Understanding Film Sensitometry: A Comprehensive Guide

Posted on Nov 11, 2024 in Electronics

Understanding Film Sensitometry

Introduction to Sensitometry

Sensitometry measures film characteristics, showing how film reacts to light, exposure, developer, and development. The process involves:

  1. Controlled exposure using a sensitometer.
  2. Normalized processing to create a sensitometric strip.
  3. Density measurement with a densitometer.

Density, Opacity, and Transparency

Density measures light transmission through film. Key concepts include:

  • Transparency (T): Ratio of transmitted light to incident light (T = Lt / Li).
  • Opacity (O): Inverse of transparency (O = 1 / T).
  • Density (D): Logarithm base 10 of opacity (D = log10O).

Logarithms simplify working with large opacity values. A common logarithm in sensitometry is 0.3 (log102), representing a one-stop exposure change.

The Characteristic Curve

The characteristic curve graphs the relationship between exposure and density. It’s S-shaped with three parts: foot (shadow detail), straight line (midtones), and shoulder (highlight detail).

Key Concepts from the Characteristic Curve

A) Film Sensitivity (ISO)

Film sensitivity (exposure index) indicates the required exposure for a given density. Higher sensitivity needs less light. ISO combines ASA and DIN scales. ASA is arithmetic (doubling the number doubles sensitivity). DIN is logarithmic (increasing by 3 doubles sensitivity). For example, 100 ASA = 21 DIN = ISO 100/21.

B) Contrast and Latitude

Exposure latitude is the acceptable exposure variation without significant image quality loss. Sensitometric contrast is the density difference between maximum and minimum density in the correct exposure range. Steeper curves mean higher contrast and lower latitude. Development time affects contrast.

C) Gamma

Gamma measures contrast as the tangent of the angle between the straight section of the characteristic curve and the log exposure axis. Higher gamma means higher contrast.