The sky is blue. Physicians give us an explanation for this: it is due to the preponderance of short wavelengths in the light diffused by the atmosphere. But why do we see it blue? Seeing the world in colour and identifying its characteristics requires processing of the image formed by the distribution of photons on the retina.
1. HOW IS THE COLOUR SENSE CREATED?
First we need to remember the various stages involved in how colour vision works.
The photons reaching the retina are absorbed by photoreceptors: cones for daytime vision and rods for vision when the light is dim, and very often both cones and rods if light is slightly reduced. The photoreceptors generate a signal when they capture a photon, whatever the wavelength involved. Due to very extensive spectral sensitivity in the field of wavelengths, almost all the photoreceptors are able to absorb short wavelength photons. It is only the rate of absorption that differentiates them. So, “S” cones (improperly named “blue”) are preferentially sensitive to short wavelengths of around 450nm, “M” cones (“green”), to medium wavelengths of around 540nm, “L” cones (“red”), to around 570nm, and rods to around 507nm. However, the probability exists that, for example, a 450nm photon hitting the retina is absorbed by a photoreceptor other than an “S” cone.
Fig. 1: Spectral sensitivity of the three groups of retinal cones.
Immediately on exit, the photoreceptors signals are recombined, and it is mainly contrast signals, of luminous or spectral origin, that enter the numerous visual paths in the retina. As for the retinal signals that head for the cortex, they are subject again to several recombinations, of variable importance, before resulting in the colour sense. In general, in these recombinations, signals from all the cone groups come into play, with variable importance. Colour is therefore an appearance attribute, constructed by our visual system. It is the tone that essentially characterises the colour of materials, and its definition is exceptionally stable within our natural environment. This phenomenon of relative stability is known as colour constancy.
With regard to the effect of spectral filtering, we note that:
In practice, every group of photoreceptors can be stimulated at short wavelengths.
An imbalance in the signals generated in cones can lead to a change in the contrasts perceived and a disturbance in colour perception which is not radical, however, as long as the three cone groups remain intact.
2. SPECIFIC CHARACTERISTICS OF BLUE VISION
In colour vision, blue, or more exactly the retinal pathway of signals issuing from the “S” cones, has a particular status. These signals contribute only slightly to luminous contrast at high spatial or temporal frequencies. Because of this fact, neither acuity nor sensitivity to flicker is based on these signals. We even speak of foveal tritanopia or small field tritanopia to indicate the reduction of colour vision due to the inability of “S” cones to process certain colour contrasts.
On the other hand, “S” cone signals contribute massively to the distinction of colours and play an essential role in identifying shades of colours. For example the difference between yellow or white,
or the distinction between warm white or cold white lights, is based on the response of “S” cones.
In summary, in terms of spectral filtering:
A strong reduction in signals from “S” cones should not affect acuity, but could lead to deterioration in the distinction of shades of colour and change colour sense.
But as long as a few “S” cone signals, even weak signals, pass through into the networks of retinal neurons, modifications to colour often go unnoticed.
Fig. 2: Illustration of the difficulty in perceiving certain colour details that are based on a variation in the signal from “S” or “blue” cones. Whereas the surface occupied by the letters in the words “Points de Vue” is less than the surface area of the rectangle, the latter stands out more.
3. WHAT WOULD BE THE IMPACT OF A BREAK IN VISIBLE SHORT WAVELENGTHS?
As long as the three groups of cones can maintain activity, colour vision, which is based on contrasts, is possible. So, everything depends on the position of the break in the visible spectrum.
A break at around 450nm, which leaves a gap at the entrance in “S” cones of almost 50% of the available photons, will have only a low impact on colour vision. Moreover, this is what happens naturally with ageing and cataract. The sky remains blue through until advanced old age. The effect of perceptive constancy, and in this case of “colour constancy”, stabilises the colours of materials in the environment, each in relation to the others, whatever the light variations.
If the break happens at around 500nm, a marked deterioration in the distinction of shades of colour is foreseeable in blue-green and purples, as well as for certain colour pairs such as yellow and white or dark blue and black. Acuity should be preserved.
On the other hand, in night vision, the subject may suffer from a notable lack of light.
Any kind of spectral filtering leads to perception deficiency.
Although colour distinction is always weakened, higher functions, that is to say the appearance and recognition of colours, are actually well preserved. In terms of colour, the visual response adjusts to the environment. As long as the light is polychromatic, the physiological adaptation capacities of humans compensate for a deficiency of light at source.