American cinematographer (Jan-Dec 1926)

July, 19'26 AMERICAN CINEM ATOGR APHER Twenty-three hundred 8" x 10" sheets of panchromatic film per gallon were developed (equivalent to 5000 feet of motion picture film). At the end of this time film desensitized in ths solutions showed an average differnce of three steps on the tablet exposures from the values obtained when fresh. This means that in these tests the effective concentration of the dye had decreased to less than one-half of its original value. In another set of similar solutions exhausted to half that extent no decrease in desensitizing action could be detected. It is probable that the margin of safety would be sufficient to cover any loss of desensitizing power during the life of the developer. In case of doubt more dye may be added occasionally. There was no indication in these or other tests that the dye affects the life of the developer. 6. Composition of the Developer. In this investigation only the elon-hydroquinone tank developer (AIQ-80 Tank) was used for exact studies of the behavior of the desensitizing dye in the developer. This is a typical dilute elon-hydroquinone developer and is widely used for motion picture work. On account of the large amount of labor involved the investigation was not extended to developers of different composition. Information in the literature, and our general experience do not indicate any marked difference in desensitizing with any developer in which the dye is sufficiently soluble to be used satisfactorily. However, desensitizers affect the speed of development to an extent which varies both with the developer and with the particular dye used. In practice, therefore, when using a desensitizer the proper development time must be found for each individual combination. Summary 1. A desensitizer is used primarily to secure greater visibility during development although it also prevents aerial oxidation fog. Greater visibility may also be obtained by so choosing a safelight that the visual intensity of the light which it transmits is a maximum and its photographic intensity in relation to the emulsion iispJ is a minimum. 2. A practical desensitizer in addition to having a satisfactory desensitizing action must not affect the latent image or the shape of the characteristic curve of the developed image. It must also not give fog or stain and should be soluble and stable in a developer. No desensitizer is known which is stable in a developer rich in hydroquinone. 3. The properties of the following commercial desensitizers have been studied in the light of the above requirements: phenosafranine, pinakryptol green, pinakryptol yellow, basic scarlet N, and aurantia. 4. The limits of safety in the use of pinakryptol green with motion picture negative and panchromatic emulsions have been determined. This desensitizer was chosen because it appeared to be the most satisfactory of the known desensitizers at the time of this investigation. 5. The comparative safety of untreated film and film desensitized for varying times with varying concentrations of pinakryptol green to different safelights has been studied. By bathing panchromatic film in a 1-10,000 solution of pinakryptol green, or after it has been in a developer containing 1-25,000 of this desensitizer for 2 or 3 minutes, inspection of the film may be conducted with safety with a Series 4 Wratten safelight containing a 25 W. bulb at a distance of 1 foot (30 cm.). Under the same conditions motion picture negative film may be safely examined with a Series O safelight. 7. A latent image on a desensitized emulsion tends to bleach out when exposed to red light. This bleaching action is greatest with non-color sensitive emulsions. With panchromatic emulsions the effect is not serious and after development has commenced no appreciable bleaching occurs. With desensitized non-color sensitive emulsions the same time of exposure to a red safelight is determined by the time required to destroy the latent image and not by the time required to produce fog. 8. Data have been obtained on the fogging action of various desensitizers with developers. 9. An exhaustive study has also been made on the effect of pinakryptol green when used in the developer instead of as a preliminary bath. Practical Importance of Desensitizers With superspeed motion picture negative film it is possible to satisfactorily inspect the image with safety during development without the use of a desensitizer. With an 8 x 10 Wratten Series 2 safelight containing a 25 W. bulb, the emulsion can be given an exposure of 2 minutes at a distance of 1 ft. (30 cm.) before a visible fog is produced, which time is far in excess of the time necessary for satisfactory inspection of the film. With this film therefore the use of desensitizers is unnecessary. With panchromatic motion picture negative film, under the above conditions an objectionable fog is produced in 10 seconds. Inspection of this film during development is therefore dangerous and unless a desensitizer is used development should always be carried out in the dark for a predetermined time at a given temperature as determined by the preliminary development of test strips. The use of pinakryptol green either as a preliminary bath or when added to the developer will permit of the safe inspection of panchormatic film with a Wratten Series 4 safelight containing a 25 W. bulb at a distance of 1 foot (30 cm.). The film should not be exposed to this light until the film has been immersed in the desensitizing solution for at least 3 minutes. For use dissolve 2-3 ounce of pinakryptol green in 50 gallons of water (20 grams per 200 liters) as a preliminary bath. When used in the developer dissolve 120 grains per 50 gallons (8 grams per 200 liters). It is usually impossible to add the desensitizer to a developer rich in hydroquinone because the desensitizer is precipitated. The dye should first be dissolved in as small a quantity of hot water as possible and then diluted with cold water or added to the developer. Desensitizers are valuable insofar as they permit of greater visibility during development and prevent aerial oxidation fog. They are not indispensable, however, and there is always a danger of accidentally fogging an emulsion in the bright light before the desensitizing solution has had sufficient time to act. With panchromatic emulsions their use permits of inspection of the image during development which is otherwise not possible.