5.13. [ Light ] section¶

This section defines the generation, transport, and boundary conditions of light (scintillation light, Cherenkov light). Only one [ Light ] section can be defined per input file. Multiple set-region and set-boundary blocks can be used to assign different optical properties to different regions and boundaries.

Note

Light is specified by the particle name light or the kf code -22.

5.13.1. Overall Structure¶

[ Light ]
yield-factor = 1e-4

set-region
  reg = 101 102
  n = 1.5
  light-yield = 10000
  table-scintillation-spectrum
    unit1 = nm
    300  0
    400  0.1
    500  0.2
    600  0.3
    700  0.1
    800  0
  end-table

set-boundary
  reg-from = 101
  reg-to   = 98
  fraction-absorption = 0.3

5.13.2. Global Parameters¶

Parameters specified at the beginning of the section (before set-region / set-boundary).

Parameter	Default	Description
yield-factor	1.0	Scaling factor for light yield. The number of generated photons is multiplied by yield-factor, and each photon’s weight is set to 1/yield-factor to preserve statistics. For example, yield-factor = 0.001 reduces the number of generated photons to 1/1000, lowering computational cost.
lmin-light	250	Lower limit of light wavelength [nm].
lmax-light	800	Upper limit of light wavelength [nm].
emin-light	Corresponds to lmax-light (1.55e-6 MeV)	Lower limit of light energy [MeV]. Can be used instead of lmax-light.
emax-light	Corresponds to lmin-light (4.96e-6 MeV)	Upper limit of light energy [MeV]. Can be used instead of lmin-light.
transport	1	Whether to transport light. = 0: No transport (useful for scoring emission only) = 1: Transport
stock-max	100000	Maximum number of light generation steps stocked per history. Increase if overflow warnings appear.

5.13.3. Table Units¶

Available units for tables are listed below.

unit1 (wavelength/energy axis)

Value	Description
nm	Nanometer
um	Micrometer
mm	Millimeter
cm	Centimeter
eV	Electron volt
keV	Kilo electron volt
MeV	Mega electron volt

unit2 (absorption/scattering coefficient or length)

For coefficient tables (-coeff):

Value	Description
1/km	1/kilometer
1/m	1/meter
1/cm	1/centimeter
1/mm	1/millimeter

For length tables (-length):

Value	Description
km	Kilometer
m	Meter
cm	Centimeter
mm	Millimeter

Note

An error will be displayed if a length unit is specified for a *-coeff parameter (or vice versa).

5.13.4. set-region Block¶

set-region starts a new region block to define optical properties. Multiple set-region blocks can be used to assign different properties to different regions.

5.13.4.1. Region Specification¶

Parameter	Description
reg	Region number(s) to assign optical properties. Multiple regions can be specified separated by spaces.

5.13.4.2. Refractive Index¶

Cherenkov light is generated in regions where a refractive index is set.

Parameter	Default	Description
n	1	Refractive index (scalar). Used when there is no wavelength dependence.
table-n	(none)	Wavelength-dependent refractive index table.

Table format example:

table-n
  unit1 = nm
  300  1.52
  500  1.50
  800  1.49
end-table

5.13.4.3. Scintillation¶

Parameters related to scintillation light generation. Both light-yield and an emission spectrum (table-scintillation-spectrum or scintillation-energy / scintillation-lambda) are required for emission.

Parameter	Default	Description
light-yield	0	Light yield [light/MeV].
table-scintillation-spectrum	(none)	Scintillation emission spectrum (wavelength-dependent relative emission probability). The table defines only the spectral shape; it does not affect the number of emitted photons (determined by light-yield). Only relative ratios of the values are meaningful, as they are internally normalized.
scintillation-energy	(none)	Scintillation light energy [MeV]. Used for single-energy emission. Alternative to table-scintillation-spectrum.
scintillation-lambda	(none)	Scintillation light wavelength [nm]. Used for single-wavelength emission. Alternative to table-scintillation-spectrum.

Table format example:

table-scintillation-spectrum
  unit1 = nm
  300  0
  400  0.1
  500  0.2
  600  0.3
  700  0.1
  800  0
end-table

Birks’ Law

Accounts for the non-linearity (quenching) of light yield with respect to energy loss of charged particles. In Birks’ law, the light output per unit path length is expressed as:

\[\frac{dL}{dx} = S \frac{dE/dx}{1 + k_B \cdot dE/dx}\]

where $S$ is the scintillation efficiency (light-yield), $dE/dx$ is the stopping power, and $k_B$ is the Birks constant. Quenching becomes stronger (and scintillation efficiency decreases) as $dE/dx$ increases (heavier particles or lower energies).

