Re: Darkfield condenser


"GTO" <gregor_o@xxxxxxxxxxxxxxx> wrote in message
news:TEsVe.3940$6e1.2213@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> 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
>
Great stuff! Another point about dark field. It will teach you the
difference between detection and resolution.

Resolution is seeing that there are two items. A technique I use in
teaching is to pick a brick building in the area and ask the class "Were is
the point you detect that the building isn't a red building but a brick
building?". That's the limit of resolution.

Now detection is like driving in the American mid-west. At night you can
see lights but you don't know what is coming at you since its so far away..
Is it a car, a truck, a tractor, a motorcycle? With dark field you can
detect stuff but you can't determine its size, shape or anything else. You
just know somethings there.

This is real fun, but very difficult to use or come to any conclusions with.
But for fun its really cool.

Kevin Cunningham
SMS


.

Relevant Pages

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