Impact accelerometer

3" LED light does nothing for us at all...
https://en.wikipedia.org/wiki/Inertial_switch
There are two kind of sensors you can use... one is the type used on car airbag systems, and the other is a simple spring with a metal post in the middle. During impact the spring moves and touches the post completing the circuit.
Any any case... what kind of battery life? What kind of impact?

 

Now that would be a different sensor altogether xD
If that steel plate was hit hard enough, the sensors I mentioned earlier would work... although the car air-bag sensor would only trigger once and would need to be reset.
If the plates 'move' a lot, stick with the spring type sensor I mentioned. If the plates don't 'move' that much, but vibrate a lot, you should look into piezo elements. They generate a voltage when exposed to vibration that you can use to trigger your light. The circuit used to monitor this will need to condition the signal though... because the piezo element can generate a high voltage if struck hard enough... (They are used in some BBQ lighters... a hammer strikes an element and it makes a high enough voltage to make a spark...) Of course, your application will not (or should not) create a spark xD

 

It would help a lot if you described what you are trying to DO so we could propose a solution. This sounds like it could be a carnival game, where you strike a steel plate with a sledge hammer and light up an LED. For that type of application you can place the impulse sensor in the hammer head, and this is actually done in the materials testing field using special hammers with piezoelectric sensors in the hammer head.

So, what is producing the impulse? What are the two objects involved? Why is the distance 100 feet? How often do impulses occur? What is the range of impulse in N·s? There is some information of impulses and how to measure them here.