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<div style="color: rgb(0, 0, 0); font-size: 14px;"><font face="Calibri">Hi Ben</font></div>
<div style="color: rgb(0, 0, 0); font-size: 14px;"><font face="Calibri">Why not use Metre angles (MA)? </font><span style="font-family: Calibri; orphans: 2; widows: 2;">This is useful as gives you a comparable metric with
</span><span style="font-family: Calibri; orphans: 2; widows: 2; background-color: rgb(255, 255, 255);">accommodation, and is the reciprocal of the viewing distance (m). </span><span style="font-family: Calibri; background-color: rgb(255, 255, 255); orphans: 2; widows: 2;">So
2 MA = vergence at 50cm, 3MA = vergence at 33cm etc. Not a linear, but at least a dioptric scale.</span></div>
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<div style="orphans: 2; widows: 2;"><span style="orphans: 2; widows: 2; background-color: rgb(255, 255, 255);"><font face="Calibri">Metre angles (MA) are calculated by dividing the Prism diopter value by the inter-pupillary distance of the person in cm.</font></span></div>
<div style="orphans: 2; widows: 2;"><font face="Calibri">MA = Prism Diopters/IPD(cm)</font></div>
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<span style="font-weight:bold">From: </span>visionlist <<a href="mailto:visionlist-bounces@visionscience.com">visionlist-bounces@visionscience.com</a>> on behalf of Ben Backus <<a href="mailto:ben@seevividly.com">ben@seevividly.com</a>><br>
<span style="font-weight:bold">Date: </span>Thursday, 3 May 2018 at 21:14<br>
<span style="font-weight:bold">To: </span>"<a href="mailto:visionlist@visionscience.com">visionlist@visionscience.com</a>" <<a href="mailto:visionlist@visionscience.com">visionlist@visionscience.com</a>><br>
<span style="font-weight:bold">Subject: </span>[visionlist] prism diopters<br>
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<div>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.<br>
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Is there any unit out there--or should we define a new one, the "constant prism diopter"--that is equal to 1/100 radians?
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<div>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?<br>
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Thanks,<br>
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Ben<br clear="all">
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<div dir="ltr">Benjamin T. Backus, PhD
<div>Chief Science Officer,<a href="https://seevividly.com" target="_blank"> Vivid Vision, Inc.</a>
<div>525 York St., San Francisco, CA 94110, USA<br>
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<div>Tel. +1-415-787-7830<br>
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<div>Associate Professor, SUNY College of Optometry, New York, NY</div>
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