I’ve been using their $40 desktop kit for months and recommend it.
Smartphone users have other choices available.
http://store.publiclab.org/collections/spectrometry
(20 percent off through September, code at the page link)
I notice they now have an infrared camera kit as well — don’t know if it’d be useful for the heat range flashlight modders care about, but worth a look: http://store.publiclab.org/collections/diy-infrared-photography/products/infragram-webcam