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Radiology Essentials 102 : Dark room Construction


By Dr Khanak Nandoliya

We are back with a new topic discussion for Radiology Residents with Dr Khanak Nandoliya. Dr Khank is a second year resident in DNB Radiology at Dr Rajendra Prasad Government Medical College , Kangra, Himachal Pradesh. He will crisply describe about Dark room construction which is a very important but boring topic in radiology. You can discuss the topic in the comment section below this blog and also at Forums Here 


  • X-ray films require handling and storage at a place devoid of white light to avoid accidental exposures.
  • Darkroom serves as film storage and processing station.
  • However, daylight film processing systems and LASER imagers are gradually replacing darkrooms.
  • Objectives of a darkroom:
    • Good film handling and storage conditions
    • Satisfactory working conditions for the staff
    • Efficient performance of equipment 

Learning Points

  • Design of a darkroom
  • Darkroom illumination
  • Various darkroom equipment
  • Health and safety issues 

Location of a darkroom

  • Centrally sited in the radiology department
  • Connected with adjacent imaging rooms by means of cassette hatches
  • Accessible in terms of electricity and water supply
  • Close to viewing and sorting area for quality check operations

Dark room design

  • A well planned darkroom has
    • Effective separation of wet and dry working areas.
    • Orderly sequence of successive stages of work.
  • A darkroom can be divided into 4 broad areas
    • Area to load and unload cassettes.
    • Film storage.
    • Processing apparatus.
    • Other essential accessories.

Operating machineries are to be kept at a distance from walls to maintain air circulation and prevent overheating.

It also protects walls from chemical splashes and spillages from the processing units.

Radiation protection by walls

  • Darkroom contains x-ray sensitive materials.
  • As darkroom is closely situated by x-ray rooms, personnel and sensitive materials inside require protection from ionizing radiation.
  • Walls are mainly concerned. But if x-ray rooms or other facilities utilizing ionizing radiation are situated at other storeys, floors and ceilings also require proper attention.
  • Same precautions are to be applied to film storage areas if they are situated outside the darkroom.

Lead equivalency of 2 mm is required

  • Structure of a room where films are stored or handled should have a lead equivalency of 2 mm. following appropriate materials can be used:
  • 25 mm or 1 inch layer of high quality barium plaster containing equal mixture of fine and coarse barium sulphate.
  • Single brick wall, that is of 225 mm of solid brick.
  • Half a brick thick wall with half inch layer of high quality barium plaster.
  • 150 mm thickness of concrete.
  • Lead boards of 2 mm thick lead. 

       Floor properties

  • Durable
  • Light coloured
  • Stain resistant
  • Easy to wipe and clean
  • Corrosion resistant
  • Non slippery
  • Non porous
  • Non absorbent
  • Ideal candidates – asphalt, ceramic tiles, porcelain tiles.


  • Why non-absorbent and non- porous material?
  • Porous and absorbent materials like cement and bricks absorb spilled chemicals and then gradually release fumes upon drying. Absorbed chemicals also make them prone to corrosive destruction.

Entrance designs

  • Darkroom entrances need special designs to allow entry and exit without admitting white light.
  • Various designs –
    • Single door
    • Double door
    • Labyrinth
    • Rotatory door
  • Besides main entrance, an alternative emergency exit must be provided.

Single door system

  • Inexpensive and least space occupying.
  • Door must be totally capable of excluding light once bolted.
  • To avoid accidental exposure of films, door locking mechanism is linked with safe lighting circuits. door is locked from inside when safe lights are on. This prevents accidental opening of door when films are being handled.
  • Disadvantages –
  • Safety concern in events of accidents.
  • Failure of door locking mechanism can lead to lock down of staff inside the dark room.


HVAC solutions

  • HVAC component comprises of co-ordinated heating, ventilation and air conditioning of the facility.
  • Long working hours in enclosed environment necessitate adequate HVAC provisions.
  • HVAC is important to provide :
    • Satisfactory working conditions for the staff.
    • Good film handling and storage conditions.
    • Efficient automatic processor performance.
  • Several recommendation stated provision of operable windows in darkroom walls which can be left open time to time to allow air cycling.
  • But associated light leakage poses great risk to x-ray films inside the dark room.
  • Air cycling – the air in a darkroom must be completely changed at least 10 times an hour.
  • Two means of air circulation – 1) exhaust – inlet fan system. 2) central air conditioning system.
  • Machineries running inside darkroom radiate heat and chemical fumes both. So air cycling is necessary.

