Figure 1 text description

How our rods and cones relate to the visible colour spectrum in terms of wavelengths

Although sensitivity cones are informally referred to as blue, green and red, these labels are only approximate in terms of the spectrum. Our rods, which only see light and dark, are receptive to a broad part of the spectrum also.
Units are in nanometres (nm).

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The visible spectrum

The normal eye is able to receive light at wavelengths from about 380 - 780 nm, which covers the colour spectrum of violet, blue, green, yellow, orange, and red. These are basically the colours of the rainbow, although a 7th colour 'indigo' is sometimes put between violet and blue. Wavelengths below 380 nm are referred to as ultra violet, and above 780 nm as infra red, and are not visible to the naked eye.

Table 1: The wavelengths resulting in the visible spectrum of colour

Wavelength (nm)	Colour range
Below 380	Ultra violet
380 - 450	Violet
450 - 490	Blue
490 - 560	Green
560 - 590	Yellow
590 - 630	Orange
630 - 780	Red
Above 780	Infra-red

Red

The part of the spectrum referred to as red is roughly 630 - 780 nm, at one end of the visible spectrum. However, the cones which are most sensitive to light in that part of the spectrum cover 470 - 700 nm (green, yellow, orange and red), and peak sensitivity is at 585 nm, at the orange end of yellow.

If red receptors are missing, some colours are not only distorted but also darker, as the eye is no longer receiving any light above 650 nm, even though the normal visible spectrum extends to around 780 nm.

Green

The Green receptors cover light from the middle part of the visible spectrum, roughly 440 - 630 nm (blue, green, yellow and orange), and peak sensitivity is about 540 nm, towards the yellow end of green.

The range 480 - 640 nm (green, yellow, orange) is light received by both red and green cones, but interpreted differently.

Rods

The Rods receive light from the broadest part of the visible spectrum 400- 600 nm (violet, blue, green, yellow), but do not interpret colour from any of it - only light and dark. Rods are not sensitive to light in the orange and red ranges (above 600 nm), so in low light situations red will tend to look black.

Blue

The Blue cones are sensitive to wavelengths in the range 380 - 530 nm (violet, blue, green) and are most sensitive at 440 nm, at the blue end of the violet range.

The light range 480 - 530 nm (blue, green) is visible by the rods and all the cones.

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