Flow Glossary

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Barometric Pressure

  • is the level of the atmospheric pressure above a perfect vacuum.

Bernoulli Theorem

  • is a means of expressing the application of the law of conservation of energy to the flow of fluids in a conduit.

Critical Flow

  • the maximum possible velocity of a compressible liquid through a pipe or an orifice is its sonic velocity.

Critical Pressure Ratio

  • is the pressure ratio of upstream pressure over downstream pressure capable of producing sonic velocity.

Darcy Formula

  • shows the relation of potential energy, kinetic energy, and loss of energy due to friction flow.

Effective Flow Diameter

  • The diameter of an orifice that will pass a specific amount of air that is proportional to the square of the diameter As an example a one micro-meter orifice will pass .01749 cubic centimeters of air per minute when the inlet pressure is 29.7 PSIA and the outlet pressure is 14.7 PSIA or less.

Gas Constant

  • is 1544 divided by the molecular weight of the gas.

Gauge Pressure

  • is pressure measured above atmospheric pressure, while absolute pressure is pressure measured above perfect vacuum as a base.

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Kinetic Energy

  • the energy of bodies of mass in motion; typical units of measure are mass and velocity. K.E. = ½ mass x velocity squared.

Potential Energy

  • an energy form not directly associated with motion; chemical energy, compressed gas, or a body by virtue of its position is able to do work; units of measure are weight x height = foot - pounds or similar units of measure.

Specific Volume

  • is the reciprocal of the weight density.

Specific Gravity

  • of a liquid is the ratio of its weight density to that of water at 68°F. The specific gravity of gases is defined as the ratio of the molecular weight of the gas to that of air.

"Standard" Atmospheric Pressure

  • is 14.696 pounds per square inch, or 760 millimeters of

Standard Conditions

  • are 68° (528°R) or 20°C (293°C).


  • usually expressed in inches of mercury, is the depression of pressure below the atmospheric level. Reference to vacuum conditions is often made by expressing the absolute pressure in inches of mercury; also millimeters of mercury and microns of mercury.

Weight Density

  • Specific weight of a substance is its weight per unit volume.

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Optical Glossary

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  • Correctness, exactness; degree of conformity of a measure to a standard or true value; 3.14162 is accurate to 5 figures, precise to 6 figures.


  • Nominal or average atmospheric environmental surrounding, with reference to temperature, relative humidity and barometric pressure.


  • Qualitatively, any opening through which radiation, particle fluxes, liquids or gases may flow.

Black Body

  • A body, unit, surface or element that absorbs all the radiation that falls upon it.

Black Body Radiator

  • A hollow cavity that is a perfect absorber and perfect radiator of all the radiation incident upon it. It's shape is generally, but not limited to the blackened inside surface of a cone.


  • Differential Absorption LIDAR.


  • Opticians unit of measure for focal length. The reciprocal of the focal length in meters; the unit of "power of a lens".


  • The ratio of the radiation emitted by a surface (sample) to the radiation emitted by a blackbody (standard) at the same temperature and under similar conditions. The emissivity (ratio) can closely approach, but never be unity (1.0) or greater than unity. In summary, the emissivity value describes the reflective and absorptive characteristic of a blackened surface in the infrared wavelengths.


  • The ability to do work; the work done by a force; it is a conserved quantity, which can be neither created nor destroyed; it may be exchanged or converted from one form to another.

Energy, Laser

  • Radiant energy; energy traveling in the form of electromagnetic waves, measured in units of joules. For example:
    1 joule = 1 watt x 1.0 second
    1 joule = 10 watts x 0.1 second
    1 joule = 100 watts x 0.01 second

"F" Number

  • In optics, the F number is the ratio of the focal length to the diameter of the optical element. F 2 = a focal length of twice the optical element diameter, a ratio of 2:1. Numerically small F numbers are called "fast" and larger F numbers are called "slow." Small F number systems have a focused beam with a large subtended angle. Large F number systems have a focused beam with a small subtended angle.

Field Stop

  • Geometrical optical angular limitation.

Focal Length, Mirror

  • The focal length of a first surface concave mirror is equal to one half the radius of curvature (F = 1/2 R).


  • Gas Transport Laser.

Hole, Round

  • A cylindrical surface that extends between the front surface and back surface of the hole substrate sheet.

Hole, Square

  • The surface that extends between the front surface and back surface of the hole substrate sheet, which is described as a cube with two opposite sides removed.

Infrared Wavelength

  • Adjacent to the visible, from 0.750 microns (750 nanometers) to 1000 microns (RF/microwaves). Near infrared = 0.750 to 3.0 microns. Middle infrared = 3.0 to 30 microns. Far infrared = 30 to 1000 microns.


  • Radiant flux, the power per unit area incident on a surface (W/M2).

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  • 1 watt per second, a watt-second.


  • Light Amplification by Stimulated Emission of Radiation.


  • Light Detection and Ranging


  • Microwave Amplification by Stimulated Emission of Radiation


  • A closed perimeter opening of any size or shape with walls that extend from the front surface to the back surface of the material through which the opening extends and allows the flow of gas and liquids.


  • See Hole, Round


  • The time rate of doing work; activity; unit = watts.

Power, Electrical

  • The voltage X the amperes (current) in units of watts (P=EI).

Power, Laser

  • The rate at which energy is produced, in terms of watts. For example:
    1 watt = 1 joule/sec = 10 joule/10 sec = 100 joule/100 sec
    10 watts = 1 joule/0.1 sec = 10 joule/sec = 100 joule/10 sec
    100 watts = 1 joule/0.01sec = 10 joule/0.1 sec = 100 joule/sec

Power, Lens

  • See Diopter.

Power, Optical

  • Optical bandwidth X the radiance of the source.


  • The degree of exactness with which a quantity is stated; 3.14162 is precise to 6 figures, accurate to 5 figures.


  • Emittance, steradiancy; the rate of radiant energy emission per unit solid angle per unit of projected area of a source in a stated angular direction from the surface per wavelength; for example, in units, watts per square meter per solid angle per wavelength (W/M2/steradian/ nm).

Slit, Optical, Air

  • A long, narrow opening with sharp longitudinal edges, through which radiation enters or exits certain optical instruments. At the microscopic level, an opening that may be described as a rectangular four sided box that extends from the front surface to the back surface of the substrate sheet.

Stop, Aperture

  • A cylindrical surface that extends between the front surface and back surface of the hole substrate sheet.


  • Transverse Excited Atmospheric (CO2 Laser).

Throughput, Optical

  • Area/Solid Angle function.

Ultra-Violet Wavelength

  • Wavelength range adjacent to the visible, from 400 nanometers to 40 nanometers. Damage to the human eye and skin increases as a function of wavelength below 380 nanometers. 40 nanometers is at the X-ray boundary.


  • Any pressure less than atmospheric pressure.

Visible Wavelength

  • Wavelength limits of the human eye, from 400 nanometers (violet) to 750 nanometers (red). Average sensivity is from 420 nanometers to 700 nanometers. Maximum sensitivity is at 555 nanometers (green).


  • (Nd:YAG) Neodymium: yttrium-aluminum-garnet.