Key Terminology

The following summarises and defines in outline the most important terms used in photo therapy and related areas of application.


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Absorption Reduction of radiant energy (e.g. light) by interaction with matter therefore creating a decrease in the power of light passing through a substance.

Absorption spectrum The relationship of absorption depending on the wavelength.

Action spectrum A plot of the relative efficiencies of different wavelengths of light in their ability to create a biological response.

Activation energy The energy that must be provided in to a molecule to allow a chemical reaction to proceed or result.

Aerobic Cell respiration in which molecular oxygen is used to take part in the oxidation or organic substrates.

Anaerobic Cell respiration which no oxygen is used to break down organic substrates during metabolism.

ATP Adenosin triphosphate. A high energy phosphate ester which serves as the principal energy-storage compound of the cell.

Attenuation Reduction of radiant energy (light) by interaction with matter, including absorption and scattering.

Average power The average amount of power (usually measured in watts) emitted by a light source.

B lymphocytes are small lymphocytes which synthesize and secrete hormonal antibodies upon antigenic stimulation.

Bandwidth The range of continuous frequencies or wavelengths in an electrical, optical or acoustic signal.

Beam diameter The diameter of a light beam extends to a point where the intensity of the light in the beam had dropped to 1/e2 (0.135) of the peak intensity. Note that according to this definition, some light in the beam falls outside of Gaussian profile of intensity.

Biological Clocks Mechanisms which allow the expression of certain biological structure's at periodic intervals.

Cell The fundamental unit of life; the smallest body capable of independent reproduction. Cells vary considerably in size and shape. But all have the common features of metabolism.

Cell cycle The timed sequence of events occurring in a cell in the period between mitotic divisions.

Cell differentiation The process whereby descendants of a common parental cell achieve and maintain specialization of structure and function.

Cell line Cultured cells of single origin capable of stable growth for many generations.

Cell-mediated immunity Immune response directly mediated by T lymphocytes rather than circulating (humoural) antibody molecules.

Cell respiration A complicated series of chemical reactions in which molecular oxygen is used to oxidize carbohydrates or other organic substances, releasing energy and producing carbon dioxide and water. During the process ADP is changed to ATP, and ATP provides the energy for biological processes.

Chemo-osmotic theory A theory which accounts for the coupling of electron transfer and ATP formation by populating the formation of H* gradients across the mitochondria membrane.

Chromatids The two daughter strands of a duplicated chromosome that are still joined by a single centromere.

Chromatin The nucleoprotein fibers of which eukaryotic chromosomes are composed.

Chromphore A light absorbing atom, molecule or molecular unit.

Chromosome A long, thin thread like body in the dividing nucleus of an eukaryotic cell. They are only observable during nuclear division. Contains DNA, RNA and protein and these constitute the genetic material of the cell.

Chromosomal aberrations During meioses the chromosomes or chromatids may become broken and subsequently rejoined and the result is a mutation of the genres in the chromosomes.

Clone A group of cells all descended from a single common ancestor.

Coenzymes Small molecules which associate with protein to form active enzymes (e.g. NAD, NADP).

Coherence refers to the inherent 'synchronicity' of light; such light is described as both spatially and temporarily coherent, signifying that the light is monochromatic, in phase/ in step or out of phase. See also Temporal coherence, Spatial coherence

Collagen A tough, inelastic,fibrous protein, formed and maintained in tissues by fibroblasts.

Colour filter A filter which absorbs some of the incident light and transmits the rest.

Collimation is a term which describes the high degree of parallelity light. Divergence is more commonly specified by manufacturers and researchers; this is essentially the opposite of collimation.

Constituent enzymes Enzymes that are synthesised in fixed amounts irrespective of the growth condition.

Continuous wave (CW) describes the output of those therapeutic units where radiant power output is relatively constant over time.

Cytochromes Iron containing proteins taking part in cellular respiration. They are oxidized by dissolved oxygen in a cell and reduced by oxidizable substances in a cell.

Diffraction Divergence of a light wave by an object of dimensions comparable to the wavelength.


Dosage is sometimes used when describing light therapy treatments as an alternative to either radiant exposure or energy. See fluence

DNA (deoxyribonucleic acid) A polymer of deoxyribonucleotids. The genetic material for all cells.

