Why Eyes Use the Light They Use

"What sets this range ultimately is not its availability, but its suitability to perform the tasks demanded of it." — George Wald, Scientific American, 1959¹

"The eye does not appear to be optimized for detection of the available sunlight, including the surprisingly large amount of infrared radiation in the environment." — Bernard Soffer and David Lynch, American Journal of Physics, 1999²

Summary

Human eyes see from about 380 to 750 nanometres (nm), just under one octave of a spectrum that runs from gamma rays to radio waves. Richard Dawkins called it "a narrow slit."³ The biologist Nathan Lents asked why a designer would give birds ultraviolet vision and deny it to people.⁴ The physicists Bernard Soffer and David Lynch computed that the eye detects only 19% of the sunlight between 320 and 1,400 nm.²

Photon energy. Visible photons carry 1.65 to 3.26 electronvolts (eV), the scale of a single chemical bond.¹ The popular figure of 1 to 10 eV is wrong.⁵
The Sun's peak depends on the units. Per nanometre, sunlight peaks in blue-green; per unit of frequency, near 1,000 nm in the infrared.²⁶
Air and water pass more than the visible band. Together they pass light from about 300 to 1,100 nm.⁷ They set outer walls, not the eye's edges.
The red edge is noise in the pigment. Pigments tuned to longer waves fire more often from heat alone. Dong-Gen Luo and colleagues concluded in 2011 that this helps explain why natural pigments peak only between about 360 and 620 nm.⁸ The body's own glow matters only beyond about 1,400 nm.²
The violet edge is damage and blur, not ionisation. A near-ultraviolet photon carries 3.1 to 3.9 eV; ionising water takes 12.6 eV.⁹ The human lens absorbs ultraviolet almost completely by age 25.¹⁰
Animals that see outside the band pay for it. The eagles Lents praises for acuity block ultraviolet.¹¹ Mammals with ultraviolet-clear lenses lack high-acuity retinas.¹² Snakes sense heat, not light, and form a "poorly focused" image.¹³
Michael Denton reads the band as fine-tuning. Wald made the same physical argument in 1959 without drawing a conclusion about purpose. Denton's version adds a unit error and an overstated ratio.

Verdict: the narrow band is not bad design. It is roughly the band any camera eye built of water and carbon compounds, under a Sun-like star, is pushed into. Lents's comparison fails because ultraviolet vision and top acuity do not occur in the same eye. Soffer and Lynch are partly right: the physical optimum lies redder, and nobody has priced the trade. The band is not evidence that the eye was designed either, because evolution predicts the same band. What remains is a question about physics: why sunlight, clear air, clear water and chemical bonds line up.

Part 1: The complaint in its own words

A narrow slit

Dawkins used the image in the last chapter of The God Delusion: "Our eyes see the world through a narrow slit in the electromagnetic spectrum. Visible light is a chink of brightness in the vast dark spectrum, from radio waves at the long end to gamma rays at the short end."³ His point is that science widens evolved senses. He does not call the slit a design flaw.

Birds see ultraviolet

Lents, a professor of biology at John Jay College, made it a design charge in 2015. He praised the acuity of "birds of prey, such as eagles and condors," and added: "Many birds can see a broader range of wavelengths than we can also, including ultraviolet light."⁴ He concluded: "Alternatively, why an intelligent designer would deny his favorite creatures the excellent vision that he provided lowly birds is quite a mystery."⁴

The eye catches a fifth of the light

The strongest version comes from physics, and it was aimed at a naive evolutionary story rather than at design. Soffer and Lynch integrated the eye's sensitivity against sunlight between 320 nm, the atmosphere's cutoff, and 1,400 nm: "It is only 19%, hardly optimal in a spectral sense."² They asked, "could we be mired in some evolutionary backwater, where once we were indeed better adapted?" They concluded that "the eye is only weakly optimized to take full advantage of the available solar spectrum."²²

The strongest form

Put together, the complaint reads: sunlight, air and water all supply usable light from about 300 to 1,400 nm; birds prove ultraviolet vision can be built; the human eye uses a fifth of what is available; so a designer who cared about human sight would have given a wider band. The parent article's test applies. Name the purpose, name the constraint, and show a build that beats the existing one on every axis that matters.¹⁴ The rest of this article supplies the constraints.

Part 2: What sets the outer walls

Photon energies

A photon's energy in electronvolts is 1,239.8 divided by its wavelength in nanometres.⁵

Wavelength	Photon energy	What happens there
124 nm	10.0 eV	Vacuum ultraviolet, absorbed high in the atmosphere
300 nm	4.13 eV	Ozone cutoff at the ground
380 nm	3.26 eV	Violet limit of normal human vision
500 nm	2.48 eV	Peak of rod rhodopsin
750 nm	1.65 eV	Red limit of normal human vision
1,000 nm	1.24 eV	Near infrared
9,300 nm	0.13 eV	Peak glow of a body at 37 °C

Wald set these energies beside chemistry in 1959. Single covalent bonds "fall between 40 and 90 kilocalories per mole, corresponding to radiation of wavelengths 710 to 320 millimicrons," and the excitations of photochemistry take "about 20 to 100 kilocalories per mole, corresponding to the absorption of light of wavelengths 1,430 to 280 millimicrons."¹ His conversions check, and visible photons are bond-sized. A popular answer on Reddit's AskScience says visible light "has energies of around 1-10 eV."¹⁵ That is wrong: 10 eV is a 124 nm photon, and one to ten electronvolts is roughly the range of bond energies.

Where the Sun peaks

A spectrum is a density, so its peak moves when the axis changes. Soffer and Lynch: "The peak brightness of the solar spectrum is in the green when plotted in wavelength units. It peaks in the near-infrared when plotted in frequency units."² The standard ASTM reference spectra give the numbers.¹⁶

Plotted as	Above the atmosphere	At the ground, direct beam
Power per nanometre	451 to 471 nm	531 to 545 nm
Power per unit frequency	933 nm	1,001 nm
Photons per nanometre	583 nm	669 nm

The ranges show raw and smoothed peaks. What survives any choice of axis is the share: the band from 380 to 750 nm holds 48.8% of the direct sunlight power reaching the ground and 31.7% of its photons, and 300 to 1,100 nm holds 78.9% of the power.⁶ Wald's 1959 figures were close.¹

Air

The ozone layer is the short wall. Wald: ozone "begins to absorb the sun's radiation strongly at 320 millimicrons, and at 290 millimicrons forms a virtually opaque screen."¹ The ASTM direct beam agrees: the atmosphere passes 0.1% at 300 nm, 15% at 320 nm, 50% at 400 nm, 82% at 700 nm and 93% at 1,000 nm, with water-vapour bands near 1,400 and 1,900 nm passing almost nothing.⁶ Air sets the eye's violet edge only for the shortest ultraviolet, and its red edge not at all.

