[Deanna01]

Hi, all.

I'm confused about how to choose proper lighting for my freshwater NPTs.

I have one 20L tank that I bought a 30" T5 HO fixture for. The lamps were 24" and 24W. I thought, since this translated into a bit more than 2 watts per gallon, that I would be fine--but it was clearly far too much light. The single T8 Sunlight bulb I got just does not light up the tank the same way, though.

Now I need to choose lighting for a 55-gallon freshwater NPT. How do I know the relationship between watts and K, and how do I choose the best light? While I would love to know a great fixture in specific, I also want to know how to choose a fixture myself, for future reference.

Thank you!

 

[Deanna01]

Also, surely the distance away from the tank will make a difference? So, for instance, could I keep my T5HO lights on my 20-gallon long if I hang them a certain distance from the top? How do I calculate how far? I had them sitting directly on the glass canopy before.

 

[Quantum]

How do I know the relationship between watts and K

there is none, watts relates to the amount of electrical energy input into the system, which determines the amount of light ultimately produced by the lamp

K, or Kelvin, in this context refers to the type of light, specifically the appearance, which is related to spectral output

surely the distance away from the tank will make a difference

yes, the decrease in intensity due solely to distance is determined by the inverse-square law, which states that the intensity is inversely proportional to the square of the distance, so doubling the distance from the source would result in only one quarter of the intensity

 

[Deanna01]

there is none, watts relates to the amount of electrical energy input into the system, which determines the amount of light ultimately produced by the lamp

K, or Kelvin, in this context refers to the type of light, specifically the appearance, which is related to spectral output

Thanks, Quantum. I apologize for the syntactic ambiguity: I don't mean the relationship of watts and K to each other; I mean their relationship to the choice of lighting fixtures. I have understood from other posts on here that a higher K lets one choose a lower wattage. Is that incorrect?

yes, the decrease in intensity due solely to distance is determined by the inverse-square law, which states that the intensity is inversely proportional to the square of the distance, so doubling the distance from the source would result in only one quarter of the intensity

Thanks again. Isn't the inverse-square law for point sources, though? It seems highly unlikely that if I had my light an inch from the water, moving it an additional inch away would result in a decrease to a quarter of the intensity.

 

[JDM]

Thanks, Quantum. I apologize for the syntactic ambiguity: I don't mean the relationship of watts and K to each other; I mean their relationship to the choice of lighting fixtures. I have understood from other posts on here that a higher K lets one choose a lower wattage. Is that incorrect?

That is incorrect... unless you have no plants. 6500K is around the ideal light temperature for plants in the aquarium. Going to 10,000K changes the light temperature or colour spectrum (far more blue) which is now mostly outside the ideal range for plants. While it is somewhat true is that you can up the wattage to compensate it's not ideal. What happens is that the light colour for a 10,000K light does have some of the 6500K colours in the light at a much lower "amount" so over driving the total light output will increase the amount of 6500K light available. With no fish, I think the 10,000K light illuminates them better and brings out their colours but it is really aimed at corals in saltwater setups.

Thanks again. Isn't the inverse-square law for point sources, though? It seems highly unlikely that if I had my light an inch from the water, moving it an additional inch away would result in a decrease to a quarter of the intensity.

The farther the subject is from the light and the more efficient the reflector is, the closer to theory the result will be. Treating the tube as a row of point sources means that there is a lot of overlapping and the only real increase of the illuminated area or light footprint in the 1" example is at the outer fringes of the footprint. I see this particularly in my lighting as I actually have multiple points of illumination from my LED fixture... 8 focused bulbs, so I can see the full effect of the overlapping "cones" of light.

I actually measured the lux at the bottom at varying fixture heights and even at 20" the light intensity does not truely follow the inverse square formula.

I like LEDs as you get what you get in a predictable fashion rather than having to juggle bulbs and reflectors. I'm just waiting on an order for a new aquatic LED system... should be in tomorrow.

Jeff.

 

[Deanna01]

This article by the Calgary Aquarium Society explains why the inverse square law does not apply to aquarium lighting. It isn't simply common sense that it doesn't.

All of which doesn't help me at all in deciding on the best lighting for my tank. Anyone have any help there?

 

[Quantum]

yes, the inverse square law applies to point source light and a fluorescent tube is not a point source

so considering the tube as a series of points as JDM suggests, one can see that the irradiation of one particular area from one point source may decrease as the distance increases, but that same area could be irradiated by an adjacent point source

the reflector issue also plays a role in making the law not perfectly applicable, but the main point remains, and that is that light intensity will decrease by some amount as the distance between the source and irradiated surface increases

 

[JDM]

This article by the Calgary Aquarium Society explains why the inverse square law does not apply to aquarium lighting. It isn't simply common sense that it doesn't.

All of which doesn't help me at all in deciding on the best lighting for my tank. Anyone have any help there?

Wish I could help but if you are looking at fluorescents, I can't much. Unless you were looking at higher end fixtures that also used certain bulbs and have the specifications, particularly lux levels at certain ranges or the PAR ratings, there are just a lot of variations.

You're pretty much stuck with anecdotal advice... the ol' "this is what worked for me"... at least there is a good amount of that here .

Jeff.