As more people use LEDs and get into the distinct wavelengths of light used by plants, this $40 US DIY spectrometer is an amazing gadget. It is supposed to have 5-10nm accuracy. Amazing when you consider real spectrometers cost $2-$3k US.
See the product page: http://publiclab.org/wiki/spectrometer
This page explains how it is calibrated: http://publiclab.org/wiki/spectral-workbench-calibration
The order page shows a couple other models for adapting a smartphone into a spectrometer: http://store.publiclab.org/products/desktop-spectrometry-kit
This could be useful not only for LED growers but CFL and T5 growers to measure exact spectrum of their Kelvin-rated bulbs.
See the product page: http://publiclab.org/wiki/spectrometer
This page explains how it is calibrated: http://publiclab.org/wiki/spectral-workbench-calibration
The order page shows a couple other models for adapting a smartphone into a spectrometer: http://store.publiclab.org/products/desktop-spectrometry-kit
This could be useful not only for LED growers but CFL and T5 growers to measure exact spectrum of their Kelvin-rated bulbs.