Doesn't the light have to go through the Ronchi screen to get to the optic under test?
The Ronchi test in general is the most sensitive when the slit or pin hole is the same size as the width between the lines. The slit as has to parallel to the lines on the grating. So when you simply place an LED directly behind the grating, the light is shining through the clear spacing between the lines and forming it's own slits. The slits are always the perfect width and always perfectly parallel to the lines on the grating. So by simply placing the LED directly behind the grating and slightly off center you have the grating/light source as always being self aligning and at it's highest sensitivity. So you have removed a source off errors and making the test more robust.