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Discussion Starter #1
We have been recommending using ordinary window screen to reduce the light intensity when you can't raise the light fixture high enough, but I had no idea how much the reduction would be. So, I bought a roll of fiberglas, black window screen from Home Depot, less than $5 for a huge piece, and tested it with a PAR meter on two tanks with two different lights.

Here is what I bought:



I set the PAR probe on the bottom of the tank, in the middle, and took a reading. Then I put one layer of screen on top of the tank, and took another reading, followed by two layers and another reading. Finally, with only one layer I lifted the layer up to the bottom of the light fixture to see what effect that would have.

Tank----no screen----one screen----one screen at light----two screens
15H------31-------------19-----------------19-----------------11--------
10-------43--------------26-----------------26-----------------16--------

You can see that one layer of screen reduced the PAR reading by about 40% and two layers reduced it by about 64%. And, it made no difference whether the screen was at the face of the light fixture or on top of the tank.

A different brand, type, or opening size screen might give different results, but this was the cheapest one HD had, so it's the only one I tested.
 

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Discussion Starter #2
Another way to look at this is, if you are getting high light with your fixture, and can't remove a bulb and still run the fixture, and can't raise the fixture, or don't want to, putting one layer of this window screen on top of the tank or over the bottom of the light, will reduce your light to medium intensity. Two layers will reduce the light to low light.
 

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Good info, but...

I'd suggest to always look into lower wattage bulbs or less bulbs first. To me, adding window screening or covering up bulbs or even raising up fixtures seems environmentally irresponsible & wasteful. Not to mention energy bills and added heat output.

Reducing the light period is another option for new tanks if you want higher light levels later on.

Best is to run your tank with the minimum amount of light necessary to get sufficient plant growth.

Just my opinion... I know others think differently. :smile:
 

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Discussion Starter #5
There are a few advantages to having the light hanging a foot or so above the tank. First, it lets you get your hands in the tank easily, so you tend to do more of the day to day maintenance that is so necessary to keep algae away from the tank. Second, the higher the light is above the top of the tank, the lower the intensity at the top of the tank. Otherwise, any tank with a light sitting on the top, has high light intensity at the top. Remember, the sun isn't right on top of our gardens, it is raised quite a bit, so the light intensity is the same from the soil up to the top of even Jack's famous beanstalk. That must be good design technique?:smile:

But, no question that it does waste electricity.
 

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Thanks Hoppy. I appreciate you taking the time to perform these experiments and post about them. It will definitely help me out when I have a tank where I can suspend the lights.
 

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i have used this method in the past, did not like the lights becoming dim, its not good for those who prefer brighter light.
 

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I think we're missing the point.... This would be to reduce the intensity if lights you already have or if you had to light odd size tanks.
 

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I think we're missing the point.... This would be to reduce the intensity if lights you already have or if you had to light odd size tanks.
i know the point, but IMO reducing the light intensity by using window screen also reduce the brightness, when you could still have the brighter light without using the window screen. am talking about using PC bulbs which are brighter but give you almost the same results when you reduce the t5 lights with window screen.
 

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Perhaps I am missing something, but how do you get "brighter" light will reducing par?

I think perhaps you are confusing color temp, which could be done with any light configuration. Or perhaps your experience with PC bulbs involved a whiter spectrum than the bulbs you've seen in t5ho.

I do not know of any way possible that light could actually be "brighter" while being less intense.

"brightness" is either a reflection of color temp(which can be changed with any type) or par. Either way, the connection to PC bulbs being "brighter" doesn't make any sense.
 

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Perhaps I am missing something, but how do you get "brighter" light will reducing par?

I think perhaps you are confusing color temp, which could be done with any light configuration. Or perhaps your experience with PC bulbs involved a whiter spectrum than the bulbs you've seen in t5ho.

I do not know of any way possible that light could actually be "brighter" while being less intense.

"brightness" is either a reflection of color temp(which can be changed with any type) or par. Either way, the connection to PC bulbs being "brighter" doesn't make any sense.

all i could say is that i had 6 wpg of PC 6700K over my 20g in the past and there were no algae issue and tank looks 10x brighter than compare to my 50g tank which have 2x54w T5HO bulbs 6700K. no matter what i do in this tank, plants are always covered with black spot algae, i can take off the algae simply by rubbing it but i cannot do this to the whole entire tank. i never had this problem with the PC lights. all am saying is T5HO even at dim lights cause these problems for me, the whole purpose of reducing the light is to reduce the algae. isn't it? why make the lights look dim when you could have brighter lights while keeping the algae away with PC type bulbs.
its is also in Hoppy's light comparison chart, where PC is still considered low light while having very high wattage compare to T5.
 

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Without a doubt, you are seeing an optical illusion or perceived difference that is not reality.

The light comparison you mention is the point that PC lights are low efficiency, nothing to do with brightness. What it means is that T5HO has more output per wattage. And by more output, "brightness" would be par. It is not that it somehow is brighter but has less usable light.

To further understand, compare incandescent lights. 100 watts of incandescent is even less efficient than PC or T8.

The reason your PC lights had less algae is simple. Less efficient bulbs and reflectors.

too much light, too little co2, and too little ferts are the cause of algae, 99.5% of the time. Once algae is established ina particular time, the time it takes to eradicate it can be very long.
 

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There are some times that lower PAR can appear "brighter" to our eyes than some "higher" PAR situations, but it's never comparing apples to apples and there are always intervening variables that make a mess of trying to predict it.

My 11g and 45g are both LED lit but using very different LED combos and driven at different drive currents.

The 11g has a PAR of about 30 on the sandbed and it appears quite bright to my eyes. Yet my 45g, which also has around 30 on the sandbed, looks pitifully dim.


I'm not entirely sure of why this is. If I crank the LEDs up on the 45g such that it looks bright to my eyes, I get algae hell.



I'm not sure how "green" registers on a PAR meter, but our eyes are way more sensitive to green light than plants are, so a green light that's ludicrously bright to us may not be all that useful to a plant. You'd think that this means "lower PAR" on lights high in the green area, but this is just a guess.
 

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One benefit that I use the window screen for is to shade part of my tank from the High light that some of my plant like / need and give my low light fish a place to hide from it.

Best wishes,
Wes
 

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I am so pleased I bought the window screen the other day, thanks to Hoppy. And, according to him I reduced my lights about 30 to 40%. My 55g is looking a little dimer now but, who cares. Btw, I use 2 54w Geisseman on this tank, about 10" from top of tank.
 

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Hi guys, I found this thread so I just continue here...

Something is odd about the spectrum of my Finnex 36" LED light using the window screen. And I just notice I bought the grey instead of black screen. Do you guys think its because I'm using grey screen the light spectrum went out of wack like this?



Two screen on


Two screen off




The same thing happened when I only use 1 screen.
 

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Discussion Starter #20
It looks like you are getting a diffraction effect that makes those bars of light show up. I haven't tried the screen with a LED light, so it may not work the same way. All I know is how it works with fluorescent lights. But, when you start shining light from near point sources through any form of "grating" you can get unexpected results (unexpected only to a non-physicist).
 
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