The sensitivity of the imaging device to different frequencies of light. This is usually in the range of 300-1000 nanometres. Visible light is up to 730nm, dull red infrared lighting from 700-850nm, invisible infrared is over 850nm.
Using the example of the human eye, the spectral response curves map the wavelength of light against the fraction of light absorbed by each type of eye cone (red, green, and blue sensitive cones). It is the sensitivity of the eye or a device to different wavelengths of light.
The response of an image sensor to the various wavelengths of light. Human spectral sensitivity ranges from approximately 420 to 720 nanometres. In terms of colour, this is from bluish red-purple through to green, yellow, orange and red with the eye most sensitive to green and yellow. Vidicon tubes and CCD devices extend past this to the infra-red regions around 850nm. Newvicons have a good response in the ultra violet end of the spectrum.