Why Do a Bright Room and a Sunny Yard Feel About Equally Bright?
― Your Eyes Squeeze a 200-Fold Gap Down to About 6-Fold
A sunny midday outdoors and a room with the lights on both feel simply "normally bright". Yet a light meter says they differ about 200-fold. The gap is not missing. Your eyes are shrinking it before you see it.
It is daytime and you are reading a book indoors. The lights are on, and you have no complaints about the brightness.
You walk out the front door and stand in the sun. It is dazzling, but after a squint you soon get used to it.
To you, it feels at most "a few times brighter". In reality, the room and the sunny spot are about 200 times apart.
Two things make the gap seem to vanish
The opening at the centre of the dark part of your eye, the pupil, shrinks in bright places and widens in dark ones. It changes about 4-fold in diameter, which is roughly 16-fold in the amount of light let in, all on its own.
But that only covers 16-fold. The rest is handled deeper in the eye and by the brain. Even when the light gets 200 times stronger, the brightness you feel rises only about 6-fold. That is how the squeezing works.
Because both work at once, we experience everything from a dim room to a dazzling yard as one continuous, ordinary world. It is a handy system, but it makes us very poor at guessing how bright things really are.
Real brightness differs by orders of magnitude
Brightness of light is measured in a unit called the lux. A clear sunny day outdoors is about 100,000 lux, and even a cloudy day is about 10,000. An office desk is around 500, and a home living room is said to be 100 to 200. At night under a full moon it is about 0.25.
The top and bottom of that list are about 400,000 times apart. On an ordinary scale the low end would be squashed flat and could not be drawn at all. So in Figure 1, each step along the scale means 10 times more.
This poor judgement causes trouble around us
Having our eyes shrink the gap makes life easier, but it also makes us misjudge whether there is enough light. The trouble shows up mostly in three places.
The first is a houseplant by the window that somehow loses its vigour. It looks bright enough to us, but the light the plant receives is only a few tens of times weaker than outdoors. The second is photography. Photos taken indoors come out blurred or dark because the real light is much dimmer than your eyes make it seem.
The third is the body clock. Morning light is said to need a certain brightness to set your body's rhythm. Indoor lighting often falls short of it. Though both are "bright", it is better to think of them as different things for your body.
Even on a gloomy overcast day, outdoors is about 10,000 lux. That is about 20 times a well-lit room. You might feel "it's dark today, so going out makes no difference", but for getting light on you, stepping outside beats sitting by a window by a wide margin.
If what we felt matched real brightness exactly, then the moment you stepped outside, every step and shade of lettering you could see indoors would wash out to plain white. Squeezing the gap is thought to be a trick that lets us pick out the shapes of things at any time, in a natural world where conditions change by orders of magnitude.
Summary
A room and a sunny spot differ about 200-fold in brightness. The opening of the pupil evens out about 16-fold of that, and the rest is handled by compressing the sensation itself. That is how we live without much noticing the gap. It is handy, but it makes our judgement of whether there is enough light rather unreliable.
Your eye is not a tool for measuring brightness.
It is a tool for shrinking gaps, so that you can see shapes even in a world that changes by orders of magnitude.
How long the eyes take to adjust when you move toward the dark is covered in Why Can't You See Anything for a While When You Go from a Bright Place into a Dark Room?, and the surprisingly strong light of a cloudy day is covered in Why Can You Get Sunburned Even on a Cloudy Day?
- Install an app on your smartphone that shows brightness in lux. Apps that use the camera or the light sensor work as a rough guide.
- First measure on your desk in the room and remember the number. Then guess "how many times brighter will it be outside?", step out the front door and measure in the sun.
- Compare your guess with the result. Many people answer within 10-fold, but the real figure should be over 100-fold. Try it on a cloudy day, 1 m from a window, and in a room at night too, and you will see clearly how the gap shrinks.
Please do not point the camera straight at the sun to measure. The number will not be right, and it is bad for both your eyes and the device.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled by level, from middle-school science to university specialist courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Biology"
- High school+Advanced high-school content, or textbook sidebars
- UniversityUniversity specialist content (vision science, psychophysics) not taught in high school
- ResearchNot taught even at university as settled fact; researchers are still investigating it
Middle schoolTerms: this phenomenon has names
- Illuminance: how much light reaches a surface. Its unit is the lux. It is the number this article has been calling "real brightness".
- Adaptation: the eye's sensitivity resetting to suit the surrounding brightness. It includes both the change in pupil opening and the change in sensitivity of cells deep in the eye.
- Logarithmic scale: a way of lining things up so each step means 10 times more. It is used to fit numbers that differ by orders of magnitude into one picture. The top band of Figure 1 is one.
