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Lens hood for Microscope Objective
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DQE



Joined: 08 Jul 2008
Posts: 1653
Location: near Portland, Maine, USA

PostPosted: Mon Jan 09, 2012 8:52 am    Post subject: Reply with quote

Rik,

Thanks for the link to the NIST report. As one would expect, their articles and reports are in a class all of their own.

I gather that consumer flat panel TVs are using a mostly meaningless measure of contrast ratio these days. They're reporting ratios up into the millions-to-one as you may have noticed.

Much research has gone into making digital image displays suitable for medical and industrial radiography and other forms of diagnostic imaging. One of the many issues they face includes the need to wear dark clothing if you're going to carefully examine a digital image. In the early days of this work, only CRT-based displays were available, with thick glass cover plates, making the issues more complex due to the spacing the plate introduced between the phosphor layer and any instrumentation that one might introduce.

X-ray film, the primary competitor to digital back then, was unusual in that it was usually not very susceptible to flare light or reflections. The surface of the film was often roughened by tiny matte beads, and in dark areas there wasn't too much flare light wandering around within the layers of the developed film. As with any imaging medium, once you get into its details, things become very complex.

My impression is that the best display devices now incorporate fairly good anti-flare and anti-reflection technologies. I notice that my fairly new LG HDTV doesn't much reflect my LED flashlight beam.

Curiously, some prominent digital displays are very shiny, indicating a high specular reflectance. Apple's computer displays are mostly that way, as are some HDTVs.
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johan



Joined: 06 Sep 2011
Posts: 1004

PostPosted: Mon Jan 09, 2012 10:17 am    Post subject: Reply with quote

I don't suppose that a lenshood making minimal difference could also be partially something to do with the lack of redundant glass in objectives could it? Ie as opposed to reversed or enlarger lenses which are generally FF 'plus a little bit more to ensure good corner performance'?
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BugEZ



Joined: 26 Mar 2011
Posts: 793
Location: Loves Park Illinois

PostPosted: Mon Jan 09, 2012 12:44 pm    Post subject: Reply with quote

Rik wrote:
Quote:
Dead on 90° seems perfect.


When I make my next lens hood I shall use 90 deg. I see the wisdom of this.

Keith.
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 20182
Location: Richland, Washington State, USA

PostPosted: Mon Jan 09, 2012 1:31 pm    Post subject: Reply with quote

BugEZ wrote:
Rik wrote:
Quote:
Dead on 90° seems perfect.

When I make my next lens hood I shall use 90 deg. I see the wisdom of this.

Please let us know how it works out. I had not really thought about the effect of cone angle until I read the NIST article and spent some time sketching reflections. I'm still not sure I understand everything that can go on, and I have the strong feeling that the issues are different for shiny and matte materials. It could be that shiny material works great but only in very specific configurations, and for matte material other configurations may work better than 90 degrees. There's a lot of opportunity for mischief when light gets to bouncing around.

johan wrote:
I don't suppose that a lenshood making minimal difference could also be partially something to do with the lack of redundant glass in objectives could it? Ie as opposed to reversed or enlarger lenses which are generally FF 'plus a little bit more to ensure good corner performance'?

I suspect it's the internal barrel design and the lens edgings more than the refractive surfaces. Microscope objectives have to be designed to handle lots of stray light coming in the front, because that's what condensers produce unless they have a field diaphragm that happens to be adjusted correctly. Camera lenses have to be protected against lots of stray light too, but that would be stray light coming the other direction, into the end that becomes the rear when reversed. They don't need any protection the other direction. Enlarger lenses don't need much protection at all, because the film holder masks a large area around the FOV. As a result, the designers of camera and enlarger lenses don't have to design baffling that's optimized in the same way that's needed for macro photography.

--Rik
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ChrisR
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Joined: 14 Mar 2009
Posts: 8406
Location: Near London, UK

PostPosted: Mon Jan 09, 2012 2:28 pm    Post subject: Reply with quote

The black surface has to be flat, and smooth, too. I'm not sure how easy that is to achieve at very small level, in a home-made conic surace!
If there are asperities (humps) with sides at 45º, then light coming from the lens direction (having bounced off its front surface) gets reflected back into the lens.


In other words a rough shiny (reflective) surface would be worse than a flat matt (non shiny) one which absorbed all the light which hit it. I think this could be more important with a closed cone because there's more opportuity for the "light to get back in".

The NIST article speaks of "black vinyl plastic with a gloss surface", which could cover a multitude of surfaces Sad .

(Flat and smooth may not be intuitively understood - here's a typical presentation:
http://www.grahamoptical.com/flatrough.html )

The cone's an interesting idea but I don't feel at all satisfied that the article does it justice. Where it speaks of "glossy, matt, felt...", I believe those are all referring to a flat mask, they only actually used one material for the cone?

Some of their comparisons between their cone and eg "no mask", really don't help a lot.
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