Diaphragm of a Microscope – Definition, Types, Mechanism, Functions

Diaphragm of a microscope is a thin opaque disc like part having a circular opening (aperture) which can be adjusted. It is fitted in the light path of microscope, mostly below the stage and near the condenser. It controls the passage of light that will go to the specimen.

The main work of diaphragm is to regulate the amount, intensity and angle of light. When the aperture is opened, more light beam passes through the specimen and image becomes bright. When the aperture is closed, less light passes and the image becomes comparatively dark.

By changing the diaphragm opening, the brightness, contrast, resolution and depth of field of the image are affected. If it is opened too much, glare is produced and contrast becomes washed out. If it is closed too much, diffraction like effect can occur and fine details are not seen properly.

Proper adjustment of diaphragm is needed for clear viewing. It is generally adjusted along with condenser so that the light beam diameter matches with the objective requirement for best image.

History of Diaphragm

1567
Daniele Barbaro described the camera obscura with biconvex lens. He noted image becomes more vivid when lens is covered leaving only a circular opening in middle.

1762
Leonhard Euler documented the need of diaphragms with small circular aperture inside telescope tubes. This was for excluding extraneous light.

1820s
Some historians credit Joseph Nicéphore Niépce for an early device resembling iris diaphragm. It was not the modern iris but similar idea of adjustable opening.

1840s–1850s
Chevalier and Jamin demonstrated early examples of iris diaphragms. During this period pivoted stops were also used, metal plates with apertures that swivel in front of lens.

1856
M. Noton invented an adjustable “cat eye” diaphragm made of two overlapping sliding squares with diamond shaped opening. In same year John Dancer patented a wheel stop which is a rotating disc having series of different sized apertures.

1858
John Waterhouse introduced Waterhouse stops. It was interchangeable metal plates with drilled holes inserted into a slot on lens barrel for changing aperture.

1858
Charles C. Harrison and Joseph Schnitzer patented an adjustable stop having six movable blades. This type of blade system laid base for multi-blade iris diaphragm.

1858
Pierre Maugey patented a diaphragm made of elastic membrane with central hole. Aperture size changes when membrane is stretched.

1867
J. H. Brown (Royal Microscopical Society) invented a popular improved version of iris diaphragm. In same year Désiré van Monckhoven separated the terms “stops” and “diaphragms” in photographic optics, later Abbe theories linked these concepts.

1870s
Ernst Abbe improved microscopy optics by designing focusable condenser with integrated iris diaphragm. This allowed control of light cone to match numerical aperture of objective lens.

1886
Lancaster was among first manufacturers to commercially fit iris diaphragms to Rectigraph lenses. It became more common feature and not only expensive special option.

1893
August Köhler introduced Köhler illumination. It used dual diaphragm system (field iris diaphragm and aperture iris diaphragm) to give even glare free illumination, light source not focused at specimen plane.

1900s
Multi-blade iris diaphragm replaced Waterhouse stops in most lenses and optical devices. It became standard aperture control method.

1916
Treitschke patented a semi-automatic diaphragm operation mechanism. This supported faster aperture control in use.

Late 1940s–1960s
Camera mechanical design improved to pre-set diaphragms for focusing wide open and closing just before exposure. Later fully automatic diaphragms (FAD) came, spring mechanism snaps iris to pre-set value only during exposure.

Definition of Diaphragm of a Microscope

The diaphragm of a microscope is a component that controls the amount of light that passes through the specimen being observed. It is typically located between the light source and the stage, and consists of a series of adjustable blades or a rotating disc with different-sized apertures. By adjusting the diaphragm, the user can regulate the intensity and size of the light beam, which in turn affects the contrast, resolution, and depth of field of the image seen through the microscope. This makes it an essential tool for optimizing the visibility of different types of specimens.