Water

The eye is filled with water, which has a sharp window. In George Hale and Marvin Querry's measurements, absorption falls from 12,694 per centimetre at 2,950 nm to 0.000247 per centimetre at 475 nm, a range of nearly eight orders of magnitude.¹⁷ Robin Pope and Edward Fry put the minimum at 417.5 nm, where light travels 226 metres before losing 63% of its strength.¹⁸ Through 2.4 cm of water, about the length of an adult eyeball,¹⁹ the Segelstein data give 85% transmission at 200 nm, over 99% from 300 to 700 nm, 41% at 1,000 nm, 5% at 1,200 nm, and effectively zero beyond 1,400 nm.²⁰⁷

So water sets a long wall near 1,000 to 1,100 nm. It sets no short wall at all: pure water is clear into the ultraviolet.

The best octave

If an eye may use one octave of light, which octave should it use? For ground sunlight after 2.4 cm of water, the octave holding the most power is 450 to 900 nm, with 76.7% of the total; the octave holding the most photons is 465 to 930 nm, with 76.5%. The human octave, 380 to 760 nm, holds 68.1% of the power and 59.1% of the photons.²¹ The human band sits near the physical optimum but about 80 nm to the blue of it. At the red end, the pigment pulls it there.

Part 3: The red edge is noise in the pigment

Heat mimics a photon

Vision starts when a photon bends the retinal molecule inside a visual pigment into a new shape. Heat occasionally does the same, and a pigment that flips in the dark sends a false signal. Luo and colleagues: "Thermal energy can complement photon energy to drive photoisomerization, but it also triggers spontaneous pigment activation as noise that interferes with light detection."⁸ They found that the energy barrier falls as the pigment's peak moves to longer waves, as Horace Barlow had proposed in 1957.⁸ Assuming that heat and light must cross the same barrier, they put it at 48.03 kilocalories per mole for rod rhodopsin, which peaks at 500 nm. That is about 2.1 eV, the energy of a 595 nm photon.⁸

A pigment tuned further red has a lower barrier and more false alarms. Luo's group extrapolated their measurements into the infrared: the thermal rate constant for cone pigments "approaches an asymptote of ~1.1 × 10−4 s−1 in the infrared, or 360 times higher than that of the 620-nm A2 red cone pigment." In a salamander red cone, "the noise rate would be 29,700 s−1 cell−1, which would reduce the cell's already low sensitivity by another 26-fold."⁸ Their conclusion: these considerations "probably explain, at least partially, why the λmax values of native visual pigments are confined to the narrow bandwidth of ~360 to 620 nm."⁸ Soffer and Lynch named the same constraint: longer-wave pigments need "larger molecules with longer conjugated chains," and "such large molecules are unstable and subject to dissociation and bleaching by thermal processes at body temperature."²

The limit is steep but not absolute. Luo's paper says so: "at least in principle, λmax > 698 nm is still physically possible."⁸ The cost is noise, and noise matters most in dim light.

The body's glow comes later

A popular answer on AskScience says of infrared, "Your own body heat obscures it, if you're not careful."²² That is true only for the thermal infrared. A body at 37 °C glows most strongly near 9,300 nm. The number of its own thermal photons per mode is about 7 × 10⁻²¹ at 1,000 nm and 1 × 10⁻² at 10,000 nm.²³ Soffer and Lynch, following Michael Pirenne and Albert Rose: "Thermal excitations in the retina only begin to compete with any incoming photons at wavelengths longer than 1.4 μm at the absolute threshold of vision."² The glow explains why no warm-blooded eye could image at 10 micrometres. It does not explain the stop at 750 nm. Pigment noise does.

The retina can be pushed

Wald reported that "Specific investigations have pursued human vision to about 312 millimicrons in the near ultraviolet, and to about 1,050 in the near infrared."¹ In 2014 Grazyna Palczewska and colleagues showed that people see pulsed infrared lasers as colour at "slightly longer than half of the stimulating IR wavelength," because two infrared photons together deliver one visible photon's energy.²⁴ The pigment answers to a visible photon's energy.

The deep-sea dragonfish Malacosteus niger sees far-red light, but not with a far-red pigment. R. H. Douglas and colleagues found ordinary pigments peaking "around 520 and 540 nm," plus derivatives of bacteriochlorophyll "used as a photosensitiser to enhance its sensitivity to longwave radiation."²⁵ The fish borrows an antenna instead of building a noisy long-wave pigment.

Part 4: The violet edge is damage and blur, not ionisation

Near-ultraviolet does not ionise

The same answer says ultraviolet "starts to be ionizing radiation."²² For the ultraviolet that reaches the ground, it does not. Ionising a water molecule takes 12.621 eV, a photon of 98 nm.⁹ Guanine, the most easily ionised base of DNA, takes about 7.9 to 8.2 eV in the gas phase.²⁶ A 320 nm photon carries 3.87 eV and a 400 nm photon 3.10 eV.⁵ Shaun Doyle's review of Denton for Creation Ministries International repeats the error: "Higher frequency EMR, such as UV, X-rays, and gamma rays, strip electrons from atoms completely."²⁷ X-rays and gamma rays do. Near-ultraviolet does not.

