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Condenser / Diaphragm / Brightness questions

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#1 Tom Stock

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Posted 27 August 2019 - 01:34 PM

So, I assumed the diaphragm was simply for brightness control, but the microscope also has a lamp brightness control.

 

I read somewhere that the diaphragm controls the angle of the light, so I suppose this would be like changing the F stop.

 

I did notice that reducing the aperture size with the diaphragm slider did seem to enhance the contrast more than changing lamp brightness alone.

 

I don't fully understand why.

 

But what about the condenser?  Why is it moveable, up and down?

 

Could someone explain the actual theory behind the moveable condenser + diaphragm?

 

Is it only to control brightness or is there more to it?

 

Thanks



#2 ssantia

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Posted 27 August 2019 - 03:32 PM

The simplest answer is:

Condenser focus the light beam on the specimen instead of being scattered all over.

Diaphragm adjusts the beam width to match the aperture of the objective.

 

Both these things help to render a sharper image and improve contrast.

 

Hope that helps.

 

Sam


Edited by ssantia, 27 August 2019 - 03:33 PM.


#3 EJN

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Posted 29 August 2019 - 09:53 PM

The iris not only controls light but resolution. While the contrast improves when closing it

on transparent specimens, closing it too far will impair resolution.

 

The ideal adjustment is determined by removing the eyepiece and looking at

the back of the objective, adjust the iris until it is about 3/4 illuminated.

 

The height adjustment of the condenser can usually be left as high as it can go

without hitting the slide. The only time you might need to adjust it is with darkfield

illumination, or when using a 4x objective where you might need to lower it to

fully illuminate the field.


Edited by EJN, 29 August 2019 - 09:55 PM.


#4 Microscopy

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Posted 05 September 2019 - 06:02 AM

I suppose (hope) that this will answer some of your questions (bare with me: I'm not a native English speaker):

Microscopes with build in or build on illuminators, as well as free standing microscope illuminators designed to provide illumination according to Köhler, have an extra diaphragm to control the beam of light: it's situated close to the illuminators (preferably focussable!) optical system (the "collector") and called the "field diaphragm".

The diaphragm at the bottom side of the condenser is called the "aperture diaphragm".

Illumination according to Köhler serves several purposes: solving the problem of uneven illumination of the field of view, due to the incoherent and rather crude bulb filaments of that era, preventing as much as possible stray light, thus preventing haze, shattering, lack of contrast etc. and achieving the highest possible resolution in the image. Up to this day, "Köhler" still is some kind of a golden standard in microscopy.


Setting up "Köhler" sounds like a hughe thing, but in practice it only takes a minute, anyway far less time than setting up a telescope:

  1. project the beam of light towards the aperture diaphragm
  2. focus the collector until the sharpest possible image of the bulb filament is projected unto the aperture diaphragm. The size of the bulb filament's image schould be as large as the diameter of the aperture diaphragm, to be able to use the condenser's N.A. fully
  3. focus on a slide with the 10x objective
  4. close the field diaphragm, raise or lower the condenser untill the sharpest possible image of the field diaphragm can be seen in the microcopic image (hence the need for a focussable condenser)
  5. open the field diaphragm untill it's just out the field of view, resulting in only the part of the object under examination being lit: no stray light entering the objective
  6. Take the eyepiece out of the tube and look at the back plane of the objective (a phase contrast centering telescope comes very handy here). Open/close the aperture diaphragm between 75% - 100% of the back plane image, not more! The cone of light from the condenser is limited to the diameter of the objective's front lens. Again: no stray light

When changing magnifications: repeat 5 and 6.

 

When the aperture diaphragm is closed (too much), it impears the resolution capability of the objective. That resolution capability is defined by the numerical aperture "N.A." of the objective, which is the sinus of half the angular aperture of the objective*the refractive index between front lens and specimen. In short: N.A. = N*sinµ.

 

For a microscope with a condenser the formula 0.61*wavelength of light used/N.A. is often used to determin resolution.

 

Take for example an objective 20x, N.A. = 0.40. Resolution = 0.61 * 550nm (white light)/0.40 = 839nm = 0.84µm.

 

Now let's say you close the aperture diaphragm too much, resulting in a restriction of the cone of light entering the objective, to an effective N.A. of 0.20. Resolution in that case will be around: 0.61 * 550nm (white light)/0.20 = 1678nm = 1.68µm, half the resolution of what the objective is capable of.

 

On the other hand: contrast and depth of field will be higher, which is sometimes an advantage, for example when observing low contrast, showing hardly any color specimens such as (often) live samples.

 

On a sidenote: I read often in high school biology courses and such that the aperture diaphragm's function is to "control image brightness", an absolute NONO. And I know plenty of high school biology teachers who can't set up a microscope the way it should...


