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>Published : 11th february 2005

>Hi, all!

>Remember astronomical photographic plates are designed for very long exposure times (hours), during which, the automatic mechanisms in the telescopes keep them pointed to a celestial body to compensate for the earth's motion.

>Cooling the plates during exposure times certainly reduces fog level=background noise.

>Baking film was an option in the old, old days when latensification was also discovered. Baking, as was already quoted, works in very low sensitivity films, and is very "iffy" in results...

>Latensification or as more recently called, flashing, is however used quite often, if mostly for modifying the contrast of a stock. During the reversal film period of TV news, flashing or latensification in the developing machines was used to raise the shadow level in low light situations.

>Jose Llufrio
Chemist
Technicolor East Coast
New York City


>Jose Llufrio writes:

class="style9">>Remember astronomical photographic plates are designed for very >long exposure times (hours), during which, the automatic mechanisms >in the telescopes keep them pointed to a celestial body to compensate >for the earth's motion.

>Astronomers do "bake" film for astrophotographic purposes, but not to alter the color. It is done to remove moisture, nitrogen and oxygen from the emulsion. If these remain in the emulsion they have a tendency to increase reciprocity failure. That is, they tend to de-sensitise the film, making very long exposure astro photography very difficult as lightly exposed grain tends to revert back to an unexposed condition.

>To prevent the water vapour, oxygen and nitrogen from being re-absorbed, the film is immersed in hydrogen gas which is extremely dry. This replaces the water vapour, etc. Don't try this at home kids. The film is then said to be hyposensitized or hypered for short. I think hypered film is available commercially

>Astro cameras are cooled with nitrogen in order to reduce the "thermal agitation" that normally takes place in film at normal temperatures. Basically, thermal agitation increases fog level and reciprocity failure. Chilling the film slows down this action, but still allows the silver halide crystals to react to light. Astronomers actually use slower films because of the finer grain and lower fog. By hypering, they can overcome the obstacles to very long -- 3 to 5 hour -- exposures.

>There's a university observatory up the street from my home. I've found that during these very long exposure times astronomers usually have little to do and very little company, and will be more than happy to explain just about anything about astronomy to anyone during that time.

Anyone remember Bernie TVC and Chem-Tone?

>Brian Heller
IA 600 DP


class="style9">>Astronomers do "bake" film for astrophotographic purposes, but not to >alter the color.

>I think I was responding to someone who suggested baking film was a way to hypersensitive it. I wasn't referring to color change.

Art Adams, DP [film|hidef|video]
Mountain View, California
San Francisco Bay Area - "Silicon Valley"

http://www.artadams.net/


>Art Adams writes :

class="style9">>I think I was responding to someone who suggested baking film was a >way to hypersensitive it. I wasn't referring to color change.

>Me neither, but someone was.

>However, the heating of the film only drives out the water vapour, the nitrogen and oxygen -- essentially what's in the atmosphere, contaminants which if left in the film promote "thermal agitation". Their removal doesn't increase the film's sensitivity per , but instead reduces low light reciprocity failure. At the extremely low light levels encountered in astrophotography this may amount to something very similar, but from my understanding, it's not the same thing . For instance, it doesn't work with normal exposure times encountered in our type of work.

>The hydrogen bath doesn't increase the speed either, it simply prevents the film from re-absorbing the atmosphere for limited time. If the interval between hypering and exposing is too long, the hydrogen will be replaced by the atmospheric gases and the film reverts to its original state.

>Sorry if I didn't make that clear.

>Some of the confusion my come from a couple of places that sell hypered film. They may talk about an increase in film speed, but again from my understanding, it's not a real increase.

>Brian Heller
IA 600 DP


>Brian Heller writes :

class="style9">>The hydrogen bath doesn't increase the speed either, it simply >prevents the film from re-absorbing the atmosphere for limited time.

>True or False?

>This should be a question on the DP "test" like the old grip and electric "test" from NABET.

>Really Brian where do you get this stuff? Always amazed..

>Nick Hoffman 600D


>Brian "Mr. Wizard" Heller wrote:

class="style9">>If the interval between hypering and exposing is too long, the hydrogen >will be replaced by the atmospheric gases and the film reverts to its >original state

>Which is why latensification is a much more useful technique -- it's done right before processing, and only effects those grains that have some exposure but not quite enough. Doesn't add to the base fog level, like flashing.

>Jeff "working on something interesting" Kreines


> Nick Hoffman writes :

class="style9">>This should be a question on the DP "test" like the old grip and electric >"test" from NABET

>Then I would have failed for sure.

class="style9">>Really Brian where do you get this stuff? Always amazed.

>From a previous post :

class="style9">>There's a university observatory up the street from my home. I've found >that during these very long exposure times astronomers usually have >little to do and very little company

>I asked an astronomer about compensating for very low light and reciprocity failure and got a three hour dissertation on astrophotography and hypering.

>I can only remember about three minutes of it.

>Brian Heller
IA 600 DP


>Jeff "Mad Scientist" Kreines writes:

class="style9">>Which is why latensification is a much more useful technique -- it's >done right before processing, and only effects those grains that have >some exposure but not quite enough. Doesn't add to the base fog level, >like flashing.

>It may be much more useful technique in some circumstances, but apparently astrophotography isn't one of them.

>Previous objections to the technique were based on the difficulty in predicting results accurately. A little latensification can go a long way, and a lot can sometimes show little effect.

>Of course there's always room for improvements.

