Simulation of the normal (above) and dichromatic (below) perception of red and green apples In almost all cases,<unk>  blind people retain blue–yellow discrimination, and most<unk>  blind individuals are anomalous trichromats rather than complete dichromats. In practice, this means that they often retain a limited discrimination along the red–green axis of<unk>  space, although their ability to separate<unk> s in this dimension is reduced.<unk>  very rarely refers to complete monochromatism. Dichromats often confuse red and green items. For example, they may find it difficult to distinguish a Braeburn apple from a Granny Smith or red from green of traffic lights without other clues—for example, shape or position. Dichromats tend to learn to use texture and shape clues and so may be able to penetrate camouflage that has been designed to deceive individuals with normal<unk>  vision. Horizontal traffic light in Halifax, Nova Scotia, Canada<unk> s of traffic lights are confusing to some dichromats as there is insufficient apparent difference between the red/amber traffic lights and sodium street lamps; also, the green can be confused with a dirty white lamp. This is a risk on high-speed undulating roads where angular cues cannot be used. British Rail<unk>  lamp signals use more easily identifiable<unk> s: the red is blood red, the amber is yellow and the green is a bluish<unk> . Most British road traffic lights are mounted vertically on a black rectangle with a white border (forming a "sighting board") and so dichromats can more easily look for the position of the light within the rectangle—top, middle or bottom. In the eastern provinces of Canada horizontally mounted traffic lights are generally differentiated by shape to facilitate identification for those with<unk> : square for red, 45° tilted square ("diamond") for yellow, and circle for green (see included image of horizontal traffic light from Nova Scotia). In the United States, this is not done by shape but by position, as the red light is always on the left if the light is horizontal, or on top if the light is vertical. However, a lone flashing light (e.g. red for stop, yellow for caution) is still problematic. File:Вечір на "інтегралі" - річка Південний Буг.Normal sight File:Deuteranopia sight.Deuteranopia sight File:Tritanopia sight.Tritanopia sight File:Monochromacy sight.Monochromacy sight
Color blindness
Flash blindness
Tunnel vision
Myopia
Cortical blindness
Amblyopia
Red eye 
Retinopathy