Playing around with a lux meter

[Derry]

I recently changed out all of the bulbs in my light fixture (two 250W MH and four T5 actinics), and I'm in the process of acclimating the tank to the new lights. However, it's always bothered me that I've only been able to "eyeball" the light intensity, which I know isn't real reliable, so I bought a $20 lux meter off of Amazon and have been playing around with it. I prefer to use layers of screen when acclimating new lights instead of messing with the photoperiod, mainly because I find it easier than dorking with my light timers. Below are some numbers I pulled using various common screening materials. The percentage is the reduction in light caused by each screening material, using incandescent light sources directly overhead. I make no claim to these numbers being uber-scientific, but I got roughly the same percentages using two different overhead lights of different intensities. All numbers are rounded approximations.

• window/solar screen - 40%
• 1/2" white egg crate - 15-20%, increasing as the angle of incidence of the light increases
• 1/2" deer/bird screen - 5%

The number that surprised me was the egg crate - I wasn't expecting it to block quite that much light. My plan at this point is to figure out a way to run layers of deer screen across the top of the tank without using egg crate as a support. Then, the next time I change out some bulbs, I'll take a reading with the lux meter using both the old bulbs and the new ones. From there, I can calculate the percent increase in light from the new bulbs, and therefore how many layers of deer screen I need to reduce the light from the new bulbs to the level of the old bulbs. Once the screens are in place, it's just a matter of removing a layer every five to seven days until all of the screens are gone.

Now, before I get pounced on, I do understand the differences between lux, foot candles and PAR. Basic lux meters are easily afforable; PAR meters are not. I was simply looking for an inexpensive way of determining a rule of thumb that would help me more effectively reduce the risk of light shock to my corals when I replaced bulbs. Hopefully this provides some ideas for others to chew on and generates some interesting dialog regarding light management strategies.

[Timfish]

I use a lux meter for exactly the same thing. Like you said not a PAR meter but a lot cheaper. I made an acrylic box for the sensor so I can stick it under water and help deciding where I put corals when I move them. It's surprising about the eggcrate and if you notice it has two different sides the edges are thicker on one side than the other and you'll get different readings depending on which side is up.