I have a question which I think should be very simple for you optometry students, but I cannot get a clue. I'm a physics PhD student, currently concentrating into the simulation of retinoscopy. I have calculated the simulation for a streak retinoscope and now have some trouble on spot retinocope. I chose the parameters of the product from Welch Allyn. From my reading of manual, I know the Welch Allyn streak retinoscope can be easily converted to spot retinoscope by replacing the linear light source with a spot light source. However, I could not find any description about the slit aperture in front of the light source. Should this aperture be removed for a spot retinoscope? Also, what is the dimension of the spot light source? I've never seen a real one. Thank you for any comments.