It damages by chemistry

Ultraviolet harms tissue because its photons are just energetic enough to break bonds. Wald: "Radiations below 300 millimicrons (95 kilocalories per mole) are incompatible with the orderly existence of such large, highly organized molecules as proteins and nucleic acids."¹ Soffer and Lynch: "all organic molecules are susceptible to direct UV damage or indirect damage from UV-generated free radicals in their proximity."² Frederik van Kuijk's 1991 review found that retinal damage rises sharply below 510 nm, so that "the blue portion of the visible region (400-500 nm) is much more hazardous than the green and red portion (500-700 nm)."¹⁰ He reported one study in which the odds of age-related macular degeneration rose 200% among people whose lens had been removed, and another, of 838 Maryland watermen, that found no link to ultraviolet exposure.¹⁰

It blurs the image

A lens made of one material bends short waves more than long ones, so blue and ultraviolet come to focus in front of red. Wald saw the human yellow lens as a fix: "In default of color correction the lens seems to do the next best thing; it eliminates the short wavelengths of the spectrum for which the color error is greatest."¹ The human retina can respond to ultraviolet. People whose lens is removed and not replaced "report vivid and detailed vision in the UV."¹² The block is in the lens, and newborn lenses still pass ultraviolet.¹⁰

Part 5: Animals that see outside the band, and what it costs

Animal	Extra range	How	What it costs
Small songbirds, parrots	Ultraviolet to about 323 nm	UV pigment and clear eye media	Only small eyes pass UV; large-eyed raptors block it
Mice, hedgehogs, many nocturnal mammals	Ultraviolet A	Clear lens	Retinas without high-acuity zones
Insects	Ultraviolet	Compound eye with no image-forming lens	Low resolution
Mantis shrimp	300 to 720 nm	12 receptor types, UV filters made of sunscreen compounds	Poor wavelength discrimination
Dragonfish	Far red	Chlorophyll-derived antenna on ordinary pigments	Works in the dark deep sea
Pit vipers, pythons, boas, vampire bats	Thermal infrared	Heated membrane and heat-sensing nerve channels	Blurred, low-contrast image, range about a metre

Birds. Olle Lind and colleagues measured the eye media of 38 bird species. Ultraviolet-sensitive birds pass ultraviolet down to a half-transmission point of 323 nm, against 358 nm for violet-sensitive birds, and transmission falls as eyes get larger. "Therefore, only small birds gain the full advantage from shifting pigment sensitivity from VS to UVS."¹¹ Raptors block ultraviolet. The authors give two reasons: raptors "have the highest visual acuity of all animals and suffer the most from the strong lens aberrations at short wavelengths," and "the damaging effect of UV radiation on the retina may be strong in birds hunting on the wing in the open."¹¹ Lents's eagles and Lents's ultraviolet-seeing birds are different birds.

Mammals. R. H. Douglas and G. Jeffery measured lenses from 38 mammal species in 2014: "All species whose lenses removed short wavelengths had retinae specialized for high spatial resolution and relatively high cone numbers, suggesting that UV removal is primarily linked to increased acuity."¹² Blocking ultraviolet may also protect the retina, "especially in long-lived diurnal species"; grey squirrels whose lenses were removed suffered more retinal damage.¹² There are exceptions. Reindeer live up to 20 years in "an extremely UV-rich environment" with "no ill effects," and some ultraviolet-sensitive parrots "live to be over 50 years old with no apparent damage."¹² Humans are long-lived, active by day and built for acuity, the profile of the animals that block ultraviolet.

Insects. Wald explained why insects use ultraviolet freely. In a compound eye, "Projection by a lens plays no part in this system, and chromatic aberration is of no account."¹ Compound eyes pay for that freedom in resolution.

Mantis shrimp. Some species have "12 different photoreceptor types, each sampling a narrow set of wavelengths ranging from deep ultraviolet to far red (300 to 720 nanometers)," yet in behavioural tests they showed "a surprisingly poor performance" at telling wavelengths apart.²⁸ Their ultraviolet receptors share one pigment and are tuned by filters of mycosporine-like amino acids, compounds "usually employed for UV photoprotection."²⁹ The animal most often cited for seeing more colours still works inside 300 to 720 nm.

Heat-sensing snakes and bats. Elena Gracheva and colleagues found that pit vipers, pythons and boas "detect infrared signals through a mechanism involving radiant heating of the pit organ, rather than photochemical transduction," with prey detected "at distances up to 1 meter."³⁰ Vampire bats do the same with a heat channel tuned to about 30 °C.³¹ George Bakken and Aaron Krochmal modelled the pit viper's pit and found "the image on the membrane to be poorly focused and of very low temperature contrast."¹³ Luo's group suggests this is why snakes use a heat channel rather than a visual pigment.⁸ Infrared sensing in vertebrates is closer to touch at a distance than to sight.

Part 6: Is the band fine-tuned?

Denton's argument

Michael Denton, a biochemist and Discovery Institute senior fellow, made the band a centrepiece of Nature's Destiny in 1998. His chapter on light opens: the sun's radiation "is uniquely fit for carbon-based life," and "the energy levels within the visual spectrum are precisely fit for photochemistry."³² He argues that visible light "is the only type of electromagnetic radiation that has the appropriate energy level for detection by biological systems," that ultraviolet and shorter waves "are too energetic and are highly destructive, while infrared and radio waves are too weak to be detected," and that other bands would need enormous eyes.³²³² In his 2018 book Children of Light, as quoted by Doyle, he puts it sharply: "it is not that life adapted to the right light but that the right light is the only light that provides the correct energy levels for photochemistry."²⁷

Wald said it first

The physics in Denton's argument is Wald's, and Denton cites him. Wald wrote in 1959 that "Life depends on a narrow band in the electromagnetic spectrum. This is a consequence of the way in which molecules react to radiation, and must hold true not only on earth but elsewhere in the universe."¹ Wald, who shared the 1967 Nobel Prize in Physiology or Medicine, drew no theological conclusion from it. His physical claim and Denton's are the same claim. The evidence in Parts 2 to 5 mostly supports it.

Where Denton overstates

A unit error. Denton writes that the activation energies of biological reactions, "between 15 and 65 kilocalories per mole," are "provided by electromagnetic radiation between 0.80 microns and 0.32 microns, more or less exactly that provided by visible light."³² Wald, from whom the numbers come, converted the same range correctly: "between 1,900 and 440 millimicrons."¹ The match Denton states does not exist; the one Wald states for bond energies does.³³
A rhetorical ratio. Denton calls the band from 0.3 to 1.5 micrometres "approximately one part in 10^25.[ the entire electromagnetic spectrum."³² On a linear scale the band is about one part in a billion of the range he gives; on a logarithmic scale it is about 3%. Neither gives one part in 10^25.[note-denton-conversion]
Infrared is detectable. Snakes and vampire bats detect infrared by heating. Denton's point survives in a narrower form: infrared cannot drive a quiet photochemical pigment at body temperature.

