Light is a form of energy. The transmission of this energy takes place as radiation. Light is only a small part of the spectrum of all radiation, ranging from short-wave cosmic rays to human-generated electrical energy.

Earth’s atmosphere transmits the optical radiation range of light and that ranges from λ = 100 nm (ultraviolet) to λ = 1 mm (infrared) wavelengths.

The range of light visible to us humans (rainbow) is between λ = 380 and 780 nm wavelength, where we see best at λ = 555 nm.

rainbow spectrum

Colored light shows only portions of the spectrum, white light contains many or all portions from the spectrum.

Sodium vapor lamp spectrum © Netzwerk Licht

The spectrum of a sodium vapor lamp appears yellowish-orange to our eye. Color temperature 2’000K.

LED 2200K © Network Light

The spectrum of this LED lamp appears white to our eye with a hint of light yellow. color temperature 2’200K. By definition, warm white means ≈ 3’000K. This lamp is warmer.

The spectrum of a cold-white LED lamp appears white to our eye with a dazzling or glaring perceived tone into blue. Color temperature 5’200K.

The radiation waves of incandescent light sources oscillate in all directions. When light oscillates in only one plane, it is completely polarized. Polarized light is created by interaction with surfaces on which it reflects or scatters. Thus there is a natural polarization of reflected light on water surfaces and the scattered blue light in the sky. This is used by some animals (z.B. honey bees) for orientation. And photographers use a polarizing filter to suppress polarized light so that the contrasts of the sky or shiny surfaces appear more beautiful.

Wave or particle?

Not all properties of light can be explained by radiation waves. Therefore there is another conception of light as particles. It is emitted in quanta (a quantum of light is a packet of light of a certain frequency, resp. energy) and is called photon. Light particles, i.e. the photons themselves, are invisible. Photons are only made visible by interaction with a receiver (retina, sensor chip, film, photographic paper, matter, gas). That is, we never see light in a vacuum, and in air only when it scatters off nitrogen or water and hits the retina in the eye.

Photons fly straight on until they hit an obstacle and interact with something else. Light rays are straight and not curved. According to Albert Einstein there is one exception: In the presence of large masses (z.B. of our sun), photons are deflected by gravity. They can be assigned a relativistic mass, even though they have no rest mass.

Blue light has a low wavelength and a high frequency.
red light has a long wavelength and a low frequency.
Blue light has therefore a higher energy than red light.

How to measure and compare light?

If you want to measure light, you basically have to capture and count photons. Every digital camera does nothing else in the sensor chip (mostly CCD = Charged-Coupled Device). This works similar to a solar cell. Instead of turning light directly into electricity, however, it stores the electrons and can later read them out per pixel. In the past it was the analog photographic film that captured the photons in a chemical reaction with silver ions.

Luminous flux

The luminous flux Φ is the radiant or luminous power of a light source and is expressed in Lumen [lm] measured. You find today with the lamp purchase also on the packing the lumens indicated.

luminous flux

Luminous intensity

From a point light source you can measure the Light intensity in candela [cd] Specify. One candela (lat. for candle) is a basic unit in the SI system.


If a point light source has one candela and emits it within the solid angle of 1 steradian [sr] (d.h. over a square meter on a sphere with a radius of one meter) is constant, then this produces exactly the luminous flux of 1 lm = 1 cd-sr.


If a luminous flux hits a plane surface A, the incoming Illuminance E in the unit lux [lx] as E = Φ / A defined, so [lm/m 2 ].


A luxmeter measures the illuminance (usually over a large angle of incidence up to a maximum of 180°) and is used by photographers to measure the incoming light at the target object.

In standards for the assessment of requirements or immissions, often Eh or Ev requires. This refers to the illuminance on the horizontal (z.B. pedestrian crossing) or vertical area (z.B. bedroom window).

The light that falls on the illuminated surface, i.e. the illuminance, is not identical to the light that is reflected from the plane. For example, a football turf has a reflectance of approx. 0.2, d.h. only one fifth of the light prescribed in the standard is visible in the grass.

Luminance [cd]

If we want to measure how bright the plane appears, we measure the Luminance L with the unit [cd/m 2 ] of the grass, i.e Light intensity per area.


The luminance meter can target a light reflecting or emitting source and evaluate it within a small angle (1°). Repeating the measurement increases statistical accuracy. It must also be excluded that not, for example, a lot of extraneous light interferes with the measurement.

The Efficiency of a light source is expressed in lumens per watt [lm/watt] given. It only says how much light is gained per electrical power from the socket. Unfortunately, higher efficiency does not automatically mean that the light is better suited for the environment. It is only true for electricity consumption if you can produce the same amount of light as before with less electrical energy. It’s often not true on the spectrum though, as the more efficient LED produces a colder, blue light.

Human vision

The healthy human eye has photoreceptors for different colors of light (red, green, blue). Color blind people do not have all the receptors, so they see the world less colorful and can distinguish some colors worse.

Of the rainbow spectrum, however, even the healthy eye does not see all colors equally well.


The green receptor is more sensitive than the red and blue receptor. Most of the vital and vital nature surrounding us is green. Maybe this helped us to discover fertile land or green food more easily.

Light meters for use in lighting engineering and lighting design are usually calibrated for this curve of daylight sensitivity of the human eye.

Other sensors, z.B. Camera chips, often pick up more of the longer wavelength part of the spectrum, i.e. infrared radiation (like night vision devices).

So you always have to make sure that measurement data is collected, filtered and compared correctly.

When does light bother?

Primarily when there is direct glare (dazzle).
Secondarily when it indirectly dazzles (veil reflection).


Vision is perceived as unpleasant (psychological glare) or visual performance is irreversibly reduced (physiological glare). Causes are an unfavorable luminance distribution or too high contrasts.

Physiological glare

Physiological glare reduces visual function without necessarily causing an unpleasant sensation. An extreme example is eye damage caused by looking directly into sunlight or into a laser beam, which irreparably damages the retina and results in measurable vision loss.

Psychological BLEND

The psychological glare causes an unpleasant sensation without necessarily being accompanied by a measurable difference in vision. However, the basic problem is the high contrast luminance difference of light sources, which causes too much excitation of the receptors on the retina and creates afterimages. All people know that when they have accidentally looked into the sun, close the eyelid and still see an afterimage of the sun shining in different colors in the eye until it fades away at some point.

Veil reflection

Reflective surfaces sometimes cause unwanted glare, so that the reflective object can no longer be seen properly at all.

Glare is also a big issue with oncoming vehicles. If this impairs one’s own vision, this endangers the Traffic safety.

Light interferes with falling asleep, as we need deep darkness around us to sleep well and healthy. Most complaints come to Dark-Sky, because the own bedroom is concerned. There it goes to the health and affects one the most.

Both in terms of Glare (pupil reflex) as well as in terms of Sleep (internal clock) we are Humans and mammals are sensitive to blue light.

Nocturnal animals orient themselves more strongly to blue light, because it corresponds more to the natural darkness (sky, moon and stars).

Blue light also scatters more in the atmosphere (during the day the sky appears blue).

Blue light is more disturbing than red light in many aspects.

However, it may well be that other organisms are sensitive to different light through different vision. For example, female fireflies glow yellow-greenish. It is therefore the case that males are most attracted to low-intensity green light, but that they avoid bright light completely, as they no longer see females (see Ineichen, Lusti 2016).

Dark-Sky Switzerland: Therefore, shutting down, i.e. no light in the wrong place at the wrong time, is the simplest and best solution to the environmental problem of light pollution.

We would like to thank our board member Roland Bodenmann from Netzwerk Licht for the graphs and for providing the basis for this page.

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