Electromagnetic radiation Energy flow formed by any oscillating electric and magnetic field at right angles and lying transverse to the deflection of energy flow. Examples are X-rays, ultraviolet light, visible light, infrared radiation and radio waves, all of which occupy various portions of the electromagnetic spectrum and differ in frequency, wavelength and energy of a quantum.

Energy can be thought of as the amount of work one or photons delivered by a given unit; it is measured in J and is given by multiplying the average radiant power output by the irradiation in s

Energy density Energy content per unit volume see Radiant exposure

Epithelium Tissue which acts as a covering or lining for any organ or organism.

Enzymes Protein molecules capable of catalyzing chemical reactions.

Erythrocyte Haemoglobin-rich nucleated red blood cell involved in oxygen transportation.

Excitation Any process that increases the internal energy of an atom or molecule.

Fibroblasts Differentiated cells which have a spindle shape and growth rate of connective tissue cells.

Filter A material put in the pat of the light allowing only certain wavelengths to pass.

Fluence (dose) Radiant energy per unit area.

Ganglion A mass of nervous tissue that is encapsulated in connective tissue.

Gene A stretch along a chromosome the codes for function production (either RNA or its translation product, a polypeptide) A unit factor of heredity.

Generation time The Time necessary for growing cells to double their mass under specific conditions.

Genome The minimum complete set of all different chromosomes found in each nucleus of a given species.

Glia (neuroglia) A tissue mainly composed of a network of protoplasmic and long fibrous branching dendrons and axons of nerve cells; it provides support for nerve cell bodies and nerve fibers, especially in the brain and spinal cord.

Glycolysis (Embden-Meyerhoff pathway) The fermentation of glucose to lactic acid with the production of ATP. Glycolysis takes place in the cytoplasm.

Hertz (Hz) The frequency of a periodic oscillation given in cycles per second.

Infrared radiation Electromagnetic radiation for which the wavelengths are within the spectrum range of 0.7 µm up to 100 µm. this portion of the spectrum is often separated into three bands by wavelength: IR-A (0.7µm - 0.78µm to 1.4µm ). or ear-infrared; OR - B (1.4µm to 3µm ) and IR-C (3µm to 100µm ) or far - infrared.

Intensity (power density) The amount of light energy incident per unit area per second. For human viewing of visible light, the usual term is illuminance; for electromagnetic radiation in general. The term is usually radiant flux.

Interphase (G0) The resting stage of a cell between two mitotic divisions.

Irradiance or power density is the incident power per unit area. It is typically specified in milliwatts per square centimeter (mW/cm2). For the calculation of irradiance for in-contact treatments, the unit's radiant power output is divided by the spot size.



Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. Lasers generate or amplify electromagnetic oscillations at wavelengths from the far infrared (submillimeter) to the ultraviolet. The laser oscillator needs two basic elements: an amplifying medium and a regeneration or feedback mechanism (resonant cavity). the amplifying medium can be a variety of substances, such as gas, semiconductor, dye solution etc.

Lectins Cell-aggulating proteins. Often used for mitogenic activation of lymphocytes.

LED A light-emitting diode. A semiconductor device in which light is produced when current carriers combine as a pnjunction. The emission is spontaneous and there are no feedback mirrors, unlike in diode lasers and specialist PhotoTherapy products of a specific standard.

Leukocyte (white blood cell) A nucleated motile colourless cell found in the blood and lymph.

Light Visible electromagnetic radiation (400 nm to 700 nm).

Linear absorption coefficient The reciprocal of the distance (in units cm2) in a thick, nonscattering layer, in which the flux density or intensity in the direction of an incident collimated light beam decreases by a factor of 1/e; also the absorption cross section (in units cm2).

Low-intensity laser therapy (LILT) is the recommended terms to describe the use of laser devices as a therapeutic modality. Alternative terms such as 'cold laser therapy', 'soft laser therapy' and 'low power laser therapy' are inappropriate and misleading.

Low (reactive) level laser therapy (LLLT) is an acceptable alternative to low-intensity laser therapy (LILT).

Lymphokines Factors released by antifenically stimulated T lymphocytes which attract phagocytic monocytes.

