# Re: Lux and lumens

```Simone askked about converting lumens to Lux.  That's easy: Lux = lumens per square meter.
That means if you 1000 lumens hitting a surface area of 1 square meter, the
intensity is 1000 Lux.  You can think of lumens as the amount of light available
and Lux as the lumens that are actually being used.  Lux is a measure of
intensity.

Simone also asked how much (many?) Lux 3000 lumens would produce.  That's
impossible to answer.  Obviously, you need to know the surface area involved.
More importantly, you need to know how many lumens are actually getting to the
surface.  Factors such as the shape of the reflector and the efficiency of the
reflector are critical. If the reflector is not perfect, the distance from the
bulb to the water may be important, since some lumens will be misdirected
elsewhere, never to be seen again.  And you should also take into account any
dense fog in your room <g>.

If 100% of the lumens coming from the bulb were being perfectly directed to the
surface, the calculation is easy (Lux=lumens/m^2). In most practical cases, this
is not true and it is almost impossible to calculate a true Lux value.

Some of us have Lux meters (about \$120 US) that we use to measure values.  This
is useful because it takes everything into account including uneven lighting of
the surface (there is more intensity from the middle of a FL bulb than from the
ends). They can also be used to check the intensity of light reaching specific
plants, taking into account reflections from the glass sides, shading by other
plants and turbidity in the water.

One caveat: they are "calibrated" to how the human eye sees brightness and not
to PAR (radiation useful for photosynthesis).

George Booth in Ft. Collins, Colorado (booth at frii_com)
Need Info?  http://www.frii.com/~booth/AquaticConcepts.htm
```