HUMAN COLOR VISION DEFECTS

THE BASIC NORMAL HUMAN COLOR VISION SYSTEM

The visible Spectrum

The visible spectrum is the portion of the electromagnetic spectrum with wavelengths between 380 nm and 760 nm. These frequencies are sensed as the following colors (wavelengths are the centers of the colors):

•       400 nm violet
•       440 nm blue
•       490 nm cyan
•       530 nm green
•       575 nm yellow
•       590 nm orange
•       650 nm red

Four kinds of sensors:

Use the chart of light sensitivity curves at right.

•       Rods

  Rods are primarily used at night, because they are much more sensitive than the cones. But they also provide motion and edge detection in brighter light. They are sensitive to a range of wavelengths between 380 nm and 650 nm, peaking at 530 nm. This covers the colors violet, blue, cyan, green, yellow, and orange. But rods are encoded as white, not as any specific color.

•       Blue Sensitive Cones

  The blue sensitive cones are sensitive to a range of wavelengths between 380 nm and 550 nm, peaking at 450 nm. This includes violet, blue, and cyan light.

•       Green Sensitive Cones

  The green sensitive cones are sensitive to a range of wavelengths between 430 nm and 670 nm, peaking at 550 nm. This includes cyan, green, yellow, and orange light.

•       Red Sensitive Cones

  The red sensitive cones are sensitive to two ranges of wavelengths. The major range is between 500 nm and 760 nm, peaking at 600 nm. This includes green, yellow, orange, and red light. The minor range is between 380 nm and 450 nm, peaking at 430 nm. This includes violet and some blue. The minor range is what makes the hues appear to form a circle instead of a straight line.

Seeing other colors:

•       Violet activates the blue cone, and partially activates the red cone.
•       Blue activates the blue cone.
•       Cyan activates the blue cone and the green cone.
•       Green activates the green cone, and slightly activates the red and blue cones.
•       Yellow activates the green cone and the red cone.
•       Orange activates the red cone, and partially activates the green cone.
•       Red activates the red cone.
•       Magenta activates the red cone and the blue cone.
•       White activates the red cone, the green cone, and the blue cone.

Colors not listed here are seen due to varying strengths of light activating the red, green, and blue cones.

Note that some people with protanomaly, deuteranomaly, or tritanomaly may see the correct colors, rather than the distorted colors they normally see, when viewing the above colors on a 3-color monitor.

After the cones have received the colors, the color information is encoded in the bipolar and ganglion cells in the retina before it is passed on to the brain. Three different encodings are used:

1. The primary encoding is luminance (brightness). It is the sum of the signals coming from the red cones, the green cones, the blue cones, and the rods. These provide the fine detail of the image in black and white.
2. A second form of encoding is used to separate blue and yellow (yellow being the sum of red and green). The signal changes one way if blue is stronger than yellow, and changes in the opposite way if yellow is stronger than blue.
3. The third form of encoding is used to separate red and green. The signal changes one way if green is stronger than red, and changes the other way if red is stronger than green.

The second and third kind of encoding do not have quite the fine resolution the luminance encoding has. Black and white vision has finer detail than color vision.

In the fovea, where central vision occurs, each luminance ganglion cell receives signal from only one cone cell of each color. There are no rod cells in the fovea, so it is night blind.

It is interesting that the light has to pass through the ganglion and bipolar cells to get to the rods and cones. The retina seems to be made backward. But this might protect the light sensitive cells from damage.

Note that the genes for the cone cells have two segments.

• One segment encodes a protein making an alternating grid with the proper spacing to be affected by certain colors of light. The rods and the different kinds of cone cells have different grid spacings, so they are sensitive to different colors.

  A rough diagram of the grid is shown at right. Notice the equally spaced rows of alpha-helix configured amino acids in the molecule. The spacings line up with the wavelength of the light of the color the molecule is sensitive too. Light in the right color band effectively rattles the entire molecule.

• The other segment of the gene tells the cone cell which bipolar cell nerve endings to connect to, so the color signals are properly encoded to be sent to the brain.

For more on this, see Human Color Vision.

DESIGNING SYSTEMS TO COMPENSATE FOR DEFECTIVE COLOR VISION

Signs, Especially Traffic Signs

Since most signs are made for luminance contrast, reading the sign is not a problem. Only if some kind of color code is built into the sign does defective color vision come into play.

