Animation of the dispersion of light as it travels through a triangular prism

Spectroscopy was originally the study of the interaction between radiation and matter as a function of wavelength (λ). In Optics, dispersion is the phenomenon in which the Phase velocity of a wave depends on its frequency A triangle is one of the basic Shapes of Geometry: a Polygon with three corners or vertices and three sides or edges which are Line Radiation, as in Physics, is Energy in the form of waves or moving Subatomic particles emitted by an atom or other body as it changes from a higher energy Matter is commonly defined as being anything that has mass and that takes up space. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. In fact, historically, spectroscopy referred to the use of visible light dispersed according to its wavelength, e. g. by a prism. In Optics, a prism is a transparent optical element with flat polished surfaces that refract Light. Later the concept was expanded greatly to comprise any measurement of a quantity as function of either wavelength or frequency. Frequency is a measure of the number of occurrences of a repeating event per unit Time. Thus it also can refer to interactions with particle radiation or to a response to an alternating field or varying frequency (ν). Particle radiation is the radiation of Energy by means of fast-moving Subatomic particles. A further extension of the scope of the definition added energy (E) as a variable, once the very close relationship E=hν for photons was realized. In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός The Planck constant (denoted h\ is a Physical constant used to describe the sizes of quanta. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena A plot of the response as a function of wavelength — or more commonly frequency — is referred to as a spectrum; see also spectral linewidth. A spectrum (plural spectra or spectrums) is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The spectral linewidth characterizes the width of a Spectral line, such as in the electromagnetic emission spectrum of an atom or the Frequency spectrum

Spectrometry refers to when a spectroscopic technique is used to assess the concentration or amount of a given species. In those cases, the instrument that performs such measurements is a spectrometer or spectrograph. A spectrometer is an Optical instrument used to measure properties of Light over a specific portion of the Electromagnetic spectrum, typically used

Spectroscopy/spectrometry is often used in physical and analytical chemistry for the identification of substances through the spectrum emitted from or absorbed by them. Physical chemistry, is the application of Physics to macroscopic microscopic atomic subatomic and particulate phenomena in chemical systems It is mostly defined as a large Analytical chemistry is the study of the Chemical composition of natural and artificial Materials.

Spectroscopy/spectrometry is also heavily used in astronomy and remote sensing. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study Remote sensing is the small or large-scale acquisition of information of an object or phenomenon by the use of either recording or real-time sensing device(s that is not in physical Most large telescopes have spectrometers, which are used either to measure the chemical composition and physical properties of astronomical objects or to measure their velocities from the Doppler shift of their spectral lines. A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared

## Classification of methods

Extremely high resolution spectrum of the Sun showing thousands of elemental absorption lines (fraunhofer lines)

### Nature of excitation measured

The type of spectroscopy depends on the physical quantity measured. The Sun (Sol is the Star at the center of the Solar System. In Physics and Optics, the Fraunhofer lines are a set of Spectral lines named for the German physicist Joseph von Fraunhofer ( 1787 Normally, the quantity that is measured is an intensity, either of energy absorbed or produced.

