ABSTRACT
The speed of an imaging system designates the relative amount of light needed to produce an image with a defined 'quality' and is expressed as a number which may be related to exposure. The exposure required to produce a given density fog may be used as an effective speed point. Threshold speed systems work at the very bottom of the toe of the characteristic curve, while systems based on inertia ignore the toe completely. The shape of each curve depends not only on the sensitivity of the material, but also on the quality of the light employed. The principles of speed determination for colour negative films follow those described for monochrome materials. However, speed determination is complicated by the fact that colour materials contain layers sensitive to blue, green and red light, and the standards involve averaging the speeds of the three sensitive layers.
