×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Does a pentaprism REALLY maintain beam perpendicularity regardless of orientation?

Does a pentaprism REALLY maintain beam perpendicularity regardless of orientation?

Does a pentaprism REALLY maintain beam perpendicularity regardless of orientation?

(OP)
I have found lots of references that claim that a pentaprism always bends incoming beams 90 degrees, regardless of beam orientation relative to the entry face ("within small angles").

Is this really true? Or is it one of those Physics 101 lies that are convenient approximations for people who don't really care?

I have worked through this "by hand" using Solidworks to sketch out the beam path in 2D and in 3D.

In 2D, everything works just fine. Any beam that enters the prism will exit at 90 degrees. Small angle seems to be defined as "beam path stays inside the glass."

In 3D, where the beam enters out-of-plane, things don't go so well. Maybe it's my 3D sketching skills? I find that as the beam deviates from in-plane, the entry and exit beams become more and more skewed (no surprise) but also at less and less of a right angle (further from 90 degrees). This part is a surprise. I do find that the error is small, as long as the entering beam is within a few degrees of true. But a small error is still an error. A beam that is out-of-true by 0.05 degrees will move 0.105" at 120". Not a big deal? It is if you are trying to align some linear bearings over that distance!

NOTE: I measured beam perpendicularity in SolidWorks by drawing a line segment perpendicular to entering and exiting beams, creating a sketch plane normal to that line (The resulting plane should be parallel to both entering and exit lines), projecting both beams onto it, and measuring the resulting angle. I think this is an accurate way to measure "perpendicularity" of two skew lines.

Can someone smart either confirm this non-ideal behavior or tell me what I'm missing?

Thanks!

William.

RE: Does a pentaprism REALLY maintain beam perpendicularity regardless of orientation?

I think the assertion that the pentaprism always bends the beam by 90 degrees is only true within a plane parallel to its major plane of symmetry and confined to the locus of the prism, e.g. in a 2D model, and is further subject to manufacturing errors. I.e. all bets are off for skew rays.

Mike Halloran
Pembroke Pines, FL, USA

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close