• Electromagnetic spectroscopy involves interactions of matter with electromagnetic radiation, such as light. Electromagnetic spectroscopy is the Spectroscopy of electromagnetic spectra which arise out of Atoms absorbing and emitting quanta of Electromagnetic Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700
• Electron spectroscopy involves interactions with electron beams. Electron Spectroscopy is an analytical technique to study the electronic structure and its dynamics in Atoms and Molecules In general an excitation source Cathode rays (also called an electron beam or e-beam) are streams of Electrons observed in Vacuum tubes i Auger spectroscopy involves inducing the Auger effect with an electron beam. Auger electron spectroscopy (AES Auger oːʒeː in French is a common analytical technique used specifically in the study of surfaces and more generally in the area of The Auger effect (ˈɔːʒɚ or Oh' jeh is a phenomenon in Physics in which the emission of an Electron from an Atom causes the emission of a second In this case the measurement typically involves the kinetic energy of the electron as variable.
• Mass spectrometry involves the interaction of charged species with magnetic and/or electric fields, giving rise to a mass spectrum. Mass spectrometry is an analytical technique that identifies the chemical composition of a compound or sample based on the Mass-to-charge ratio of charged particles An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge The term "mass spectroscopy" is deprecated, for the technique is primarily a form of measurement, though it does produce a spectrum for observation. This spectrum has the mass m as variable, but the measurement is essentially one of the kinetic energy of the particle.
• Acoustic spectroscopy involves the frequency of sound.
• Dielectric spectroscopy involves the frequency of an external electrical field
• Mechanical spectroscopy involves the frequency of an external mechanical stress, e. Dielectric Spectroscopy (sometimes called impedance spectroscopy) measures the Dielectric properties of a medium as a function of Frequency g. a torsion applied to a piece of material.

### Measurement process

Most spectroscopic methods are differentiated as either atomic or molecular based on whether or not they apply to atoms or molecules. History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by Along with that distinction, they can be classified on the nature of their interaction:

• Absorption spectroscopy uses the range of the electromagnetic spectra in which a substance absorbs. Absorption spectroscopy refers to a range of techniques employing the interaction of electromagnetic radiation with matter This includes atomic absorption spectroscopy and various molecular techniques, such as infrared spectroscopy in that region and nuclear magnetic resonance (NMR) spectroscopy in the radio region. In Analytical chemistry, atomic absorption spectroscopy is a technique for determining the concentration of a particular metal element in a sample Infrared spectroscopy (IR spectroscopy is the subset of Spectroscopy that deals with the Infrared region of the Electromagnetic spectrum. Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is the name given to a technique which exploits the magnetic properties of certain nuclei Radio frequency ( RF) is a Frequency or rate of Oscillation within the range of about 3 Hz to 300 GHz
• Emission spectroscopy uses the range of electromagnetic spectra in which a substance radiates (emits). Emission spectroscopy is a spectroscopic technique which examines the wavelengths of Photons emitted by atoms or molecules during their transition from an The substance first must absorb energy. This energy can be from a variety of sources, which determines the name of the subsequent emission, like luminescence. Luminescence is also the title of an album by singer Anggun. Luminescence is Light not generated by high temperatures alone Molecular luminescence techniques include spectrofluorimetry. Fluorescence spectroscopy aka fluorometry or spectrofluorometry is a type of Electromagnetic spectroscopy which analyzes Fluorescence from a sample
• Scattering spectroscopy measures the amount of light that a substance scatters at certain wavelengths, incident angles, and polarization angles. The scattering process is much faster than the absorption/emission process. One of the most useful applications of light scattering spectroscopy is Raman spectroscopy. Raman spectroscopy (pronounced S— is a spectroscopic technique used in Condensed matter physics and Chemistry to study vibrational rotational and

## Common types

### Absorption

Absorption spectroscopy is a technique in which the power of a beam of light measured before and after interaction with a sample is compared. Absorption spectroscopy refers to a range of techniques employing the interaction of electromagnetic radiation with matter When performed with tunable diode laser, it is often referred to as Tunable diode laser absorption spectroscopy (TDLAS). Tunable diode laser absorption spectroscopy ( TDLAS) is a technique for measuring the concentration of certain species such as Methane, Water vapor Tunable diode laser absorption spectroscopy ( TDLAS) is a technique for measuring the concentration of certain species such as Methane, Water vapor It is also often combined with a modulation technique, most often wavelength modulation spectrometry (WMS) and occasionally frequence modulation spectrometry (FMS) in order to reduce the noise in the system.

