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Biomedical subjects

D G Falconer

Publications and source records attributed to D G Falconer.

6 recordsLinked to original sources
PubMed1980

Digital measurement of pallor-disc ratio.

In an effort to quantitate optic nerve head pallor, a method has been devised to measure pallor-disc ratio using optic nerve head photographs. After electronically scanning photographs through various Wratten filters, it was observed that a red filter produces good contrast between the disc and the retinal surface, while a green filter produces good contrast between the pale area of the disc and the remaining normal-appearing nerve head. A computer program fitted an elliptical form to both the disc region of red image and the pale area of green image. Pallor-disc ratios were calculated on both area and radial bases. Reproducibility of 3.5% for the area ratios and 7% for the radial ratios were obtained.

Computers↗
PubMed1980

Photogrammetry experiments with a model eye.

Digital photogrammetry was performed on stereophotographs of the optic nerve head of a modified Zeiss model eye in which optic cups of varying depths could be simulated. Experiments were undertaken to determine the impact of both photographic and ocular variables on the photogrammetric measurements of cup depth. The photogrammetric procedure tolerates refocusing, repositioning, and realignment as well as small variations in the geometric position of the camera. Progressive underestimation of cup depth was observed with increasing myopia, while progressive overestimation was noted with increasing hyperopia. High cylindrical errors at axis 90 degrees led to significant errors in cup depth estimates, while high cylindrical errors at axis 180 degrees did not materially affect the accuracy of the analysis. Finally, cup depths were seriously underestimated when the pupil diameter was less than 5.0 mm.

Astigmatism↗
PubMed1977

Comparative reproducibility of the digital photogrammetric procedure utilizing three methods of stereophotography.

The reproducibility of digital photogrammetry of optic nervehead photographs taken with the Zeiss fundus camera with Allen stereoseparator, the Donaldson fundus camera, and the Zeiss fundus camera with twin-prism separator were evaluated. Reproducibility of the various quantitative factors was significantly better when the two types of simultantic cups. Larger per cent errors were encountered with shallower optic cups. Substantial improvements in parameter reproducibility were obtained when finer sampling intervals were used during film digitization.

Computers↗
PubMed1976

Twin-prism stereo-separator.

We describe a twin-prism stereoseparator for the Zeiss fundus camera which gives high-quality stereophotographs of the optic disc and simultaneously records both images on the same frame of film. The stereobase of photographs taken with this system ranges from 2.0 to 4.5 mm. It varies linearly with the entrance pupil which ranges from 4.0 to 6.5 mm. The instrument provides a reliable method for monitoring changes in optic disc topography.

Fundus Oculi↗
PubMed1976

Analog vs. digital photogrammetry for optic cup analysis.

This report compares the reproducibility of analog and digital photogrammetry for topographical analysis of the optic cup. Ten simultaneous stereophotographs of each of three optic discs were taken with the Zeiss fundus camera plus a twin-prism stereoseparator. Digital processing produced photogrammetric contours of the optic cup and estimates of its geometry. Optic cup contours from the same stereopairs were also drawn with an analog stereoplotter and were analyzed by manual planimetry. The reproducibility of digital photogrammetry and analysis for estimation of cup depth, cup width, cup cross sectional area, and cup volume was concluded to be generally superior to the reproducibility of analog photogrammetry and manual analysis.

Analog-Digital Conversion↗