www.photomacrography.net :: View topic - Numerical aperture of a naked light bulb?
www.photomacrography.net Forum Index
An online community dedicated to the practices of photomacrography, close-up and macro photography, and photomicrography.
Photomacrography Front Page Amateurmicrography Front Page
Old Forums/Galleries
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 
Numerical aperture of a naked light bulb?

 
Post new topic   Reply to topic    www.photomacrography.net Forum Index -> Macro and Micro Technique and Technical Discussions
View previous topic :: View next topic  
Author Message
Beatsy



Joined: 05 Jul 2013
Posts: 911
Location: Malvern, UK

PostPosted: Thu May 07, 2015 5:41 pm    Post subject: Numerical aperture of a naked light bulb? Reply with quote

If I shine a halogen desk lamp down onto a specimen sat on the stage of an inverted microscope (no intermediate lenses), what is the effective N.A. of the light source?

If I place half a table tennis ball over the specimen in the above configuration, does that increase the effective N.A. of the light source?
Back to top
View user's profile Send private message
rjlittlefield
Site Admin


Joined: 01 Aug 2006
Posts: 18244
Location: Richland, Washington State, USA

PostPosted: Thu May 07, 2015 8:27 pm    Post subject: Reply with quote

Without the diffuser, the illumination NA is equal to half the lamp width divided by distance to subject. This can approach 0.0 .

With the diffuser, the illumination NA gets much larger. It would be 1.0 if the diffuser were evenly illuminated. But because the sides of the diffuser are not illuminated so brightly, falling off to dark along the sides, the sort-of-effective NA is smaller, perhaps 0.7 .

The effect of the diffuser should be blatantly obvious on out-of-focus detail.

Here is a dusty and badly laminated stage micrometer, shot once illuminated with a bare Jansjö LED die at about 14 inches, and again with a half pingpong ball placed around the subject. The rule is focused. Note the dramatic rings around the out-of-focus dust on the left side, versus the almost complete invisibility of dust on the right side.



Note also that the effect on focused detail is not as large as we might guess at first thought. This is because focused detail is governed by some rule along the lines of (NA_obj + min(NA_obj,NA_cond)), while out-of-focus detail is more governed by the condenser alone.

If I defocus the rule slightly, this is what happens (bare LED on the left, pingpong diffuser on the right):



Does this help?

--Rik

Further details: Shot with Nikon CF N Plan Achromat 10X NA 0.25, direct projection onto a Canon T1i, displayed by Photoshop from high-quality JPEGs straight from camera.
Back to top
View user's profile Send private message Visit poster's website
Beatsy



Joined: 05 Jul 2013
Posts: 911
Location: Malvern, UK

PostPosted: Fri May 08, 2015 3:32 am    Post subject: Reply with quote

Very useful info, thanks Rik.

I need to (temporarily) increase the working distance between light source and specimen on my Zeiss inverted, while preserving reasonable resolution (about N.A. 0.5-0.6 will do - with up to 100mm clearance). I just wanted to put some numbers to the results I was seeing with setups not involving the condenser. The table tennis ball isn't really an option, but a largish piece of white card spotlit from below should do.

Thanks again for the comprehensive response.

Cheers
Back to top
View user's profile Send private message
Asha



Joined: 09 Apr 2013
Posts: 70

PostPosted: Fri May 08, 2015 10:25 am    Post subject: Reply with quote

Rik, do you think you're getting those dust effects due to partial-coherence in the system? Diffraction ringing around the dust is like what I've seen in interferometry setups. In the lab it is usually a full coherence effect since monochromatic sources are used. In the lab, diffusion also solves the problem, even though it is laser light.
Back to top
View user's profile Send private message
ChrisR
Site Admin


Joined: 14 Mar 2009
Posts: 7254
Location: Near London, UK

PostPosted: Fri May 08, 2015 1:22 pm    Post subject: Reply with quote

Asha - I think it's just like you see on your sensor from dust particles if you use a small enough aperture. Try a pinhole "lens" to make something around f/100.
Any old light will do it.
Back to top
View user's profile Send private message
Asha



Joined: 09 Apr 2013
Posts: 70

PostPosted: Fri May 08, 2015 2:21 pm    Post subject: Reply with quote

Chris, but it disappears when the light is diffused, assuming the optics and aperture stay the same in both shots.
Back to top
View user's profile Send private message
rjlittlefield
Site Admin


Joined: 01 Aug 2006
Posts: 18244
Location: Richland, Washington State, USA

PostPosted: Fri May 08, 2015 2:43 pm    Post subject: Reply with quote

Yes, I think it is just the same as dust with a small aperture.

I normally think of the ringing in terms of classic diffraction, not explicitly in terms of coherence although of course classic diffraction implicitly assumes coherence within a temporal window size corresponding to the maximum wavefront error.

But it seems like the behavior would also make sense in terms of partial coherence, since the NA of the illumination source is much smaller than that of the imaging system. Maybe revisit the Goodman reference as discussed at http://www.photomacrography.net/forum/viewtopic.php?p=126689#126689?

Anyway, for more "fun with pinholes", see Rainbow Rings regarding Fresnel diffraction.

--Rik
Back to top
View user's profile Send private message Visit poster's website
Display posts from previous:   
Post new topic   Reply to topic    www.photomacrography.net Forum Index -> Macro and Micro Technique and Technical Discussions All times are GMT - 7 Hours
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group