### Fluorescence

Spectrum of light from a fluorescent lamp showing prominent mercury peaks

Fluorescence spectroscopy uses higher energy photons to excite a sample, which will then emit lower energy photons. A fluorescent lamp or fluorescent tube is a Gas-discharge lamp that uses Electricity to excite mercury Vapor. Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum Fluorescence spectroscopy aka fluorometry or spectrofluorometry is a type of Electromagnetic spectroscopy which analyzes Fluorescence from a sample In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena This technique has become popular for its biochemical and medical applications, and can be used for confocal microscopy, fluorescence resonance energy transfer, and fluorescence lifetime imaging. Biochemistry is the study of the chemical processes in living Organisms It deals with the Structure and function of cellular components such as Confocal microscopy is an optical imaging technique used to increase Micrograph contrast and/or to Reconstruct three-dimensional Images by Förster resonance energy transfer (abbreviated FRET) also known as Fluoresence resonance energy transfer or resonance energy transfer ( RET Fluorescence lifetime imaging or FLIM is a powerful tool for producing an image based on the differences in the exponential decay rate of the Fluorescence from a fluorescent

### X-ray

When X-rays of sufficient frequency (energy) interact with a substance, inner shell electrons in the atom are excited to outer empty orbitals, or they may be removed completely, ionizing the atom. X-ray spectroscopy is a gathering name for several spectroscopic techniques for determining the electronic structure of materials by using X-ray excitation X-ray crystallography is a method of determining the arrangement of Atoms within a Crystal, in which a beam of X-rays strikes a crystal and scatters The inner shell "hole" will then be filled by electrons from outer orbitals. The energy available in this de-excitation process is emitted as radiation (fluorescence) or will remove other less-bound electrons from the atom (Auger effect). The absorption or emission frequencies (energies) are characteristic of the specific atom. In addition, for a specific atom small frequency (energy) variations occur which are characteristic of the chemical bonding. With a suitable apparatus, these characteristic X-ray frequencies or Auger electron energies can be measured. X-ray absorption and emission spectroscopy is used in chemistry and material sciences to determine elemental composition and chemical bonding.

X-ray crystallography is a scattering process; crystalline materials scatter X-rays at well-defined angles. If the wavelength of the incident X-rays is known, this allows calculation of the distances between planes of atoms within the crystal. The intensities of the scattered X-rays give information about the atomic positions and allow the arrangement of the atoms within the crystal structure to be calculated.

### Flame

Liquid solution samples are aspirated into a burner or nebulizer/burner combination, desolvated, atomized, and sometimes excited to a higher energy electronic state. The use of a flame during analysis requires fuel and oxidant, typically in the form of gases. Common fuel gases used are acetylene (ethyne) or hydrogen. Acetylene ( IUPAC name ethyne), C2H2 is a Hydrocarbon belonging to the group of Alkynes It is the simplest of all alkynes Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Common oxidant gases used are oxygen, air, or nitrous oxide. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five Nitrous oxide, commonly known as " laughing gas," is a Chemical compound with the Chemical formula N 2 O. These methods are often capable of analyzing metallic element analytes in the part per million, billion, or possibly lower concentration ranges. "Parts-per" notation is used especially in Science and Engineering, to denote Ratios (relative proportions in measured quantities particularly In Chemistry, concentration is the measure of how much of a given substance there is mixed with another substance Light detectors are needed to detect light with the analysis information coming from the flame.