What the band does and does not show

It shows a real constraint. Any camera eye made of carbon compounds and water, working at body temperature under a Sun-like star, faces pigment noise to the red, damage and blur to the violet, and water beyond 1,100 nm. No proposed build beats the human band on every axis: extending into the ultraviolet costs acuity and risks the retina, and extending into the red costs sensitivity. By the parent article's test, the complaint fails.

It does not show that the eye was designed. Evolution also predicts that eyes will use this band, because the same physics filters every lineage, and Wald predicted it for life anywhere. The fine-tuning question moves off the eye and onto physics: why the photons of the Sun, the windows of air and water, and the energies of chemical bonds line up. That depends partly on the kind of star. Treated as a rough blackbody, a 3,000 K red dwarf puts only 3.9% of its photons in the human band, against 26.6% for the Sun.³⁴ Denton's real argument lives at that cosmic level, and this article does not settle it.

Objections and responses

Birds see ultraviolet, so a better eye is possible. A different eye is possible. Lind's data show that ultraviolet transmission falls with eye size and that the highest-acuity birds block it.¹¹¹¹ Lents compares humans with a composite bird that has the eagle's acuity and the songbird's ultraviolet, and no such bird exists.
The eye catches only 19% of the sunlight from 320 to 1,400 nm, so it is poorly optimised. Partly right. The best-octave calculation agrees that the physical optimum lies redder than human vision.²¹ But the 19% counts light, not the noise cost of catching it, and Luo's measurements show that cost is steep for red-shifted pigments. Whether a 380 to 900 nm human eye would see better at dusk has not been calculated.
The Sun peaks in green and so does the eye, which is the fingerprint of evolution, or of design. Neither. The green peak is a choice of axis.² The robust fact is that the eye sits inside the window of air and water, near the best octave.
Ultraviolet is ionising, so of course eyes avoid it. Near-ultraviolet is not ionising.⁹ Its harm is photochemical, which makes a stronger point: the photons able to drive vision are the photons able to break bonds.
Snakes see in infrared, so infrared eyes are possible. Snakes sense heat, with a blurred, low-contrast image.¹³ No known visual pigment peaks in the infrared.⁸
The band is fine-tuned, so God designed the eye. The band is evidence of a constraint, not of a designer of eyes. Both creation and evolution predict eyes in this band. Any force the argument has lies in cosmic fine-tuning, which needs a separate case.
Before the Fall, human eyes may have seen more. Nothing in the evidence here points to it. A proposal of original design followed by loss needs evidence of the original condition, as the parent article says. None is offered.

Issues and unanswered questions

Nobody has priced the redder eye. Soffer and Lynch show a lot of unused light between 700 and 1,400 nm. Luo shows the noise cost of reaching for it. A calculation weighing the two for a human cone at dusk would settle whether the human red edge is optimal, and this article has not done it.
The protective role of ultraviolet-blocking lenses is not universal. Reindeer and some parrots take ultraviolet into the retina for decades without apparent harm.¹² The chromatic-blur reason is also "difficult to prove experimentally," in Douglas and Jeffery's words.¹² The acuity correlation is measured; the mechanism is partly inferred.
The evidence on ultraviolet and macular degeneration is mixed. One study van Kuijk cites found higher odds in people without a lens, and another found no link to ultraviolet exposure.¹⁰
Several sources were read only as abstracts. The mantis shrimp, dragonfish and pit-viper optics papers were paywalled, and only their abstracts are quoted. Denton's 2018 book was not read directly; its key sentence is quoted through Doyle.
Some source claims are historical inferences. Douglas and Jeffery's view that ultraviolet-sensitive pigments are ancestral, and Soffer and Lynch's history of vision evolving in water, are reconstructions. This article uses their measurements, not those histories.

Verdict

The narrow band of human vision is not a design flaw. Its red edge is set by noise in the pigment, its violet edge by photochemical damage and lens blur, and its outer walls by water and ozone. Every animal that sees outside it pays a measurable price. The complaint from bird vision fails, because ultraviolet vision and top acuity do not occur in the same eye. The complaint from physics succeeds in part: the human band sits blueward of the photon optimum, and nobody has shown whether that trade is the best available. The band is equally poor evidence that the eye was designed, because evolution predicts the same band. Denton's fitness argument is sound where it repeats Wald, and its real force, if any, lies in the physics of stars and molecules rather than in the eye.

