http://www.chrony.ca/english.mv
http://www.pact.com/chrono.html
http://www.oehler-research.com/
All of them work the same the screen is a photoelectic cell. Same thing as a solar battery charger. The photoelectic cell converts light into electicity. The shadow of the bullet going over the first cell causes a interuption in the electric out put of the cell which starts the timer. The shadow of the bullet interupts the electric out put of the second cell and stops the timer as it goes over it. The amount of time in miliseconds it took the the bullet to pass between the 2 cells is converted and displayed in feet per second or meters per second depending on which model/brand of chronograph you have.