2.1 Sources of Light Emission

    There are many possible explanations to the sources of ultra-weak light emission from biological samples such as fluorescence[12], phosphorescence[12], bioluminescence[13], chemiluminescence[13], superradiance[14] and biophoton[12], etc. These sources have different mechanisms of light emission and they have been the subjectxx of many researches both on its origin and on its applications.

    Photon emission from organisms has been a subject of study for many years. Luminescence is a general term applied to all forms of cool light, i.e., light emitted by sources other than a hot, incandescent body.  Such a cool source is  a black body radiator. Luminescence is caused by the movement of electrons within a substance from more energetic states to less energetic states.

There are many types of luminescence, including: chemiluminescence, produced by certain chemical reactions, chiefly oxidations, at low temperatures; electroluminescence, produced by electric discharges, which may appear when silk or fur is stroked or when adhesive surfaces are separated; and triboluminescence, produced by rubbing or crushing crystals. Bioluminescence is luminescence produced by living organisms and is thought to be a type of chemiluminescence. The luminescence observed in the sea is produced by living organisms. Other examples of bioluminescence include glowworms, fireflies, and various fungi and bacteria found on rotting wood or decomposing flesh. If the luminescence is caused by absorption of some form of radiant energy, such as ultraviolet radiation or X rays (or by some other form of energy, such as mechanical pressure), and ceases as soon as (or very shortly after) the radiation causing it ceases, then it is known as fluorescence. If the luminescence continues after the radiation causing it has stopped, then it is known as phosphorescence.[Xiao Lu, you need to be careful to place quotation marks around statements that are reproduced verbatim.]

2.2 Sonoluminescence
Sonoluminescence is the emission of light by bubbles in a liquid excited by sound. It was first discovered by scientists at the University of Cologne in 1934, but was not considered very interesting at the time. In recent years, sonoluminescence has created a stir in the physics community. The mystery of how a low-energy-density sound wave can concentrate enough energy in a small enough volume to cause the emission of light is still unsolved. It requires a concentration of energy by about a factor of one trillion. To make matters more complicated the wavelength of the emitted light is very short--the spectrum extends well into the ultraviolet range. Shorter wavelength light has higher energy. The observed spectrum of emitted light indicates the temperature in the bubble of at least 10,000 degrees Celsius.[8] But the light flashes from the bubbles are xx less than 12 Ps. The extremely short intervals of high temperatures can create some exotic chemistry that holds promise for industrial applications. [9]

Here is a summary of what we know about sonoluminescence[18]:

The light flashes from the bubbles are extremely short - less than 12 pico-seconds (trillionths of a second) long.

The bubbles are very small when they emit the light - about 1 micrometer (thousandth of a millimeter) in diameter.

Single-bubble sonoluminescence pulses can have very stable periods and positions. In fact, the frequency of light flashes can be more stable than the rated frequency stability of the oscillator making the sound waves driving them.

For unknown reasons, the addition of a small amount of noble gas (such as helium, argon, or xenon) to the gas in the bubble increases the intensity of the emitted light dramatically.

2.3 Phosphorescence

Phosphorescence is luminescence produced by certain substances after absorbing radiant energy or other types of energy. Phosphorescence is distinguished from fluorescence in that it continues even after the radiation causing it has ceased. Phosphorescence was first observed in the 17th century. But was not studied scientifically until the 19th century. According to the theory first advanced by Philipp Lenard, energy is absorbed by a phosphorescent substance, causing some of the electrons of the crystal to be displaced. These electrons become trapped in potential troughs from which they are eventually freed by temperature-related energy fluctuations within the crystal. As they fall back to their original energy levels, they release their excess energy in the form of light. Impurities in the crystal can play an important role, some serving as activators or coactivators, others as sensitizers, and still others as inhibitors of phosphorescence. Organo-phosphors are organic dyes that fluoresce in liquid solution and phosphoresce in solid solution or when adsorbed on gels. Their phosphorescence, however, is not temperature-related, as ordinary phosphorescence is, and instead some consider it to be a type of fluorescence that dies slowly.

