RPS Imaging Science Group

The eye response functions

One of the creative attributes of Infrared photography is that it provides a different view on a scene to that perceived by the human visual system. The very term infrared (after red) suggests that it is the region of the spectrum that starts where the visual red finishes.
So where does infrared start and the visual region finish? The clue to this comes in the chart above. This shows the eye response function to 2 different light levels. The “photopic” line shows the combined response of the human eye under normal daylight light levels. It can be seen that the photopic response falls effectively to zero around 680nm so infrared occupies the region after this.
As a result of this pictures taken through filters that allow wavelengths to pass that are longer than 680nm record phenomena not noted by the naked eye, which is what makes it so interesting!
The chart above also reveals an interesting issues. At low light levels the eye fails to see in colour and moves to scotopic vision. As the curve in the chart reveals the eye sensitivity response moves to shorter wavelengths. If we were to consider the onset of “visual infrared” in this case it would be wavelengths above 600nm!
It would be an interesting project to take some images at wavelengths below 600nm and see how close they look to moonlight when rendered into a monochrome image. I feel a project coming on…
These curves are plotted from CIE standard data that can be found in a really useful resource from University College London – see http://www.cvrl.org/