Abstract
The requirements are discussed in terms of geometrical optics and isoncandela diagrams. IT is deduced that the maximum intensity should be at 1° below the cut-off level, which should be visible to the driver. Width and uniformity of beam are discussed, also luminous efficiency where an axial filament with prism optics are better than a transverse filament with fluting. Details are given of photometry to determine four quantities for a log lamp, namely useful flux, width and thickness of beam, and penetration distance in fog.