Re: diffraction (warning, long post!)

[Tom <chowt@xxxxxxxxxxxxxxxxxxx>]

Hi,

I'm not an optical engineer, but just a meer physicist who works with
optics. Besides, diffration doesn't just occur in optics. So here is a
paired down simplified (hopefully!) non-mathematical explanation of
optical diffraction (if you want the math, I can give it to you, but you
might find it easier to look it up in a text book...).  If you are not
interested in a rather long post on this subject, just hit your "delete"
key and continue on your happy way! 

Diffraction is basically the bending of light around an obstacle, like the
edge of a diaphragm. It is the same whether you are talking about a single
edge or a diffraction grating.  It is often described as "Fresnel
diffraction" or "Fraunhofer diffraction". "Fraunhofer diffraction" is a
special case of "Fresnel diffraction", and is usually used to describe
diffraction of light from a single round aperture in optical systems, ie:
diffraction from the iris in your lens. 

(Another simple case is a single slit. A diffraction grating is just many
slits using the same theory as a single slit. Similarly you can have and
array of small apertures, which is what you see when you look through your
umbrella)

If you project an image of a small bright spot using mono-chromatic light,
Fraunhofer diffraction from a small aperture in a lens shows up as
concentric rings about that spot.  The size and brightness of each ring
depends on the size of the aperture with respect to the wavelength of the
light and the focal length of the lens. The closer the aperture is to the
wavelength, the more distinct each ring is. Different wavelengths of light
(ie: different colours) result in different size rings. Since white light
is 'poly-chromatic', diffraction results in a smearing of the point image
(point spread). 

If the aperture is small (within an order of magnitude of the wavelength)
then the majority of the diffracted light is concentrated about the point,
resulting in a diffuse spot instead of a point image. As the aperture
increases the diffracted light becomes more evenly distributed, so your
image of the spot becomes more distinct (as the undifracted light defining
the spot remains unchanged). Much of this has to do with the way the
diffracted light from one part of the diaphragm interferes (constructively
or destructively) with light from another part. 

For typical apertures used in photography (ie f22-f8 for 35mm, f64 for
large format), the diameter of the aperture is much larger than the
optical wavelength, thus you do not see this ring or point spread effect. 
The diffracted light is generally uniformly distributed on the film plane
and thus contributes only to a decrease in the image contrast. 

You can think of optical diffraction as a dim point source of light
occurring at every point that a ray of light touched an edge (this analogy
describes diffraction gratings as well). Thus there is a ring of diffuse
light originating from the edge of the diaphragm, which occurs at all
apertures. As you decrease the size of the aperture, the open area
decreases faster than the diameter of the aperture (if r=radius, the area
deceases proportionally with r*r while the diameter decreases
proportionally with r ). Since the amount of light for your image reaching
the film depends on the area of the open aperture, and the amount of
diffuse diffracted light depends on the diameter (length of the edge that
light is diffracting from), as you go to smaller apertures, the diffracted
light becomes a more significant amount of the total light that hits the
film.  In addition, as the aperture becomes very small, the diffracted
light becomes more concentrated (or directed) towards the original path,
resulting now in a decrease in image sharpness (and not just a decrease in
contrast). This is the reason that photographic lenses are limited in the
minimum aperture (besides the mechanical difficulties in making an
adjustable iris close that small...). 

Since IR light has a longer wavelength than visible, the minimum aperture
to see diffraction effects become larger, thus the warning not to use too
small an aperture. I don't think this has been a problem in practice. 

As for the halo effect on Kodak HIE film, this is due to the fact that
there is no antihalation layer on that film. The anithalation layer is an
opaque layer that prevents light from scattering around in the film base. 
It is the only film I know that does not have an antihalation layer (there
might be others, but I don't know about them). Light from a bright spot on
the film scatters in the film base out towards the darker parts of the
image exposing the emulsion there, resulting in light that 'leaks' into
the shadows. Sharp lines still look sharp, but have less contrast and a
light halo. Diffraction from a small aperture would soften a sharp line,
and result in none, or very little noticeable halo. (as I have only used
35mm Kodak HIE, the halo and graininess seems to mask any diffraction
effects)

The antihalation layer in a film resides between the emulsion and the film
base, preventing any light from reaching the film base. It is removed
during fixing, which is why your films are still opaque after developing
(and stop bath) but become clear after fixing. 

As for reversal processing (making slides out of negative films) it can be
done with any B&W film, the popular one out here is TMAX 100. It is all in
the processing. 

There was another comment about refocusing view camera lenses (not on this
thread, but it is optics after all...). The amount of focus correction
required for IR light depends completely on the optical design of the
lens. Varriations from lens to lens can be huge. Manufacture information
is required (or do your own test). The other solution is to buy an
apochromat that is corrected into the IR range, very $$$$! (Hasselblad
makes a 250mm, and I believe Schneider makes some view camera lenses, or
use to)

I'd be happy to answer, discuss, be corrected, etc... on anything
mentioned above, but just a warning, don't expect too prompt an answer, I
find it hard enough to get time to check my Email. I don't know how Willem
manages so many post! ;-}

Cheers,

Tom




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End of INFRARED-PHOTOGRAPHY Digest 12
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