US3570383A

US3570383A - Apparatus for developing and fixing a thermodevelopable photographic medium

Info

Publication number: US3570383A
Application number: US680873A
Authority: US
Inventors: Eugene Berg
Original assignee: Scott Paper Co
Current assignee: Warren SD Co
Priority date: 1967-11-06
Filing date: 1967-11-06
Publication date: 1971-03-16
Anticipated expiration: 1988-03-16
Also published as: GB1241421A; NL6815136A; CH490697A; DE1805797A1; BE723383A; FR1590486A

Abstract

An apparatus for developing a photographic medium by heating the medium to volatilize the chemical components on the surface of the medium and remove the volatilized components from the surface. The developing apparatus comprises a developing chamber, including appropriate heating elements, a gas-dispersing system to supply entraining gases to the surface of the photographic medium and an exhaust system to remove the volatilized components and transporting means for conveying the photographic medium through and out of the developing chamber.

Description

United States Patent [72] Inventor Eugene Berg 2,058,983 10/1936 Horn 95/94gas Westbrook, Maine 2,811,909 11/1957 Bungay 95/89gas [21] App1.No. 680,873 3,233,338 2/1966 S1ipson... 34/155(X) [22] Filed Nov. 6, 1967 3,280,717 10/1966 Bungay 95/89 [45] Patented Mar. 16,1971 3,435,751 4/1969 Goodman et a1. 95/94 [73] Assignee Scott Paper Co. 3,440,944 4/1969 Endermann et a1 95/89 Delaware County Primary Examiner-Samuel S. Matthews Assistant Examiner- Robert P. Greiner Attrneys-Stanton T. Hadley, John A. Weygandt, William J. [54] APPARATUS FOR DEVELOPING AND FIXING A y h W- Kane, J Anthony J McNulty and THERMODEVELOPABLE PHOTOGRAPHIC M311"! Falgus MEDIUM 4C1 ,6D F

alms rawmg ABSTRACT: An apparatus for developing a photographic [52] U.S.Cl 95/89, medium by heating the medium to volatilize the chemical 34/155 components on the surface of the medium and remove the volatllized components from the urface The developing ap Fleld ofSearch .1 93, aratus comprises a developing hamber including ap- 34/155, 160, 41 propriate heating elements, a gas-dispersing system to supply entraining gases to the surface of the photographic medium [56] References cued and an exhaust system to remove the volatilized components UNlTED STATES PATENTS and transporting means for conveying the photographic medi- 3,403,456 /1968 Smith, Jr. 34/160 um through and out ofthe developing chamber.

/2 94 I mz 94 22\ 26 72 Eli/i "5 Z\ 2 o o o o ,7 [/4 l 4o Ill z 24 m4 l r 1 52:21" I 66 i 4 m 63 l Patented March 16, 1971 4 Sheets-Sheet 2 Patented March 16, 1971 3,570,383

4 Sheets-Sheet 5 Patented March 16, 1971 3,570,383

4 Sheets-Sheet 4 AP?ARATUS Milt DEVELOPTNG AND FTXING A THERMUEEVELQPABLE PHQTQGRAPHHC MEDIUM BACKGROUND OF THE lNVENTlON The present invention relates to photographic equipment in general, and more particularly, to an apparatus for developing and stabilizing a thermodevelopable photographic medium.

The apparatus of the present invention provides a means for developing an exposed photographic medium of film by uniformly heating the film to drive off volatile chemical components of the film. in many instances, the films volatilized chemical components can cause fogging or other damage to the developing or developed film surface if allowed to remain in contact with the surface for any appreciable length of time. in heating a photographic medium or film to develop and fix the image, for example as described in copending patent application Ser. No. 641,720, filed Apr. 21, 1967, some of the chemical components of the imaged surface are volatilized. If this volatilized chemicals or the particulate matter which sometimes forms during development are allowed to remain in contact with the imaged surface of the film, the surface can become fogged and soiled thereby impairing the image quality, contrast and resolution. Therefore, it is desirable that the volatilized chemical components be removed from the vicinity of the film surface before they have an opportunity to contaminate the surface.