2.4 Fluorescence

    Fluorescence is luminescence in which light of a visible color is emitted from a substance under stimulation or excitation by light or other forms of electromagnetic radiation or by certain other means. The light is given off only while the stimulation continues; in this the phenomenon differs from phosphorescence, in which light continues to be emitted after the excitation by other radiation has ceased.

    If an atom is excited by external energy or light, after a certain amount of timex the energy absorbed by the atom will be released, this emission is called fluorescence. Fluorescence of certain rocks and other substances had been observed for hundreds of years before its nature was understood. Probably the first to explain it was the British scientist Sir George G. Stokes, who named the phenomenon after fluorite, a strongly fluorescent mineral. Stokes is credited with the discovery (1852) that fluorescence can be induced in certain substances by stimulation with ultraviolet light. He formulated Stokes's law, which states that the wavelength of the fluorescent light is always greater than that of the exciting radiation, but exceptions to this law have been found. Later it was discovered that certain organic and inorganic substances can be made to fluoresce by activation not only with ultraviolet light but also with visible light, infrared radiation, X rays, radio waves, cathode rays, friction, heat, pressure, and some other excitants. Fluorescent substances, sometimes also known as phosphors, are used in paints and coatings, but their chief use is in fluorescent lighting.

2.5 Bioluminescence

    Bioluminescence is the ability of certain living things in the environment to give off light, which is the result of chemical processes that occur in the tissues of organisms. This process does not produce heat like every other light source, the light produced remains cold. The Bioluminescent process is used by scientists to discover how light is produced without heat.

    Most Bioluminescing occurs in deep sea/ocean waters in sea creatures/plants and on land in fireflies. It is a known fact that more than 90% of fish/xmarine organisms from 300 to 3,000 feet deep glow.

   Bioluminescent animals include such organisms as ctenophores, annelid worms, mollusks, insects such as fireflies, and fish. The production of light in bioluminescent organisms results from the conversion of chemical energy to light energy. In fireflies, one type of a group of substances known collectively as luciferin combines with oxygen to form an oxyluciferin in an excited state, which quickly decays, emitting light as it does. The reaction is mediated by an enzyme, luciferase, which is normally bound to ATP in an inactive form. (ATP - adenosine triphosphate, is the energy storing molecule of all living organisms.)  When the signal that stimulates the specialized bioluminescent cells to flash is receive, the luciferase is liberated from the ATP, causing the luciferin to oxidize, and then somehow recombining with ATP.  Different organisms produce different bioluminescent substances.  Bioluminescent fish are common in the ocean depths; the light probably aids in species recognition in the darkness. Other animals seem to use luminescence in courtship and mating, and to divert predators or attract prey.

2.6 Superradiance

    "Superradiance" is an effect which can convert disordered energy of various kinds into coherent electromagnetic energy. This was apparently first described by R.H. Dicke in 1954, in an article in Physical Review.[25]  Superradiant photons are emitted in a coherent burst. This coherent burst is only possible if all of the atoms or molecules emit cooperatively.

2.7 Chemiluminescence

    Chemiluminescence encompasses a wide range of phenomena that involve light emission during a chemical reaction or process. It is different from the well known fluorescence phenomenon where external light excites a molecule such as a dye to generate light on its decay to a ground state.

    In chemiluminescence, the excited species (such as free radicals or reactive oxygen species), are either produced during a chemical reaction or when the energy released during chemical reactions is transferred to a probe, such as luminol that in turn emits light.  One of the commonly known chemiluminescence phenomena that is visible to the human eye and occurs in living organisms is the "Bioluminescence" of fireflies and photobacteria. Another emerging phenomenon, where the light is not visible to the human eye, and where the excited species that emit light are endogenously and spontaneously produced in living systems, is called the "Biophoton emission".

