[visionlist] prism diopters
Sheedy, James E.
jsheedy at pacificu.edu
Fri May 4 14:53:27 -05 2018
Ben et al,
I would use Centrads-- they are 1/100 of a radian. They have tbe following advantages:
1. They are equal to Prism Diopters (that ODs use when thinking about angles) - becuz they only deal with small angles.
2. Technically the Centrads are more accurate and are linear with angle.
3. Centrads have been taught to ODs - at least they have heard the term.
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------ Original message------
From: Little, Julie-Anne
Date: Fri, May 4, 2018 12:38 PM
To: Ben Backus;visionlist at visionscience.com;
Subject:Re: [visionlist] prism diopters
Why not use Metre angles (MA)? This is useful as gives you a comparable metric with accommodation, and is the reciprocal of the viewing distance (m). So 2 MA = vergence at 50cm, 3MA = vergence at 33cm etc. Not a linear, but at least a dioptric scale.
Metre angles (MA) are calculated by dividing the Prism diopter value by the inter-pupillary distance of the person in cm.
MA = Prism Diopters/IPD(cm)
Julie-Anne Little PhD MCOptom FHEA FEAOO
Senior Lecturer in Optometry
Ulster University, Northern Ireland, UK
ORCID ID: orcid.org/0000-0001-5242-8066
From: visionlist <visionlist-bounces at visionscience.com<mailto:visionlist-bounces at visionscience.com>> on behalf of Ben Backus <ben at seevividly.com<mailto:ben at seevividly.com>>
Date: Thursday, 3 May 2018 at 21:14
To: "visionlist at visionscience.com<mailto:visionlist at visionscience.com>" <visionlist at visionscience.com<mailto:visionlist at visionscience.com>>
Subject: [visionlist] prism diopters
I'm helping to build some virtual reality vision tests for optometrists to use. Generally optometrists want visual angle to be measured in prism diopters (PD). But PD are not a nice unit: they are nonlinear. For example, 40 PD = atan(40/100) = 0.38 radian, but 40 * (1 PD) = 0.40 radian.
Is there any unit out there--or should we define a new one, the "constant prism diopter"--that is equal to 1/100 radians?
Has anyone tried simply declaring that, for your use, 1 PD will be equal to ~0.573 deg? How did your optometrist friends react to that?
Benjamin T. Backus, PhD
Chief Science Officer, Vivid Vision, Inc.<https://seevividly.com>
525 York St., San Francisco, CA 94110, USA
Associate Professor, SUNY College of Optometry, New York, NY
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