Simple Ratio 695/420 Carter1

Carter1 (Ctr1) is one of the most effective plant stress indices developed by Gregory A. Carter. It uses the ratio of reflectance at 695nm (red-edge) to 420nm (blue) to detect various types of plant stress. This ratio was found to be significantly greater in stressed compared to non-stressed leaves for all stress agents tested. 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. Ctr1 is particularly suited for plant stress detection, early stress warning, water stress assessment. The general formula is 695nm / 420nm, which requires 420 and 695 spectral bands.

Ctr1 is supported by 3 satellite sensors in our database, including WorldView 2, WorldView 3, WorldView Legion. Each sensor uses different band designations — for example, WorldView 2 uses the formula Red_Edge / Coastal, while WorldView 3 uses Red Edge / Coastal. Select a sensor above to see its specific band mapping.

Ctr1 requires 420 (420), 695 (695). 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, Ctr1 provides complementary information that NDVI cannot capture on its own. The choice of index depends on your application, sensor availability, and atmospheric conditions.