Sources

1.
Wald, George. “Life and Light.” Scientific American 201(4):92–108. Oct 1959. Wald shared the 1967 Nobel Prize in Physiology or Medicine for work on visual pigments. Read through archive.org search-inside on a lending-restricted scan. Printed page = leaf − 1 from the scan's page map; checked against the article's first page, 92, and the text's reference to "illustration at top of pages 94 and 95". Mirror: Archive.org
1. 1a.
  Page 92: “Life depends on a narrow band in the electromagnetic spectrum. This is a consequence of the way in which molecules react to radiation, and must hold true not only on earth but elsewhere in the universe”. ↩
2. 1b.
  Page 92: “What sets this range ultimately is not its availability, but its suitability to perform the tasks demanded of it. There cannot be a planet on which photosynthesis or vision occurs in the far infrared or far ultraviolet, because these radiations are not appropriate to perform these functions.” ↩
3. 1c.
  Page 94: “reactions involve energies of activation between 15 and 65 kilogram calories (kilocalories) per mole. This is equivalent energetically to radiation of wavelengths between 1,900 and 440 millimicrons.” ↩
4. 1d.
  Page 94: “fall between 40 and 90 kilocalories per mole, corresponding to radiation of wavelengths 710 to 320 millimicrons.” “about 20 to 100 kilocalories per mole, corresponding to the absorption of light of wavelengths 1,430 to 280 millimicrons.” The first quotation concerns the energies needed to break single covalent bonds; the second, the excitation of valence electrons in photochemistry. ↩ ↩
5. 1e.
  Page 94: “Radiations below 300 millimicrons (95 kilocalories per mole) are incompatible with the orderly existence of such large, highly organized molecules as proteins and nucleic acids.” ↩
6. 1f.
  Page 94: “The commonly stated limit of human vision—400 to 700 millimicrons—already includes 41 per cent of the sun's radiant energy before it reaches our atmosphere, and 46 per cent of that arriving at the earth's surface. The entire photobiological range—300 to 1,100 millimicrons—includes about 75 per cent of the sun's radiant energy, and about 83 per cent of that reaching the earth.” ↩
7. 1g.
  Page 94: “begins to absorb the sun's radiation strongly at 320 millimicrons, and at 290 millimicrons forms a virtually opaque screen.” ↩
8. 1h.
  Page 102: “Specific investigations have pursued human vision to about 312 millimicrons in the near ultraviolet, and to about 1,050 in the near infrared.” ↩
9. 1i.
  Page 102: “Ordinarily our vision is excluded from the ultraviolet, not primarily because the retina or its visual pigments are insensitive to that portion of the spectrum, but because ultraviolet light is absorbed by the lens of the eye.” Page 103: “In default of color correction the lens seems to do the next best thing; it eliminates the short wavelengths of the spectrum for which the color error is greatest.” ↩
10. 1j.
  Page 103: “One group of animals, however, makes important use of the ultraviolet in vision. These are the insects.” “Projection by a lens plays no part in this system, and chromatic aberration is of no account.” ↩
2.
Soffer, Bernard H., and David K. Lynch. “Some paradoxes, errors, and resolutions concerning the spectral optimization of human vision.” American Journal of Physics 67(11):946–953. Nov 1999. Publisher's PDF posted on eScholarship (UCLA). PDF page n is printed page 944 + n, checked against the running footers. The text layer prints parentheses as ~ and !.
1. 2a.
  Page 946: “The peak brightness of the solar spectrum is in the green when plotted in wavelength units. It peaks in the near-infrared when plotted in frequency units. Therefore the oft-quoted notion that evolution led to an optimized eye whose sensitivity peaks where there is most available sunlight is misleading and erroneous.” “The eye does not appear to be optimized for detection of the available sunlight, including the surprisingly large amount of infrared radiation in the environment.” ↩ ↩ ↩ ↩
2. 2b.
  Page 950: “fraction of the available light between 320 nm (the atmospheric transmission cutoff) and 1400 nm (the thermodynamic noise limit) detected by the eye. It is only 19%, hardly optimal in a spectral sense.” ↩ ↩
3. 2c.
  Page 951: “could we be mired in some evolutionary backwater, where once we were indeed better adapted?” ↩
4. 2d.
  Page 951: “Thermal excitations in the retina only begin to compete with any incoming photons at wavelengths longer than 1.4 μm at the absolute threshold of vision.” “Pirenne has suggested that strong infrared sensitivity would be undesirable because the infrared in the eyeball, the body's own heat radiation, would cause the external world to be obscured by a luminous fog of this radiation. This is precisely why we do not expect to see beyond 1.4 microns.” The text layer prints 1.4 μm as "1.4 mm". The authors cite Albert Rose, Vision Human and Electronic (1973), p. 50, and M. H. Pirenne, J. Physiol. 106 (1947). ↩ ↩
5. 2e.
  Page 951: “To get sensitivities further in the infrared via photoisomerization, larger molecules with longer conjugated chains, which would have their first electronic excited states at lower energies, would be needed. But such large molecules are unstable and subject to dissociation and bleaching by thermal processes at body temperature”. ↩
6. 2f.
  Page 951: “At the short wavelength side, all organic molecules are susceptible to direct UV damage or indirect damage from UV-generated free radicals in their proximity.” ↩
7. 2g.
  Page 952: “the eye is only weakly optimized to take full advantage of the available solar spectrum. The erroneous belief often arises from a blind faith in the power of evolution to optimize absolutely”. ↩
3.
Dawkins, Richard. The God Delusion. Houghton Mifflin. 2006. Read from the source library PDF. The passage sits on the page headed "362 THE GOD DELUSION".
1. 3a.
  Page 362: “Our eyes see the world through a narrow slit in the electromagnetic spectrum. Visible light is a chink of brightness in the vast dark spectrum, from radio waves at the long end to gamma rays at the short end.” ↩ ↩
4.
Lents, Nathan H. “The Poor Design of the Human Eye.” The Human Evolution Blog. Jan 12, 2015. Retrieved Sep 27, 2026. The page's author box describes Lents as Professor of Biology at John Jay College of The City University of New York. Saved in the source library.
1. 4a.
  Paragraph Compare this to the excellent vision of most birds: “Compare this to the excellent vision of most birds, especially birds of prey, such as eagles and condors. Their visual acuity at great distances puts even the best human eyes to shame. Many birds can see a broader range of wavelengths than we can also, including ultraviolet light.” ↩