Middle schoolHigh schoolChecking with a formula: how 200-fold becomes 6-fold
We set up a formula linking felt brightness to real brightness, then put numbers into it and follow along.
| In symbols | S ＝ k × E0.33 |
| In words | Felt brightness ＝ constant × cube root of illuminance |
| Where it comes from | Stevens' power law, which links stimulus strength to sensation. It is an empirical rule from experiments in which many people were asked to rate brightness, and the exponent for brightness is said to be about 0.33 to 0.5. |
| Meaning of the symbols | S is felt brightness (a relative value with no unit), E is illuminance (in lux), k is a constant of proportionality |
| Illuminance outdoors on a clear day | about 100000 lux |
| Illuminance in a well-lit room (on an office desk) | about 500 lux |
| Diameter of the pupil | about 2 mm in bright places, about 8 mm in dark places |
| The number that gives about 200 when cubed | said to be about 5.8 |
| Ratio of real brightness, outdoors to indoors | 100000 ÷ 500 ＝ 200 |
| Ratio of pupil diameters | 8 ÷ 2 ＝ 4 |
| Ratio of light let in (set by the area of the opening) | 4 × 4 ＝ 16 |
| Take the felt ratio as 5.8 and cube it to check | 5.8 × 5.8 ＝ 33.64 |
| Multiply once more | 33.64 × 5.8 ≒ 195 |
The final 195 came out almost the same as the 200 we started with. So the estimate that "the feeling is about 6-fold" fits the real 200-fold gap through the cube-root formula. And the pupil can only earn 16-fold. Most of the 200-fold must therefore be squeezed by the parts of the eye behind the pupil and beyond.
High schoolHigh school+The pupil alone is nowhere near enough
High schoolThe pupil is opened and closed by a ring-shaped muscle called the iris. It shrinks in the light and opens in the dark, as a reflex that happens regardless of your will. It is covered in Basic Biology as an example of how nerves work.
High school+The range of brightness people experience, from starlight to a snowfield at noon, is said to span about 10 billion-fold. The 16-fold the pupil can earn is very small against that range. The rest is covered by the sensitivity of the light-receiving cells themselves changing over time, and by the way the circuits are built to report the "ratio to the surroundings" rather than absolute light and dark.
UniversityWhy can sensation be written as a power?
The idea of expressing the link between stimulus strength and sensation as a power is called Stevens' power law. Separately there is the Weber–Fechner law, which expresses sensation as a logarithm, and which fits better depends on the type of sense. The exponent for brightness is said to be about 0.33 to 0.5, and this article treated it as a cube root. But the value shifts with how it is measured. The same person gives different values for a small spot of light seen in darkness and for a wide surface seen in a bright room. It is practical to think of laws of sensation as approximations for particular conditions.
📖 The form of the formulas, and further reading: Japanese Wikipedia, "Psychophysics" (explains the power law and the logarithmic law side by side)
ResearchWhat is still not clear
- Where to draw the line on brightness for the body clock It is widely accepted that morning light sets the body's rhythm. But how bright, and for how many minutes, is enough varies with age and with how the previous evening was spent, and no single standard has been settled.
- Outdoor light and short-sightedness Reports from many countries say that children who spend more time outdoors are less likely to become short-sighted. Whether the brightness itself, looking into the distance, or the colour content of the light is what matters is still debated.
- Artificial light at night How far lighting at night affects sleep and health varies widely between studies, and the link between amount and effect is still being sorted out.
In other words, the content of this article is also "an explanation within what is known today". Please treat the illuminance figures as rough guides that vary widely with place and time of measurement.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: properties of light / stimulus and response | The unit of brightness, and how the eye receives light |
| High school | Basic Biology: nerves and regulation of the body | The pupil opening changing by reflex |
| High school+ | Maths: logarithms / Physics: photometry | Figure 1, lining up quantities that differ by orders of magnitude on a log scale |
| University | Vision science, psychophysics | Stevens' power law |
| Research | Chronobiology, ophthalmology | Brightness needed for the body clock, outdoor light and short-sightedness |
| ― | Links to daily life | Indoor plants, failed photos, the benefit of going outside in the morning |
- Japanese Industrial Standards (日本産業規格) JIS Z 9110, "General rules of recommended lighting levels" ― a standard setting recommended illuminance for offices, homes and other places
- S. S. Stevens, "On the psychophysical law", Psychological Review (1957) ― the original source of the idea of expressing stimulus strength and sensation as a power
- Illuminating Engineering Institute of Japan (照明学会), ed., Lighting Handbook (『照明ハンドブック』) ― measured indoor and outdoor illuminance values, and an explanation of eye adaptation
- Tadashi Oyama et al. (大山正ほか), An Invitation to Visual Psychology (『視覚心理学への招待』) ― an introductory book on the perception of brightness and adaptation
※This article is a popular-science explanation for general readers. The figures given are rough guides to help you understand how things work. Illuminance varies widely with place, time and weather. If you have concerns about light and your eyes or health, please consult a specialist such as an eye doctor.