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Theoretical Introduction

The Sun generates about 4.1020 megawatts of power. Only negligible part of this unimaginable amount of energy - in the form of electromagnetic radiation (including light) - reaches the Earth"s upper atmosphere and further Earth"s and ocean"s surface and yet smaller part illuminates coral reefs, enabling photosynthesis.

Version 2015-I

Reef Aquarium Lighting - Theoretical Introduction

Summary

Basic underwater light conditions (Illuminance and Spectral Irradiance) change greatly over a day, month and year, depending on:

  • properties of light reaching the water surface which further depends on:
    • region
    • solar elevation
    • weather conditions
  • water surface character (smooth / rough)
  • water turbidity
  • water depth
 

A Journey from the Sun to the Reef

The Sun

Sun

The Sun generates about 4.1020 megawatts of power.

Only negligible part of this unimaginable amount of energy - in the form of electromagnetic radiation (including light) - reaches the Earth's upper atmosphere and further Earth's and ocean's surface and yet smaller part illuminates coral reefs, enabling photosynthesis.

The Earth's upper atmosphere

Earth

The value of Total Solar Irradiance (the amount of solar radiative energy incident on the Earth's upper atmosphere) is 1.361 W/m2 in following wavelengths:

  • Ultraviolet - 100 - 380 nm (UV)
  • Visible - 380 - 780 nm (about 50% of total)
  • Infrared - 780 – 1.000.000 nm (IR)

Even the Sun looks yellow it is making light all across the spectrum - see Spectral Irradiance further in this article.

The Earth's surface

Surface

Relatively large portion (~30%) of solar radiation is reflected and absorbed by the atmosphere (major part of UV and part of IR). Thus, the Solar Irradinace at sealevel is about 1.000 W/m2 on the average (all-day average is 250 W/m2) and largely depends on the region and weather conditions. Therefore the time cumulation is more predicative and practical. It is expressed as Annual Insolation and the value may vary from 500 to 2.500 kWh/m2 by region:

Insolation

Source:
www.greenrhinoenergy.com/solar/radiation/empiricalevidence.php

 

A light incident on ocean's surface in tropical regions at midday under clear sky has following key properties:

  • Illuminance of about 110.000 lux (bright sunlight)
  • Spectral Irradiance - see graph bellow
    • X-axis - wavelength (UV - ultraviolet, V - visible, IR - infrared)
    • Y-axis - Spectral Irradiance
    • Grey area - Total Solar Spectral Irradiance at upper atmosphere
    • Colored area - Spectral Irradiance at sea level

Spectral

Ocean water surface & column

Level

Water surface and water column substantially affects the underwater light conditions due to the following effects:

  • Reflection (1) - effect of air-water interface
    • diminution of light intensity
  • Scattering (2) - effect of suspended particles reflecting light
    • light diffusion
    • diminution of light intensity
    • changes of spectral irradiance
  • Absorption (3) - function of depth
    • changes of spectral irradiance
    • diminution of light intensity

Level

Underwater

Underwater
Illuminance

Diminution of illuminance as a function of depth for coastal waters is shown if following graph:

X-axis - illuminance in % of suface
Y-axis - water depth in meters

Depth

Further, direct illuminance is a linear function of solar elevation.

Spectral Irradiance

Changes of underwater spectral irradiance is caused by light absorption which is a function of depth and different for each wavelength. Following graph shows light transmission in pure seawater:

X-axis - wavelength
Y-axis - water depth

InDepth

The graph above would look guite differently for coastal turbid waters (maximum penetration shifted to green part of spectra) and for brackish waters (maximum penetration shifted to red part of spectra).

Coral Reef

Underwater

Except all parameters mentioned above there is other important one restraining the amount of light received by corals - the position:

Position

  • Rock-shade position (1)
    • affects the amount of light received mainly during sunrise or sunset (out of the noon in general)
  • Overlapping position (2)
    • affects the amount of light received throughout a day

Practical Example

Following example was created for Egypt reef areas with the aim to estimate average underwater light conditions throughout a year. Following parameters has been considered:

  • geographical specifics
    • sun elevation
    • light intensity (estimation)
    • weather effect (rough estimation)
  • light reflection by water surface (estimation)
  • effect of water turbidity (estimation)
  • water depth
 
Sun elevation - June 21st Sun elevation - December 22nd
Jun Dec

Sunrise
Sunset
Daylength
Max. altitude

05:54
19:59
14:05
83°

Sunrise
Sunset
Daylength
Max. altitude

06:47
17:00
10:12
36°

Calculated illuminance Calculated illuminance
Depth [m]
Max [lx]
RHAD [lx] *
Depth [m]
Max [lx]
RHAD [lx] *
1
5
10
20.000
6.700
3.300
9.000
3.000
1.500
1
5
10
8.600
2.900
1.400
3.800
1.300
600
* RHAD = Rounded hourly average throughout a day

For later use in reefkeeping practice also hourly average throughout a year has been calculated with following results for illuminance in various water depths:

  • 01 m - 6.400 lux
  • 05 m - 2.100 lux
  • 10 m - 1.100 lux

Aquarium Lighting - Reef Tank vs Nature

Key Differences between Reef Tank and Nature

Even its clear there is no practical way to reach natural Illuminance of tropical regions (ca 110.000 lux - midday bright sunlight) in reef aquarium, the good thing is it is not needed, because all tank - nature differences will help:

  • No elevation changes of illuminator throughout a day
  • No weather effects
  • Usually smooth water surface
  • Usually very low water turbidity
  • Low water depth
  • Adjustable daylength
 

The advantageous tank-nature differences mentioned above allow to mimick natural underwater lighting conditions in reefkeeping practice.

 
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