In drag racing there are 2 different ways that the starter can set the Christmas tree. One is called a ‘sportsman’ tree, and the other is called a ‘pro’ tree. The way the sportsman tree works is when the starter hits the button, it activates 3 separate ambers prior to the green light. These amber lights have a half second .500 interval between them, as well as a half second delay between the last amber light and the green light. A Perfect light on a sportsman tree is a .500 light. What this means, is when the last amber comes on, there is a .500 second interval until the green light is activated. If you front tire broke the beam exactly when the green came on, it would be considered a perfect light, or a .500 light. If you left the beam 1/10th of a second after the green, it would be a .600 light. A good light for sportsman is typically a .520-.530 or better.

Thats correct for a  track still using incandescent bulbs. Now MOST NHRA tracks use LED bulbs, with the change to the new bulb they also changed the reaction time. now a .000 is perfect. At all NHRA events .000 is a perfect time reaction time for both trees.

Why was it .500 before?  The thinking was that because there was some delay in the time between the bulb getting juice and actually lighting , so .500 would be a perfect light for sportsman and .400 for Pro trees.

The NHRA realized that it was some confusion over the .5 and .4 reaction time, so with the new LED bulbs that light instantly, they changed the reaction time to .000
The way a pro tree works is once a car is staged (front tires in both the pre staged and staged beams), the starter hits the button. At that point all three amber lights light up at once, and then the green comes on. The interval between the 3 ambers coming on and the green is .400 seconds, or 4/10ths of a second. -So a perfect light on a pro tree is a .400 . One big difference in these 2 different ‘trees’ is on a sportsman tree you are anticipating the green light, because you get to see 3 ambers spaced 1/2 second apart prior to the green. So, you know when the green is coming as the 3 ambers start to light up. On a pro tree, you are reacting to the flash of the 3 ambers prior to the green light coming on. So, on a pro tree, you are having to react to the ambers, but on a sportsman tree you have to anticipate the green from the 3 prior amber intervals.
So, on a sportsman tree you are trying to time your launch so the tires break the beam when the green light comes on, and the 3 half second intervals of the ambers allow you to try to ‘time’ your launch. But on a pro tree you start to launch the car as soon as you see the ambers flash, .4 seconds before the green comes.