Detectors¶

Detectors are used to collect the weights of photon packets that escape the sample at the top or bottom sample surface or to collect the weight of photon packets that is specularly reflected when launching with the selected source. The three detector locations can be populated and passed to the Monte Carlo simulator through the xopto.mcml.mcdetector.base.Detectors. All the detector types that can be used in the Monte Carlo simulator are implemented by subclassing xopto.mcml.mcdetector.base.Detector. The xopto.mcml.mcdetector module includes a number different detectors:

xopto.mcml.mcdetector.total.Total implements a detector that collects the weight of photon packets into a single accumulator.
xopto.mcml.mcsdetector.radial.Radial implements a radial detector that collects the photon packets through concentric annular rings.
xopto.mcml.mcdetector.cartesian.Cartesian implements a Cartesian detector that collects the photon packets through a grid of rectangular detectors.
xopto.mcml.mcdetector.symmetric.SymmetricX implements a Cartesian detector that is symmetric across the x axis (uses the absolute value of y coordinate).
xopto.mcml.mcdetector.probe implements several several standard and custom parameterized layouts of optical fibers in optical fiber probes.
- xopto.mcml.mcdetector.probe.sixaroundone.SixAroundOne implements the common six-around-one layout of optical fibers.
- xopto.mcml.mcdetector.probe.lineararray.LinearArray implements a linear layout of optical fibers, assuming that all the fibers are equal.
- xopto.mcml.mcdetector.probe.fiberarray.FiberArray implements a general layout of optical fibers, where each optical fiber can have different properties.

The individual detectors are conveniently imported into the xopto.mcml.mc and xopto.mcml.mcdetector modules.

Note

The first and last accumulator of the detectors along each axis will also collect/accumulate the weights of photon packets that exceed the lower and upper limit of the corresponding axis.

Radial¶

The following example shows how to create a Radial detector centered at $(x,y)=(0,0)$ that collects photon packets from $r=0$ mm to $r=1$ mm through concentric annular rings of thickness 10 μm. Note that the distribution of detectors along the radial direction is defined by Axis.

from xopto.mcml import mc

det = mc.mcdetector.Radial(mc.mcdetector.Axis(0.0, 1.0e-3, 100))

To use a logarithmic spacing of annular rings, set the input parameter logscale=True.

from xopto.mcml import mc

det = mc.mcdetector.Radial(mc.mcdetector.Axis(0.0, 1.0e-3, 100, logscale=True))

The acceptance cone of the detector can be limited by specifying the minimum acceptance angle cosine. The angle is by default computed relative to the top or bottom sample surface normal. This behavior can be customized by setting the reference direction through the direction input argument to a custom unit-length vector.

Note

The acceptance cone is applied after the photon packet leaves the sample and is refracted into the surrounding medium.

The following example creates a radial detector that collects photon packets within ±10 ^o of sample surface normal.

To use a logarithmic spacing of annular rings, set the input parameter logscale=True.

from xopto.mcml import mc
import numpy as np

cosmin = np.cos(np.deg2rad(10.0))
det = mc.mcdetector.Radial(
    mc.mcdetector.Axis(0.0, 1.0e-3, 100, logscale=True),
    cosmin=cosmin
)

Cartesian¶

In the next example, we create a Cartesian detector that collects photon packets through a grid of rectangular accumulators that span $[-1, 1]$ mm along the x axis and $[-2, 2]$ mm along the y axis. The size of the rectangular accumulators is $(x,y)=(10,10)$ μm. Note that parameters cosmin and direction can be optionally used to limit the acceptance cone of the detector.

from xopto.mcml import mc

det = mc.mcdetector.Cartesian(
    xaxis = mc.mcdetector.Axis(-1.0, 1.0, 200),
    yaxis = mc.mcdetector.Axis(-2.0, 2.0, 400),
)

If a Cartesian detector is created with a single axis, the axis configuration is applied to the x and y axis.

