Alper Ülkü

To me it does not seem correct. I think you got the solid angle wrong. 0.2 steradian solid angle defines a detector with apex angle 14.5 degrees, quite uncommon. Illuminance = Omega. Luminance . cos (theta) If Omega as you say is 0.2 then Ev = 0.2 * Lv * cos (theta) = 19.28 lm / m2 (Ev/Lv factor is not 2/25:0.08 but is 0.1285) If 0.2 degree is the angle subtended by the detector which is generally the case; then Omega becomes = 2pi (1 - cos 0.2/2) = 9.5e-6 sr, Ev = 150 x 9.5e-6 x cos (50) = 9.16e-4 lm/m2

Here is the solution: Sun subtends about 0.5 degree on earth surface solid angle Omega becomes = 2pi (1- cos (0.5/2)) = 5,98e-5 steradians At bright day light and at 90 degree normal incident, illuminance of sun at earth's surface is around 10.000 lux = lm/m2 Luminance = illuminance / Omega = 10.000/5,89e-5 =1.67e9 cd/m2 QED