<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto">For angles up to 45pd the approximation of 2 pd per deg works pretty well. See this excellent paper by Irsch<br><br><a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4502167/#!po=13.2353">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4502167/#!po=13.2353</a><div><br><br><div id="AppleMailSignature">Eileen Birch<div>Sent from my iPhone</div></div><div><br>On May 3, 2018, at 10:14 AM, Ben Backus <<a href="mailto:ben@seevividly.com">ben@seevividly.com</a>> wrote:<br><br></div><blockquote type="cite"><div><div dir="ltr">
<div><div><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><br></div>Is there any unit out there--or should we define a new one, the "constant prism diopter"--that is equal to 1/100 radians? <br></div><div><br></div><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></div><div><br></div>Thanks,<br></div>Ben<br clear="all"><div><div><div><div><div><div><div><div><div class="gmail-m_7090171537637337073m_-4884412161004783853gmail_signature"><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"></div><div dir="ltr"><br></div><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></div><div>Tel. +1-415-787-7830<br></div><div>and<br></div><div>Associate Professor, SUNY College of Optometry, New York, NY</div></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div>
<div><div class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div><div></div><div><br></div></div></div></div></div></div></div></div></div></div></div></div></div></div></div>
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