One place where a color code is built into the sign is a traffic sign. The following color code is used:

•     White - Regulatory sign, lettering on all signs except orange, yellow, yellow-green. and white
•     Black - Special Regulatory sign, lettering on signs that are orange, yellow, yellow-green or white
•     Strong Magenta - Emergency traffic control signs
•     Red - Stop, Yield, Do not enter, and Wrong way
•     Orange - Construction and temporary traffic control
•     Yellow - Warning of hazard
•     Strong yellow green - School crossing or children present
•     Green - Guidance to locations, permitted movements
•     Blue - Road user services guidance, evacuation routes
•     Violet - Automatic toll collection
•     Brown - Recreation and cultural interest guidance

Only a few cases exist where color identification might cause confusion, because traffic signs also have shape codes:

• Orange and red - No signs exist that could be confused.
• Red and magenta - No signs exist that could be confused.
• Orange and magenta - The signs have the same meanings. The only difference is the duration of the traffic event.
• Orange and yellow - The signs have the same meanings. The only difference is the duration of the traffic event.
• Yellow and yellow green - The signs have the same meanings. The only difference is the extra meaning of children present.
• Orange and green - No signs exist that could be confused.
• Yellow and green - No signs exist that could be confused.
• Brown and green - The signs have the same meanings. The only difference is the recreational or cultural content.
• Brown and blue - The signs have the same meanings. The only difference is the service, recreational, or cultural content.

Traffic Signals

Because recognizing the aspect of the traffic signal is so important to traffic safety, special engineering is needed to ensure that drivers with defective color vision receive the correct message. Several methods are used:

1. For anomalous color vision, use the special glasses or filters below.
2. Position of the light in the signal face.
3. Selection of colors that can still be distinguished from each other by most people with defective color vision. This has been done in the US DOT standards.
4. Other methods, such as different shapes for different colors.

For more on this, see Defective Color Vision and Traffic Signals.

Color Filters, glasses, or lamps for Identifying Basic Colors

A trick that can be used (but not while driving a car) is to have color filters to hold in front of the eye. Choice of filters depends on the type of color blindness:

• For protanopia and protanomaly, use a red filter and a green filter.
  Only magenta, red, orange, yellow, white, and some violet pass through the red filter.
  Only cyan, green, yellow, white, and some orange pass through the green filter.
  A magenta and a cyan filter can be substituted for the red and green filter.
• For deuteranopia, deuteranomaly, and red-green indistinction, use a red filter and a green filter.
  Only magenta, red, orange, yellow, white, and some violet pass through the red filter.
  Only cyan, green, yellow, white, and some orange pass through the green filter.
  A magenta and a cyan filter can be substituted for the red and green filter.
• For tritanopia and tritanomaly, use a blue filter and a yellow filter.
  Only magenta, red, orange, yellow, white, green, and cyan pass through the yellow filter.
  Only cyan, blue, violet, magenta, and white pass through the blue filter.
• For monochromatism, use a red filter, a green filter, and a blue filter.
  Only magenta, red, orange, yellow, and some violet pass through the red filter. The red may be very dark.
  Only cyan, green, yellow, and some orange pass through the green filter.
  Only cyan, blue, violet, magenta, and white pass through the blue filter.
  A magenta, a yellow, and a cyan filter can be substituted.
• Filters of different colors can be used in the left and right lenses, sacrificing depth perception for color differentiation.

A trick that can be used is to have colored lights to shine on object to help discern its color. Choice of lamp colors depends on the type of color blindness:

• Use colored lights that can be switched on to make certain colors stand out.
• Use lights of different colors on opposite sides of the work station.
• Use a blinking colored light to make certain colors flash.
• Place a rotating color wheel in front of a light to make certain colors blink differently than other colors.

Special Lamps for Anomalous Color Vision

This trick can be used with protanomaly, deuteranomaly, and tritanomaly. A special lamp can be used to let the person see what others see, under certain conditions:

• The special lamp must emit only red, green, and blue light, but no other colors.
• The lamp can contain separate bulbs for red, green, and blue.
• A tricolor (red, green, blue) LED can be used.
• The special lamps must be the only illumination in the area.
• The person may have to learn what each of the colors looks like under the special lamp.
• This works because most objects that are orange, yellow, cyan, magenta, or white reflect the colors red, green, and blue in the proper ratios.
• The colors seen here will be the same colors seen on a tricolor color TV screen.
• This method might be useful for identifying colors in color coding systems.
• Build Special Lamps for Defective Color Vision.

