Simple Ratio 550/420 Carter6

Carter6 (Ctr6) is a simple ratio vegetation stress index developed by Gregory A. Carter. It uses the ratio of reflectance at 550nm (green) to 420nm (blue) to detect plant stress. The index is based on the principle that stress factors interfere with photosynthesis and alter the reflectance spectrum, particularly in the blue and green regions. Vegetation indices quantify plant health, biomass, and photosynthetic activity by exploiting the contrast between how plants absorb visible light for photosynthesis and reflect near-infrared radiation from their cellular structure. Ctr6 is particularly suited for vegetation stress detection, plant health monitoring, water stress assessment. The general formula is 550nm / 420nm, which requires 420 and 550 spectral bands.

Ctr6 is supported by 4 satellite sensors in our database, including NAIP, WorldView 2, WorldView 3, WorldView Legion. Each sensor uses different band designations — for example, NAIP uses the formula Green / Blue, while WorldView 2 uses Green / Coastal. Select a sensor above to see its specific band mapping.

Ctr6 requires 420 (420), 550 (550). Vegetation strongly absorbs red light for photosynthesis while reflecting near-infrared light from its mesophyll cell structure, making this contrast a reliable indicator of plant vigour.

Both Python and R code samples are provided above. In Python, use rasterio to load individual band GeoTIFF files and numpy for the arithmetic. In R, the terra package handles raster operations efficiently. The key is to load bands as floating-point arrays to avoid integer division, and to handle division-by-zero cases where the denominator equals zero. For production use, consider applying a valid data mask to exclude no-data pixels before calculation.

While NDVI is the most common vegetation index, Ctr6 provides complementary information that NDVI cannot capture on its own. The choice of index depends on your application, sensor availability, and atmospheric conditions.