Sensitometers

A sensitometer is a device used to measure the sensitivity of photographic emulsions and other light-sensitive materials. It functions by exposing a strip of film or paper to a series of precisely controlled light intensities. This is typically achieved through a step tablet, which has a range of neutral density filters of varying thicknesses, or by moving the material past a slit illuminated by a light source of constant intensity. The resulting exposed strip then forms a calibrated density scale.

After development, the optical density of each step on the exposed material is measured using a densitometer. This density data is then plotted against the logarithm of the exposure it received. This plot, known as a characteristic curve or Hurter-Driffield (H&D) curve, reveals crucial information about the material's response to light. Key parameters derived from this curve include the toe, which represents low-exposure sensitivity, the straight-line portion, indicating linear response, and the shoulder, where the emulsion begins to saturate.

The sensitivity of the emulsion can be quantified from the H&D curve, often by determining the exposure required to achieve a specific density. Sensitometers are essential tools in photographic research, quality control, and the development of new imaging materials, ensuring consistent performance and predictable results. They are also utilized in industries beyond traditional photography, such as in the calibration of materials used in printing plates and photolithography.