Checking a book, Applied Photography by C.R. Arnold, P.J. Rolles, J.C.J.
Stewart published by Focal Press, 1971.
"2.5.3.1 Halation. Halation is a special case of image spread which is
caused by scattered light which is totally reflected within the base
material. This adversley affects the emulsion MTF and in some cases
prevents 100% modulation at any spatial frequency. "
or in simpler terms from M.J. Langford "Basic Photography" 3rd Ed 1975
Focal Press
"Emulsion Resolving Power
The ability of an emulsion to resolve fine detail is of equal practical
importance to lens quality. Emulsion resolving power is noticeably reduced
as (a) grain size becomes coarser, and (b) emulsion thickness becomes
greater.
Silver halide crystals are exceedingly small-one-tenth of a square inch of
emulsion may easily contain more grains than the world contains people. But
they are also coated up to fifty or more grains thick. Light passing into
the emulsion is scattered by the thickness of this overlapping grain
population so that it effects a larger area of the film, slightly
`spreading' the recorded image. The effect of this `irradiation' is to make
poorer the detail resolved in highlight areas, particularly if these are
overexposed.
Some of the irradiated light passes through the emulsion and may be
reflected back by the rear of the base as halation. Scatter may then occur
again as the ring of reflected light is irradiated within the emulsion.
-Definitions: 'Irradiation'- light scatter within the emulsion layer,
causing spread of the recorded image. Irradiation is made worse by
overexposure and the use of thickly coated emulsions.
'Halation'- light, noticeably from image highlights, reflected from the rear
of the base back to the emulsion-forming `halos' around each highlight.
Halation is largely eliminated by anti-halation backing dye which either
absorbs most of this light, or reflects back only wavelengths to which the
emulsion is insensitive.
With most generally lit scenes the effects of strong irradiation and
halation are simply to give a grey, degraded and poorly resolved negative.
However, with subjects containing intense individual highlights, such as
night scenes containing street lamps, these may record as 'cotton-wool
balls' surrounded by geometrically perfect halos. If you are purposely
aiming for this in an attempt to symbolise intensely bright light sources,
use a glass plate with a thick emulsion-having wiped off the anti-halation
dye and replaced it with a sheet of aluminium foil.
Whenever we require maximum emulsion resolving power-as when making '
detailed record photographs on small-format negatives-a fine-grained, thinly
coated, efficiently backed emulsion is desirable. We should take care not to
overexpose. Because of this influence of grain size and coating thickness on
resolving power, high resolution emulsions are inevitably `slow' in their
sensitivity to light."
Or, Ansel Adams, "The Negative" Little Brown 1981
"Layers may be added to the back of the film base to prevent scratching and
curl of the film, and to avoid halation. Halation occurs when light passes
through the emulsion and base, and reflects off the rear surface of the
base, back into the emulsion, causing exposure effects usually visible as a
halo around any bright points. The antihalation dye on the back of the
support prevents such reflection during exposure, and then usually washes
out during processing. Other means of avoiding halation include a coating
between the emulsion and the base or a slight dye density within the base
itself. Both of these are permanent, but contribute only a slight overall
density which does not affect the image-density relationships in printing.
An antiscratch layer may be added over both the emulsion and base, and both
sides are sometimes treated to permit retouching."
For Blooming, try M.J. Langford "Advanced Photography", Focal Press 3rd
Ed, 1974
"Manufacturers now reduce reflection losses by coating, or `blooming', a
thin film of transparent material on each glass surface. The anti-reflection
coating must have a refractive index equal to the square root of the
refractive index of the glass. (Calculations are shown on page 405.) This
produces two reflections of equal intensity (see Fig. 1.9). It should also
be coated to a thickness equal to one quarter of the wavelength of the
incident light. In this way light reflected from the underside of the
coating travels half a wavelength further than light reflected from the top.
The result is two rays of reflected light which are `out of phase'- i.e. the
troughs of the waveform of one ray correspond to the crests of the other
ray. Due to the phenomena of light `interference' rays combining in this way
cancel each other out. Light energy previously reflected follows the path of
refraction. The result is an image of improved brightness range and
intensity."
The above explains the Halo, which clearly can only occur with film, with
blooming a product of the lens.
Hope the explanations help.
Jim Thyer
----- Original Message -----
From: "Elson T. Elizaga" <elson@xxxxxxxxxxx>
To: "List for Photo/Imaging Educators - Professionals - Students"
<photoforum@xxxxxxxxxxxxxxxxxx>
Sent: Sunday, December 30, 2007 3:23 AM
Subject: Re: Halo and blooming
I made a google on "film camera haloing" and found one image that's
described as a halo. The page is