In addition to the problem of film surface contamination, thermodevelopable photographic films are prone to uneven thermal development. The photographic medium should be uniformly heated at a given temperature for a predetermined length of time in order to obtain even development of the exposed image. Uneven development can also becaused by a concentrated stream of cold entraining gases being brought in contact with the surface of the developing film.

it is accordingly a general object of the present invention to provide an apparatus for developing and fixing a thermodevelopable photographic medium.

it is a specific object of the present invention to provide a developing apparatus which uniformly heats an imaged film to a selected temperature within a predetermined developing temperature range.

it is another object of the present invention to provide a developing apparatus wherein a uniformly dispersed flow of gases sweep the surface of the developing film in order to immediately entrain the material driven off by the heating step and remove such material from the vicinity of the film surface.

it is still another object of the present invention to provide a uniform well-dispersed gas flow across the surface of the developing film which will remove the chemical components driven from the surface of the film without causing cold spots on the developing film.

These and other objects and features of the present invention will become apparent from the following description of a preferred embodiment selected for purposes of illustration and shown in the accompanying drawing in which:

FlG. l is an isometric view of the developing apparatus of the present invention;

FlG. 2 is a plan view of the present invention with the top cover removed showing the developing chamber, the film transport means and the gas-dispersing and exhaust systems;

, H6. 3 is a view in cross section taken along line 3-3 of FIG.

FlG. 4 is an isometric cross-sectional view of the developing chamber showing the path of the entraining gases as they pass through the dispersing means, across the film surfaces and through the exhaust ducts;

EKG. is an isometric view of the film support means; and

H6. (6 is an isometric view of the developing chamber, exhaust conduits and exhaust chamber.

The present invention is housed in a chassis 1 shown in FlGS. l, 2 and 3 having a front member 2 containing an upper horizontal film intake slot 3 and a lower horizontal film return slot 45, a bottom member 6, first and second side members 8 and 9 with the second side member9 having a vent aperture 10, a back member ill and a top member 12 hingably mounted on to the back member. A film feed tray M is mounted on the front member to guide film into the film intake slot.

The rectangular-shaped developing chamber 20 in which the exposed photographic medium 21 is developed has a front wall 22 having a horizontal film intake slot 23, a bottom 24, a backwall 26 containing a horizontal film return slot 27 positioned opposite the film intake slot 23, a top 28, a first sidewall 30 and a second sidewall 32. As is shown in FIG. 6, the second sidewall 32 contains a lower exhaust port 3 and an upper exhaust port 36. The developing chamber 20 is divided into an upper unit 2% and a lower unit 20b as illustrated by FIG. 6 by dividing the walls in half. The upper unit 20a is removably mounted on the lower unit 20b to'allow the upper unit to be easily removed for cleaning or repairing purposes.

The film transport system conveys the photographic medium through the developing chamber. in particular, the exposed photographic medium 21 is engaged by a first pair of horizontal nip rolls comprised of a lower driving roll 40 and an upper driven roll 42 and convexed into the developing chamber 20 through the film intake slot 23. Rolls 40 and 42 are positioned in front of the film intake slot 23 and adjacent thereto. The lower driving roll 40 is journaled between

vertical support members

43 and 44 by means of frictionless bearings and is operably connected to a drive means 45. The upper driven roll 42 is journaled between

vertical supports

43 and 44 by means of bearing blocks 47 which are slidably mounted in vertical slots cut in the support members. The lower roll 40 is driven towards the intake slot 23 so as to convey the photographic medium 21 into the developing chamber. The upper roll 42 is biased against roll 40 or the film 21 by its own weight or other known biasing means. The upper roll 62 coacts with the lower roll it) in transporting the film into and through the developing chamber. The slidably mounted bearing blocks allow the upper roll 42 to be vertically displaced to accommodate films of various thicknesses.

As the photographic medium or film is being conveyed through the developing chamber 20, it is guided by a lower film support grid 50 and an upper film-retaining grid 52. The lower film support grid extends from the lower edge of the film intake slot 23 to the lower edge of the film exit slot 27. The film-retaining grid 52 positioned above and approximately parallel to the lower film support grid 50 extends from the upper edge of the film intake slot 23 to the upper edge of the film return slot 27. The upper and lower grids 50 and 52 are made up of a series of filaments 53 stretched between the film intake and film exit slots. The filaments in the grids 50 and 52 on each side of the center of the film slots are substantially parallel to each other and diverge away from the centerline of the film feed path when viewed from a downstream direction as shown in FIG. 5. The filaments'slanted in this manner prevent film from jamming in the developing chamber. The filaments can be made of glass fibers, polymer coated fibers, wire and the like.

