MOTION MOUNTAIN

142 4 images and the eye – optics
2.50
Solar Radiation Spectrum
Irradiance (W m-2 nm-1)
2.00
1.50
1.00
0.50
O 3
O 3
H 2 O
O 2,
H 2 O
AM0 Direct normal (black body at 5780 K)
AM0 Direct normal (Gueymard 2004)
AM1.5 Direct normal (ASTM G 173)
H 2 O
H 2 O, CO 2
H 2 O, CO 2 H2 O, CO 2
H
0.00
2 O
200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600
Wavelength (nm)
F I G U R E 90 A black body spectrum at 5780 K, the solar spectrum above the atmosphere in direction of
the Sun, with 1350 W/m 2 , and the spectrum with 1.5 air masses, or atmospheric thicknesses, in
between, with 844 W/m 2 . The latter roughly describes the spectrum of a typical sunny day at sea level.
The gases responsible for the absorption bands are also shown (© Chris Gueymard).
light sources
Without radiation sources, there would be no images. All imaging techniques need
sources of radiation. In the domain of visible light optics, the most important light
sources of visible and infrared light are hot objects, such as candles, the Sun or flashlamps.
Physically speaking, these light sources are approximations of black bodies. Let
usseewhytheyareused.Coldlightsources,suchaslightemittingsemiconductordiodes,
firefliesorlasers,areexploredlater on.
Why can we see each other? Black bodies and the temperature of
light
Physicistshave a strange use of the term ‘black’. A body that glows perfectly is called a
blackbody. In this domain, ‘perfect’ means that the surface of the body hasno effect on
its colour.
⊳ Ablackbody is a body that absorbs all radiation impinging on it.
In other words, a black body is a body without reflection or transmission of radiation.
Black bodies are an idealization; above all, they are only black at low temperature. With
increasing temperature, black bodies glow or shine in black, brown, red, orange, yellow,
white or light blue.
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net