# Im

**Image Circle**- The circular image field over which image quality is acceptable, can be defined in terms of its angular subtense.
**Image Frequency**- An undesired frequency capable of producing the desired frequency through heterodyning.
**Image Inversion**- Change in the orientation of an image in one meridian.
**Image Plane**- The plane perpendicular to the optical axis at the image point.
**Image Rejection**- The decrease in response of a superheterodyne receiver to the image frequency as compared with its response to the desired signal, usually expressed in decibels.
**Imaginary Axis**- The y-axis of an Argand diagram.
**Imaginary Number**- i denotes the imaginary number formed by square rooting negative one.
**Imaginary Part**- The imaginary part of a complex number.
**Imbalance**- Unequal radial weight distribution on a rotor system.
**IMEP**- During the cycle of an engine useful work is only done on the power stroke.
**Immiscible**- Incapable of being mixed.
**Immobilised Electrolyte**- An electrolyte made motionless by use of a gel additive or AGM separator.
**Impact**- A collision of a mass in motion with a second mass.
**Impact Energy**- A measure of the energy absorbed during the fracture of a specimen of standard dimensions and geometry when subjected to very rapid loading
**Impact Harshness**- The interior sound and vibration resulting from tyre interactions with discrete road disturbances.
**Impact Isolation Class**- A measure or specification of isolation effectiveness of building structures from impact noises such as slammed doors, dropped objects, footfalls, shuffled furniture, etc. The higher the IIC rating, the better such isolation.
**Impact Pressure**- Another name for Dynamic Pressure.
**Impact Sound**- The sound produced by the collision of two solid objects.
**Impact Test**- A test to determine the behavior of materials when subjected to high rates of loading, usually in bending, tension or torsion.
**Impact Testing**- A method of measuring the frequency response function of a structure by hitting it with a calibrated hammer and measuring the system′s response.
**Impedance**- Mechanical impedance is the ratio of a force-like quantity to a velocity like quantity when the arguments of the real (or imaginary) parts of the quantities increase linearly with time.
**Impedance Matching**- Maximum power is transferred from one circuit to another when the output impedance of the one is matched to the input impedance of the other.
**Impedance of Free Space**- Z
_{0}= 376.7304 Ohm **Impeller**- A device that imparts motion to a fluid.
**Implosion**- A violent inward collapse.
**Improper Fractions**- A fraction whose numerator is of greater absolute value than it's denominator.
**Impulse**- The product of the force and the time during which it acts.
**Impulse Response**- The response of a system to a unit impulse or Dirac′s delta function. The Fourier transform of the impulse response is the frequency response function.
**Impulsive Noise**- A noise level that fluctuates over a range greater than 10 dB during observation is classified as impulsive.
**Impurities**- Undesirable foreign material in a pure substance or mixture.
**Impurity**- A foreign atom in a crystal.