Visual Fields
Automated perimetry strategy selection, reliability analysis, and recognizing field-defect patterns.
The Stimulus, the Decibel Scale, and Choosing the Right Test
At the assistant level perimetry is a test you set a patient up for and start. At the technician level it is a psychophysical measurement whose units you can convert, whose stimulus you can specify, and whose pattern you can justify.
What the machine actually measures. Perimetry maps the hill of vision: sensitivity is highest at fixation and falls off toward the periphery, and it declines with age at every location. A threshold is the dimmest stimulus a patient detects about half the time at that spot — a statistical quantity, not a fixed one, which is why repeat values scatter and why that scatter grows in damaged retina.
Decibels are attenuation, not brightness. The scale is anchored to the machine's maximum output, conventionally 10,000 apostilbs, with dB = 10 × log10(maximum ÷ stimulus). Ten decibels is one log unit.
In the full COT guide
- The Stimulus, the Decibel Scale, and Choosing the Right Test
- Setup, Reliability, and the Artifact Catalogue
- Reading the Printout: Indices, Glaucoma Patterns, and Neuro Localization
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