Lymphocyte White blood cell, found in large numbers in higher vertebrates, which is characterised by a seemingly inert nucleus surrounded by only traces of an endoplasatic reticulum - poor cytoplasm (Small lymphocytes). they all into two groups, B and T cells, which are indistinguishable under the light microscope.

Lysome A membrane bounded particle, smaller than a mitochondrion, occurring in cytoplasm. They contain hydrolic enzymes which are liberated when the cell is damaged.

Macrophage Large, phagocytic white blood cell with one nucleus found at the sites of corpuscle formation, e.g. in bone marrow, lymph nodes, spleen etc. Their function is to remove foreign particles from blood and lymph. Macrophages are also found in loose connective tissue, but they only become active when tissue is damaged.

Medium is the central component if a device; the medium determines the wavelength(s) of light or near infra-red radiation emitted by the device.

Metabolic pathway A set of consecutive intracellular enzymatic reactions that convert one molecule to another.

Metabolism The sum total of various chemical reactions occurring in a living cell.

Mitochondrion Organelle about 0.5 µm in width and up to 10 µm in length found in the cytoplasm of all aerobic eukaryotic cells; the centre of ATP generation through oxidative phorphorylation.

Mitogen Substance which provokes cell division (mitosis). Examples are lectins like phytohemagglutinin and concanavalin A.

Mitosis Process whereby chromosomes duplicate and segregate accompanied by cell division. The process takes place in four phases: prophase, metaphase, anaphase and telophase.

Monochromatic Light with narrow spectral bandwidth.

Monochromicity literally means 'single coloured'; monochromatic light is 'practically all at one wavelength' with very narrow bandwidth.

Monocyte the largest leukocyte (macrophage) found in the blood. They are phagocyte cells.

Monolayer A layer of cells which is informally one cell thick.

mRNA Messenger RNA that serves as a template for protein synthesis.

Multiple scattering Interactions involving light, in which the photons are scattered more than once before being absorbed or escaping from the medium.

Mutagenes Physical and chemical agents, such as radiation, heat, or alkylating or dominating agents, which raise the frequency of mutations greatly above the spontaneous background level.

Myelocyte Marrow cell in the long bones which are possibly the precursor of leucocytes.

NAD Nicotinamide adenine dinucleotide. A coenzyme taking part in redox reactions. It oxidizes substrates by removing NADH. The reduced form reduces substrates and is itself oxidized to reconstitute NAD. This cyclical process is of fundamental importance and the ratio of NAD to NADP in a cell plays an important part in controlling the rate of energy production.

NADP Nicotinamide adenine dinucleotide phosphate. A coenzyme taking part in redox reactions. It oxidizes substrates by removing hydrogen and is itself changed to the reduced form of NADPH. The reduced form reduces substrates and is itself oxidized to reconstitute NADP.

Nerve cell A uninucleated cell, with cytoplasm containing fibris and with long thread like processes (axons and dendrites) arising from the cell body. Nerve cells are usually taken to exclude the process but the term is sometimes used instead of neuron.

Nonlinear effects Optical interactions which are proportional to the square or higher powers of light intensities. Passive optical transmission of light, such as by a lens, is linear transmission. Nonlinear effects generate harmonies of optical frequencies and sum and difference frequencies when two light waves are mixed. Nonlinear materials are materials in which nonlinear effects are likely to occur at moderate power levels.

Nucleous Round granular structures found in nucleus of eukaryotic cells usually associated with specific chromsomal site. Involved in rRNA synthesis and ribosome formation.

Nucleus A spherical or ovoid body present in almost all living cells of animals and plants but absent in bacteria and blue green algae. They contain the structure controlling the reproduction and functioning of the cells.

Oncogene The gene responsible for inducing the transformed phenotype.

Optical density A measurement of transmission equal to the base-10 logarithm of the reciprocal of transmittance (the fraction of the incident light transmitted). An object with optical density of zero is transparent; an optical density of one corresponds to 10 percent transmission.

Output is lower in power reflecting the use of very very low operating current.

Oxidative phosphorylation Coupled electron transfer and ATP formation which occurs on the mitochondrial membrane.

