How Light Works

The two visual functions of the human eye—scotopic and photopic, commonly called day and night -- are plotted here, showing sensitivity by the color of light received.

NPS

Light, known as electromagnetic radiation, includes radio wave, microwave, infrared, visible, ultraviolet, x-rays, and gamma rays. What we call visible light forms the visible spectrum, or colors of the rainbow, and represents a very narrow band in the entire electromagnetic spectrum. Blue and violet light contain more energy and have a shorter wavelength than orange and red light. In our discussions of night sky brightness and light pollution, we are focusing on the visible light only.

Human Vision

Under well-lit conditions, the photopic curve describes the response of the human eye over the visible light spectrum. The peak of this curve is in the yellow-green, at a wavelength of 555 nanometers. Under very dark conditions, scotopic, or dark-adapted, vision is more blue-sensitive, but it is perceived as a black-and-white image by the human brain. While scotopic vision is important in observing the sky and the landscape at night, light measurement is often based upon the daytime photopic curve.

The human eye naturally measures light in a logarithmic scale. For example, if four objects were seen side by side with an actual brightness of 2, 4, 8, and 16, they would be perceived by the eye as 1, 2, 3, and 4 in brightness. Visual contrast is also seen in a non-linear manner and is dependent on the angular size of the object.

Measuring Light

Light is usually measured as photon flux, proportional to the number of photons per second striking the human eye or a light meter. Photon flux is called illuminance, and its engineering units are lux (metric) or footcandles (English); both are linear scales. The human eye can observe a very wide range of photon flux, from about 6 photons per second of blue light (10-9 lux) to brilliant sunlight reflecting off snow (104 lux), a nearly 10 trillion to one range.

In astronomy, illuminance is measured in visual magnitudes, an inverse logarithmic scale where smaller numbers mean brighter objects. The sun has a visual magnitude of -26.7 (producing an illuminance of 108,000 lux) at the top Earth's atmosphere, while the faintest stars visible to the human eye without optical aid are about magnitude 7.2 (0.000000003 lux).

Another way to compare the measuring of light in lux is easily seen in the graphic below:

Light trespass into the natural environment from an unshielded or improperly shielded bright source.

NPS / Dan Duriscoe

Bright light sources are very noticeable at night even at a great distance. For example, a typical unshielded streetlamp produces about 0.0005 lux of illuminance to an observer at 500 meters (0.25 mile). This illumination seems like a small amount, but the crescent moon produces only 0.01 lux, and the planet Venus at its brightest produces 0.0001 lux of illuminance. Therefore, this light would be brighter than any natural object in the night sky other than the moon. Also, a small, bright source of light will impair dark adaptation of the human eye, further restricting the observer's ability to enjoy the natural night environment.

The sun's surface as seen from earth measures an apparent 0.5 degrees in diameter and has a luminance of about 1,600,000,000 cd/m2. It cannot be observed directly with the human eye without damaging the retina. Conversely, the darkest part of the natural night sky is not "pitch black," but in fact can be measured at about 0.00017 cd/m2. It is easily seen as luminous to the dark-adapted human eye, especially if objects like trees are silhouetted against it. In astronomy, luminance may be expressed as visual magnitudes per square arc second (MSA), and 0.00017 cd/m2 is equivalent to 22.0 MSA.

The Milky Way rises over the Green River in Dinosaur National Monument. NPS / Dan Duriscoe

A moonlight hike on the John Muir Trail in Kings Canyon National Park.
National Geographic Traveler / Dmitri Alexander