2. 4b.
  Paragraph The superiority of the bird eye: “The superiority of the bird eye shows that whatever designed the human eye, be it nature or a deity, is capable of producing eyes that are much better than the human eye. The question of why nature didn't provide humans with better eyes is easily answered by evolutionary theory: it wasn't strongly selected for. Alternatively, why an intelligent designer would deny his favorite creatures the excellent vision that he provided lowly birds is quite a mystery.” ↩ ↩
5.
Photon energy E = hc/λ. With hc = 1,239.84 eV·nm, 380 nm gives 3.26 eV, 400 nm 3.10 eV, 500 nm 2.48 eV, 700 nm 1.77 eV, 750 nm 1.65 eV, 1,000 nm 1.24 eV, and 124 nm gives 10.0 eV. One kilocalorie per mole is 0.04336 eV. Computed in research/solar-spectrum.ts. ↩ ↩ ↩
6.
Computed from the ASTM G173-03 reference spectra¹⁶ with research/solar-spectrum.ts (Deno). Peaks are taken on the raw 0.5 to 5 nm grid and after a 21-point running mean. Band shares are trapezoid integrals over 280 to 4,000 nm. Direct-beam transmission is the direct-normal column divided by the extraterrestrial column. "Per unit frequency" multiplies each value by λ²; "photons per nanometre" multiplies by λ. ↩ ↩ ↩
7.
Transmission through 2.4 cm is exp(−2.4α), with α the absorption coefficient in the Segelstein 1981 table, interpolated on log scales.²⁰ The window above 50% runs from 190 nm to about 1,110 nm, with a dip near 970 nm. Computed in research/solar-spectrum.ts. ↩ ↩
8.
Luo, Dong-Gen et al. “Activation of visual pigments by light and heat.” Science 332(6035):1307–1312. 2011. Read as the PMC author manuscript, which has no printed page numbers; places are given by paragraph. Mirror: PMC
1. 8a.
  Section Abstract: “Thermal energy can complement photon energy to drive photoisomerization, but it also triggers spontaneous pigment activation as noise that interferes with light detection.” “The predicted high noise for pigments with λmax in the infrared presumably explains why they apparently do not exist in nature.” ↩ ↩
2. 8b.
  Paragraph Our visual system has an extremely high sensitivity: “More than 50 years ago, Barlow (8) proposed that pigments with longer peak-absorption wavelengths (λmax) are noisier and thus less suitable for dim-light detection.” Reference 8 is H. B. Barlow, Nature 179:255 (1957). ↩
3. 8c.
  Paragraph We began calculations with α = 1: “We began calculations with α = 1; thus, EaT=EaP=0.84hc/λmax. For rhodopsin with λmax = 500 nm, we obtain EaT=48.03 kcal mol−1.” α = 1 is the paper's assumption that thermal and photic activation cross the same barrier. 48.03 kcal/mol is 2.08 eV, the energy of a 595 nm photon. ↩
4. 8d.
  Paragraph The visual pigment with the longest λmax known so far: “The rate constant for cone pigments approaches an asymptote of ~1.1 × 10−4 s−1 in the infrared, or 360 times higher than that of the 620-nm A2 red cone pigment.” “the noise rate would be 29,700 s−1 cell−1, which would reduce the cell's already low sensitivity by another 26-fold”. “Perhaps for this reason, the viper pit organ detects infrared radiation with a heat-sensing ion channel rather than a visual pigment”. ↩ ↩
5. 8e.
  Paragraph The visual pigment with the longest λmax known so far: “The visual pigment with the longest λmax known so far in nature is the A2 red cone pigment (λmax ~ 620 nm).” “These considerations probably explain, at least partially, why the λmax values of native visual pigments are confined to the narrow bandwidth of ~360 to 620 nm, limiting color vision accordingly.” ↩ ↩
6. 8f.
  Paragraph The visual pigment with the longest λmax known so far: “Thus, at least in principle, λmax > 698 nm is still physically possible.” ↩
9.
National Institute of Standards and Technology. “Water: gas phase ion energetics data.” NIST Chemistry WebBook, Standard Reference Database 69. n.d. Retrieved Sep 27, 2026.
1. 9a.
  Table Gas phase ion energetics data: “IE (evaluated) 12.621 ± 0.002 eV”. 12.621 eV is the energy of a 98 nm photon. ↩ ↩ ↩
10.
van Kuijk, Frederik J. G. M. “Effects of ultraviolet light on the eye: role of protective glasses.” Environmental Health Perspectives 96:177–184. 1991. Read from the PMC scan. Soffer and Lynch cite it for ultraviolet damage in people without a lens. Mirror: PMC
1. 10a.
  Page 179: “Light below this wavelength has a photon energy of > 2.43 eV, and a sudden rise occurs in the amount of damage produced in the retina as a function of exposure to light of different wavelengths”. “Therefore, the blue portion of the visible region (400-500 nm) is much more hazardous than the green and red portion (500-700 nm).” "This wavelength" is 510 nm, from the preceding sentence. ↩
2. 10b.
  Page 179: “Lerman reported that after birth, nearly all of the UV light is transmitted by the lens (77). During childhood, lens transmittance decreases, and by the age of 25, the lens absorbs UV light almost completely”. ↩ ↩
3. 10c.
  Page 179: “They found that the odds for having ARMD may be increased by 50% among persons with cataracts, 80% in persons with opacities that do not decrease visual acuity, and 200% among aphakic persons (80).” “West et al. found no association between ARMD and cumulative exposure to either UV-A or UV-B light (81).” ARMD is age-related macular degeneration; aphakic persons have had the lens removed. The West study was of 838 Maryland watermen. ↩ ↩
11.
Lind, Olle et al. “Ultraviolet vision in birds: the importance of transparent eye media.” Proceedings of the Royal Society B 281(1774):20132209. 2014. CC BY 3.0. Read as Europe PMC full text; places are section headings. Mirror: PMC
1. 11a.
  Section Abstract: “Birds with UVS pigments have ocular media that transmit more UV light (wavelength of 50% transmittance, λT0.5, 323 nm) than birds with VS pigments (λT0.5, 358 nm).” “Therefore, only small birds gain the full advantage from shifting pigment sensitivity from VS to UVS.” ↩
2. 11b.
  Section Eye size and the variation of ocular media transmittance in lens eyes: “Our data (figure 5) are, to our knowledge, the first to support the hypothesis that bird sensitivity to short UV wavelengths is constrained by the size of the eye [7].” ↩
3. 11c.
  Section Raptors and other birds with low ultraviolet transmittance: “First, raptors have the highest visual acuity of all animals [48] and suffer the most from the strong lens aberrations at short wavelengths. Specifically, chromatic aberration may deteriorate the image for the short wavelength sensitive cone too much if short wavelength light is transmitted. Second, the damaging effect of UV radiation on the retina [46] may be strong in birds hunting on the wing in the open.” ↩ ↩ ↩