Before the film is released by the first set of nip rolls, it is engaged by a second set of nip rolls having a second lower driving roll 60 and a second driven roll 62. Rolls 60 and 62 are journaled between the vertical supports 63 and 64 behind the film exit slot in a manner similar to that described for the first set of nip rolls. The second driving roll 60 is operably connected to the driving means 45 and driven in the same direction as the first driving roll 40. In view of the fact that the second set of nip rolls engages the sheet of photographic medium while it is still being acted upon by the first set of nip rolls the surface speed of the second driving roll must be the same as the surface speed of the first driving roll as.

The second set of nip rolls propel the photographic medium down a film return chute. The film return chute as has a leading edge 67 as shown in H6. 3 which is positioned behind slightly above the nip created by the rolls 60 and 62. The film chute depends rearwardly and downwardly in a smooth continuous arc until is has reversed direction whereupon it, the chute, depends downwardly and forwardly through the film return slot to terminate with stop member 64 as illustrated by W6. 3.

in many cases, the photographic medium picks up an electrical charge while. it is passing through the developing chamber. This electrical charge often causes the developed photographic medium to stick to the film return chute. Therefore, in a most preferred embodiment, the film is propelled down the film return chute by a third set of nip rolls. The third set of nip rolls is made up of a third lower driving roll 80 and a third driven upper roll having a shaft 31 and rollers 82 and is positioned behind and below the level of the second set of nip rolis. The third set of nip rolls is journaled by means of frictionless bearings between

horizontal support members

84 and 85 with the third lower driving roll 8t operably connected to the drive means and driven in the same direction and at the same surface speed as the first and second driving rolls. The upper shaft 81 is journaled between floating bearing blocks. The film return chute 66 contains slots 86 through which the rollers 82 coact with the surface of the lower roll 80 or the film to propel the film down the return chute and prevent it from sticking and blocking the chute. The floating bearing blocks allow the size of the nip between the rollers 82 and the driving roll lit) to vary in order to accommodate films of various thicknesses.

As the exposed photographic medium is conveyed through the developing chamber 20, it is uniformly heated to'drive off volatile chemical components and thereby develop and fix the image. The uniform heat required to fully develop the photographic medium is provided by a plurality of electric heaters 88. The heaters run the length of the developing chamber and are positioned below the level of the film support grid In order to obtain uniform heating of the developing chamber and the film to be developed, it has been found necessary that the individual heaters exhibit nearly uniform heat gradient characteristics along the entire length of the heating section of the heater element. I-leaters exhibiting hot spots or showing heat taper off near the ends or center of the heater element do not uniformly heat and develop the film. If portions of the film are not heated to within the developing temperature range, uneven development results. Furthermore, in order to provide uniform development along the edges of the film, the heater elements should extend beyond each edge of the film. An extension of one inch has been found to be sufficient. In cases where the heating element has not been extended beyond the edge of the film, underdeveloped areas have been found along the edges of the film. In a 22-inch developing chamber designed to develop 18- inch wide film, heaters having a 24-inch stainless steel sheath and a 22-inch black oxide heating surface have given good results. These heaters were obtained from Vulcan Electric, Danvers, Mass. As there are many types of heaters on the market which have uniform heat gradients along their length and as these various heaters are well known to those skilled in the art, we do not intend to be limited by the recitation of the above heaters.

The entraining gases, to be effective as previously explained, must be well dispersed to provide a uniform curtain of flowing gases which sweep the entire surface of the developing photographic medium or film. Entraining gases which are not well dispersed may strike the film surface as a concentrated stream of gas and cause a cold spot or underdeveloped area on the surface of the developing film. Uniform dispersion is particularly important when the entraining gases such as air are brought into the developing chamber from the surrounding environment. The uniform or even dispersion of the entraining gases is provided by the gas dispersing means which is comprised of a plurality of gas dispersing ducts Ml, 91, 92 and 93. These gas-dispersing ducts are located within the developing chamber and run the length thereof. The entraining gases enter the dispersing ducts through the gas intake apertures 94. in a convenient arrangement, the gas intake apertures 94 for each duct comprise a row of evenly spaced apertures. The gas dispersing ducts are in gaseous communication with the interior of the developing chamber by means of a plurality of dispersing apertures 95. It has been found that the most uniform and effective dispersion of the entraining gases in order to accomplish the objectives of this invention is obtained when the dispersing apertures in each dispersing duct are arranged in two or more parallel rows 97 and 9b as illustrated in FlG. 2. In such an arrangement, the area of the apertures in the row nearest the outside wall of the developing chamber, should be greater than the area of the interior rows for reasons which will be explained more fully in the discussion of the flow pattern of the entraining gases.

