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# Re: Lighting Intensity

```>> >>From a previous post...
>> >> Second Question: If question one is true, does one form of light
>> >> penetrate water better than the other?  IOW, should a tank that is lit
>> >> by flourescents be more intensly lit at the same depth as light from a
>> >> MH bulb?

>> The light penetration of a MH with proper reflector can reach up to 36
>> inches easily, however, no matter how many fluorescent lamps you have,
>> the maximum penetration is about 18 inches.  Again, if the bulbs are
>> placed at the surface of the water.  This can be easily measured, as
>> you have mentioned, with a lux meter.

> From: George Booth <booth at hpmtlgb1_lvld.hp.com>
>I'm sorry, my Confusion Alarm just went off.  Could you go into more
>detail on why FL light is different than MH light?  Are the photons
>somehow weaker?
>Also, I know you didn't mean this, but what you said was that if a 30"
>deep tank was lit with FL bulbs, the lower 14" would be pitch black.
> don't think this happens in real life.

Okay, sorry for the delay in this reply, we just had 4 days of festive holidays
and the mail is piling up.
Let's do some calculations.
Metal Halide bulb 150W Efficiacy=90 lm/W Lux = 13500 lm
Fluroscent  TLD 30W /86 Efficiacy=90 lm/W Lux = 10800 lm (4 tubes)

We choose a 1 m x 1m square area, hence, we have only 4 fluroscent tubes of 3ft
length (imagine that over a 3 ft tank!).
The light from a fluroscent tube is spread evenly across the 1m2 (1 metre
square) tank, assuming the tubes are about 1 inch above the water.  In actual
fact, I think you would get less intensity from the fluroscent lights as in
close proximity with another tube, adsorption effects occur.
The MH tube with reflector would be about 1~2 foot above the tank.

From the above, we can see that the MH is a more powerful lamp (in terms of
lumens) and also has a smaller "footprint" with a much higher intensity.

In terms of lighting, there is the Inverse Square Law of lighting - the
illuminance varies with inversely with the square of the distance i.e. the
further away you are from a light source the lower the illuminance.  Also, the
water absorbs light.

Hence, because of the smaller footprint, the MH penetration is better, giving
illumination to deeper tanks.  I don't think that for a 30" tank, only the
upper 14" would be illuminated, it's just that the amount of light at those
levels are too low to support plant life.  I understand, too, that the blues
are absorbed faster, hence the use of actinic tubes which are rich in blue to
counter this loss.  Actinic tubes are in heavy use in marine reef tanks.

Hopes this makes sense

Rodney
```