Humidity and temperature control

  • Ideal humidity level – 40 to 60% relative humidity.
  • Temperature – 18 to 20° C.
  • Failure of temperature and humidity regulation may cause rise in processor temperature. This leads to increased density and fogging of processed films.

Water supply to dark room

  • Hot and cold water supply, both are needed for film processing.
  • Water can be heated using radiant heaters.

Darkroom illumination

  • The most important component of a darkroom system.
  • It will be discussed under two headings:
    • Ordinary white lighting.
    • Safe lighting.
  • Electric switches and wirings must be kept away from wet area of the dark room to prevent electrocution and damage to electric work by chemicals.
  • Cord pull switches are recommended.
  • All lightings must be connected to earthing system.

Ordinary white lighting

  • Ordinary lighting is necessary for :
    • Inspection and maintenance of cassettes and screens.
    • Cleaning of work surfaces and floors
    • Equipment servicing
  • White lighting should be:
    • Sited close to the ceiling to avoid casting of strong shadows
    • Moderate in intensity. High intensity lighting makes it difficult to accommodate under low safe lighting.
  • Centrally placed.

Safe lighting

  • Exposure to white light would cause instant fogging of films.
  • However, no lighting is absolutely safe.
  • We use illumination which is relatively safe for x-ray films.
  • So safe lighting is necessary to provide sufficient illumination to handle, manipulate and process films without causing significant fogging.

What is safeÍ› lighting?

  • Ordinary white light is composed of separate colours.
  • The response of the photographic film to the colours of the visible spectrum is quite different.
  • Maximum sensitivity of photographic emulsion is in the blue part of the spectrum.
  • The sensitivity of photographic film to blue light and their lack of sensitivity to the other colours mean that they will not record any image when exposed to the other colour lights.
  • Films used in radiography are blue-sensitive or non-colour sensitive films.
  • Special dyes can be used to impart sensitivity to other colors.

Effect of safelight on exposed films

  • Unexposed films are relatively insensitive to safe light exposure. Safe light exposure is insufficient to reach the threshold level required by the Ag-halide crystals to trigger any response.
  • In exposed films, Ag-halide crystals have already reached or exceeded required threshold level to produce any response. All exposures are cumulative.
  • So any small additional exposure from even safe lighting may result in noticeable overall increase in density. That leads to visible fogging.

Dark room equipment

  • Loading bench:
    • Large enough to accommodate 3-4 largest sized cassettes.
    • Made of non-static material such as wood and stone. Electrostatic charge can damage films and cassettes. Plastic, formica are to be avoided.
  • Film storage:
    • Unexposed films for immediate use are stored in hopper. Films are arranged according to sizes.
    • Films for long term storage are stored in lightproof cupboards.

Storage of unexposed radiographs

  • Storage conditions: 16-26 C with 30-50% relative humidity.
  • Issues:
    • Too high temperature and humidity – increased fog levels
    • Too low temperature and humidity – increased static discharge

Film processing

  • Film processing renders invisible latent image on exposed film into visible manifest image.
  • Film processing is done either manually or by automated processors.
  • Exposed film undergo processing cycle in both the methods.
    • Development
    • Fixing
    • Washing
    • Drying
  • Time taken by automatic processor at 33°C is 90 seconds & 3.5 minutes at 27 °C.


  • Primary function – conversion of latent image into a manifest image.
  • Film density is produced by converting silver ions into metallic silver, which causes each processed grain to become black.
  • The developer solution supplies electrons that migrate into the sensitized grains and convert the other silver ions into black metallic silver. This causes the grains to become visible black specks in the emulsion.
  • Thus developer solution can amplify amount of silver atoms by donating electrons.