The exhaust system of the present invention is extremely important to the effectiveness of the present invention. Films having good image quality, contrast and resolution are not obtained when the chemicals driven from the film surface during the heating procedure are allowed to remain in contact with the film surface for any appreciable length of time. The removal of these volatilized chemical components is the function of the exhaust system.

The exhaust system is comprised of an exhaust fan res, upper and

lower exhaust ducts

102 and 104, respectively, means for operably connecting the exhaust ducts to the exhaust fan and exhaust vent means 106.

The exhaust fan we is mounted on an exhaust chamber 108 and in operable communication therewith by means of exhaust port 109; The centrally positioned upper and

lower exhaust ducts

102 and 104 are located in the upper and lower units of the developing chamber between the gas dispersing ducts. The upper and lower exhaust ducts which run the length of the developing chamber and terminate in the

exhaust ports

34 and 36, respectively, are in gaseous communication with the interior of the developing chamber exclusively by means of the upper and lower exhaust

duct intake slots

112 and 114 as shown in FIG. 3. The width of the

intake slots

112 and 114 is adjusted by means of

closure plates

116 and 118 slidably mounted to the exhaust ducts my means of elongatedfastening slots 115 as shown in FIG. 4. The

exhaust ducts

102 and 104 are centrally positioned within the developing chamber and conveniently located between the dispersing ducts in order to allow the radiant heat from the heaters 33 to heat the walls of the exhaust ducts. If the walls are not heated, the volatilized, chemical components swept from the film surface by the entraining gases tend to precipitate or plate out on the walls of the duct, obstructing the flow of exhaust gases. In

, a most preferred embodiment, the upper and lower exhaust ducts are provided with

removable plates

120 and 122,

respectively. These plates can be easily removed to facilitate cleaning of the exhaust ducts.

Upper and

lower exhaust conduits

124 and 126, respectively, shown in FIG. 6, bring the upper and lower exhaust ducts in operable gaseous communication with the exhaust chamber 108. One end of the lower exhaust conduit 126 is mounted to the second sidewall 32 of the developing chamber 24) enclosing the lower exhaust port 34. The other end is attached to the exhaust chamber 108 around exhaust chamber intake port 128. The upper exhaust conduit 124 is also mounted to the second side wall 32 enclosing the upper exhaust port 36 and is removably mounted on the exhaust chamber ltlii around exhaust chamber intake port 129 thereby allowing the upper unit of the developing chamber 20a to be removed for cleaning.

FIG. 4 illustrates the path of entraining gases through the developing chamber. The entraining gases, usually air, although any gas which would not damage the film can be employed, are brought into the developing chamber through the intake apertures @4. The entraining gases are then drawn out the dispersing ducts through the apertures 95. In order to compensate for the greater distance from the exhaust ducts and the attendant reduction of pressure acting on the apertures in rows t at near the outer walls of the developing chamber, the area of these apertures is greater than the area of the interior dispersing apertures in rows 97. This insures a uniform flow of entraining gases through the outer apertures thereby providing a flow of entraining gases along the entire film surface and throughout the entire developing chamber.

The entraining gases drawn through lower dispersing ducts 9 2 and @3 are heated as they flow past heaters 8h. The entraingases pulled through the upper dispersing ducts 90 and 9t. are pulled down along the surface of the developed or developing film. it is essential to the obtainment of the objectives of the present invention that the entraining gases issuing from the upper dispersing ducts hit and 91 be well dispersed so that they can be heated by radiant heat from the heaters 88. During their sweep across the surface of the photographic medium, these gases entrain the volatilized and particulate chemicals driven off the film surface. The gases flowing along the lower surface of the photographic medium pick up any development by products which have flowed over the edges of the photographic medium. it has been found that a ratio between the area of the exhaust slot and the cross-sectional area of the

respective exhaust port

34 or 36 of from about 1:3 to 1:10 provides sufficient airflow to entrain the chemical components and remove them from the developing chamber thereby preventing damage to the film surface. In a preferred embodiment, the upper intake slot 112 is slightly wider than the lower intake slot 114. The exhaust gases are then pulled through the exhaust ducts, through the

exhaust ports

34 and 36 into the

exhaust conduits

124 and 126 and into the exhaust chamber. From the exhaust chamber the exhaust gases are pulled through the exhaust fan 100 and pushed into and through the exhaust vent MP6. The exhaust gases are either vented to the outside or to a filter which will absorb the entrained chemicals.