Edited by Microscopy, 05 September 2019 - 06:17 AM.

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#5 Tom Stock

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Posted 05 September 2019 - 07:58 AM

Thanks for the detailed and informative reply.  Interesting point about resolution. I will give this a try.



#6 bumm

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Posted 17 September 2019 - 10:26 AM

I suppose (hope) that this will answer some of your questions (bare with me: I'm not a native English speaker):

Microscopes with build in or build on illuminators, as well as free standing microscope illuminators designed to provide illumination according to Köhler, have an extra diaphragm to control the beam of light: it's situated close to the illuminators (preferably focussable!) optical system (the "collector") and called the "field diaphragm".

The diaphragm at the bottom side of the condenser is called the "aperture diaphragm".

Illumination according to Köhler serves several purposes: solving the problem of uneven illumination of the field of view, due to the incoherent and rather crude bulb filaments of that era, preventing as much as possible stray light, thus preventing haze, shattering, lack of contrast etc. and achieving the highest possible resolution in the image. Up to this day, "Köhler" still is some kind of a golden standard in microscopy.


Setting up "Köhler" sounds like a hughe thing, but in practice it only takes a minute, anyway far less time than setting up a telescope:

  1. project the beam of light towards the aperture diaphragm
  2. focus the collector until the sharpest possible image of the bulb filament is projected unto the aperture diaphragm. The size of the bulb filament's image schould be as large as the diameter of the aperture diaphragm, to be able to use the condenser's N.A. fully
  3. focus on a slide with the 10x objective
  4. close the field diaphragm, raise or lower the condenser untill the sharpest possible image of the field diaphragm can be seen in the microcopic image (hence the need for a focussable condenser)
  5. open the field diaphragm untill it's just out the field of view, resulting in only the part of the object under examination being lit: no stray light entering the objective
  6. Take the eyepiece out of the tube and look at the back plane of the objective (a phase contrast centering telescope comes very handy here). Open/close the aperture diaphragm between 75% - 100% of the back plane image, not more! The cone of light from the condenser is limited to the diameter of the objective's front lens. Again: no stray light

When changing magnifications: repeat 5 and 6.

 

When the aperture diaphragm is closed (too much), it impears the resolution capability of the objective. That resolution capability is defined by the numerical aperture "N.A." of the objective, which is the sinus of half the angular aperture of the objective*the refractive index between front lens and specimen. In short: N.A. = N*sinµ.

 

For a microscope with a condenser the formula 0.61*wavelength of light used/N.A. is often used to determin resolution.

 

Take for example an objective 20x, N.A. = 0.40. Resolution = 0.61 * 550nm (white light)/0.40 = 839nm = 0.84µm.

 

Now let's say you close the aperture diaphragm too much, resulting in a restriction of the cone of light entering the objective, to an effective N.A. of 0.20. Resolution in that case will be around: 0.61 * 550nm (white light)/0.20 = 1678nm = 1.68µm, half the resolution of what the objective is capable of.

 

On the other hand: contrast and depth of field will be higher, which is sometimes an advantage, for example when observing low contrast, showing hardly any color specimens such as (often) live samples.

 

On a sidenote: I read often in high school biology courses and such that the aperture diaphragm's function is to "control image brightness", an absolute NONO. And I know plenty of high school biology teachers who can't set up a microscope the way it should...

Very clear and concise!  Thank you!  I've read much of this before, but generally overly technically presented with a considerable amount of obscuring gobbletygook.

                                          Marty



#7 Microscopy

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Posted 22 September 2019 - 06:46 AM

Here's a picture of such an illuminator, intended to provide illumination according to Köhler. It can serve as kind of a model for many free standing, build-in or build on microscope illuminators.

 

This is a vintage, free standing, 1950's Reichert "Lux FNI" illuminator, intended to be used with the now obsolete Osram incandescent 6V, 5A bulb 8100 and equivalents, pre-centered and soldered in a proprietary Reichert adapter (which didn't came cheap...):

Lux FNI-ann.jpg

 

The Reichert illuminator has a swing in/out frosted screen. The handle is on the other side of the illuminator, so not visible in the picture above but it can be seen here in a LUX FNI as part of the large Binolux II illuminator for the Zetopan:

BinoluxII.jpg

And this is a picture of the build on Lux US 12V, 100W halogen illuminator on a Zetopan microscope. Notice the small field diaphragm handle on top of the mechanical coupling between microscope and illuminator, the curled ring (collector focussing mechanism) and the lever on the side to push/pull in/out the frosted screen:

Lux US.jpg

 

In a good design a frosted screen is not necessary and if it's unremovable it makes it impossible to achieve #2 in the protocol I described above, where it is necesary to project an enlarged bulb filament image onto the aperture diaphragm, something like this:

Lux FNI filament.jpg


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