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, so they a ...
s of
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
that have traveled through a
scattering (
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 water quality.
Fluids can ...
)
medium
Medium may refer to:
Science and technology
Aviation
*Medium bomber, a class of war plane
*Tecma Medium, a French hang glider design
Communication
* Media (communication), tools used to store and deliver information or data
* Medium of ...
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" is an acronym for "light amplification by stimulated emission of radiation". The fi ...
light pass through a turbid medium such as
fog or
body tissue
In biology, tissue is a biological organizational level between cells and a complete organ. A tissue is an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function. Organs are ...
, 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 perfectly collimated light beam, with no divergence, would not disperse with distance. However, diffraction ...
, 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).
Ballistic photons have many applications, especially in high-
resolution medical imaging systems. Ballistic scanners (using ultrafast time gates) and
optical coherence tomography
Optical coherence tomography (OCT) is an imaging technique that uses low-coherence light to capture micrometer-resolution, two- and three-dimensional images from within optical scattering media (e.g., biological tissue). It is used for medica ...
(OCT) (using the
interferometry principle) are just two popular imaging systems that rely on ballistic photon detection to create
diffraction-limited
The resolution of an optical imaging system a microscope, telescope, or camera can be limited by factors such as imperfections in the lenses or misalignment. However, there is a principal limit to the resolution of any optical system, due to t ...
images. Advantages over other existing imaging modalities (e.g.,
ultrasound
Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies ...
and
magnetic resonance imaging) 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.
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 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).
*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