Industrial Radiography Testing: Darkroom Processes and Technical Calculations

Industrial Radiography Testing: Darkroom Processes and Technical Calculations

Dr. Hardev V. Singh, Dr. Rashmi Shrivastava

IES University, Bhopal, Ph.D. (Chemistry)

Abstract – Industrial Radiography Testing (IRT) is a fundamental Non-Destructive Testing (NDT) technique used to detect internal flaws in metals, welds, and composite materials without causing any damage. The darkroom process plays a vital role in ensuring radiographic films capture high-quality images, free from fogging and artifacts. This paper presents a comprehensive overview of the step-by-step darkroom procedures, essential technical calculationsincluding exposure time, source-to-film distance, and film sensitivityalong with troubleshooting, quality control measures, and radiation safety considerations. Additionally, practical examples demonstrate calculation methods, making this paper a useful reference for industrial radiographers and NDT engineers.

1. INTRODUCTION

   Industrial Radiography Testing (IRT) employs penetrating radiation, such as X-rays or gamma rays, to inspect the internal structure of materials. Applications include aerospace components, pipeline inspection, construction welds, and nuclear industry equipment.

   The darkroom is a controlled environment that protects radiographic films from unintended exposure to light. Proper darkroom procedures are critical for:

   • Preventing fogging
   • Achieving optimal image contrast and density
   • Ensuring accurate detection of defects such as cracks, porosity, inclusions, or corrosion

   Precise technical calculations optimize exposure, reduce radiation hazards, and enhance defect detectability. This paper integrates practical darkroom workflow with theoretical calculations to provide a complete guide for industrial radiography.

2. MATERIALS AND EQUIPMENT
   1. Radiographic Films
      • Types: Blue-sensitive (orthogonal films), green-sensitive, and double-emulsion films for high resolution
      • Characteristics: High density resolution, appropriate film speed for selected radiation source
   2. Chemicals
      • Developer: Converts latent image on the film to visible image
        • Composition: Metol, Hydroquinone, Sodium sulfite, Potassium bromide
        • Temperature: Typically 2024°C for optimal reaction
      • Fixer: Removes unexposed silver halide, making the image permanent
        • Composition: Sodium thiosulfate, Acetic acid, Hardening agents
      • Stop Bath: Diluted acetic acid (12%) to halt developer activity
   3. Darkroom Equipment
      • Safe light with red/orange filter (low intensity to avoid fogging)
      • Film hangers, reels, drying racks
      • Thermometer and hygrometer to monitor temperature and humidity
      • Ventilation system to remove chemical fumes
   4. Safety Gear
      • Lead aprons, gloves, and goggles for handling radioactive sources
      • Radiation monitoring badges for personnel
3. DARKROOM PROCESSES
   1. Film Handling
      • Always handle by edges; avoid touching the emulsion side
      • Maintain clean hands or wear gloves
      • Store films in a light-tight container until ready for exposure
   2. Film Development
      1. Immerse film in developer solution for prescribed time
      2. Gently agitate to ensure uniform development
      3. Example: Blue-sensitive film at 22°C development time = 56 minutes

      Chemical Reaction:

      + +

      Silver halide crystals exposed to radiation are reduced to metallic silver forming the latent image.

   3. Stop Bath
      • Immerse film in dilute acetic acid for 3060 seconds
      • Stops the development reaction and prevents overdevelopment
   4. Fixing
      • Fixing time: 510 minutes
      • Removes unexposed silver halide
      • Ensures permanence of the radiograph
   5. Washing and Drying
      • Wash film in running water 1015 minutes
      • Dry in a dust-free environment
      • Prevents chemical residues, stains, or fogging
   6. Inspection
      • Use view boxes to evaluate density, contrast, and detect defects
      • Ideal radiographs exhibit high contrast and uniform density across the image
4. Technical Calculations in Industrial Radiography
   1. Exposure Time (t)

      =

      × 2

      ×

      Where:

      • = exposure time (seconds)
      • = material-specific constant
      • = source-to-film distance (cm)
      • = radiation intensity (R/min)
      • = film sensitivity factor

        Example Calculation:

      • Steel plate, = 0.5, = 50 cm, = 2 R/min, = 0.8

        =

        0.5 × 502

        2 × 0.8

        1250

        =

        1.6

        781.25 sec 13 min

   2. Source-to-Film Distance (SFD)

      Using the inverse square law:

      12 = 22

      1 2

      • Adjust exposure time if distance changes
      • Ensures proper density without overexposure
   3. Film Density & Contrast
      • Density (D):

        = log

        0

         

        10( )

        Where 0= incident light, = transmitted light

      • Contrast: Slope of density vs. log exposure curve
      • High contrast improves defect visibility
   4. Radiation Safety
      • Maximum permissible dose (MPD) for industrial radiographers: 50 mSv/year
      • Exposure time must be controlled using shielding and distance
      • Personnel should wear dosimeters for monitoring
5. QUALITY CONTROL & TROUBLESHOOTING
   

   Problem	Cause	Solution
   Fogging	Light leak / chemical contamination	Check darkroom, replace chemicals
   Under/Overexposure	Incorrect exposure time or distance	Recalculat exposure, adjust source distance
   Scratches on film	Mishandling	Handle by edges, use reels
   Chemical Residue	Inadequate washing	Extend washing duration, check water quality

6. CONCLUSION
   1. • Accurate darkroom procedures and handling ensure high-quality radiographs
      • Proper calculations optimize exposure, improve defect detection, and enhance safety
      • Digital radiography may reduce chemical usage, but traditional film-based methods remain critical in many industries
7. REFERENCES

1. ASTM E94 Standard Guide for Radiographic Examination
2. ISO 5579 Non-destructive testing Industrial radiographic film
3. Welding Inspection Technology Textbooks (NDT Resources)
4. Hellier, C. Handbook of Nondestructive Evaluation, 3rd Edition
5. Industrial Radiography Testing NDT-2025