The light-emitting hole of the photocell should be located at the focal point of the lens in front of it, and its diameter (d) should depend on the focal length (f) of the lens so that d/f is less than 0.04;
Aperture The iska Ifto Yuuna of phototube waa in uu ku yaalaa meesha ugu focal of gashto ee hore waxaa ka mid ah, iyo dhexroor (d) waa in la go'aamiyo dhererkiisu wuxuu focal (f) ee la gashto si d / f ka yar yahay 0.04.