The overall Canon 40D spectral sensitivity is shown below. The response curves show the response of a camera we have modified to see UV+Visible+Infrared light. The color dots at the top of the Adjust RBG Curves graph are actual pictures taken by a Canon 40D at the corresponding wavelengths. This response curve is not the response of a stock, factory issued Canon 40D which has a response limited to under 700nm.
We measured the linearity of the sensor response the RGB peaks of 450nm, 550nm and 625nm. To measure linearity, we set the monochromator at the measurement frequency and varied the shutter speed from 1 to 30 seconds. The raw linearity graphs show that the sensor response is not completely linear. Note that the red response peaks at and clips at our set shutter speeds.
At 15 seconds exposure, the red channel is clipping on the Canon 40D. This is not a camera problem. The clipping occurs because the light source used to test has a lot of output at 625nm (Red). 255 is the maximum value for red, so once a count of 255 is hit, the camera can not record anything higher.