Special Glasses and filters for Anomalous Color Vision

This trick can be used with protanomaly, deuteranomaly, and tritanopia. A special filter can be used to let the person see what others see, under certain conditions:

• The special filter removes spectral yellow and spectral cyan from the light seen by the viewer.
• The best filter is a notch filter with sharp edges.
• The illumination must contain the colors the filters pass.
• The person may have to learn what each of the colors looks like using the glasses.
• This works because most objects that are orange, yellow, cyan, magenta, or white reflect the colors red, green, and blue in the proper ratios.
• The colors seen here will be the same colors seen on a color TV screen.
• This method is useful for identifying colors in color coding systems.
• This method will probably work for identifying signal lights for trains, automobiles, boats, ships, and aircraft.
• As long as the driver can identify the signals with them on, it is safe to drive a car with these glasses.
• This will NOT work for protanopia, deuteranopia, tritanopia, red-green indistinction, or total color blindness.
• The glasses must be removed when driving under low-pressure sodium street lights. The filters remove all of the yellow light these streetlights make.
• A company named EnChroma makes these glasses.

Photography and Television for Anomalous Color Vision

This trick can be used with protanomaly, deuteranomaly, and tritanomaly. The process of tricolor color photography or television can be used to let the person see what others see, under certain conditions:

• The method of color separation in tricolor photography and color television divides the response into red, green, and blue signals.
• Display of the image on a TV set, a camera screen, or a monitor provides the spectral gaps necessary for producing separate red, green, and blue stimuli that can work with the anomalous cone cells.
• A Quattron 4-color screen can NOT be used for this.
• This would NOT work with any color photography system that relies on spectral interference or separation instead of color separation (e.g. Lippman Photochrome).
• The cyan-magenta-yellow printing process may or may not work with individual people. This depends on the nature of the pigments printed and the specific kind of anomalous vision present. But it should work with the glasses found above.
• The person may have to learn what each of the colors looks like on the screen and in the prints.
• This works because most objects that are orange, yellow, cyan, magenta, or white reflect the colors red, green, and blue in the proper ratios.
• This could be useful for identifying colors in color coding systems. Point a camera at the color to be identified and look at the monitor or the screen on a camera.

Color Coded Wiring and Other Color Codes

Various tricks work to make color codes work:

• Choose sets of colors that are easily distinguished with protanopic, deuteranopic, and tritanopic vision. Examples:

  Colors	Magenta Yellow Cyan Black Gray	Orange White Azure Black Gray
  Normal
  Protan
  Deutan
  Tritan

• Use the color filter trick above.
• Use a colored mirror.
• Use colored lights that can be switched on to make certain colors stand out.
• Use lights of different colors on opposite sides of the work station.
• For anomalous color vision, use the special lamp above.
• For anomalous color vision, use the special glasses or filters above.
• For anomalous color vision, use the camera method above.
• Use a blinking colored light to make certain colors flash.
• Place a rotating color wheel in front of a light to make certain colors blink differently than other colors.
• If only certain colors are in use, maybe all that is needed are samples of those colors on a card with their names.

Colored Lights on Equipment

Various tricks work to distinguish the colors:

• Place a small colored mirror where it will reflect the signal light.
• For anomalous color vision, use the special glasses or filters above.
• Use the color filter trick above.

Colored Lights on Aircraft, Airport Signals, Boats, Ships, and Coast Guard Markers

Various tricks work to distinguish the colors:

• For anomalous color vision, use the special glasses or filters above.
• Place a small colored mirror nearby, where it can reflect the light from the source into the eye of the operator.
• Use the color filter trick above. A flat filter mounted on eyeglass frames that can swivel down in a hurry is useful here.

Suggested modifications to the light colors:

• Since no aviation or marine lights are yellow, the red vs yellow difficulty with automotive signals and deuteranopia is not a problem. The only light colors used are red, white, and green.
• The color standards for aircraft and marine uses should be the same ones used for traffic signals.
  • The white should be white, neither bluish nor yellowish. It must be a continuous spectrum, not a high intensity discharge or LED source with gaps in the spectrum.
  • The red must contain orange, but no yellow, green, or blue. This looks yellow to people with protanopia and deuteranopia or red to people with tritanopia.
  • The green must contain cyan and blue, but no red, orange, or yellow. This looks blue to people with protanopia and deuteranopia, or green to people with tritanopia.
  If the lamps do not follow this standard, government is committing a serious error by not adopting it.
• Because the lights on these objects can be a considerable distance away, shapes and colored insertions used on traffic signals will not work here.
• The only people not addressed by this standard are those with monochromatism.

Colored Lights on Trains, Train Signals, and Other Ground Transportation Equipment

Various tricks work to distinguish the colors:

• For anomalous color vision, use the special glasses or filters above.
• Place a small colored mirror nearby, where it can reflect the light from the source into the eye of the operator.
• Use the color filter trick above. A flat filter mounted on eyeglass frames that can swivel down in a hurry is useful here.

Suggested modifications to the light colors:

• The color standards for train and other ground transportation lights should be the same ones used for traffic signals. So the same problems and solutions apply.
• Semaphore, position light, and color position light signals do not depend on color vision.