The temperature of the developing chamber, i.e., the developing temperature, can be controlled manually or automatically. The manual control system consists of a rheostat which is connected to an electricity source and to the heaters. The rheostat is used to control the current going to the heaters and thereby control the amount of heat given off by the heaters. The temperature of the developing chamber is detected by means of a thermocouple placed within the developing chamber or exhaust system and read from a temperature gauge P.5d attached to the housing.

in the automatic system a thermistor or temperature sensing means can be positioned in the lower exhaust conduit, in the exhaust chamber or in the exhaust fan. Sensitive and accurate control of the developing chamber temperature is obtained by placing the thermistor 15.55 in the sidewall of the developing chamber as indicated in FIG. 6. When so positioning the thermistor, care must be taken not to block the passage of photographic medium through the developing chamber. in addition, the thermistor should not be placed so near the heaters that it responds directly to the surface temperature of the heaters rather than to the temperature of the interior of the developing chamber. The thermistor is operably connected to a temperature control means 156. The desired developing temperature is selected and the temperature control means 156 set at that temperature. The temperature control means is also operably connected to the heater elements 88 and to an on-off switch 115% which is connected to the electric current supply source. When the temperature of the exhaust air flowing through the lower exhaust channel goes above or below the selected temperature, the current passing to the heaters M is varied, thereby increasing or decreasing the amount of heat given off by the heaters. if desired, a temperature gauge connected to a thermocouple located within the developing chamber or exhaust system can be employed to check the temperature of the developing chamber.

The operating temperature employed will depend upon the type of film being developed and the length of the time than the film is subjected to the heat of the developing chamber. W hen developing the film material described in copending application Ser. No. 641,720, a temperature of from 300 to 400 F. can be employed. The residence time of the photographic medium within the developing chamber will depend upon the particular photographic medium being developed and the degree of development desired.

The operable connections between the lower driving rolls 40, till and hill and the drive means 45 as illustrated as in H6. 2 are endless drive chains res and 170. However, as there are many different types of drive systems known to skilled in the art such as belts, gear boxes and the like is not intended that the present invention should be limited to any particular drive system.

The terms photographic medium or film used herein interchangeably designate any film, paper or the like, containing photoresponsive components which is developed or fixed by heat.

it is intended to cover all changes and modifications of the examples of the invention herein chosen for purposes of the disclosure which do not constitute departures from the spirit and scope of the invention.

lclaim:

1. An apparatus for developing photographic mediums comprised of:

a developing chamber having:

a front horizontal film intake slot;

an oppositely positioned back film exit slot;

upper and lower exhaust ports; and

an upper and a lower closure plate slidably mounted on the upper and lower exhaust ducts over the exhaust duct intake slots to allow the width of the exhaust duct intake slots'to be adjusted;

a film transport system comprised of:

a first pair of nip rolls operably connected to a drive means and positioned in front of and adjacent to the film intake slot to convey the photographic medium into and through the developing chamber;

a film support means and a film-retaining means extending between the film intake slot and film exit slot to guide the photographic medium through the developing chamber;

a second set of nip rolls operably connected to the drive means and positioned behind the film exit slot and adjacent thereto so as to engage the photographic medium before it is released by the first set of nip rolls;

a film return chute; and

a third set of nip rolls positioned behind the second set of nip rolls with the third set of nip rolls being comprised of a lower driving roll and an upper shaft bearing a plurality of rollers spaced along the shaft, said film return chute having a plurality of slots through which the rollers contact the lower roll;

a heating means comprised of a plurality of electric heaters mounted in the developing chamber below the film support means;

a plurality of gas-dispersing ducts located within the developing chamber, said ducts having a plurality of gas intake apertures by means of which the entraining gases are brought into the dispensing ducts; and

a plurality of gas-dispersing apertures which being the interiors of the ducts in operable gaseous communication with the interior of the developing chamber; and

an exhaust system comprised of:

upper and lower exhaust ducts positioned centrally within the developing chamber and in gaseous communication therewith exclusively by means of exhaust duct intake slots, said exhaust ducts being positioned therein so as to be heated by said heating means to prevent precipitation of the removed chemical components;

an exhaust fan; and

an exhaust transport means connecting the exhaust ducts and the exhaust fan and bringing them in operable gaseous communication.

