Natural Daylight through the
Atmosphere around Earth

P. Mullins   2014


Natural Light Comparison with White Light Emitting Diodes (LEDs)

Less blue means less Rayleigh scattering from streetlights at night, and less blue is healthier for us too. 
This graphic from Wikipedia with added reference lines helps to map atmospheric color levels in natural daylight.

% Scattered  (Sun illuminance becomes luminance in atmosphere ′Sky′)
Health Affecting
Spectra Area
-17% Blue
-4% Red  
 Outside the Atmosphere 

Comparison Measurements
Along the visible spectrum this scatter effect is inversely proportional to wavelength.

The red 650nm is scattered by 4%, and the blue 450nm is scattered by 17% with resultant CCT of approximately 5780K

  1. Another perception may be, 94% of the red and 83% of the blue may reach sea level.
    To compare illumination among light types, we combine the color range values to see a ratio.
    Given any value in Lux or FC, natural light has more red than blue and typically CRI (color rendering index) is 100.
  2. Light Rays are a point-source where Illuminance Lux or FC=[candela÷distance2]:  These light rays are basically collimated outside of atmosphere till about 75 miles from Earth.
    Lighted Area is Luminance [candela/meter2]:  This is for Sky brightness without direct Sun′s rays.
  3. Luminance is derived from  Illuminance × area reflectance ÷ PI × ω(solid-angle) to where it is viewed.
    Our vision (retina & Pupil-Iris) behaves differently to these two quantities; Luminance can never appear as bright as, nor effect our health as much as Illuminance.
  4. With our atmosphere, not all Sun radiation reaches the Earth′s surface.  Four paths are often illustrated: one diagrams a first bounce as directly back to space, then a multiple of two bounce reflections for all angles back to space, showing why Earth appears blue from the outside at all Sun-side angles.  Some energy is absorbed in the atmospheric nitrogen, and finally the remaining radiation reaches sea level.
  5. Earth atmosphere limits Circadian Spectra below the normal level of non-Circadian Spectra.
    Common cool-white LEDs have chips producing 450nm radiation to excite nearby phosphor.  This phosphor produces the additional green through red radiation.  Chip and phosphor emission levels vary among CCT ranges, and some LED blue:red ratios exceed 2:1. 
  6. Natural daylight has a typical 450nm blue at 87% of 650nm red.  (blue:red = 450nm:650nm = 0.87:1) 
    With a cool-white LED providing a 2:1 blue:red ratio, that′s 3 times our natural healthy blue level.
Two (2) necessary test routine continuations:
 1) Continue the Acute Nocturnal Melatonin Suppression from "White Light" testing with non-blue-peak type LEDs, and compare that difference.
 2) Determine time required for Melatonin to return to normal after exposure of both types to permit public practice.

Beware that some atmospheric refraction and dispersion graphics have been inverted and misused as illumination in error.  This science is well known for hundreds of years, and you should confirm all details using multiple sources in agreement.

Information Links:
Visit here for many valid references Rayleigh scattering
Math UCR Edu. Why is the Sky Blue? See that your blue cones are 50% more sensitive than your green and red.
Hyperphysics GSU Edu. Blue Sky and Rayleigh Scattering
Incandescent Replacement Lamps and Health 360K pdf
Carcinogenesis & Mutagenesis:  A Working Threshold for Acute Nocturnal Melatonin Suppression from "White Light"  810K pdf
A test of the S/P ratio as a correlate for brightness perception

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