Attached is an illustration of an image being demagnified using a double convex lens. I want to determine the intensity of the light (mW/cm2) if I were to put a lux meter at various locations after the convex lens given that the object is at a fixed location. The object is a monochromatic light projected by a projection system which goes through demagnification due to the double convex lens. My assumption would be that be that as you move closer to the focal plane, the intensity will increase. I know that the intensity is inversely related to the the magnification (M = -di/do). Thus, if M = -1/2, then intensity at the focal plane is I2 = I0/M = 2*I0. But what if I put a lux meter before and after the focal plane? How can I determine the intensity of the light which is out of focus? How can I derive the relationship between the light intensity of the object (I0) and the intensity of the image before the focal plane (I1), at the focal plane (I2), and after the focal plane (I3)?