A spectroscopy-based detector is developed to measure the nitrogen and chlorophyll content of tomato leaves and then to
predict the growth condition of tomato plants in greenhouse. The detector uses two wavebands, 527 nm and 762 nm,
since it is proved that these wavebands are sensitive to nitrogen and chlorophyll content in plant leaves by previous field
test. The detector contains: A Y-type optic fiber, two silicon photocells, a signal processing unit, and a MCU. Light
reflection from tomato leaves is transmitted by the Y-type optic fiber to the surface of the silicon photo cells, which
transfer optical signal into electrical signal. Then the analog signal is amplified to conform to the TTL level signal
standard and finally converted to digital signal by MAX186. After that, the MCU carries on a series of actions, including
data calculating, displaying and storage. Using the measured data, the Normalized Difference Vegetation Index (NDVI)
is calculated to estimate the nitrogen and chlorophyll content in plant leaves. The result is directly displayed on an LCD
screen. Users have an option in saving data, either into a USB-memory stick or into a database over the PC serial port.
The detector is portable, inexpensive, and convenient, which make it meet farmers' need in China. The performance test
shows that the growth model works very well, and the device has high accuracy in predicting the growth condition of
tomato plants in greenhouse.