Optical coherence tomography (OCT) has developed rapidly since its first realisation in medicine and is currently an emerging technology in the diagnosis of skin disease. OCT is an interferometric technique that detects reflected and backscattered light from tissue and is often described as the optical analogue to ultrasound. The inherent safety of the technology allows for in vivo use of OCT in patients. The main strength of OCT is the depth resolution. In dermatology, most OCT research has turned on non-melanoma skin cancer (NMSC) and non-invasive monitoring of morphological changes in a number of skin diseases based on pattern recognition, and studies have found good agreement between OCT images and histopathological architecture. OCT has shown high accuracy in distinguishing lesions from normal skin, which is of great importance in identifying tumour borders or residual neoplastic tissue after therapy. The OCT images provide an advantageous combination of resolution and penetration depth, but specific studies of diagnostic sensitivity and specificity in dermatology are sparse. In order to improve OCT image quality and expand the potential of OCT, technical developments are necessary. It is suggested that the technology will be of particular interest to the routine follow-up of patients undergoing non-invasive therapy of malignant or premalignant keratinocyte tumours. It is speculated that the continued technological development can propel the method to a greater level of dermatological use.

https://link.springer.com/content/pdf/bfm%3A978-3-319-06419-2%2F1.pdf

Optical Coherence Tomography

The diffraction tomography theorem is adapted to one-dimensional length measurement. The resulting spectral interferometry technique is described and the first length measurements using this technique on a model eye and on a human eye in vivo are presented.

Measurement of intraocular distances by backscattering spectral interferometry

https://stuff.mit.edu/afs/athena/course/2/2.717/www/sp_oct_fercher_95.pdf

Applied Numerical Analysis. By C.F. Gerald. Pp. xii, 340. £3·60. 1970. (Addison-Wesley.)

Bessel-Gauss beams

The concept of a quasihomogeneous source is introduced. Unlike a source that is strictly homogeneous in its statistical properties, a quasihomogeneous source may be finite. Many physical sources, both primary and secondary ones, are adequately approximated by this model. Coherence and radiometric properties of light generated by such sources (assumed, for simplicity, to be planar) are discussed and an important reciprocity relation is shown to exist between light in the far zone and in the source plane. This relation implies that the degree of coherence in the far zone is given by the classic form of the van Cittert-Zernike theorem, even though the source may have a high degree of spatial coherence over arbitrarily large areas. The reciprocity relation also provides a generalization of a recently derived result that expresses the angular dependence of the radiant intensity in terms of the degree of spatial coherence of light in the source plane. The dependence of all the basic radiometric quantities on the distribution of the optical intensity across the source and on the degree of spatial coherence of the light emerging from the source is discussed and is illustrated, for some typical sources, by computed curves.

Coherence and radiometry with quasihomogeneous planar sources

The electromagnetic field associated with a generalized beam is analyzed theoretically, where the beam may take on an irradiance cross section described by a gaussian function with arbitrary elliptical symmetry. For this analysis, the field is represented by an expansion into an angular spectrum of plane waves. Expressions for the field components throughout a medium that is free of sources are found by use of an asymptotic approximation that is common in gaussian-beam analysis. In addition, more-precise expressions for these components are found, which are valid outside the neighborhood of focus. Near focus, gaussian beams may have only approximately gaussian cross sections, and in this region the behavior of beams without circular symmetry is greatly complicated. The effects of noncircular symmetry are discussed in some detail, and a method for correcting a beam to produce circular symmetry is described.

Electromagnetic Field of a Gaussian Beam with an Elliptical Cross Section

Spot size and divergence for Hermite Gaussian beams of any order

Some general theorems are derived about spatial coherence properties of a two-dimensional statistically homogeneous source that gives rise to any prescribed angular distribution of radiant intensity. The results are applied to sources that radiate in accordance with Lambert’s law. It is found that lambertian sources cannot be spatially strictly incoherent and that they have, in fact, certain unique coherence properties. This result is illustrated by calculations for a blackbody source. The idealized case of a spatially completely incoherent source is discussed for comparison.

Coherence properties of lambertian and non-lambertian sources*

The actual focal plane for a laser beam under frequently encountered conditions is shifted from the plane given by geometrical optics. The concept of effective Fresnel number for an unapertured Gaussian laser beam is defined and used to examine focal shifts. It is found that the effective Fresnel number has a simple physical interpretation that makes clear its relationship to the focal shift phenomena. Expressions are obtained that relate the important Gaussian beam parameters to the effective Fresnel number, and a method of correcting for the focal shift in optical systems design is described.

William H. Carter

Papers

Coherence and radiometry with quasihomogeneous planar sources

Electromagnetic Field of a Gaussian Beam with an Elliptical Cross Section

Spot size and divergence for Hermite Gaussian beams of any order

Coherence properties of lambertian and non-lambertian sources*

Focal shift and concept of effective Fresnel number for a Gaussian laser beam