LIGHT INTENSITY AS MEANS FOR MEASURING LENS POWERS

Abstract

Abstract This study was carried out to prove the possibility of using light intensity as means in the measurement of lens power. Seven different procedures were used for measuring power of lenses selected randomly from three international trial sets (English, Indian. and Chinese). Powers of (185) lenses were measured; (137) were spherical lenses and (48) were cylindrical ones. 6 to 10 readings were taken at variable distances between the tested lens and the light meter, to gain various intensities of light. Some experiments were carried out by using an optical bench and others by a modified optical bench. A linear equation relating the light intensity to the detector distance was applied to obtain the results. Parallel ray method for concave lenses showed good results with a high intensity; when ophthalmoscope and 12volt lamp were used. Here the uses of the modified optical bench gave better results than the ordinary optical bench; because the tilting error was increased with the latter. However, when the light intensity was low (3.5volt), the modified optical bench results were worse, due to the aperture factor, which increases the aperture error. Hence the light disc incident on the detector becomes larger with non parallel method and that decreases the aperture error. Convex lenses had got good results with this method especially with the use of new battery rather than parallel ray method. Upon increasing the lens power, the light disc on the detector was decreased and this increased the aperture error. Generally, cylindrical lenses showed good results. Convex cylindrical lenses with parallel rays exhibited more accurate results than spherical convex lenses with the same method. Although the intensity of light was higher for the latter (ophthalmoscope), the aperture error in cylindrical lenses is less severe and that because it is confined to only one meridian. Key words: - Light — intensity — lenses - power ~ optical bench.