Rod monochromacy is a very rare form of color deficiency, affecting only 1 in every 30,000 people (Carroll, Neitz, Hofer, Neitz, & Williams, 2004). Rod monochromats have no functioning cones of any kind and therefore can be described as truly color blind. As a result, they see the world in shades of gray—high-reflectance objects are white, low-reflectance objects are black, and intermediate-reflectance objects are various shades of gray. In metamer matching, only one color is required to match another—as all the rod monochromat will be doing is adjusting the percentage of reflected light—that is, how gray the surface is.
Because they have no cones, rod monochromats have many other visual problems in addition to color blindness. Rod monochromats are dependent on their rod vision in both bright and dim light. This has serious disadvantages during daylight conditions—as these individuals are highly sensitive to light but have poor visual acuity. Because they are using scotopic vision all the time, rod monochromats are extremely sensitive to bright lights. For example, in a room that would be considered normally illuminated for people with intact cone systems, a rod monochromat will find it too bright. As such, rod monochromats often must wear sunglasses indoors. Going outside on a bright sunny day can be overwhelming and requires very strong eye protection. In reality, most rod monochromats will avoid bright outdoor conditions even with very strong sunglasses. Moreover, because the rods do not support acuity, rod monochromats have very poor visual acuity and must wear very strong lenses in order to read. Even then, most rod monochromats require large-print typefaces in order to read normally. To summarize, unlike those with color deficiencies, rod monochromats are at a serious disadvantage relative to normally sighted people.
In this illustration, you can simulate how a rod monochromat might see a scene.
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:
Eye: the image on the right will either be as a person with
normal vision sees it or as a person with rod monochromacy sees the image.
Depending on your browser this might take a while.
Use Your Own Image: allows you to upload your own image to simulate the rod monochromacy.
Pressing this button restores the settings to their default values.