Hi, I am setting up a Lambertian light source by placing a 'source rectangular' behind a pre-defined 'slide'. This approach is explained in Zemax document. However, there is not much information about how far the 'source rectangular' should be behind the 'slide'. In my setup, there are also some lenses and apertures. I have experimented a bit, and the different distance between these two can yield quite different images. for instance, with the distance of 1e-3 mm, the image looks like , whild with the distance of 1e-4 mm, the image is  and the radiance intensity is also one order higher. 

How can I decided the distance between the source rectangular and the slide, and which one above I should trust?

Thank you.