Starter solution:

  • When developer tank has been refilled after emptying for cleaning purposes, the developer replenisher will be overactive resulting in chemical fogging in first few films.
  • A starter solution is added to fresh replenisher in developing tank to prevent this.
  • It acts as an acid restrainer.

Factors affecting development process

  • Constitution of developer
  1. Concentration of developing agents:
    • Higher concentration results in greater activity but eventually lead to chemical fog. Therefore dilution is needed.
  2. pH of developer solution:
    • When pH decreases development gets inhibited. So alkalinizer is needed.
  3. Concentration of restrainer & anti-foggant
  • Temperature of developer: developer activity increases with rise in temperature. Increase in temperature increases fogging, image contrast and image density. Reduction in temperature reduces them.
  • 20° C is optimum temperature.
  • Development time depends upon the quality of developer solution, type of emulsion and agitation of the solution.


  • Removes unexposed Ag-halide from the film to make the remaining image permanent.
  • Secondary functions – to stop the development process and to harden the emulsion.
  • Removal of unexposed Ag-halide clears the image and renders unexposed part transparent. Removal is achieved by dissolving Ag- halide into fixing solution.

•Removes fixing solution from the surface.
•Poor washing results in persistent thiosulfate on the emulsion, causing brown stains on the film upon storage.
•Optimal removal of fixing agent improves archival quality of the radiograph.
•Washing is based on the principle of diffusion.
•Water in washing tank has lower concentration of thiosulfate. So thiosulfate in the emulsion diffuses out into washer solution. So washer solution needs more frequent change than the previous two solutions.
•Continuous flow and increased temperature of water improves washing efficacy.
•Drying temperature is maintained by thermostats.
•Drying removes 85-90% moisture from emulsion while retaining 10-15% moisture.
•Over-drying renders emulsion too brittle and prone to cracking on long term storage.
•While retention of more moisture makes it delicate and prone to physical damage.
Replenishment systems
•Developer and fixer solutions weaken with time and use.
•Developer is exhausted by both use and exposure to air.
•Fixer solution is weakened by accumulation of Ag-halides and developer solution removed from film.
•Per film, more fixer solution replenishment is required than developer solution.
•Two auto-replenishment systems are used: microswitch system and flooding system.
  • Micro-switches incorporated in the input tray of a processor detect film sizes and replenish the solutions according to size and number of films.
  • Flood replenishment system replaces a particular pre-set volume at pre-determined time period, independent of size or number of films processed.

Recirculation system: mixes used solution with unused solution in reservoirs of individual solution. It maintains solution activity and agitation of the solution.

Silver recovery
•Fixer solution is used to remove unexposed Ag-halide from the film. So used fixer solution contains high concentration of accumulated silver.
•Disposal of silver is to be regulated because silver is a heavy metal, and a precious metal too.
•Silver recovery refers to process of silver removal from used fixer solution.
•The most basic process is to drain fixer into a holding tank to be processed by a recycler later or by connecting a silver recovery unit with fixer tank.
Metallic replacement method:
•Fixer solution is filtered by steel wool filter. Silver replaces iron in the steel wool and gets deposited in the filter. A filter can be later processed after accumulation of significant amount of silver.
•Inexpensive method.
•Yields unrefined silver.
•Electrolytic method:
•The most efficient and the most expensive method. Yields pure silver.
•Electrically charged drums or discs are used which attract silver ions. Silver gets
coated on the charged electrode surface and can be removed later by scraping.
•Silver sulphide formation in the solution after recovery is hazardous for film processing. Sulphide compounds give foul smelling toxic gas fumes.
Safety issues
•Electrical appliances in a wet working area.
•Toxic substances used in processing solutions: acetic acid, glutaraldehyde, hydroquinone, phendione, sodium sulphite, ammonium thiosulfate.
•Safety equipments: face mask ,protective goggles and gloves, aprons.
COSHH (Control Of Substances Hazardous To Health) guidelines
•Assess health risks to determine the hazards.
•Decide necessary precautions.
•Introduce control measures to eliminate or minimize the risks.
•Monitor control measures and working procedures for effectiveness.
•Conduct health surveillance for early detection of occupational exposure related illness.
•Train all the staff and provide adequate information.

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