• Atomic Emission Spectroscopy - This method uses flame excitation; atoms are excited from the heat of the flame to emit light. This method commonly uses a total consumption burner with a round burning outlet. A higher temperature flame than atomic absorption spectroscopy (AA) is typically used to produce excitation of analyte atoms. Since analyte atoms are excited by the heat of the flame, no special elemental lamps to shine into the flame are needed. A high resolution polychromator can be used to produce an emission intensity vs. A polychromator is an Optical device that is used to disperse Light into different directions to isolate parts of the Spectrum of the light wavelength spectrum over a range of wavelengths showing multiple element excitation lines, meaning multiple elements can be detected in one run. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. Alternatively, a monochromator can be set at one wavelength to concentrate on analysis of a single element at a certain emission line. A monochromator is an optical device that transmits a mechanically selectable narrow band of Wavelengths of Light or other Radiation chosen from Plasma emission spectroscopy is a more modern version of this method. See Flame emission spectroscopy for more details. Emission spectroscopy is a spectroscopic technique which examines the wavelengths of Photons emitted by atoms or molecules during their transition from an
• Atomic absorption spectroscopy (often called AA) - This method commonly uses a pre-burner nebulizer (or nebulizing chamber) to create a sample mist and a slot-shaped burner which gives a longer pathlength flame. In Analytical chemistry, atomic absorption spectroscopy is a technique for determining the concentration of a particular metal element in a sample The temperature of the flame is low enough that the flame itself does not excite sample atoms from their ground state. The nebulizer and flame are used to desolvate and atomize the sample, but the excitation of the analyte atoms is done by the use of lamps shining through the flame at various wavelengths for each type of analyte. In AA, the amount of light absorbed after going through the flame determines the amount of analyte in the sample. A graphite furnace for heating the sample to desolvate and atomize is commonly used for greater sensitivity. The graphite furnace method can also analyze some solid or slurry samples. Because of its good sensitivity and selectivity, it is still a commonly used method of analysis for certain trace elements in aqueous (and other liquid) samples.
• Atomic Fluorescence Spectroscopy - This method commonly uses a burner with a round burning outlet. The flame is used to solvate and atomize the sample, but a lamp shines light at a specific wavelength into the flame to excite the analyte atoms in the flame. The atoms of certain elements can then fluoresce emitting light in a different direction. Fluorescence is a Luminescence that is mostly found as an The intensity of this fluorescing light is used for quantifying the amount of analyte element in the sample. A graphite furnace can also be used for atomic fluorescence spectroscopy. This method is not as commonly used as atomic absorption or plasma emission spectroscopy.

Plasma Emission Spectroscopy In some ways similar to flame atomic emission spectroscopy, it has largely replaced it.

• Direct-current plasma (DCP)

A direct-current plasma (DCP) is created by an electrical discharge between two electrodes. A plasma support gas is necessary, and Ar is common. Samples can be deposited on one of the electrodes, or if conducting can make up one electrode.

• Glow discharge-optical emission spectrometry (GD-OES)
• Microwave-induced plasma (MIP)

Spark or arc (emission) spectroscopy - is used for the analysis of metallic elements in solid samples. Inductively coupled plasma atomic emission spectroscopy (ICP-AES also referred to as inductively coupled plasma optical emission spectrometry (ICP-OES is an analytical technique used Laser-induced breakdown spectroscopy (LIBS is a type of atomic emission Spectroscopy which utilises a highly energetic Laser pulse as the excitation source For non-conductive materials, a sample is ground with graphite powder to make it conductive. In traditional arc spectroscopy methods, a sample of the solid was commonly ground up and destroyed during analysis. An electric arc or spark is passed through the sample, heating the sample to a high temperature to excite the atoms in it. The excited analyte atoms glow emitting light at various wavelengths which could be detected by common spectroscopic methods. Since the conditions producing the arc emission typically are not controlled quantitatively, the analysis for the elements is qualitative. Nowadays, the spark sources with controlled discharges under an argon atmosphere allow that this method can be considered eminently quantitative, and its use is widely expanded worldwide through production control laboratories of foundries and steel mills.

### Visible

Many atoms emit or absorb visible light. In order to obtain a fine line spectrum, the atoms must be in a gas phase. This means that the substance has to be vaporised. The spectrum is studied in absorption or emission. Visible absorption spectroscopy is often combined with UV absorption spectroscopy in UV/Vis spectroscopy. Ultraviolet-visible spectroscopy or ultraviolet-visible spectrophotometry ( UV/ VIS) involves the Spectroscopy of Photons in the UV-visible Although this form may be uncommon as the human eye is a similar indicator, it still proves useful when distinguishing colours.

