in the light's properties, make your own with Custom appearance..
I cant remember exactly, but i think A = fully dark Z=fully bright.. i'm not sure though.
I think there is a tutorial on it somewhere on TWHL. i'll have a search.
Found it.
Custom Appearance: Used for designing your own custom effect or to give a named light an appearance other than Normal.
If you want to design your own light effect, you need to give the light a Name. The brightness will vary according to the strings of letters you enter in this property. If you want, you can trigger a light with a custom appearance and toggle the light on and off.
You give the light the appearance you want by specifying a string of letters (a-z) where a is completely dark and z is maximum brightness, the 4th number in the Brightness property. Letters in between a and z represent brightness levels in between. Each letter will give you about 1/10th of a second at that brightness. If you want more than a 10th of a second at that brightness, repeat the same letter. The letters do not have to go in alphabetical sequence and can be in any order that varies the light the way you want it. There is a maximum number of alpha characters you can enter of 142. So for a really slow pulse you could try: aaaabbbbccccddddeeeeffffgggghhhhggggffffeeeeddddccccbbbbbaaaa and this will give you about a six second dark to bright to dark effect.
Note: You can create some major funkiness by using capital letters, numbers or other characters instead of lower case letters in the custom appearance property! Since you're using Zoner's Half Life Tools and the most recent .fgd file (right?), you will have noticed that your light entity has a few extra values: ZHLT Fade and ZHLT Falloff.
The brightness of a real point source of light in nature will decrease as the inverse square of the distance it travels. That is, the brightness at 2 meters will be 1/4 of the brightness at 1 meter. If the brightness at 1 meter is 1/(1*1) = 1, the brightness at 2 meters is 1/(2*2), or 1/4. This "falloff" of brightness, the spreading out of light from a source, looks natural to us 'cause that's, like, how nature actually works. The ZHLT Fade and Falloff properties allow us to change the way the compiler calculates the brightness of a light as it gets farther and farther from its source.