Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Objective type
Patent
1995-07-24
1998-07-21
Dang, Hung X.
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Objective type
351246, A61B 310
Patent
active
057841454
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a pupillometer, that is to say apparatus for determining pupil dimensions for the eyes of a subject, in particular, although not necessarily exclusively, a human subject.
There are many medical applications for the ability accurately to measure pupil size. For example, there are many studies in which measurement of anisocoria (inequality of pupil sizes) has played an important role as a diagnostic test for disease. It is known, for example, that subjects with corneal abrasions, marginal keratitis and acute anterior uveitis demonstrate significantly smaller mean pupillary diameters in the affected eye. Other applications for anisocoria measurement include the diagnosis and localisation of the lesion(s) in Horner's syndrome, as a means for detecting neuro-ophthalmic and ophthalmic disease in newborn infants and in subjects exhibiting Pigmentary Dispersion Syndrome.
In more general terms, observation of the delicately controlled pupil provides a method for observing the mechanisms of the autonomic nervous system. It is known for example that the dynamic parameters of pupillary reflexes (e.g. peak velocity of constriction, latency and amplitude) can provide a useful method of detecting the physiological effects of drug or opiate addiction, e.g. using the Naloxone eye drop test described by Creighton et al. in The Lancet (Apr. 8, 1989), pp 748-750.
(2) Description of the Prior Art
One of the simplest methods of determining pupil size relies upon comparison of the pupil with a series of black dots of various sizes. The inherent subjectivity of this method means that it is suitable only for making crude estimations. Photographic techniques, in which the measurement of the pupil diameter is obtained from an enlarged print or negative, offer a more objective method, but there may still be a degree of subjectivity associated with such manual determinations.
Electronic systems for pupil measurement are known, the simplest of which use a photocell to detect the amount of infra-red radiation reflected by the iris from a nearby source. Such systems allow measurements to be made in total darkness or under controlled levels of illumination. They do, however, require individual calibration for every subject, since the reflectivity of the iris varies with pupil size, and pupil area is also not linearly related to the amount of reflected radiation.
Scanning systems which do not require this recalibration have been devised. Early `flying spot` scanners mechanically directed a narrow spot of intra-red radiation across the eye, with a photocell detecting the reflected radiation. More recently, electronic image scanners have become popular: instead of scanning the eye itself, its image is projected by a lens onto a detector which is electronically scanned. The simplest of these image scanners comprise single-dimensional arrays of photodiodes which have to be accurately aligned with the pupil diameter. The use of two single-dimensional arrays, mounted orthogonally in the image plane, may obviate this necessity. Although these systems assume that the pupil is circular and have limited capabilities for dealing with eye movements, their fast processing time (e.g. with temporal resolution of up to 10 milliseconds) and comparatively low cost render them attractive tools for many research applications.
Television systems are now most commonly used for image scanning in pupillometry. A modern instrument typically employs a two-dimensional charge-coupled device (CCD) array (usually with 512.times.512 detector elements) as an image sensor. Each element produces a voltage dependent on its exposure to e.g. infra-red radiation, with detector element voltages being individually interrogated and read out serially onto an output line as a video signal. Older instruments use an electron beam image scanning tube such as the `Vidicon`, where an electron beam is generated at the rear of the tube and deflected across the image electromagnetically or electrostatically a
REFERENCES:
patent: 3782364 (1974-01-01), Watt
patent: 4834528 (1989-05-01), Howland et al.
patent: 5036347 (1991-07-01), Tsunekawa et al.
patent: 5042937 (1991-08-01), Cornsweet
Britten Alan John
Ghodse Abdol Hamid
Gibson George Henry
Taylor Daniel Robert Stafford
Dang Hung X.
St. George's Enterprises Ltd.
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