Invisible Signal (Spell)

[Lysander]

A good spying/secret attack plan spell:

Invisible Signal
Evocation [Light]
Level: Sor/Wiz 1
Components: S, M
Casting Time: 1 standard action
Range: Long (400 ft. + 40 ft./level)
Effect: Invisible signal flare
Duration: One round

You fire an invisible glowing bolt of light that flies until it strikes an object, then explodes into a brilliant fireworks display that lasts one round. If it doesn't strike an object it explodes at the spell's maximum range. This spell is completely silent and its visual effects are invisible, making it ideal for secretly signaling allies that have the ability to see the invisible. Despite the bright glow of the signal it casts no light. Assuming you can see the flare at all, it is easy to spot. When fired straight up, unless there are additional obstacles such as cloudy weather or obstructing terrain, it only requires a DC10 spot check to view it from a mile away. Add an addition 5 to the spot check for each further mile. Halve the spot DC at night. The invisible signal does no damage to any creature or object it strikes, in fact they cannot even feel it. A caster can also choose to cast this spell as a visible effect.

Material
A firefly or a piece of phosphorescent moss

[Meirnon]

All I can say is redo the wording a bit. It requires a tiniest bit of work by the reader to understand that it's only glowing if you can see invisibility, and some readers would dislike the initial confusion. Just some better wording to indicate such in the start of the text would be helpful.

Other than that, I'm already seeing uses for this spell, especially in campaigns that include mass-battles. Maybe an option to change the color of the illusion?