Parameter	Default	Description
Birks-kB-cm/MeV	0 (disabled)	Birks constant kB [cm/MeV]
Birks-kB-mm/MeV	0 (disabled)	Birks constant kB [mm/MeV]
Birks-kB-g/cm2/MeV	0 (disabled)	Birks constant kB [g/cm²/MeV]. Internally converted to cm/MeV using the cell density.

Time Structure

Specifies the time distribution of scintillation light.

Parameter	Default	Description
decay-time	0 (instant emission)	Decay time constant [ns].
decay-time2	0	Decay time constant of the 2nd component [ns]. Used for two-component decay.
rise-time	0	Rise time constant [ns]. Must be smaller than decay-time.
decay-fraction	(none)	Fraction of the 1st component. Required when using decay-time2.
decay-fraction2	(none)	Fraction of the 2nd component. Required when using decay-time2.

Note

When decay-time2 is set, both decay-fraction and decay-fraction2 must also be set. When rise-time is set, decay-time must also be set.

5.13.4.4. Absorption Coefficient¶

Defines light absorption in the medium. Either the absorption coefficient (unit: 1/cm) or absorption length (unit: cm) can be specified. Internally converted to absorption coefficient (coefficient = 1 / length).

Scalar (no wavelength dependence)

Parameter	Description
absorption-coeff	Absorption coefficient [1/cm]
absorption-length	Absorption length [cm]. Must be greater than 0.

Table (wavelength-dependent)

Parameter	Description
table-absorption-coeff	Wavelength-dependent absorption coefficient table. unit1: wavelength or energy unit (nm, etc.) unit2: coefficient unit (1/cm, etc.)
table-absorption-length	Wavelength-dependent absorption length table. unit1: wavelength or energy unit (nm, etc.) unit2: length unit (cm, etc.)

Conversion from Internal Transmittance

Absorption coefficients can be automatically calculated from internal transmittance values found in optical glass catalogs. Based on the Beer-Lambert law: $\alpha = -\ln(\tau) / d$.

Parameter	Description
table-tau1	Internal transmittance table for 1 mm (0.1 cm) thick sample
table-tau10	Internal transmittance table for 10 mm (1.0 cm) thick sample
table-tau25	Internal transmittance table for 25 mm (2.5 cm) thick sample

Table format example (absorption coefficient):

table-absorption-coeff
  unit1 = nm
  unit2 = 1/cm
  300  0.5
  500  0.1
  800  0.05
end-table

Table format example (internal transmittance):

table-tau25
  unit1 = nm
  300  0.01
  500  0.90
  800  0.95
end-table

5.13.4.5. Scattering Coefficient¶

Defines light scattering in the medium. Rayleigh and Mie scattering can be set independently. As with absorption, either coefficient or length can be specified.

Rayleigh Scattering

Parameter	Description
table-scattering-coeff-rayleigh	Wavelength-dependent Rayleigh scattering coefficient table. unit1: wavelength or energy unit unit2: coefficient unit (1/cm, etc.)
table-scattering-length-rayleigh	Wavelength-dependent Rayleigh scattering length table. unit1: wavelength or energy unit unit2: length unit (cm, etc.)

Table format example:

table-scattering-length-rayleigh
  unit1 = nm
  unit2 = cm
  300  5
  500  50
  800  200
end-table

Mie Scattering

Parameter	Description
table-scattering-coeff-mie	Wavelength-dependent Mie scattering coefficient table. unit1: wavelength or energy unit unit2: coefficient unit (1/cm, etc.)
table-scattering-length-mie	Wavelength-dependent Mie scattering length table. unit1: wavelength or energy unit unit2: length unit (cm, etc.)

Mie Scattering Anisotropy

Parameter	Default	Description
anisotropy	0	Mie scattering anisotropy parameter g (scalar). The g parameter in the Henyey-Greenstein phase function. g = 0: isotropic, g > 0: forward scattering dominant, g < 0: backward scattering dominant. Range: -1 ≤ g ≤ 1.
table-anisotropy	(none)	Wavelength-dependent anisotropy parameter table. Only unit1 is needed.

