• The physical properties of light

    For a long time we did not know what light actually was. In the middle ages it was even believed by some that the eyes emitted rays and that sight was the effect of those rays touching the world around. The questions associated with light seem only to have been solved in the last 100 years.

What you should know about light

What actually is light?
For the physicist, light is a form of electromagnetic radiation – but only the small range (between 380 and 750 nm), which is visible to humans. There are also gamma rays, X-rays, ultra-violet, infrared, microwaves and radio waves. The forms of radiation which border on the visible range, such as infrared and ultraviolet, are also often referred to as “light”.

What is the colour spectrum?
Light occurs in different wavelengths which we perceive – through a prism, for example, or a raincloud – as colours (rainbow colours). White only looks like a colour to us (e.g. sunlight) when colours of the spectrum are combined. In the case of artificial sources of light, it is possible to influence the colour spectrum; individual colours can be emphasised.

What’s the effect of the light colours?
Blue and red are the wavelengths that can absorb chlorophyll most effectively and, for that reason, these colours are the most beneficial for photosynthesis. Green is least useful and is therefore reflected or allowed to pass through. That’s why leaves look green. If the red element in light is predominant plants will grow to an enormous size, but they will be delicate. If the light is predominantly blue, on the other hand, their growth will  be extremely slow and they will be compact. Optimum plant growth therefore occurs if there is a balance between the blue and red spectrum.  

What does full spectrum mean?
Sunlight, or daylight, has the full colour spectrum and can be effectively simulated using what are known as full-spectrum fluorescent tubes. This requires at least four different fluorescent substances or 5-band fluorescent tubes. Such tubes, however, possess a considerable smaller radiant power than 3-band fluorescent tubes. It is a cliché to say “3-band is better for plants but full-spectrum is better for the eye".

It is different with high-quality LED lamps (such as EHEIM powerLEDs for freshwater aquariums) as they satisfy both conditions. They reproduce the full sunlight spectrum and have a high radiant power. They are therefore both “good for the eye” and outstanding for plants.

How is light measured?
The units of measurement for light refer to brightness, luminous efficiency, light colour and colour reproduction.

• LUX (lx)
LUX (lx) refers to luminous intensity (the amount of light which falls on a surface), e.g.: midday sun in a tropical summer 80.000 – 100.000 lx; dull summer’s day 10.000 – 20.000 lx; full moon at night about 25 lx; bright window 4.000 – 2.000 lx

• Lumen (lm)
Lumen (lm) refers to the luminous efficiency that a lamp radiates in every direction. From an indication of luminous efficiency (lm) and wattage (W) it is possible to identify efficiency, e.g.: (incandescent) light bulb 10–20 lm/W; energy-saving bulb 50 – 85 lm/W; T5 fluorescent tube 80 – 90 lm/W; LED depending on design 20 – 120 lm/W.

• Kelvin (K)
Kelvin (K) indicates the colour temperature – i.e. the light colour. Lower figures represent the warm, reddish range, and higher figures the cold, bluish range.
The light colours of lamps are classified in three groups: warm white 2.000 – 3.000 K (pleasant moods); neutral white approx. 4.000 K (objective moods); daylight white from 5.000 (natural). In the case of fluorescent tubes approx. 5.300 K of daylight white.
In the case of freshwater aquariums, the significant figures lie somewhere between 4.000 and 7.000 K. In the case of marine water aquariums, the figures should be above 10.000 K.

• Colour reproduction index (Ra)/CRI
Another unit of measurement is the colour reproduction index (Ra)/CRI, which indicates the effect, regarding colour, that a lamp generates on coloured objects or people. The higher the figure the more natural the impression (e.g. in an aquarium).

• Band fluorescent tubes
We also refer to 3 band or 5 band fluorescent tubes if they have 3 or 5 particularly strong characteristics in their colour spectrum (see also full spectrum).

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