    The importance of the chemiluminescence phenomena as a probe for chemical reactions was realized when a better uderstanding was gained of the lucigen-luciferase reaction of the bioluminescence. Chemiluminescent probes were projected and are continuously being developed as an attractive alternative to radioisotopes. These probes absorb the energy released during chemical reactions and subsequently emit light. Through intensive research of scientists all over the world, we now have a variety of chemiluminescent probes that are extensively used in clinical diagnostics, pharmaceuticals, life sciences, etc.[19]

2.8 Biophotons

    Biophotons, or ultraweak photon emissions of biological systems, are weak electromagnetic waves in the optical range of the spectrumn, other words, light.  All living cells of plants, animals and human beings emit biophotons which cannot be seen by the naked eye but can be measured by special equipment. This light emission is an expression of the functional state of the living organism and its measurement therefore can be used to assess this state. For example, cancer cells and healthy cells of the same type can be discriminated by typical differences in biophoton emission. After an initial decade and a half of basic research on this discovery, biophysicists of various European and Asian countries are now exploring the many interesting applications such as cancer research, non-invasive early medical diagnosis, food and water quality testing, chemical and electromagnetic contamination testing, cell communication, and various applications in biotechnology.

    The ultraweak light emission, biophoton emission, of whole leaves was investigated by research group INABA Biophoton [23] with respect to experimental changes of the environment. [Xiao Lu, the preceeding sentence doesn't make sense to me.]  The leaves were exposed to UV-light, to a period of temperature variation from 5C to 30C, to anaerobic atmosphere, and to artificial infection by a virus.  The biophoton emission was found to be very sensitive to the metabolic and physical changes within the leaf that was induced by such environmental changes. A general tendency of rising biophoton activity was observed when the leaf was exposed to such stressful situations.

    Based on quantum efficiency data and light absorption data it has been estimated that mammalian cells emit approximately one photon per cell during a time period of 3-20 minutes[26].

2.9 Casimir Effect

    In 1948 Dutch physicist Casimir showed that the consequence of the zero point field is an attractive force between two uncharged, perfectly parallel, conducting plates. This is easily shown by considering two parallel plates seperated by a distance, lets say d.  By working out the zero-point energy of the vacuum, mainly[20].[Xiao Lu, should this word be "namely"?]


The 2 in front of the summation is due to 2 independent polarizations, also the prime on the summation signifies that a factor of 1/2 should be inserted if any of the polarization modes vanishes. The above quantity is infinite, hence showing that the zero point energy of the vacuum is infinite for any finite volume.

Casimir showed that the attractive force per unit area between two parallel conducting plates is

where ? is Planck's constant over 2p and c is the speed of light.

The Casimir effect is a small attractive force which acts between two close parallel uncharged conducting plates. It is due to quantum vacuum fluctuations of the electromagnetic field.(Fig.2.1)


Transfer interrupted!




Fig.2.1 Casimir Effect:

Attractive force exists between two uncharged, perfectly parallel, conducting plates

due to quantum vacuum fluctuations.

    In 1996 Steven Lamoreaux measured the tiny force.  His results were in agreement with the theory to within an experimental uncertainty of 5%.(Fig.2.2)

Fig.2.2 Casimir Effect Experiment[27]

    The plates are very close together so that only small fluctuations fit in between; the bigger modes are excluded.  They exert a total force greater than that exerted by the smaller modes and hence push the plates together. Steve K. Lamoreaux, now at Los Alamos National Laboratory, relied on a torsion pendulum. A current applied to the piezoelectric stack tried to [Xiao Lu, why the font change here?] move the Casimir plate on the pendulum; the compensator plates held the pendulum still. The voltage needed to prevent any twisting served as a measure of the Casimir effect.

2.10 Virtual Particles

    Virtual particles can spontaneously flash into existence from the energy of quantum fluctuations.  The particles, which arise as matter-antimatter twins, can interact but must, in accordance with Heisenberg's uncertainty principle, disappear within an interval set by Planck's constant, h.

Fig.2.3 Virtual Particles

2.11 The Heisenberg Uncertainty Principle

    In 1927, Heisenberg formulated a fundamental property of quantum mechanics which stated that it is impossible to measure both a particle's position AND its momentum exactly [Should this word be "simultaneously"?].  The more precisely we determine one, the less we know about the other. This is called the Heisenberg Uncertainty Principle. The mathematical relation is:

    ?X?P? ? /2 This means that the uncertainty in the position (x) times the uncertainty in the momentum (p) is greater than or equal to a constant (? divided by two.)

    This principle can also be written in terms of energy and time:

    ?E?T? ? /2 This means that the uncertainty in the energy of a particle multiplied by the uncertainty of time is greater than or equal to a constant (?/2).