Purpose of Diaphragm of a Microscope

1. Controls light amount and intensity
   It is used to regulate the amount of light passing from the illuminator to the specimen. The intensity of light can be increased or decreased by opening or closing the aperture.
2. Adjusts the angle of illumination
   It controls the diameter of the light beam and also the angle of the cone of light. The light cone that enters into the objective lens is adjusted by this part.
3. Enhances image contrast
   By altering the aperture size, the contrast and brightness of image is changed. Transparent and delicate structures are seen better when contrast is properly adjusted.
4. Optimizes depth of field
   Changing the aperture size helps in controlling the depth of field. This is useful for viewing thick, layered or three-dimensional specimens where different planes are present.
5. Regulates resolution
   It maintains the balance between contrast and resolution. Proper setting prevents image degradation and diffraction type artifacts when aperture is too small.
6. Determines numerical aperture (NA)
   The diaphragm regulates the numerical aperture of condenser. This finally affects the effective working NA and resolving power of the microscope system.
7. Reduces glare and stray light
   By restricting excess scattered light, glare is reduced. This prevents the image from getting washed out and gives clearer view.

How does Diaphragm works

1. Mechanical movement (iris diaphragm)
   Most modern microscopes have iris diaphragm made of thin overlapping metal blades. These blades are connected to an external lever or control ring. When the lever is moved, the pins drive the blades to pivot and slide on each other. The central circular opening becomes larger or smaller in smooth way.
2. Mechanical movement (disc diaphragm)
   In simpler microscopes disc diaphragm is present. It is a rotating circular plate having pre-drilled holes of different sizes. When the disc is rotated, one hole comes in the light path and it clicks in position.
3. Light regulation
   By changing the size of central opening, diaphragm works like a gatekeeper. It decides how much light will pass through the specimen. It also decides the angle of light cone that strikes the specimen and enters the objective.
4. Closing the diaphragm (smaller hole)
   When diaphragm is closed, the light amount is reduced. The contrast becomes more and depth of field also increases. But if it is closed too much, resolution decreases and diffraction type artifacts may be seen like dark halo at edges.
5. Opening the diaphragm (larger hole)
   When diaphragm is opened, brightness increases and finer details can be seen. The theoretical resolution becomes better. But if it is opened too wide, stray light and glare increases and image contrast becomes washed out.
6. Working with other diaphragm (Köhler illumination)
   In research microscopes dual diaphragm system is used. Field diaphragm controls the area of specimen illuminated and removes unnecessary light to reduce glare. Aperture (condenser) diaphragm controls the angle of light cone to balance contrast and resolution. For best viewing this aperture diaphragm is usually kept about 60% to 90% open.

Types of Diaphragms

1. Mechanical diaphragm
   It is a simple rotating disk or lever type mechanism. It has preset fixed aperture sizes. It controls the amount of light passing to the specimen.
2. Iris diaphragm
   It is an adjustable diaphragm made of overlapping metal plates or blades. These blades form a circular opening. The aperture size is continuously varied with a lever or ring to control light intensity.
3. Aperture diaphragm (condenser diaphragm)
   It is positioned within the substage condenser. It controls the angle of the light cone illuminating the specimen. It affects numerical aperture, resolution and contrast.
4. Field diaphragm (luminous-field diaphragm)
   It is located near the light source at the base of microscope. It controls the area of specimen that is illuminated. It reduces stray light and glare and it does not mainly change optical resolution.
5. Annular diaphragm
   It is a special plate having a transparent ring. It produces a hollow cone of light. It is essential in phase contrast microscopy.
6. Opaque stop (spider stop)
   It is a central light stop used in darkfield illumination. It blocks direct central light rays. Only oblique peripheral rays are allowed to strike the specimen.
7. Waterhouse stops
   It is a historical diaphragm system. It consists of interchangeable oblong metal strips or plates. Each plate has a punched hole of specific size.
8. Wheel stops (rotating stops or Dancer diaphragms)
   It is a circular disc having series of different sized apertures. The disc is rotated to bring one aperture into the light path.
9. Washer stops (pillbox stops)
   These are individual discs with punched holes. They were fitted in early landscape camera lenses behind a retaining ring.
10. Sliding stops
    It is a metal strip having multiple punched holes. The strip is slid across the lens opening to change aperture size.
11. Noton diaphragm
    It is an early adjustable “cat eye” diaphragm. It is made of two overlapping sliding plates. It has diamond-shaped opening.
12. Maugey diaphragm
    It is an elastic membrane with a small central hole. The aperture size is changed by stretching the membrane.