2. The apparatus claimed in claim ll wherein the gas intake apertures of each dispersing duct are disposed in an evenly spaced row and the gas-dispersing apertures are disposed at regular intervals in at least two parallel rows.

3. An apparatus for developing photographic medium comprised of a developing chamber having a film intake slot and a film exit slot, a film transport means for conveying the photographic medium through the developing chamber, a heating means positioned within the developing chamber for uniformly heating and developing the photographic medium, a gas-dispersing means for supplying entraining gas flow over the surface of said medium as it is conveyed through said chamber and an exhaust means operatively coupled to said gas dispersing means for maintaining said gas flow and removing the chemical components driven from the photographic medium during the heating thereof, said exhaust means including exhaust ducts positioned centrally within the developing chamber on opposite sides of the path of said photographic medium therethrough and positioned therein so as to be heated by said heating means to prevent precipitation of the removed chemical components, said exhaust ducts comprising means defining exhaust duct intake slots for exclusive communication with the developing chamber, and including means for varying the width of said exhaust duct inlet slots.

4. An apparatus for developing photographic medium comprised of a developing chamber having a film intake slot and a film exit slot, a film transport means for conveying the photographic medium through the developing chamber, a heating means positioned within the developing chamber for uniformly heating and developing the photographic medium, a gas-dispersing means for supplying entraining gas flow over the surface of said medium as it is conveyed through said chamber and an exhaust means operatively coupled to said gas-dispersing means for maintaining said gas flow and removing the chemical components driven from the photographic medium during the heating thereof, said exhaust means including exhaust ducts positioned centrally within the developing chamber on opposite sides of the path of said photographic medium therethrough and positioned therein so as to be heated by said heating means to prevent precipitation of the removed chemical components, said exhaust ducts comprising means defining exhaust duct intake slots for exclusive communication with the developing chamber, the ration between the cross-sectional area of each said exhaust duct inlet slot and its corresponding exhaust outlet port falls in the range from about 1:3 to about 1:10.

Claims

2. The apparatus claimed in claim 1 wherein the gas intake apertures of each dispersing duct are disposed in an evenly spaced row and the gas-dispersing apertures are disposed at regular intervals in at least two parallel rows.
3. An apparatus for developing photographic medium comprised of a developing chamber having a film intake slot and a film exit slot, a film transport means for conveying the photographic medium through the developing chamber, a heating means positioned within the developing chamber for uniformly heating and developing the photographic medium, a gas-dispersing means for supplying entraining gas flow over the surface of said medium as it is conveyed through said chamber and an exhaust means operatively coupled to said gas dispersing means for maintaining said gas flow and removing the chemical components driven from the photographic medium during the heating thereof, said exhaust means including exhaust ducts positioned centrally within the developing chamber on opposite sides of the path of said photographic medium therethrough and positioned therein so as to be heated by said heating means to prevent precipitation of the removed chemical components, said exhaust ducts comprising means defining exhaust duct intake slots for exclusive communication with the developing chamber, and including means for varying the width of said exhaust duct inlet slots.
4. An apparatus for developing photographic medium comprised of a developing chamber having a film intake slot and a film exit slot, a film transport means for conveying the photographic medium through the developing chamber, a heating means positioned within the developing chamber for uniformly heating and developing the photographic medium, a gas-dispersing means for supplying entraining gas flow over the surface of said medium as it is conveyed through said chamber and an exhaust means operatively coupled to said gas-dispersing means for maintaining said gas flow and removing the chemical components driven from the photographic medium during the heating thereof, said exhaust means including exhaust ducts positioned centrally within the Developing chamber on opposite sides of the path of said photographic medium therethrough and positioned therein so as to be heated by said heating means to prevent precipitation of the removed chemical components, said exhaust ducts comprising means defining exhaust duct intake slots for exclusive communication with the developing chamber, the ration between the cross-sectional area of each said exhaust duct inlet slot and its corresponding exhaust outlet port falls in the range from about 1:3 to about 1:10.