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
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
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
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:
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
Ocean water surface & column
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
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
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
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
Except all parameters mentioned above there is other important one restraining the amount of light received by corals - the 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
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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