5.13.4.6. Re-emission: Fluorescence, Phosphorescence, and Wavelength Shifting¶

Defines re-emission (fluorescence, phosphorescence, wavelength shifting) of optical photons in the medium. When an optical photon is absorbed via the re-emission absorption process, a new photon may be emitted with a probability given by the quantum yield, using the re-emission spectrum and a random isotropic direction.

This process is independent of the normal (non-radiative) absorption defined by absorption-coeff / table-absorption-coeff. Both processes contribute additively to the total absorption: the total absorption coefficient is the sum of the non-radiative absorption coefficient and the re-emission absorption coefficient. Boundary absorption (fraction-absorption) does not trigger re-emission.

Re-emission absorption (wavelength-dependent)

Parameter	Description
table-reemission-coeff	Wavelength-dependent re-emission absorption coefficient table. unit1: wavelength or energy unit (nm, etc.) unit2: coefficient unit (1/cm, etc.)
table-reemission-length	Wavelength-dependent re-emission absorption length table. unit1: wavelength or energy unit (nm, etc.) unit2: length unit (cm, etc.)

Re-emission spectrum

Parameter	Description
table-reemission-spectrum	Emission spectrum of re-emitted photons. Same format as `table-scintillation-spectrum`. Only unit1 is needed. The y-values represent relative emission probability as a function of wavelength.

Scalar parameters

Parameter	Default	Description
reemission-yield	0	Quantum yield for re-emission (0 to 1). Probability that an absorbed photon produces a re-emitted photon. = 0: No re-emission (default). Even if `table-reemission-coeff` is set, re-emission does not occur without this parameter.
reemission-time	(none)	Decay time constant [ns] for re-emission. The delay time is sampled from an exponential distribution. If not set, re-emitted photons are generated with no time delay.

Table format example:

set-region
  reg = 101
  n = 1.5
  reemission-yield = 0.85
  reemission-time = 12  $ ns

  table-reemission-coeff
    unit1 = nm
    unit2 = 1/cm
    350  1.0
    400  0.5
    450  0.1
    500  0.01
  end-table

  table-reemission-spectrum
    unit1 = nm
    400  0.0
    420  0.3
    450  1.0
    500  0.8
    550  0.3
    600  0.0
  end-table

5.13.5. set-boundary Block¶

set-boundary starts a new boundary block to define the behavior of light at boundaries between regions.

Note

Light is fully absorbed at boundaries not defined by set-boundary. Also, since the default value of fraction-absorption is 1 (100% absorption), light is fully absorbed even at defined boundaries unless transmission or reflection fractions are explicitly set.

5.13.5.1. Boundary Specification¶

Boundaries can be specified as directional (one-way) or bidirectional.

One-way specification (reg-from / reg-to)

Parameter	Description
reg-from	Origin region of light. Multiple regions can be specified separated by spaces.
reg-to	Destination region of light. Multiple regions can be specified separated by spaces.

Bidirectional specification (reg1 / reg2)

Parameter	Description
reg1	One side of the boundary. Multiple regions can be specified separated by spaces.
reg2	The other side of the boundary. Multiple regions can be specified separated by spaces.

When using reg1 / reg2, the same boundary conditions apply regardless of which direction the light passes through.

5.13.5.2. Boundary Conditions¶

When light reaches a boundary, absorption is first evaluated with probability fraction-absorption, then transmission is evaluated with probability fraction-pass. If neither occurs, the light is reflected.

Parameter	Default	Description
boundary-type	reflection	Physical model for the boundary. = fresnel: Fresnel reflection = reflection: Directly specify reflection type and fractions
fraction-absorption	1	Absorption fraction at the boundary (0 to 1).
fraction-pass	0	Transmission fraction at the boundary (0 to 1). Scalar value.
table-pass	(none)	Wavelength-dependent transmission fraction table. Only unit1 is needed.
absorb-if-not-pass	0	= 1: Absorb light that is not transmitted (instead of reflecting). = 0: Reflect light that is not transmitted (default).

Note

When boundary-type = fresnel, the refractive index (n) must be set for both adjacent regions.

Reflection Types

Specifies the reflection model when light is not transmitted.

