# List of Terms related to Strain Measurement with Optical Sensors

λ | Wavelength | Wavelength is the measured peak wavelength of a fiber Bragg grating sensor. It is normally expressed in nanometers (nm). |

λ_{0} | Reference wavelength | The reference wavelength is the peak wavelength of a fiber Bragg grating sensor at a reference condition (zero strain, at reference temperature, and so forth.. It is normally expressed in nanometers (nm). |

Δλ | Wavelength variation | The wavelength variation (also commonly referred to as shift or as change) is the difference between the wavelength and the reference wavelength (reference value): Δλ= λ- λ |

k | k-factor | The gauge factor k (also referred to as k-factor) of an optical strain gauge is the proportional change in the Bragg wavelength (Δλ/λ_{0}) and the strain variation Δε. Is is being measured as: Δλ/λ_{0} = k.Δε. This value is a dimensionless number and depends on the characteristically used optical fiber and sensor encapsulation. In the case of HBM optical strain sensors, the k-factor is identified on the data- and calibration sheets that are individually delivered with each sensor. |

ε | Strain | Strain is a dimensionless value that represents the relative change in the length of a material to its initial length. It is normally of a very small value, and hence is represented by µm/m, ppm or 10 |

S | Sensitivity | The sensitivity of an optical strain sensor is the direct ratio between the measured strain and the change in the Bragg wavelength: Δε/Δλ= S. It is normally stated as value in micro-strain per nanometer [(µm/m)/nm)] and is different for every sensor, as it depends on its initial base wavelength, that is: S=1/(k. λ |

TCS | Temperature cross-sensitivity | The temperature cross-sensitivity is a sensor measurement drift caused by temperature variation. It is the strain that is wrongly measured when there is a change of 1ºC (or 1ºK) in temperature. The value is given in (µm/m)/ºC [or (µm/m)/ºK] and can be used to compensate the effect of temperature on the optical strain sensor (not considering the compensation for the thermal expansion of the specimen). |

σ | Stress | Mechanical stress is expressed by the quotient of the force F and the cross-sectional area A of the stressed material, σ=F/A. It is normally represented in KPa. |

E | Elastic modulus | The modulus of elasticity, or Young’s modulus, is the ratio between stress and strain in a linear elastic material. It is given by Hooke’s Law (σ=E.ε). It is normally represented in GPa (10 |

v | Poisson's ratio | Poisson's ratio is defined by the division of the transverse strain ε |