PDT Photodynamic therapy. A modality which requires the presence of a photosensitizer and light.

Peak power The quantity of energy in a single light source commonly in laser the pulse multiplied by the length of the pulse duration.

Peroxisoms Intracellular organelles which contain a fine granular matrix and often have cristaelike cores. They have been found to contain four enzymes involved in hydrogen peroxide metabolism, including the H2O2 - degradative enzyme catalase. They may be important in purine degradation, hostorespiration and the metabolic pathway known as the glyoxylate cycle.

PHA mitogenic lectin, phytohaemagglutuinin.

Phagocyte A white blood cell which engulfs foreign bodies particularly pathogens. In mammals polymorponuclear leukocytes, monocytes and macrophages are phagocytes.

Phagocytosis A process for food gathering employed by phagocytes. This process involves the surrounding and subsequent digestion of cellular-sized objects by pseudopod-like projections.

Photoinhibition is the term used to describe the inhibitory effects of light upon biological functions (see also below).

Photobiostimulation is the term used to describe the stimulatory effects of light upon biological functions e.g. in clinical practice this might be various wound healing processes. The term has been inappropriately used as an alternative to the more precise generic terms such as photobiomodulation or low-intensity laser therapy (LILT)

Photodynamic action A biological photosensitization that requires the presence of molecular oxygen.

Photometric units A system of optical units based on the visual response of the standard, light adapted human eye.

Photon A quantum of electromagnetic radiation; photons travel at the speed of light and have energy hv, where he is Planck's constant (6.625x10-34 Jxs) and v is the frequency of Hz. Quantum properties of electromagnetic radiation take place during absorption, emission etc.

Phot-redox reaction A photochemical reaction that is equivalent to electron transfer from a donor molecule to an acceptor.

Photosensitization A process that requires both light and a chromophoric photosensitizing agent in order to take place.

Phototherapy a Treatment unitizing UV, visible or near IR radiation for medical purposes.

Phototoxicity Toxicity initiated by exposure of a biological object to light associated with the presence of chemical or drug.

Plasmacyte The cell differentiated from antigenically stimulated B lymphocyte. They are characterized by a very rough endoplasmatic reticulum and secretion of massive amounts of antibodies.

Plasma membrane (cell membrane, plasmalemma, plasmic membrane) Physical barrier that surrounds the cell surface and encloses the cytoplasm. The membrane is semipermiable and largely composed of lipids and proteins. About 10 nm thick.

Polarisation refers to the orientation of waves of light. The direction of the electric vector of electromagnetic radiation in the plane perpendicular to the direction of propagation.

Polymorphonuclear leukocyte A polynucleated, irregularly shaped white blood cell, exhibiting amoeboid movement. Active phagocytes are continually produced in the bone marrow and constitute about 70% of all leukocytes in humans. Polymorphonclear leukocytes are divided into eosinophils, basophils and neutrophils. All three types increase in number during infection.

Power density (intensity) The power per unit area (W/m2 for example) contained in a light beam or falling on a given target area. It is the same as the radiometric term irradiance Intensity of light is the power per unit of area.

Power / energy meter An instrument which measures the amount of optical power (typically in watts) or energy (joules). I can operate in the visible, infrared, or ultraviolet region, and detect pulsed or continuous beams.

Power output see Irradiance

Power output describes the output of those devices which allow the radiant power output to be delivered as short pulses of energy. Pulsing can be produced by chopping the output of continuous wave devices, modulating the diode's output or by using a diode which is inherently pulsed.

Primary photochemical reaction A photochemical reaction specifying the initial chemical species generated by light absorption.

Psoralen-plus UVA (PUVA) A photochemotherapy for skin disorders utilizing the combined action of a psoralen drug and UVA.

Pulse duration (pulse width) The time period between the half peak power points at the front and trailing edges of a pulse.

Pulsed laser A laser that generates energy in pulses rather than continuously.

Quantum A small portion of energy associated with discrete events (absorption, emission) of interaction of light and matter.

Quantum efficiency (quantum yield) The probability that the absorption of a photon of a specified wavelength will induce a selected molecular response; also referred to as quantum yield.

