Ballistic light, also known as ballistic photons, is
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...
s of
light
Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
that have traveled through a
scattering
In physics, scattering is a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including particles and radiat ...
(
turbid
Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of both water clarity and wate ...
)
medium in a straight line.
When pulses of
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
light pass through a turbid medium such as
fog or
body tissue
In biology, tissue is an assembly of similar cells and their extracellular matrix from the same embryonic origin that together carry out a specific function. Tissues occupy a biological organizational level between cells and a complete org ...
, most of the photons are either scattered or
absorbed. However, across short distances, a few photons pass through the scattering medium in straight lines. These coherent photons are referred to as ballistic photons. Photons that are slightly scattered, retaining some degree of
coherence, are referred to as snake photons.
The aim of ballistic imaging modalities is to efficiently detect ballistic photons that carry useful information, while rejecting non-ballistic photons. To perform this task, specific characteristics of ballistic photons vs. non-ballistic photons are used, such as
time of flight
Time of flight (ToF) is the measurement of the time taken by an object, particle or wave (be it acoustic, electromagnetic, etc.) to travel a distance through a medium. This information can then be used to measure velocity or path length, or as a w ...
through coherence-gated imaging,
collimation
A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A laser beam is an archetypical example. A perfectly collimated light beam, with no divergence, would not disp ...
, wavefront propagation, and
polarization.
Slightly scattered "quasi-ballistic" photons are often measured as well, to increase the signal 'strength' (i.e.,
signal-to-noise ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to noise power, often expressed in deci ...
).
Ballistic photons have many applications, especially in high-
resolution medical imaging
Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some organs or tissues (physiology). Medical imaging seeks to revea ...
systems. Ballistic scanners (using ultrafast time gates) and
optical coherence tomography
Optical coherence tomography (OCT) is a high-resolution imaging technique with most of its applications in medicine and biology. OCT uses coherent near-infrared light to obtain micrometer-level depth resolved images of biological tissue or oth ...
(OCT) (using the
interferometry
Interferometry is a technique which uses the ''interference (wave propagation), interference'' of Superposition principle, superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important inves ...
principle) are just two popular imaging systems that rely on ballistic photon detection to create
diffraction-limited images. Advantages over other existing imaging modalities (e.g.,
ultrasound
Ultrasound is sound with frequency, frequencies greater than 20 Hertz, kilohertz. This frequency is the approximate upper audible hearing range, limit of human hearing in healthy young adults. The physical principles of acoustic waves apply ...
and
magnetic resonance imaging
Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to generate pictures of the anatomy and the physiological processes inside the body. MRI scanners use strong magnetic fields, magnetic field gradients, and ...
) is that ballistic imaging can achieve a higher resolution in the order of 1 to 10 micro-meters, however it suffers from limited imaging depth.
Due to the exponential reduction of ballistic photons as thickness of the scattering medium increases, the images often have a low number of photons per pixel, resulting in
shot noise
Shot noise or Poisson noise is a type of noise which can be modeled by a Poisson process.
In electronics shot noise originates from the discrete nature of electric charge. Shot noise also occurs in photon counting in optical devices, where s ...
.
Digital image processing
Digital image processing is the use of a digital computer to process digital images through an algorithm. As a subcategory or field of digital signal processing, digital image processing has many advantages over analog image processing. It allo ...
and
noise reduction
Noise reduction is the process of removing noise from a signal. Noise reduction techniques exist for audio and images. Noise reduction algorithms may distort the signal to some degree. Noise rejection is the ability of a circuit to isolate an u ...
are often applied to reduce that noise.
See also
*
Medical optical imaging
References
{{Reflist
*K. Yoo and R. R. Alfano, "Time-resolved coherent and incoherent components of forward light scattering in random media", Optics Letters 15, 320–322 (1990).
*L Wang, P P Ho, C Liu, G Zhang, R R Alfano "Ballistic 2-d imaging through scattering walls using an ultrafast optical kerr gate" 1991, August 16t
*K. M. Yoo and R. R. Alfano "Time-resolved coherent and incoherent components of forward light scattering in random media" 199
*K. M. Yoo, Feng Liu, and R. R. Alfano "When does the diffusion approximation fail to describe photon transport in random media?" 28 May 199
*S. Farsiu, J. Christofferson, B. Eriksson, P. Milanfar, B. Friedlander, A. Shakouri, R. Nowak,
Statistical detection and imaging of objects hidden in turbid media using ballistic photons, ''Applied Optics'', vol. 46, no. 23, pp. 5805–5822, Aug. 2007.
Light
Optical imaging