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7 Light in Biology and Medicine
adopted definition of acuity, α V , is the inverse of the angular gap size in arc minutes
between discernible black objects on a white background (a ‘chart’), with chart
lighting of 480 lux (32 W/m 2 ). If the objects are separated by a distance d and are a
distance L from the eye, then α V = (360 × 60)/(2 π) × (d/L). Thus 20/20 vision
corresponds to a visual acuity of α V = (360 × 60)/(2 π) × 0.07 in/(20 × 12 in),
which, as we indicated above, is 1 min of arc.
This visual acuity limit depends on the density of photoreceptors in the fovea of
the retina and the number of ganglion cells connected. The fovea where sharp vision
occurs holds most of the about six million color photoreceptors (called ‘cone’ cells)
while the approximately 120 million ‘rod cells’ are distributed over the whole retina
and are more sensitive to light, but have no color discrimination. There are three
types of cones, with greatest sensitivity to red, green or blue colors. This gives the
human the ability to distinguish about 10 million colors. The peak sensitivities are
at wavelengths 564 nm (red), 534 nm (green), and 420 nm (blue).
The indication 20/20 for the acuity of a human eye means that eye must be no
greater than 20 ft from an object to resolve details that an eye with good vision can
resolve at 20 ft. A person with 20/40 vision can resolve alphabetic letters at 20 ft
that a person with good vision can resolve at 40 ft. To make a test of vision, letters
are displayed on a Snellen chart, and a set of lenses are used to see what lens power
is required to correct fuzzy vision due to near or farsightedness.
A person with 20/20 vision can only distinguish the separation of contours in
letters at 20 feet if they are separated by more than 7 hundredths of an inch. Such
resolvable contours make an angle of 1 arc minute at 20 ft. Good human vision is no
better than three times the diffraction limit.
7.27.3 The Eagle Eye
Eagle eyes each have two foveae, one pair for binocular vision and the other to spot
objects to the side of the head. They also can see into the soft ultraviolet part of the
electromagnetic spectrum. The typical eagle eye pupil is larger than a human eye
pupil, so it has a better optical resolution, with a corresponding greater density of
fovea cones. The combination means the eagle has an acuity four times as good the
best human eye, putting the eagle eye resolution close to the diffractive limit.
7.27.4 More Colorful
Rainbow colors are called ‘primary’, as each is near a single frequency. Mixtures of
two rainbow colors produce ‘secondary’ colors, such as brown. An equal intensity
mixture of all three of red, green, and blue light makes white light. A combination
of just red and green light will appear yellow. Such combinations of light are called
7 Light in Biology and Medicine
adopted definition of acuity, α V , is the inverse of the angular gap size in arc minutes
between discernible black objects on a white background (a ‘chart’), with chart
lighting of 480 lux (32 W/m 2 ). If the objects are separated by a distance d and are a
distance L from the eye, then α V = (360 × 60)/(2 π) × (d/L). Thus 20/20 vision
corresponds to a visual acuity of α V = (360 × 60)/(2 π) × 0.07 in/(20 × 12 in),
which, as we indicated above, is 1 min of arc.
This visual acuity limit depends on the density of photoreceptors in the fovea of
the retina and the number of ganglion cells connected. The fovea where sharp vision
occurs holds most of the about six million color photoreceptors (called ‘cone’ cells)
while the approximately 120 million ‘rod cells’ are distributed over the whole retina
and are more sensitive to light, but have no color discrimination. There are three
types of cones, with greatest sensitivity to red, green or blue colors. This gives the
human the ability to distinguish about 10 million colors. The peak sensitivities are
at wavelengths 564 nm (red), 534 nm (green), and 420 nm (blue).
The indication 20/20 for the acuity of a human eye means that eye must be no
greater than 20 ft from an object to resolve details that an eye with good vision can
resolve at 20 ft. A person with 20/40 vision can resolve alphabetic letters at 20 ft
that a person with good vision can resolve at 40 ft. To make a test of vision, letters
are displayed on a Snellen chart, and a set of lenses are used to see what lens power
is required to correct fuzzy vision due to near or farsightedness.
A person with 20/20 vision can only distinguish the separation of contours in
letters at 20 feet if they are separated by more than 7 hundredths of an inch. Such
resolvable contours make an angle of 1 arc minute at 20 ft. Good human vision is no
better than three times the diffraction limit.
7.27.3 The Eagle Eye
Eagle eyes each have two foveae, one pair for binocular vision and the other to spot
objects to the side of the head. They also can see into the soft ultraviolet part of the
electromagnetic spectrum. The typical eagle eye pupil is larger than a human eye
pupil, so it has a better optical resolution, with a corresponding greater density of
fovea cones. The combination means the eagle has an acuity four times as good the
best human eye, putting the eagle eye resolution close to the diffractive limit.
7.27.4 More Colorful
Rainbow colors are called ‘primary’, as each is near a single frequency. Mixtures of
two rainbow colors produce ‘secondary’ colors, such as brown. An equal intensity
mixture of all three of red, green, and blue light makes white light. A combination
of just red and green light will appear yellow. Such combinations of light are called