i dont think there is a set rule yet dude.. it also depends on the lens used also how high its being hung.. most people just do trial and error if im not mistaken

@Sico As @shihr said it depends on how high above the water your light is going to be and what lenses if any you're going to use. The best is to draw a sketch (to scale) and then doing the math if you want it to be perfect. Since you've got a 140 deg beam angle, you're working with a cone of light or to make it easier, a triangle from the side. So the higher it is above the water, the bigger the base of the triangle. The base being the spread of the light on the water surface. Mine are spaced to have a 50% overlap at the water surface. But that again will depend on your preference and which chips you are using. Trial and error is the name of the game. My configuration has changed 3 times already. You gotta do what works for your tank. That is the nature of DIY after all.

1. If the chip has a 120 degree lighting angle then you can assume the light spread is quadruple that of the distance. So, if you place the chip 5cm above the tank then it will have a diameter of 20 cm. So, if you have a 45 cm wide tank and only want a single row of chips then they have to be 11 cm above the water level. 2. The chip casts essentially a circular lighting pattern, so you can easily calculate lux by lm/ (pi x r^2) where r is in meters. So, for the 11 cm scenario above, this would pi x 0.22 x 0.22 ~ 0.15 m2. Lets assume you run at 1 A, then according to Greystoke's tables, for the LED CREE-xpe Neutral White you get 225 Lumen per chip. So that is 225 Lm/0.15 m2 = 1500 lux. This chip has 81 Lm/PPFD, so that is (1500/81 =) 18.5 PPFD per chip. 3. So, for a minimum of 10 chips to get 200 PPFD over a 0.15 m2 area. The average 1.2 m tank has a surface area of 0.54 m2, so that would mean (0.54/0.15 = ) 3.6x more chips, i.e. 36 LED CREE-xpe Neutral White chips to average 200 PPFD. Personally, I think all you need is 100 PPFD to have good looking, planted tank provided you have ample CO2. Also, the above estimate is probably an over-estimate. If you have your chips only 5 cm apart then there is a lot of overlap of the light beam and the lux over the tank is going to be much higher than 1500 lux/chip. Modeling this mathematically is a bit tedious for me so I can only guesitmate based on assuming "square" light distributions. That "square diameter" model predicts a 2 x 2 grid, (each chip 5 cm apart) spaced about every 10 cm along the tank should give give about 200 PPFD. This needs to be confirmed experimentally... Good luck... Wudze, can you give the specs for your array and, if you have it, the lux value at 15 cm?

@tyronegenade I have six rows of LED chips as per the pics I posted yesterday. The two outside rows are 9 x 3W 3200K chips, the ones just inside from those are 10 x 3W 6500K chips and the 2 centre rows are 15 x 1W Blue and Red LEDs. This reading is directly under the two rows of "white" chips. This reading is in the centre of the rig, directly under the Blue and Red chips.

Thanks guys that answers a lot of questions. I was going to go for 2x rows of 3W white (15000k) @ 17cm intervals down the length of the tank with 3W Red and 3W UV in between the rear row of lights.

Haha no problem at all... Its not so much of a hijack really, as much as it is a 'you can do what Blueneon did and get crappy results as he did, or take his advise and go single high watt chips and use the other's advise below for more details' hehe

Isnt 15000k a bit high.. Thats more geared to marine.. You might get algae at that high a kelvin. Surely a good mix of 10000k with some 6500k would do the job?

With just the 6500K chips the light is already too "white"! That's why I added the 3200K chips, to get the colour a bit closer to normal daylight. The 15000K and 10000K chips are going to be super white and look really odd. But that's just my 2c. Others might prefer the blueish white look.

I appreciate it depends on a couple of factors, but any idea what the optimum height is above water level to place the LEDs. With the ADA LED unit it's about 140 to 150mm if I remember correctly and depending how full your tank is of course ?

I'm currently using 20 000k and 14 000k strips in my tank with T5's and the light looks decent to me... The plants seem to like it.

Errata: just noticed a mistake in my model and the value and "That "square diameter" model predicts a 2 x 2 grid, (each chip 5 cm apart) spaced about every 10 cm along the tank should give give about 200 PPFD. This needs to be confirmed experimentally" should read 100 PPFD instead of 200 PPFD... Oops. @ Wudze, I'm going to guess wildly and guess you have about 170 PPFD out of that array... Can you take a measurement about 30 cm from the array? By that point there should be 100% overlap between the LEDs. I went with Natural/Nature/Neutral White (5000 K) as it looked the most natural to me. It seems to grow plants well, even at relatively low lux/PPFD (as said, mine put out 4000 Lux which is about 50 PPFD). I don't think K ratings mean much for LEDs. They all have pretty much the same spectrum and it is the spectrum that is important.

For my sins I found myself at Builders warehouse late this evening, can't seem to loose my sadomasochist trait sadly. I wanted the thinnest wood screw they had, ideally under 3mm, after much explaining to a staff member that I met in the appropriate aisle, he showed me a pack of nails, ... love this store !

i had 3 x t8 45cm bulbs. replaced it with 2 x 18w LED bulbs and 1 x 10w LED blue-white LED. less power, much more light before after 1 x 10w 9v blue-white (middle part of the LED is white) 2 x 18w LED BTW. the white leds are def. full spectrum. they even made a rainbow on the floor

Cool. Those 18W chips might be the answer to our needs. Can you take a Lux reading and then another in a few months time so we can see how they fade over time?