### Ultraviolet

All atoms absorb in the Ultraviolet (UV) region because these photons are energetic enough to excite outer electrons. Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays If the frequency is high enough, photoionisation takes place. Photoionisation is the physical process in which an incident Photon ejects one or more Electrons from an Atom, Ion or Molecule UV spectroscopy is also used in quantifying protein and DNA concentration as well as the ratio of protein to DNA concentration in a solution. Several amino acids usually found in protein, such as tryptophan, absorb light in the 280nm range and DNA absorbs light in the 260nm range. For this reason, the ratio of 260/280nm absorbance is a good general indicator of the relative purity of a solution in terms of these two macromolecules. Reasonable estimates of protein or DNA concentration can also be made this way using Beer's law. In

### Infrared

Main article: Infrared spectroscopy

Infrared spectroscopy offers the possibility to measure different types of inter atomic bond vibrations at different frequencies. Infrared spectroscopy (IR spectroscopy is the subset of Spectroscopy that deals with the Infrared region of the Electromagnetic spectrum. Especially in organic chemistry the analysis of IR absorption spectra shows what type of bonds are present in the sample. Organic chemistry is a discipline within Chemistry which involves the scientific study of the structure properties composition reactions, and preparation

### Raman

Main article: Raman spectroscopy

Raman spectroscopy uses the inelastic scattering of light to analyse vibrational and rotational modes of molecules. Raman spectroscopy (pronounced S— is a spectroscopic technique used in Condensed matter physics and Chemistry to study vibrational rotational and The resulting 'fingerprints' are an aid to analysis.

### Nuclear magnetic resonance

Main article: NMR spectroscopy

Nuclear magnetic resonance spectroscopy analyzes the magnetic properties of certain atomic nuclei to determine different electronic local environments of hydrogen, carbon, or other atoms in an organic compound or other compound. Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is the name given to a technique which exploits the magnetic properties of certain nuclei Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 An organic compound is any member of a large class of Chemical compounds whose Molecules contain Carbon. A chemical compound is a substance consisting of two or more different elements chemically bonded together in a fixed proportion by Mass. This is used to help determine the structure of the compound. Chemical structure refers to Molecular geometry, Electronic structure and Crystal structure.

### Photoemission

Main article: Photoemission

### Mössbauer

Transmission or conversion-electron (CEMS) modes of Mössbauer spectroscopy probe the properties of specific isotope nuclei in different atomic environments by analyzing the resonant absorption of characteristic energy gamma-rays known as the Mössbauer effect. Mössbauer spectroscopy (Mößbauer is a spectroscopic technique based on the Mössbauer effect. The nucleus of an Atom is the very dense region consisting of Nucleons ( Protons and Neutrons, at the center of an atom Gamma rays (denoted as &gamma) are a form of Electromagnetic radiation or light emission of frequencies produced by sub-atomic particle interactions The Mössbauer effect (Mößbauer) a physical phenomenon discovered by Rudolf Mößbauer in 1957, refers to the resonant and recoil-free emission and absorption