12.
Douglas, R. H., and G. Jeffery. “The spectral transmission of ocular media suggests ultraviolet sensitivity is widespread among mammals.” Proceedings of the Royal Society B 281(1780):20132995. 2014. CC BY 3.0. Read as Europe PMC full text; places are section headings. Mirror: PMC
1. 12a.
  Section Abstract: “All species whose lenses removed short wavelengths had retinae specialized for high spatial resolution and relatively high cone numbers, suggesting that UV removal is primarily linked to increased acuity.” ↩ ↩
2. 12b.
  Section Introduction: “if the UV-absorbing lens of humans is removed following cataract surgery or traumatic injury, and not replaced by a UV-absorbing prosthetic, they report vivid and detailed vision in the UV”. ↩
3. 12c.
  Section What is the function of preventing ultraviolet radiation…: “Removing short wavelengths, especially in long-lived diurnal species, could protect the retina as the degree of retinal light damage is considerably increased at shorter wavelengths”. “when the UV-absorbing lenses of grey squirrels were removed, the retinae of these eyes suffered more retinal damage than intact companion eyes”. ↩
4. 12d.
  Section What is the function of preventing ultraviolet radiation…: “The reindeer, for example, lives in an extremely UV-rich environment and can reach ages of up to 20 years, yet it seems to suffer no ill effects from allowing UV to reach the retina”. “some UV-sensitive parrots can live to be over 50 years old with no apparent damage”. ↩ ↩
5. 12e.
  Section What is the function of preventing ultraviolet radiation…: “Short-wave absorbing filters will also increase image quality as both the degree of Rayleigh scatter and chromatic aberration are increased in this part of the spectrum [52], although such a function is difficult to prove experimentally.” ↩
13.
Bakken, George S., and Aaron R. Krochmal. “The imaging properties and sensitivity of the facial pits of pitvipers as determined by optical and heat-transfer analysis.” Journal of Experimental Biology 210(16):2801–2810. 2007. Abstract only; the publisher's page refused the download.
1. 13a.
  Section Abstract: “We find the image on the membrane to be poorly focused and of very low temperature contrast.” “To account for observed behavioral capabilities, the sensory endings on the pit membrane apparently must respond to temperature contrasts of 0.001 degrees C or less.” ↩ ↩ ↩
14.
The test is set out in Arguments from Bad Design, Part 2: state the task and the cost, compare a feasible alternative, and price the tradeoff. ↩
15.
Astrokiwi. “Comment on "Is there anything special or discerning about 'visible light' other then the fact that we can see it?".” Reddit, r/askscience. Oct 30, 2013. Retrieved Sep 27, 2026. Anonymous Reddit user. Reddit refused the download; the text was retrieved from the PullPush archive of Reddit. Linked from Eric's research notes, which is how the figure reached the article's catalogue entry.
1. 15a.
  Paragraph 3: “Our eyes detect light with chemical reactions. So the light photons need to have a similar energy to the range of energies used in chemical reactions, and visible light has energies of around 1-10 eV, which is just right.” ↩
16.
ASTM International. ASTM G173-03, Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37° Tilted Surface. 2003. Spectra computed by Christian Gueymard with SMARTS 2.9.2: extraterrestrial, global tilt and direct normal, 280 to 4,000 nm, air mass 1.5. The NREL page timed out; the identical table was taken from the pvlib repository. Mirror: Raw.githubusercontent.com ↩ ↩ ↩
17.
Hale, George M., and Marvin R. Querry. “Optical constants of water in the 200-nm to 200-μm wavelength region.” Applied Optics 12(3):555–563. 1973. Read as the absorption-coefficient table compiled by Scott Prahl. Minimum 0.000247 per cm at 475 nm; maximum 12,694 per cm at 2,950 nm. Mirror: Omlc.org ↩
18.
Pope, Robin M., and Edward S. Fry. “Absorption spectrum (380–700 nm) of pure water. II. Integrating cavity measurements.” Applied Optics 36(33):8710–8723. 1997. Read as the table compiled by Scott Prahl. Minimum 0.0000442 per cm at 417.5 nm, a 1/e length of 226 m. Mirror: Omlc.org ↩
19.
Gray, Henry. Anatomy of the Human Body. Lea & Febiger. 20th edition. Edited by Warren H. Lewis. 1918. Public domain. Page anchored between the running heads "1001" and "1002 ORGANS OF THE SENSES AND THE COMMON INTEGUMENT" in the source library's OCR.
1. 19a.
  Page 1001: “The bulb measures rather more in its transverse and antero-posterior diameters than in its vertical diameter, the former amounting to about 24 mm., the latter to about 23.5 mm.” ↩
20.
Segelstein, David J. The complex refractive index of water. University of Missouri-Kansas City. 1981. Master's thesis, read as the absorption table (10 nm to 10 m) compiled by Scott Prahl. Absorption through 2.4 cm: 200 nm 85%, 300 nm 99.6%, 1,000 nm 41%, 1,200 nm 5%, 1,450 nm about 10^-33. Mirror: Omlc.org ↩ ↩ ↩
21.
For each lower bound x from 280 to 2,000 nm in 5 nm steps, the ASTM direct-normal ground spectrum,¹⁶ multiplied by the 2.4 cm water transmission,²⁰ was integrated from x to 2x, in power and in photons, and compared with the total from 280 to 4,000 nm. Without water, the best octave for power is 455 to 910 nm (57.5%) and for photons 655 to 1,310 nm (49.7%). Computed in research/solar-spectrum.ts. ↩ ↩
22.
RainHappens. “Comment on "Why can most animals only see a very small band of the electromagnetic spectrum?".” Reddit, r/AskScienceDiscussion. Sep 6, 2016. Retrieved Sep 27, 2026. Anonymous Reddit user. Retrieved from the PullPush archive of Reddit. Linked from Eric's research notes.
1. 22a.
  Paragraph 2: “As you go into the UV it becomes increasingly difficult to build high-resolution detectors (partially because it starts to be ionizing radiation, partially because there isn't too much UV / EUV / etc out there to begin with, and partially because UV-transparent materials are relatively hard to come by).” ↩
2. 22b.
  Paragraph 4: “But it's pretty hard to build a proper decent-resolution IR detector, for one. Your own body heat obscures it, if you're not careful.” ↩
23.
The mean number of thermal photons per mode is 1/(e^(E/kT) − 1), with kT = 0.0267 eV at 310 K. This gives 5 × 10⁻⁴¹ at 500 nm, 7 × 10⁻²¹ at 1,000 nm, 2 × 10⁻⁷ at 3,000 nm and 1 × 10⁻² at 10,000 nm. A 310 K blackbody peaks at 9,350 nm per unit wavelength. Computed in research/solar-spectrum.ts. ↩
24.
Palczewska, Grazyna et al. “Human infrared vision is triggered by two-photon chromophore isomerization.” Proceedings of the National Academy of Sciences 111(50):E5445–E5454. 2014. Mirror: PMC
