Fluence
2024-3-31
Definition:
Light energy per unit area.
In fundamental physics a flux is defined as the time-integrated quantity of some radiation or particle flow. In optics, the luminous flux F of a light pulse is defined as the light energy delivered per unit area. Its most common unit is J/cm2.
Like light intensity, luminous flux is also a position-dependent value. For lasers, the luminous flux is usually greatest at the optical axis and decreases away from the optical axis.
In some cases, the maximum value of the luminous flux is more interesting. For a Gaussian beam, the peak luminous flux is the total light energy divided by π w2/2, where w is the beam radius of the Gaussian beam.
If the time-dependent light intensity is known, the luminous flux can be obtained by integrating the pulse in the time domain.
The use of luminous flux
When a strong ultrashort pulse causes gain saturation in a laser crystal or active fiber, the pulse width is usually much smaller than the upper energy level lifetime. The degree of saturation depends only on the pulsed light flux, not the time distribution of light intensity. An important characteristic of a gain medium is its saturation flux.
A similar situation occurs with gain media for slow saturable absorbers.
In the case of laser damage caused by laser pulses, the damage threshold is also often referred to as luminous flux. However, the damage threshold is not a quantity independent of the pulse width. Normally, the critical luminous flux increases as the pulse width increases.
Light energy per unit area.
In fundamental physics a flux is defined as the time-integrated quantity of some radiation or particle flow. In optics, the luminous flux F of a light pulse is defined as the light energy delivered per unit area. Its most common unit is J/cm2.
Like light intensity, luminous flux is also a position-dependent value. For lasers, the luminous flux is usually greatest at the optical axis and decreases away from the optical axis.
In some cases, the maximum value of the luminous flux is more interesting. For a Gaussian beam, the peak luminous flux is the total light energy divided by π w2/2, where w is the beam radius of the Gaussian beam.
If the time-dependent light intensity is known, the luminous flux can be obtained by integrating the pulse in the time domain.
The use of luminous flux
When a strong ultrashort pulse causes gain saturation in a laser crystal or active fiber, the pulse width is usually much smaller than the upper energy level lifetime. The degree of saturation depends only on the pulsed light flux, not the time distribution of light intensity. An important characteristic of a gain medium is its saturation flux.
A similar situation occurs with gain media for slow saturable absorbers.
In the case of laser damage caused by laser pulses, the damage threshold is also often referred to as luminous flux. However, the damage threshold is not a quantity independent of the pulse width. Normally, the critical luminous flux increases as the pulse width increases.
