They propagate in a relatively straight line, like light, with the result that, because of the curvature of the earth, their range is determined by the height of the transmitting and receiving antennae.

As the importance of the role of the public broadcasting service in the digital system is recognised, the service will, of course, have to be reviewed and revamped in the light of changes in technology, but it will also have to perform the function of guaranteeing the quality of services to a large public, which will continue for a long time to use its common or garden television sets and wireless reception on a daily basis; therefore, strategic recognition of the role of land-based digital television, whose growth can only be considered to be an opportunity here as well.

The principle of measurement is that light is transmitted through a specific length of the smoke to be measured and that proportion of the incident light which reaches a receiver is used to assess the light obscuration properties of the medium.

The apparatus shall incorporate a light source, a support for the test piece, a photoelectric cell receiver and an indicator (see Figure 1), together with the necessary means of suppressing the effects of stray light.

A convergent achromatic lens L2 corrected for spherical aberrations, links the diaphragm DT with the receiver R; the diameter of the lens L2 must be such that it does not diaphragm the light diffused by the sample in a cone with a half top angle

Note: Receptors are classified in terms of the type of energy associated with the stimulus (light, heat, sound, etc.).