É meio literal... mas é usado assim.

"provide a a smallest half-value angle that is about 80% of the half-value ... wherein a half-value angle of said developing magnetic pole is about 22 ..."
www.patentstorm.us/patents/6449452/claims.html

"Rear projection screens may be characterized by their half value angle. ... The larger the half value angle the more suitable the screen will be for viewing ..."
www.freepatentsonline.com/4375909.html

"Decor attachment (You will find a detailed decor attachment description on pages 338 up to 345!) TC-T 18 W. Half-value angle 2 x 52°. Shielding angle 15° ..."
www.spittler.com/products/allgemein/pdf/general_rdl_125.pdf

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Note added at 5 mins (2008-07-22 00:03:47 GMT)
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Google: aproximadamente 130.000 para half-value angle.

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Note added at 7 mins (2008-07-22 00:05:08 GMT)
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ou *half-value angle*.

"TC-D 2 x 13 W. Half-value angle 2 x 50°. Shielding angle 26°. Critical radiation angle/1000 cd/m. 2. 59°. Luminaires quantity for 100 lx·m ..."
www.halachev.com/pdf/get_file.php?go=retailing_rtrd_dl165_d...

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Note added at 54 mins (2008-07-22 00:52:08 GMT) Post-grading
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I am glad I could help! :-D