Parameter	Default	Description
fraction-perfect-specular	1.0	Relative fraction of perfect specular reflection.
fraction-specular-lobe	0	Relative fraction of specular lobe reflection. Spread controlled by roughness parameter.
fraction-diffuse	0	Relative fraction of Lambertian diffuse reflection.
fraction-backscatter-lobe	0	Relative fraction of backscatter lobe reflection.
roughness	0.1	Roughness parameter for reflection lobes (0 to 1). Applied to fraction-specular-lobe and fraction-backscatter-lobe. = 0: perfect specular reflection, = 1: completely diffuse. Internally converted to Blinn-Phong exponent s = 2/roughness² - 2.

Note

The four values fraction-perfect-specular, fraction-specular-lobe, fraction-diffuse, and fraction-backscatter-lobe are relative fractions and are automatically normalized to sum to 1. For example, the following two settings are equivalent:

fraction-perfect-specular = 0.6
fraction-diffuse = 0.4

fraction-perfect-specular = 3
fraction-diffuse = 2

5.13.6. Loading External Data¶

Parameter	Description
load	Load optical properties from an external file. Place the file in the `$PHITSPATH/data/light/` directory and specify the filename without the `.inp` extension. Example: `load = CsI` loads `$PHITSPATH/data/light/CsI.inp`.

5.13.7. Others¶

Parameter	Description
print-comment	Display in the log output during calculation.

5.13.8. Examples¶

5.13.8.1. Scintillation Light Generation¶

[ Light ]
yield-factor = 1e-3

set-region
  reg = 101
  light-yield = 10000
  table-scintillation-spectrum
    unit1 = nm
    300  0
    400  0.1
    500  0.2
    600  0.3
    700  0.0
  end-table

5.13.8.2. Cherenkov Light Generation¶

[ Light ]
yield-factor = 0.1

set-region
  reg = 101
  n = 1.3

5.13.8.3. Fresnel Boundary¶

[ Light ]
set-region
  reg = 101
  n = 3.0

set-region
  reg = 900
  n = 1.0

set-boundary
  reg1 = 101
  reg2 = 900
  boundary-type = fresnel
  fraction-absorption = 0.1

5.13.8.4. Perfect Specular Reflection¶

[ Light ]
set-region
  reg = 101
  n = 1.3

set-boundary
  reg-from = 101
  reg-to   = 900
  fraction-absorption = 0.5
  fraction-perfect-specular = 1

5.13.8.5. Perfect Specular + Specular Lobe + Diffuse Reflection (1:1:1)¶

[ Light ]
set-region
  reg = 101
  n = 1.3

set-boundary
  reg-from = 101
  reg-to   = 900
  fraction-absorption = 0.5
  $ ---------------------------
  fraction-perfect-specular = 1
  fraction-specular-lobe = 1
  fraction-diffuse = 1
  roughness = 0.1
  $ ---------------------------

5.13.8.6. Absorption¶

[ Light ]
set-region
  reg = 101
  light-yield = 100
  scintillation-lambda = 400 $ nm
  absorption-length = 1 $ cm

set-boundary
  reg-from = 101
  reg-to   = 900
  fraction-absorption = 0

5.13.8.7. Scattering (Rayleigh Scattering)¶

[ Light ]
set-region
  reg = 101
  table-scattering-length-rayleigh
    unit1 = nm
    unit2 = cm
    200  5
    800  5
  end-table

set-boundary
  reg-from = 101
  reg-to   = 900

5.13. [ Light ] section¶

5.13.1. Overall Structure¶

5.13.2. Global Parameters¶

5.13.3. Table Units¶

5.13.4. set-region Block¶

5.13.4.1. Region Specification¶

5.13.4.2. Refractive Index¶

5.13.4.3. Scintillation¶

5.13.4.4. Absorption Coefficient¶

5.13.4.5. Scattering Coefficient¶

5.13.4.6. Re-emission: Fluorescence, Phosphorescence, and Wavelength Shifting¶

5.13.5. set-boundary Block¶

5.13.5.1. Boundary Specification¶

5.13.5.2. Boundary Conditions¶

5.13.6. Loading External Data¶

5.13.7. Others¶

5.13.8. Examples¶

5.13.8.1. Scintillation Light Generation¶

5.13.8.2. Cherenkov Light Generation¶

5.13.8.3. Fresnel Boundary¶

5.13.8.4. Perfect Specular Reflection¶

5.13.8.5. Perfect Specular + Specular Lobe + Diffuse Reflection (1:1:1)¶

5.13.8.6. Absorption¶

5.13.8.7. Scattering (Rayleigh Scattering)¶

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