Re: Darkfield condenser

>From [1]:

"...It is called a high-resolution darkfield condenser [Cardioid condenser].
A darkfield condenser ensures that no direct light can enter the objective.
This type of darkfield condenser accomplishes this by creating a very wide
cone of light (the inner cone has an NA of 1.20 and the outer cone has an NA
of 1.40). Such a setup requires an objective with an NA not greater than
1.10. If the NA of the objective is larger than the value for the inner cone
(here 1.20), direct light will enter the objective, which makes darkfield
illumination impossible. Since this darkfield condenser creates such a wide
and narrow light-cone, immersion oil must be put between the condenser and
the slide."

"...This condenser is not trivial to operate and the following steps must be
taken when working successfully with this unit:
1.. Use immersion oil between condenser and slide
2.. Clean surfaces of slide carefully
3.. Do not expect too much performance when using thick samples
4.. Avoid slides made out of glass with many fluorescent impurities
5.. Use centering screws to center condenser for optimal darkfield
illumination
6.. Do not use an objective with an NA of more than 1.10
7.. Do not use an objective with less than 10x magnification

Fortunately, if you plan to use a high power lens with a maximum NA of 1.25
or higher, you could just add a funnel stop in the back focal plane of the
objective to lower its maximum NA to something around 1.10. You could also
use a special high power lens with an iris."

[1] http://www.microscopy-uk.org.uk/mag/artjul03/gocompmic.html

"justbeats" <steve_beats@xxxxxxxxxxx> wrote in message
news:1126517553.936322.209240@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>I just got a second-hand Zeiss darkfield condenser. IMHO it's the
> best bang for the buck of any accessory I've ever bought for my Zeiss
> inverted! For diatom imaging at high mag (and high NA), the improvement
> in contrast compared to other techniques (e.g. BF, PC, COL or DIC) is
> massive. I'd already tried low NA darkfield with excellent results, but
> I didn't expect the hi-NA version to be quite so much better again.
> Wow!
>
> It's the one accessory I don't mind having to use immersion oil
> for! Good job too as it's wholly non-functional if not oiled to the
> back of the slide :-)
>
> Couple of questions...
>
> The condenser is marked "1.2/1.4". Obviously this is the NA, but
> why two numbers? Is it related to immersion medium (i.e water/oil or
> glycerine/oil in this case)?
>
> I have an old set of prepared slides containing many species of
> Coscinodiscus, including some extremely rare varieties and two marked
> "new sp?". Unfortunately, the slides are very thick and I can JUST
> get the illumination onto the specimen but the condenser is touching
> the slide (except for a very thin smear of oil between the two
> surfaces). I guess thick slides preclude the use of a hi-NA darkfield
> condenser? Shame, as it was those that I most wanted to use this DF
> technique on. Any way to solve this limitation?
>
> Cheers
> Beats
>


.

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