2.12 Quantum Vacuum Radiation

    The words "nothing," "void," and "vacuum" usually suggest uninteresting empty space.  To modern quantum physicists, however, the vacuum has turned out to be rich with complex and unexpected surprises.  These physicists envisaged this empty space as a state of minimum energy where quantum fluctuations, consistent with the uncertainty principle of the German physicist Werner Heisenberg, can lead to the temporary formation of particle-antiparticle pairs. If these particle-antiparticle pairs move nonlinearly, quantum vacuum radiation comes into being.

    Macroscopically neutral objects can temporarily be charged in the form of virtually emitted and absorbed particles such as electron-positron pairs. If these pairs move nonlinearly or are accelerated, light will be emitted. Based on this theory, bioradiation can also be interpreted as a kind of quantum vacuum radiation.




Fig.2.4 Radiation Emitted from

Charges Accelerated.

2.13 Cosmic Radiation

    Cosmic rays are high energy charged particles, originating in outer space, that travel at nearly the speed of light and that strike the Earth from all directions.  Most cosmic rays are the nuclei of atoms, ranging from the lightest to the heaviest elements in the periodic table.  Cosmic rays also include high energy electrons, positrons, and other subatomic particles.  The term "cosmic rays" usually refers to galactic cosmic rays, which originate in sources outside the solar system.  They are distributed throughout our Milky Way galaxy.  However, this term has also come to include other classes of energetic particles in space, including nuclei and electrons accelerated in association with energetic events on the Sun (called solar energetic particles), and particles accelerated in interplanetary space.

    Cosmic rays are composed of essentially all the elements in the periodic table; about 89% of the nuclei are hydrogen (protons), about 10% helium, and about 1% heavier elements.  The common heavier elements (such as carbon, oxygen, magnesium, silicon, and iron) are present in about the same relative abundance as in the solar system, but there are important differences in their elemental and isotopic composition that provides information about their origins and about the history of galactic cosmic rays.  For example there is a significant overabundance of the rare elements Li, Be, and B [Why not spell these names out, as you have later in this sentence.]produced when heavier cosmic rays such as carbon, nitrogen, and oxygen fragment into lighter nuclei during collisions with the interstellar gas.  The isotope 22Ne is also overabundant, showing that the nucleosynthesis of cosmic rays and of solar system material have differed. Electrons constitute about 1% of galactic cosmic rays.

2.14 Fluorescence Lifetime Measurement

    Fluorescence is a process that occurrs when excitation light fall on an object. Its atoms absorb xx energy, and after a certain amount of time the energy absorbed by the atoms will be released, or emitted. The fluorescence decay process occurs on a nanosecond or sub-nanosecond time scale.

    The determination and measurement of fluorescence lifetimes provides a mechanism that can be used to probe indirectly into the magnitude and rate of these additional processes. Fluorescence lifetimes are regularly used to obtain quenching constants [Xiao Lu, do you think you should define the term "quencing constant"?] for slow and diffusion-limited biomolecular solution processes.

    The fluorescent lifetime of a species ["species?"]is found by determining the amount of time required for n0, the number of molecules in an initially excited state to decay to the point where n, the number of molecules remaining in the excited state has been reduced by a factor of e-1. Most fluorescent species acting alone (in the absence of some quenching mechanism) will decay exponentially as expressed in following equation:


    where t is time, t0 is the lifetime, and

    when t=t0 n=n0/e.

    The fluorescence (excited-state) lifetime can be measured by a variety of alternative techniques. Common to all is some form of pulsed or modulated excitation light source. Flash lamps, steady-state arc lamps, and lasers are the most commonly used sources. In our experiment, we use an air gap source as the excitation light source.

    System Setting:[ Xiao Lu, are you sure your reader will undsrstand what "System Setting" means?  I don't.]

    PMT Voltage:1950 V

    Spark Gap Voltage: 2500 V

    Discriminator 1: 3.0

    Discriminator 2: 3.3


    Time Range:0.2m s

    Coarse Gain: 1

    Fine Gain:8.79

    Bias: 0.07

    Gate: Anti-coincidence

    Strobe: Internal

    Delay Box: 16 ns

    Filter: input 3500 nm

    Output 5100 nm

Fig.2.5 System Setup of Fluorescence
Lifetime Measurement