How much photosynthetically active radiation (=PAR) really reaches the aquarium plants?
Aquarium fans (fortunately!) know it’s better not to have colourful aquarium lighting and that bright lighting has little in common with lighting quality (e.g. fluorescent tubes in living rooms). During their evolution plants have adapted to the spectral components of sunlight and now use a range between 380 and 780 nm (blue to red) for photosynthesis. These are the wavelengths that chlorophyll and other pigments use to supply the plant with energy.
It’s very important to JBL that the JBL LED SOLAR lamps generate high PAR values to make your aquarium plants grow well. But what happens to the PAR value at increasing water depths? No one has ever measured how the PAR value changes under water and how it behaves at different colour temperatures.
To find out, the 1 m deep Koi aquarium at the JBL Research Centre was turned into a diving tank. Biologist Heiko Blessin brought along his diving suit and diving equipment to measure the lamps under water using a PAR meter. The PAR meter was placed in a waterproof mobile phone case and the sensor head was slowly moved back and forth in the direction of the light at different depths. Elena Hänle, JBL’s chemistry expert, noted the highest PAR value displayed at each depth on the outer pane.
The values were measured directly above the surface (5 cm distance to the JBL LED SOLAR NATUR ) and directly below the surface, then at distances of 10 cm, until they reached the bottom of the tank.
The tables show the exact values measured, with the reducing value caused by the mobile phone cover corrected. The illuminance in lux was always simultaneously measured with a luxmeter, so the lux values are also entered in the table.
Good news for the aquarium plants is that we were still able to measure 131 PAR (at 2700 K), 122 PAR (at 4000 K) and 150 PAR (at 6700 K) in 40 cm water depths! These are really excellent values, translating into extraordinary plant growth to please all aquarium owners!
The lux value was also measured, as it is a good index for the illuminance. The lumen number is irrelevant, because it can be increased at will, e.g. by amplifying the green part of the spectrum, without being beneficial to the photosynthesis of the plants. Only with the combination of the full spectrum and the high PAR value can we see how well lighting can promote plant growth!
Kelvin describes the light colour or colour temperature of light and is abbreviated with K. It is perceived by the human eye as yellowish-warm or bluish-cool.
The measurement
Tested LED: JBL LED SOLAR NATUR 68 W
Distance LED to water surface: 4-5 cm
Measurement: Below the centre of the LED. Measuring depth related to the distance to the water surface. At 80 cm water depth no value could be recorded due to the size of the PAR meter.
2700 K
| Measuring point | Measuring depth [cm] | PAR [μmol/m2s] | Illuminance [lx] |
|---|---|---|---|
| Above the water surface | 0 | 817.8 | 31.2 |
| Below the water surface | 0 | 788.8 | 26 |
| Below the water surface | 10 | 345.68 | 13.2 |
| Below the water surface | 20 | 225.04 | 8.8 |
| Below the water surface | 30 | 157.76 | 6.2 |
| Below the water surface | 40 | 131.08 | 5 |
| Below the water surface | 50 | 114.84 | 4.2 |
| Below the water surface | 60 | 100.92 | 3.8 |
| Below the water surface | 70 | 89.32 | 3.2 |
| Below the water surface | 80 | ND | 2.6 |
*measured PAR values + loss factor through waterproof plastic cover (~16%)
4000 K
| Measuring point | Measuring depth [cm] | PAR [μmol/m2s] | Illuminance [lx] |
|---|---|---|---|
| Above the water surface | 0 | 865.36 | 30.7 |
| Below the water surface | 0 | 788.8 | 27.8 |
| Below the water surface | 10 | 336.4 | 14.8 |
| Below the water surface | 20 | 229.68 | 9.4 |
| Below the water surface | 30 | 167.04 | 7 |
| Below the water surface | 40 | 121.8 | 5.5 |
| Below the water surface | 50 | 103.24 | 4.9 |
| Below the water surface | 60 | 99.76 | 4.1 |
| Below the water surface | 70 | 87 | 3.6 |
| Below the water surface | 80 | ND | 3.2 |
*measured PAR values + loss factor through waterproof plastic cover (~16%)
6700 K
| Measuring point | Measuring depth [cm] | PAR [μmol/m2s] | Illuminance [lx] |
|---|---|---|---|
| Above the water surface | 864.2 | 33.9 | |
| Below the water surface | 827.08 | 28.6 | |
| Below the water surface | 10 | 386.28 | 15 |
| Below the water surface | 20 | 266.8 | 9.5 |
| Below the water surface | 30 | 191.4 | 7.1 |
| Below the water surface | 40 | 149.64 | 5.7 |
| Below the water surface | 50 | 125.28 | 5.3 |
| Below the water surface | 60 | 104.4 | 4.6 |
| Below the water surface | 70 | 96.28 | 3.4 |
| Below the water surface | 80 | ND | 3.2 |
*measured PAR values + loss factor through waterproof plastic cover (~16%)