from xopto.mcml import mc

det = mc.mcdetector.Cartesian(mc.mcdetector.Axis(-1.0, 1.0, 200))

Total¶

To collect the weights of photon packets that leave the sample into a single accumulator, use the xopto.mcml.mcdetector.total.Total detector. Note that parameters cosmin and direction can be optionally used to limit the acceptance cone of the detector.

from xopto.mcml import mc

det = mc.mcdetector.Total()

Symmetric¶

To collect photon packets symmetrically across the x axis, regardless of the y coordinate use the xopto.mcml.mcdetector.symmetric.SymmetricX detector. This type of a detector is useful for sources that are tilted along the x axis and produce a reflectance or transmittance response that is symmetric along the x axis and the location along the y axis is not important. The distribution of accumulators is defined through xopto.mcbase.mcutil.axis.SymmetricAxis that also supports logarithmic spacing of the accumulators. Note that parameters cosmin and direction can be optionally used to limit the acceptance cone of the detector. In the following example we create an instance of SymmetricX detector that spans the range $[-10,10]$ mm along the axis with the accumulator size along the set to 10 μm.

from xopto.mcml import mc

det = mc.mcdetector.SymmetricX(mc.mcdetector.SymmetricAxis(0.0, 10.0, 1000))

Probe¶

Probe detectors that utilize various layouts of optical fibers are intended for use with complex surface layouts that break the radial symmetry of the reflectance / transmittance at the sample surface. Consequently, the Radial detector cannot be used to derive the reflectance collected through the individual fibers.

In the following example we create a six-around-one layout of optical fibers. All the optical fibers have a fused silica core with a 400 μm diameter, cladding diameter 420 μm, $NA$ 0.22 and are tightly packed, i.e. the distance between the cores of the central and the surrounding optical fibers is 420 μm. The refractive index of the fused silica is taken from the xopto.materials.ri module.

from xopto.mcml import mc
from xopto.mcml.mcutil import fiber
from xopto.materials import ri

fib = fiber.MultimodeFiber(
    400e-6,
    420e-6,
    ncore=ri.glass.fusedsilica.default(550e-9),
    na=0.22
)
detector = mc.mcdetector.SixAroundOne(fib)

Note that the geometry of the layout (spacing of fibers) and the properties of the optical fiber can be changed through accessing the related class properties.

detector.spacing = 800e-6
detector.fiber.na = 0.23
detector.fiber.dcore = 200e-6
detector.fiber.dcladding = 220e-6
detector.fiber.ncore = ri.glass.fusedsilica.default(400e-9)

Use in Monte Carlo simulator¶

To use the detectors in Monte Carlo simulations, we need to populate an instance of xopto.mcml.mcdetector.base.Detectors, that can independently set the detectors at the top and bottom sample surfaces and the detector for photon packets that are specularly reflected at the medium-sample boundary when launched. Note that it is not required to populate all the three detectors (top, bottom and specular) as illustrated in this example.

from xopto.mcml import mc

top = mc.mcdetector.Radial(mc.mcdetector.Axis(0.0, 1.0, 100))
bottom = mc.mcdetector.Cartesian(mc.mcdetector.Axis(-1.0, 1.0, 200))
specular = mc.mcdetector.Total()

detectors = mc.mcdetector.Detectors(top=top, bottom=bottom, specular=specular)

After completing the Monte Carlo simulations, the returned instance of Detectors can be used to access the collected reflectance (also used for transmittance) or the accumulated raw weight of the photon packets. Note that the reflectance / transmittance is computed as the raw weight in the accumulator divided by the number of launched photon packets and if applicable also divided by the surface area of the accumulator (weight/m ²).

top_reflectance = detectors.top.reflectance
top_raw weight = detectors.top.raw

bottom_reflectance = detectors.bottom.reflectance
bottom_raw weight = detectors.bottom.raw

specular_reflectance = detectors.specular.reflectance
specular_raw weight = detectors.specular.raw