Spectral sensitivity refers to the relative sensitivity of a receptor type to all of the wavelengths. Rods are relatively more sensitive to the short wavelengths, whereas the cones are relatively more sensitive to the longer wavelengths. Indeed, cones are most sensitive to about 555 nm and rods are most sensitive to just under 500 nm. This difference in spectral sensitivity is named the Purkinje Shift. This shift occurs as we transition from day vision to night vision and back again.
An interesting practical implication of the Purkinje effect is that, as nighttime conditions appear, longer wavelengths (reds) of light will appear darker, whereas shorter wavelengths (blues) will appear relatively brighter. This means that red objects become more difficult to see at night than blue or green objects with similar reflectance values. It is for this reason that, some years, ago, fire departments started shifting away from traditionally red vehicles. Because red is harder to see at night, fast-moving red fire trucks will be more difficult for other drivers to see than fast-moving blue fire trucks. However, another driving convention does have perceptual validity. Red light is less likely to affect our dark adaptation than other colors. This validates the decision to make taillights on cars red. Drivers will see the red taillights at night but the red light will not affect their dark adaptation as much as taillights of other colors would. You may have noticed how distracting a white (or broken) taillight can be at night, as it interferes with your ability to see well in the dark. This is also why the little screens of cell phones can be so distracting while in an otherwise dark movie theater.
In this illustration you can simulate how the relative brightness of different colors change as you ships from photopic (daytime) vision to scotopic (nighttime) vision.
To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.
On the Illustration tab, you can adjust the parameters and start a simulation of dark adaptation.
Below is a list of the ways that you can alter the illustration. The settings include the following:
Make Image: either in color, like photopic vision, or gray scale, black and
white, like scotopic vision; You will need to set the image to gray scale to be able to
simulate scotopic vision.
Eye State: change the eye state to photopic or scotopic and see how the relative brightness of the different colors change. Red go toward black and blues become brighter.
How Display: Single refers to showing only one image which has all your current settings; Compare will put the original photopic color on the left and the image in the current eye state on the right.
Use Your Own Image: allows you to upload your own image to simulate the Purkinje Shift.
Pressing this button restores the settings to their default values.