## Other types

• Photoacoustic spectroscopy measures the sound waves produced upon the absorption of radiation. Photoacoustic spectroscopy is based on the Photoacoustic effect.
• Photothermal spectroscopy measures heat evolved upon absorption of radiation. Photothermal spectroscopy is a group of high sensitivity Spectroscopy techniques used to measure optical absorption and thermal characteristics of a sample
• Circular Dichroism spectroscopy
• Raman optical activity spectroscopy exploits Raman scattering and optical activity effects to reveal detailed information on chiral centers in molecules. Circular dichroism (CD is a form of Spectroscopy based on the differential absorption of left- and right-handed circularly polarized Light. Raman optical activity (ROA is a vibrational spectroscopic technique that is reliant on the difference in intensity of Raman scattered right and left circularly polarised
• Terahertz spectroscopy uses wavelengths above infrared spectroscopy and below microwave or millimeter wave measurements. Terahertz frequency radiation for spectroscopy is typically generated in one of three ways Time domain terahertz spectroscopy ( TDTS) using ultrashort
• Inelastic neutron scattering works like Raman spectroscopy, with neutrons instead of photons. Inelastic neutron scattering is an experimental technique commonly used in condensed matter research to study atomic and molecular motion as well as magnetic and crystal field This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron. In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena
• Inelastic electron tunneling spectroscopy uses the changes in current due to inelastic electron-vibration interaction at specific energies which can also measure optically forbidden transitions.
• Auger Spectroscopy is a method used to study surfaces of materials on a micro-scale. Auger electron spectroscopy (AES Auger oːʒeː in French is a common analytical technique used specifically in the study of surfaces and more generally in the area of It is often used in connection with electron microscopy. An electron microscope is a type of Microscope that uses Electrons to illuminate a specimen and create an enlarged image
• Cavity ring down spectroscopy
• Fourier transform is an efficient method for processing spectra data obtained using interferometers. This article specifically discusses Fourier transformation of functions on the Real line; for other kinds of Fourier transformation see Fourier analysis and The use of Fourier transform in spectroscopy is called Fourier transform spectroscopy. Fourier transform spectroscopy is a measurement technique whereby spectra are collected based on measurements of the temporal coherence of a radiative source Nearly all infrared spectroscopy (FTIR) and Nuclear Magnetic Resonance (NMR) spectroscopy are performed with Fourier transforms.
• Spectroscopy of matter in situations where the properties are changing with time is called Time-resolved spectroscopy. In Physics and Physical chemistry, time-resolved spectroscopy is the study of dynamic processes in materials or chemical compounds by means of spectroscopic
• Mechanical spectroscopy involves interactions with macroscopic vibrations, such as phonons. In Physics, a phonon is a quantized mode of vibration occurring in a rigid crystal lattice, such as the Atomic lattice of a Solid An example is acoustic spectroscopy, involving sound waves. Sound' is Vibration transmitted through a Solid, Liquid, or Gas; particularly sound means those vibrations composed of Frequencies
• Time-resolved spectroscopy
• Spectroscopy using an AFM-based analytical technique is called Force spectroscopy. In Physics and Physical chemistry, time-resolved spectroscopy is the study of dynamic processes in materials or chemical compounds by means of spectroscopic The atomic force microscope (AFM or scanning force microscope (SFM is a very high-resolution type of scanning probe microscope, with demonstrated resolution of fractions Force spectroscopy is a dynamic analytical technique that allows the study of the mechanical properties of single Polymer Molecules or Proteins, or individual
• Dielectric spectroscopy
• Thermal infrared spectroscopy measures thermal radiation emitted from materials and surfaces and is used to determine the type of bonds present in a sample as well as their lattice environment. Dielectric Spectroscopy (sometimes called impedance spectroscopy) measures the Dielectric properties of a medium as a function of Frequency Thermal infrared spectroscopy (TIR spectroscopy is the subset of Infrared spectroscopy that deals with radiation emitted in the infrared part of the Electromagnetic spectrum The techniques are widely used by organic chemists, mineralogists, and planetary scientists. Mineralogy is an Earth Science focused around the Chemistry, Crystal structure, and physical (including optical) properties of Minerals Planetary science, also known as planetology and closely related to planetary astronomy, is the Science of Planets or Planetary systems

## Background subtraction

Background subtraction is a term typically used in spectroscopy when one explains the process of acquiring a background radiation level (or ambient radiation level) and then makes an algorithmic adjustment to the data to obtain qualitative information about any deviations from the background, even when they are an order of magnitude less decipherable than the background itself.

Background subtraction can effect a number of statistical calculations (Continuum, Compton, Bremsstrahlung) leading to improved overall system performance.