than any other organism. The light from a single organism cannot be seen but that from a colony is visible to the dark-adapted eye. Even so we must remember that the eye is an exceedingly delicate instrument which can detect very small energy changes. The “minimum radiation visually perceptible” has been calculated by Reeves (1917) to be in the neighborhood of 18 × 10-10 ergs per second and the light from a small colony of luminous bacteria represents little more radiation than this.
Lode (1904, 1908), by a modified grease spot photometer method, ascertained that the light of his brightest bacterial colony of Vibrio rumple had an intensity of 7.85 × 10-10 H.K. per sq. mm. or 0.785 H.K. per 1000 sq. metres (=0.562 German-normal candles per 1000 sq. metres). In round numbers this is about one German-normal candle per 2000 sq. metres, or two to three times this area for the light from an ordinary stearin candle. Lode calculated that the dome of St. Peter’s at Rome, if covered with bacteria, would give little more light than a common stearin candle. An ordinary room of 50 sq. metres wall and ceiling area would give out only 0.039 German-normal candle. It does not seem likely that luminous bacteria will ever come into vogue for illuminating purposes. Friedberger and Doepner (1907) by a photographic method, not entirely free from error, found that one square millimetre of lighting surface of a bouillon culture of photobacteria gave 6.8 × 10-9 German-normal candles, about ten times Lode’s value. Even at this rate commercial lighting by luminous bacteria does not appear a promising field for investors.
To sum up, we may say that light from animal sources is in no way different from light of ordinary sources, except in intensity and spectral extent. It is all visible light, containing no infra-red or ultra-violet radiation or rays which are capable of penetrating opaque objects. It is not polarized as produced, but may be polarized by passing through a Nichol prism. Like ordinary light, animal light will also cause fluorescence and phosphorescence of substances, affect a photographic plate, cause marked heliotropism of plant seedlings (Nadson, 1903) and stimulate the formation of chlorophyll (Issatschenko, 1903, 1907). Because of the weakness of bacterial light, etiolated seedlings do not become green to the eye (Molisch, 1912 book), but a small amount of chlorophyll is formed which can be recognized by the spectroscope because of its absorption bands.
Notes
3 The metre-kerze is a unit of illumination, not of intensity, and is incorrectly used by Steche.