The beam given by dioptric apparatus

Abstract

It is shown that the light in the axial direction given by a dioptric lens using a white source can only be white if the light source is above a certain limiting size. For such light sources a simple expression is deduced for the axial beam candle-power for revolving lenses. A graphical method is given for computing the axial beam candle-power for light sources of smaller size than the critical value. This makes allowance for the variation of luminosity with wave-length for the source used. The distribution of light for directions not on the optical axis is also computed in a general way. The effect of the dispersion of the glass of the lens on the maximum distance at which satisfactory candle-power readings can be made is also considered. The case of fixed lenses is then studied and the general solution obtained for the intensity at any distance and in any direction for light of any colour. From this the candle-power on the horizontal plane is derived as a special case. The rest of the paper consists of numerical results for lenses of the Fresnel section in both revolving and fixed types. Tables to enable the calculation of candle-power to be made are included.