Radiant exposure or energy density is the energy delivered per unit area. It is usually measured and specified in joules per square centimeter (J/cm2) and is given by multiplying the radiant power output (in watts) by the time of irradiation (in seconds) and dividing the product by the area of irradiation (in square centimeters).

Radiant power output of a therapeutic laser ir phototherapy unit is usually specified in milliwatts. This gives a measure of the number of photons emitted per second.

Repetition rate The rate at which pulses are repeated. For lasers, typically measured as pulses per second (Hz). Sometimes lasers are said to be repetitively pulsed when they produce pulses regularly at fixed intervals, rather than producing single pulses.

Ribosome Small cellular particles (10 to 15 nm in diameter) made up of rRNA and protein. Ribosomes are the site of protein synthesis. They occur mainly in the endoplasm of a cell, but also on the nuclear membrane.

RNA (ribonucleic acid) A polymer of riobonucleotids. A molecule of RNA consists of one strand.

rRNA Ribosmal RNA. The nucleic acid component of ribosomes, making up two thirds of the mass of the ribosome in E. coli and about one-half the mass of mammalian ribosomes.

Schawann cell A long thing uninucleated cell forming part of a the neurilemma (a think outer coat surrounding nerve fibers). rRNA Ribosomal RNA The nucleic acid component of ribosome's, making up two thirds of the mass of ribosome in E. coli and about one half the mass of mammalian ribosome's.

Singlet molecular oxygen Molecular oxygen that is in the lowest excited electronic state. gather are two singlet states of the oxygen.

Singlet energy transfer The intermolecular transfer of energy fro the excited singlet state of a donor to the singlet ground state of an acceptor, resulting in the acceptor being in the excited singlet state.

Singlet state Spectroscopic notation for an atom, a molecule, or an ion in which all electron spins are paired resulting in a total spin of zero.

Spatial coherence The coherence (or phase correlation) of light over an area of the wavefront of a beam.

Spontaneous emission of radiation occurs when an excited electron spontaneously returns to its resting level and gives off a photo of light energy as a result

Stimulated emission of radiation typically occurs when a photon of a precise quantal energy interacts with an excited atom to give rise to a second photon, which will carry exactly the same energy as the first.

Superluminious diode (SLD) is a diode source, the output of which is exactly like that of a laser diode, except for the property of coherence. The clinical relevance of coherence remains a subject of ongoing debate.

T lymphocyte Small lymphocyte which processes the surface antigen and direct cell medicated immune response.

Temporal coherence The coherence (or phase correlation) of light over time. Light is temporarily coherent when the phase change during a time interval remains constant regardless of when the interval is measured.

Threshold process A process in which the observable response does not start until a threshold or conditioning dose has accumulated. Tissue optics Trends in optics dealing with light propagation (including absorption, scattering etc) in biological tissues.

Triplet state Spectroscopic notation for an atom, molecule, or ion in which two electron spins are unpaired.

Type I photosensitization A photosensitized chemical reaction in which the initial interactions of the excited photosensitizer occur with the substrate.

Type II photosensitization A photosensitized chemical reaction in which the initial interactions of the excited photosensitizer occur with molecular oxygen, resulting in the production of singlet molecular oxygen.

Ultraviolet radiation Electromagnetic radiation that has wavelengths from soft X-rays down to visible, violet light. This spectral region is frequently categorized into three separate nads of wavelength: UV-A (315 - 400 nm) UV-B (280 - 315 nm) and UV-C (200 - 280 nm).

Visible spectrum When white light is passed through a prism of diffraction grating a contentious spectrum is formed. The centre part of this is visible from the blue to the red end.

Visible radiation (light) Electromagnetic radiation that can be sensed by the human eye. Generally used to describe wavelengths between 400 nm to 700 nm.

Watt (w) The unit of power or radiant flux - one joules per second.

Wavelength is typically expressed in nanometres (nm) (or less commonly micrometers - µm) and, expressed in terms of the wave theory of light, is the distance between one positive maximum of the electromagnetic field and the next. At a given wavelength of light, all photons carry exactly the same energy.

White light Mixture of colours - specific wavelengths of visible light that appear white to the eye. Mixture of three colours or specific wavelengths is sufficient to produce white light.







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