How to Use the Diaphragm

1. Locate the control
   First find the diaphragm control lever, dial or rotating collar. It is usually present on or just beneath the substage condenser.
2. Keep it fully open at starting
   Start the observation with diaphragm wide open. Maximum light will pass through the specimen in this position.
3. Focus the specimen first
   Keep the specimen properly focused under the objective lens. After focusing, diaphragm adjustment is done for best view.
4. Gradually close the aperture
   While looking through eyepiece, slowly close the diaphragm. This reduces the diameter of the light cone entering the objective.
5. Stop at the optimal balance
   Close it till the specimen just begins to become slightly dark. At this point contrast becomes crisp and clarity is better.
6. Follow 60% to 90% open rule
   In most cases diaphragm is kept about 60% to 90% open. This setting gives balance of contrast, resolution and depth of field.
7. Match with objective lens NA
   If NA scale or numbers are printed near the lever, it can be matched with the objective NA. This helps in getting maximum resolution for that objective.
8. Readjust after changing magnification
   Whenever objective lens is changed to higher or lower magnification, diaphragm must be adjusted again. Each objective needs different aperture setting.
9. Do not use as dimmer
   Aperture diaphragm should not be used only for making view bright or dark. Brightness control is done by lamp voltage knob or neutral density filters.
10. Avoid extreme open and extreme close
    Too much opening gives glare and specimen details get washed out. Too much closing reduces resolution and false halos or diffraction type artifacts may appear.

Functions of the Diaphragm

• Controls light intensity
  It adjusts the overall amount of light passing from the light source through the specimen. Light intensity is increased when aperture is opened and reduced when it is closed.
• Regulates the angle of illumination
  It changes the diameter and angle of the light cone coming from the condenser. The light cone that strikes the sample and enters objective is controlled.
• Enhances image contrast
  It helps in differentiating delicate structures inside the specimen. Excess light is prevented, so image does not get washed out.
• Determines resolving power
  It sets the working numerical aperture of the microscope system. This decides the ability to separate small and closely placed details clearly.
• Optimizes depth of field
  It manipulates the range of depth that remains in sharp focus. This is useful for layered and three-dimensional specimens.
• Reduces glare and optical artifacts
  It blocks stray and scattered light entering optical system. It prevents diffraction halos, fringes and other visual distortions in image.
• Modulates spatial coherence
  It controls the coherence of light hitting the specimen. Smaller aperture increases coherence and transparent edges and fine particles become more visible.

Advantages of Diaphragms

• Improves image contrast
  By regulating the amount and angle of light, diaphragm gives better differentiation between parts of specimen. Transparent and low contrast structures are seen more easily.
• Optimizes depth of field
  By adjusting the aperture size, depth of field can be controlled. It is useful for keeping selected region of thick, layered or three-dimensional specimen in sharp focus.
• Controls brightness and prevents washout
  It modulates light intensity so overexposure and washout in translucent samples is prevented. It can also be opened wider when more light is needed for thick or opaque specimens.
• Reduces glare and unwanted reflections
  Closing the aperture restricts stray light and glare. This improves the overall clarity and sharpness of image.
• Balances resolution and contrast
  It allows the user to make a compromise between resolving power and contrast. By controlling size and angle of light cone, better detail is obtained without losing contrast too much.
• Provides precise continuous adjustments
  Modern iris diaphragm gives continuously variable control, not fixed steps. It helps in matching illumination cone with numerical aperture of the objective lens being used.

Disadvantages of Diaphragms

• Loss of resolution and artifacts
  If the aperture diaphragm is closed too much, the optical resolution is reduced. Diffraction artifacts can be produced like fringes or artificial light halos around the edges.
• Glare and washout
  If the aperture diaphragm is opened too wide, stray light enters into the objective. Contrast becomes low and image details can get washed out.
• Lack of precision (disc diaphragms)
  Disc diaphragms give only fixed aperture sizes in steps. Sometimes one setting gives less contrast and the next setting gives too much glare, so fine control is not possible.
• Vulnerability to contamination
  Iris diaphragm is exposed and has moving mechanical parts in substage region. Dust, grease, wear and spilled immersion oil can affect it easily.
• Friction and sticking
  If lubricant becomes old or corrosive chemicals act on it, blades develop friction. Blades may stick together or move slowly and proper adjustment becomes difficult.
• Fragility
  The metal blades and drive pins are delicate. If a stuck lever is forced, pins can bend and blades can tear, and diaphragm failure can occur.
• Inconvenience (historical types)
  Older types like Waterhouse stops were slow to use. The plates can be lost or damaged easily during handling.

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