Hi Cthugha In my previous thread which has now been closed, you mentioned the Hong-Ou-Mandel effect. I have attempted to simulate it in an Excel spreadsheet but I have not managed to obtain the correct-looking results. I have attached a Jpeg of the spreadsheet and would be very grateful if you could have a look at it. I am hoping that you will firstly understand what I am trying to do in the spreadsheet and then let me know what I am doing wrong. I may be basing the spreadsheet on completely wrong assumptions or interpretations of the Hong-Ou-Mandel effect. Thank you very much.