Luminol: how does it help forensic investigators?

Luminol: how does it help forensic investigators?
In the reaction bellow a solution including luminol, and sodium hydroxide is mixed with a solution containing potassium ferricyanide and hydrogen peroxide.
The luminol and peroxide react to form 3-aminophthalate, although the reaction requires iron to catalyze it (potassium ferricyanide).
Only a tiny amount of iron is required, thus allowing the detection of iron based solutions (like blood) at levels as low as 1 part per million.
Typically the reaction has a half-life of around 8-9 seconds.

Unfortunately a number of other substances will also catalyze the reaction including horseradish, bleach (hydrogen peroxide), and copper.
Nonetheless an experienced forensic investigator can sometimes determine what substance catalyzed the reaction using parameters observable to the naked eye such as emission intensity, half-life and spatial distribution.

In other words if someone tried to remove the blood stains with bleach and some iron remained, then the emissions intensity for the blood and the bleach catalyzed reaction will have different brightnesses, duration and emission wavelength (perceived color).

Sources and further reading:
http://chem-faculty.lsu.edu/mccarley/new/mccarley/Chemistry%202001/Articles_SP2008/Barni_Talanta_Luminol_Forensics.pdf
http://www.explorecuriocity.org/Content.aspx?contentid=185
http://chemistry.about.com/od/glowinthedarkprojects/a/luminolblood.htm
http://www.shsu.edu/chm_tgc/JPPdir/JPP1999/

Watch video:
Luminol reaction with oxygen
Gif via reddit