[pure-silver] Halo effects with MACO' s Aura film..not...

Since it is a bit quiet on the list lately I thought I might share some results 
with MACO's IR film "Aura"..

One of the charms of Kodak's HIE film is the lack of an anti-halation layer, 
causing strong halo effects of say white trees with leaves (Wood effect) 
against a black sky, already with "normal" exposure and a 25 filter. A couple 
of years ago Kodak stopped producing HSI film in 4*5 format. I never worked 
with that size, I think it also had the distinct halo effect, perhaps less so, 
since the enlargingfactor is usually less of 4*5 film versus 35 mm.

The only "player" in the 4*5 film field is MACO (aka EFKE PL100 film doped with 
IR sensitive dyes AFAIK), some 2 years ago they brought a version of their 
standard film 820c without an anti-halation layer called AURA.

I've using the "normal" vrsion for quite some time now, and am actually quite 
happy with it, I do get nice IR effects when using a 70 filter, also some 
modest Wood effect, my main objection is that the emuslion is soft, the first 
batches had some emulsion artefacts, but most important: the working speed is 
1.5-3 asa..

I got the oppertnunity to test this AURA version, but until now I am not 
impressed: it behaves almost exactly as the normal version, only when you 
overexpose 1-3 stops (already a problem: measuring IR light is a bit of an hit 
and miss, so I tend to overexpose anyway, unlike Kodak HIE the highlights do 
not block up dramaticly, and souping it in PyrocatHD gives a nice contrast 
reduction in the highlights, but I disgress..). This over exposing gives long 
exposure times and long working hours in the darkroom.

Michael Gudzinowicz gave some suggestions to enhace the halo effect: see below. 
I tried placing alumminium foil behind the film: no effect. Only 2 months ago I 
came across a 4*5 film holder which could take glassplates, so I could test his 
suggestion to place a glassplate between the holder and the film: again no halo 
effects..

Best,

Cor

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
With most films, one doesn't see a true halo, but rather a fogging due to
scatter
in the emulsion, and occasionally, the "imprinted" pattern of the film
pressure
plate. The latter can be a real problem with EKC 35mm HSI.

The "halo" effect around bright point light sources is best demonstrated
with
glass plates. The distance between the point and the surrounding circular
halo
depends upon the refractive index of the medium and its thickness. The
refractive 
index determines the angle of maximum reflection intensity at the
air/glass interface.

R = 2 * T / (((N^2)-1) ^ 0.5)

For 2 mm thick glass with an index of 1.5, the halo radius (R) would be
3.57 mm
on the plate. 

R = 2 * 2 /((1.5)^2)-1)^0.5 = 4 / (2.25 - 1)^0.5 = 4 / (1.25)^0.5 = 3.57
mm


Of course film base is much thinner, and enlargment would be required
to see a halo if it is resolved. An air gap between the film and pressure
plate
has an index of 1.0 and doesn't really contribute, but it can reflect
light.

If you want to see the 19th century halo effect around point light
sources,
I'd use a glass plate holder with a sheet of film covering a clear glass
plate (no emulsion). I sold my glass plate holders and I don't have film
without
an anti-halation backing, so I can't do the experiment, but it should
work.

Another approach would be to use a thin glass cover slip or slide mount
over the 
pressure plate of a 35 mm camera to act as a spacer with high refractive
index. 
The thickness and halo diameter could be scaled to the printing
magnification of 
the negative.
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