>Ergo, the demise of Chem-Tone.

>KinettaTone?

>Brian Heller
IA 600 DP


class="style9">>Which is why latensification is a much more useful technique -- it's >done right before processing, and only effects those grains that have .some exposure but not quite enough.

>In a previous post someone referred to latensification as being synonymous to flashing.

>Would someone please define?

>Argyris (encyclopaedic knowledge is always good) Theos
DoP
Athens Greece


class="style9">>In a previous post someone referred to latensification as being >synonymous to flashing.
>Would someone please define?

>Try the CML reference area . . .

>http://cinematography.net/edited-pages/LATENSIF.HTM

>BTW this also comes up on Google as the first hit for latensification.

>Dominic Case
Atlab Australia


>Brian Heller writes :

class="style10">>the heating of the film only drives out the water vapour, the nitrogen and >oxygen

>Is this ever done using a high vacuum rather than heat? Seems to me that once you've driven out the baddies you could somehow keep the film vacuum-packed and not have to futz with hydrogen.

>And you'd never have to worry about the potentially damaging effects of heat, either.

>Dan Drasin
Producer/DP
Marin County, CA


>Mr. Case's Link to a CML Article is very informative... for me it says there is nothing (little) gained in a 24 fps scenario...

>Isn't this what Mr. Boyle hoped for at the institution of this Site...???

>...Thank You Mr. Boyle for this great forum !

>David Rakoczy
Emerald Coast Filmworks Inc.
Florida/ USA


>Dan Drasin writes :

class="style10">> Is this ever done using a high vacuum rather than heat?

>Yes, the vacuum reduces the time required to remove the contaminants. The hydrogen is then introduced. The higher the vacuum, the more rapidly the process can proceed, but it still takes hours.

>I've since learned that it doesn't have to be pure hydrogen, but can be a mixture of hydrogen (10%) and nitrogen (90%), as long as it is perfectly dry. This takes the hydrogen concentration below the explosive threshold and would obviously make the process a lot safer for amateur astronomers.

>Heat is still required to remove the moisture trapped in the gelatin.

class="style10">>Seems to me that once you've driven out the baddies you could >somehow keep the film vacuum-packed and not have to futz with >hydrogen.

>However, as soon as the vacuum packed film is exposed to the atmosphere it will immediately start to re-absorb moisture, etc. Apparently the hydrogen prevents re-absorption for a period of time.

>I seem to remember that if it's kept sealed, it's good for a maximum of a couple of days.

>Most astrophotographers prefer to prepare their own film just prior to shooting to be sure of their results.

class="style10">>And you'd never have to worry about the potentially damaging effects of >heat, either.

>I'm not sure of the exact temperature, but it's not very high around 120 degrees F or 50 C.

>Also, FWIW in re these discussions, it was Kodak scientists who came up with the "hypering" technique.

>It's snowing here so the observatory is closed, otherwise there would probably be someone there who could give some further info.

>Brian Heller
IA 600 DP


class="style10">>Isn't this what Mr. Boyle hoped for at the institution of this Site...???

>The free flow and exchange of information relevant to professional DP's is exactly it.

>Oh and Mr Boyle was my father, I'm Geoff!

>Cheers

>Geoff Boyle FBKS
Director of Photography
EU Based
www.cinematography.net


>Oopps... you are right Dominic!

>I just didn't think before replying.

>Argyris Theos
DoP
Athens Greece


class="style10">>Anyone remember Bernie TVC and Chem-Tone?

>Wasn't Chem-Tone used to reduce the contrast on CRI processing?

Bruce Barham
Miami


>Bruce Barham wrote :

class="style10">>Anyone remember Bernie TVC and Chem-Tone?

>Wasn't Chem-Tone used to reduce the contrast on CRI processing?

>What was the actual Chem-Tone process? I remember in the late seventies, early eighties a lot of clients liked the lower contrast look for commercials because the telecine machines at the time tended to yield very contrasty images.

>Chemically, what was done to the negative to achieve this?

>Paul Varrieur S.O.C.


>Regarding Chem-Tone:

>Latensification (flashing) in the negative developer bath, my dear friend Dan Sanberg of TVC was the front man for the process, great marketing tool.

>The negative stock of the day was grainier and had increased contrast compared to today's negative stocks, helped with 16mm especially.

>Vinny Hogan
Cineworks-Miami


>Vinny Hogan writes :

class="style10">>Latensification (flashing) in the negative developer bath, my dear friend >Dan Sanberg of TVC was the front man for the process, great marketing >tool.

>If I remember correctly, TVC also touted that Chem-Tone also increased the film speed. I also seem to remember that in its earlier days it could be very inconsistent, with a couple of jobs victimized by the "Sabbatier effect".

>But my memory may be fogged as well.

>Brian Heller
IA 600 DP


class="style10">>If I remember correctly, TVC also touted that Chem-Tone also >increased the film speed.

>It opened up the shadows a bit. (See "Taxi Driver"). I would not say really a speed increase, but given the "fragility" of 5247/7247 negative insofar as underexposure was concerned, arguably helpful. You could say it gave an alternative to a push one which 47 didn't seem to handle too well for print.

>Last time I saw "Taxi Driver" on cable the transfer did not represent the - how to put it ? - desat/ "Agfa-ish" look of the release prints, but you _can_ see the effect of Chem-Tone in the night ext's especially.

>I think we're headed for CML-Nostalgia list, today Chem-Tone would likely be a solution in search of a problem !

Sam Wells