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Lighthouse optics are usually complex designs, the lens components tailored to suit both the type of light source available at the time of installation and the navigational requirements of the locality. The original light source may have been either a paraffin flame or an incandescent mantle; modern technology now requires that electric lamps are used. It is not economical to make lamps comparable in shape to the original source and it is also not practicable to reconstruct and refocus the existing lens components. The lens designs differ widely, each to suit the needs of its station, and the beam characteristics are affected differently by the choice of electric lamp. These effects are not usually calculable by accepted methods, as for example those in BS 942. A programme of investigation was instituted to select a suitable current production lamp for a range of lens designs, most of the lamps were only available for trial on site. The criteria were adequate luminance, long life, high luminous efficacy, correct colour and a combination of shape and uniformity compatible with the lens. These criteria could only be properly judged by photometric measurement of the actual beams of light. A photometric technique for on-site measurements had to be developed. There were three principal problems; the changing atmospheric attenuation over long distances, the rapid horizontal traverse of a rotating beam, and the mechanical impossibility of tilting the optic to provide a vertical scan through the beam. The solutions are: a high-intensity standard projector (0.2 Mcd) and a sensitive telephotometer for measurements down to the visual threshold, a rapid response amplifier and recorder or a time-integrating photometer to measure the horizontal scan of the beam, and large prismatic panels in front of the lens to depress or elevate the beam through small angles (±2°) in the direction of the photometer. By these means, the fourth problem of interrupting the regularity of the signals given to mariners at night did not arise.
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