1. 24a.
  Section Significance: “humans can detect IR at wavelengths longer than 1,000 nm and perceive it as visible light”. Section Discussion: “the perceived wavelength was slightly longer than half of the stimulating IR wavelength.” ↩
25.
Douglas, R. H. et al. “Enhanced retinal longwave sensitivity using a chlorophyll-derived photosensitiser in Malacosteus niger, a deep-sea dragon fish with far red bioluminescence.” Vision Research 39(17):2817–2832. 1999. Abstract only; the full text was not read.
1. 25a.
  Section Abstract: “has two visual pigments within its retina which form a rhodopsin/porphyropsin pigment pair with lambda max values around 520 and 540 nm”. “a mixture of defarnesylated and demetallated derivatives of bacteriochlorophylls c and d, that are used as a photosensitiser to enhance its sensitivity to longwave radiation.” ↩
26.
National Institute of Standards and Technology. “Guanine: gas phase ion energetics data.” NIST Chemistry WebBook, Standard Reference Database 69. n.d. Retrieved Sep 27, 2026.
1. 26a.
  Table Ionization energy determinations: Gas-phase values listed: 7.85 eV (photoelectron, Dougherty and others, 1978), 8.0 ± 0.2 eV (electron impact, Verkin and others, 1976), 8.24 ± 0.03 eV (vertical, Hush and Cheung, 1975). ↩
27.
Doyle, Shaun. “Fine-tuned light.” Journal of Creation 38(1). 2024. Retrieved Sep 27, 2026. A review of Michael Denton, Children of Light (Discovery Institute Press, 2018), published by Creation Ministries International, a young-earth organisation. The book itself was not read.
1. 27a.
  Section The light of life: “Higher frequency EMR, such as UV, X-rays, and gamma rays, strip electrons from atoms completely and denature biological molecules.” ↩
2. 27b.
  Section The light of life: “… it is not that life adapted to the right light but that the right light is the only light that provides the correct energy levels for photochemistry [emphasis original].” Doyle quoting Denton's Children of Light; no page is given. ↩
28.
Thoen, Hanne H. et al. “A different form of color vision in mantis shrimp.” Science 343(6169):411–413. 2014. Abstract only; the full text is paywalled and was not read.
1. 28a.
  Section Abstract: “some of which have 12 different photoreceptor types, each sampling a narrow set of wavelengths ranging from deep ultraviolet to far red (300 to 720 nanometers).” “Behavioral wavelength discrimination tests (Δλ functions) in stomatopods revealed a surprisingly poor performance”. ↩
29.
Bok, Michael J. et al. “Biological sunscreens tune polychromatic ultraviolet vision in mantis shrimp.” Current Biology 24(14):1636–1642. 2014. Abstract only.
1. 29a.
  Section Abstract: “Surprisingly, five of the six UV receptor types contain the same visual pigment.” “These filters are composed of various mycosporine-like amino acid (MAA) pigments. Commonly referred to as "nature's sunscreens," MAAs are usually employed for UV photoprotection”. ↩
30.
Gracheva, Elena O. et al. “Molecular basis of infrared detection by snakes.” Nature 464(7291):1006–1011. 2010. Read as the PMC author manuscript. Mirror: PMC
1. 30a.
  Section Abstract: “Thus, snakes detect infrared signals through a mechanism involving radiant heating of the pit organ, rather than photochemical transduction.” Paragraph 1: “Biophysical studies suggest that this system is exquisitely sensitive, such that vipers can detect prey at distances up to 1 meter.” ↩
31.
Gracheva, Elena O. et al. “Ganglion-specific splicing of TRPV1 underlies infrared sensation in vampire bats.” Nature 476(7358):88–91. 2011. Abstract only.
1. 31a.
  Section Abstract: “Among vertebrates, only vampire bats, boas, pythons and pit vipers are capable of detecting infrared radiation.” “Here we show that vampire bats tune a channel that is already heat-sensitive, TRPV1, by lowering its thermal activation threshold to about 30 °C.” ↩
32.
Denton, Michael J. Nature's Destiny: How the Laws of Biology Reveal Purpose in the Universe. Free Press. 1998. Chapter 3, "The Fitness of the Light." Read through archive.org search-inside on a lending-restricted scan. Printed page = leaf − 24 from the scan's page map, confirmed by the running head "THE FITNESS OF THE LIGHT • 61" on leaf 85. Mirror: Archive.org
1. 32a.
  Page 47: “In which it is shown that the electromagnetic radiation reaching the surface of the earth is uniquely fit for carbon-based life.” “the energy levels within the visual spectrum are precisely fit for photochemistry.” “Like water, the light of the sun appears to be of optimal biological utility.” ↩
2. 32b.
  Page 51: “70 percent of the electromagnetic radiation emitted from the surface of the sun is concentrated in an exceedingly narrow radiation band extending from the near ultraviolet (0.3 microns) through the visible light range into the near infrared (1.50 microns). This minute band represents the unimaginably small fraction of approximately one part in 10^25 of the entire electromagnetic spectrum”. The scan's text layer prints the exponent as "1025"; the caption on page 48 gives the range of wavelengths as 10^25. ↩
3. 32c.
  Page 52: “For the great majority of chemical reactions which occur in living things, the activation energy of the reactions (the amount of energy needed to cause a chemical reaction between two molecules) generally lies between 15 and 65 kilocalories per mole.” “These energy levels are provided by electromagnetic radiation between 0.80 microns and 0.32 microns, more or less exactly that provided by visible light.” ↩
4. 32d.
  Pages 61–62: “Light radiation is the only type of electromagnetic radiation that has the appropriate energy level for detection by biological systems. UV, X ray, and gamma rays are too energetic and are highly destructive, while infrared and radio waves are too weak to be detected because they impart so little in energy interacting with matter.” ↩
5. 32e.
  Page 67: “To obtain the same resolving power as that of the human eye with, say, radio waves with a wavelength of 100 centimeters would require a reflector disc 10 kilometers in diameter.” ↩
33.
15 kcal/mol is 0.65 eV, a 1,907 nm photon; 65 kcal/mol is 2.82 eV, a 440 nm photon. The band 320 to 800 nm corresponds to 36 to 89 kcal/mol. Denton's range of 10^-16 to 10^9 microns spans 25 decades; 0.3 to 1.5 microns spans 0.7 decades, about 3% on a log scale, and 1.2 microns is about 10^-9 of 10^9 microns on a linear one. ↩
34.
Planck blackbody curves integrated from 10 nm to 200 µm in research/solar-spectrum.ts. 3,000 K: peak 966 nm, 10.9% of power and 3.9% of photons in 380 to 750 nm. 4,500 K: 32.8% and 16.1%. 5,772 K: 43.9% and 26.6%. 10,000 K: 39.2% and 38.5%. For scale only. ↩