Using a LUX Meter for PAR readings. How to calculate PARS estimates?

[Wally.B]

I posted a thread about my 250W bulbs being to strong.

However this is a separate topic about using my Lux Meter to figure out if my lights are too strong. (I don't have real PAR meter).

Has anyone used a LUX meter to figure out their lighting PAR levels?

The LUX meter I have is a (Milwaukee MW700). http://www.milwaukeeinst.com/site/component/content/article/36-standard-portable-meters/88-products-g-standard-portable-meters-g-mw700-wp

I didn't want to spend a more expensive Lux Meter since the goal was to simply use it to place corals around (ie I find a good spot for a coral, then move it to a similar spot).

However I think my 250W lights are too intense. I want to raise them to a proper level, so getting a PAR reading on my lights would be ideal.


This link was provided but I find a bit confusing http://www.advancedaquarist.com/2013/2/equipment

In the article is says you can convert LUX to PAR as stated below. (estimate conversion).

...it is possible to convert lux measurements to PAR values. Use these results with some caution - in most cases it would be safe to assume the results will be low.
•Divide blue (450nm) LED Lux by 69
•Divide white (7,300 K) LED Lux by 45
•Divide blue (450nm)/white (7,300 K) combination LED (2:1 white/blue ratio) Lux by 67

This appears to be for figuring LED lighting and combining BLUE with White LUX readings.

I want to simply check my MH Hailade PAR levels for my 14500K bulbs, which I think can be consider white.

If true, would I take my MH Lux reading and divide by 45, to get an PAR estimate? (Probably a bit higher divisor, since 45 is for 7300k, I have 14000k bulbs.

 

[Wally.B]

I just did some more digging and found a good article.

Appears the LUX to PAR conversion is more complicated for LED lights.

However for MH lights is pretty simple conversion as per this link http://www.egc.com/useful_info_lighting.php

Just divide LUX reading by 3.05 to get PARS.

 

[Narwhal82]

I wouldn't put any stock in that conversion equation.

The sensor used in a lux meter is different than the sensor used in a PAR meter. The lux meter is going to have a sensor that is more sensitive to the wavelengths most visible to the human eye and will weight the values between 500 and 700 more highly than the wavelengths below 500 nm.

An ideal PAR meter would give equal weight to all wavelengths between 400 and 700 nm. Apogee PAR meters are ok but Licor are better.

Any light with a high amount of blue energy (wavelengths below 500 nm) is going to be severely underrepresented by a lux meter.

That equation may work for warm white lighting (where the light output more closely matches that of the spectrum for lux measurements) but it's going to be useless for measuring aquarium lamps with a lot of blue in them.

Andy

 

[Wally.B]

I wouldn't put any stock in that conversion equation.

The sensor used in a lux meter is different than the sensor used in a PAR meter. The lux meter is going to have a sensor that is more sensitive to the wavelengths most visible to the human eye and will weight the values between 500 and 700 more highly than the wavelengths below 500 nm.

An ideal PAR meter would give equal weight to all wavelengths between 400 and 700 nm. Apogee PAR meters are ok but Licor are better.

Any light with a high amount of blue energy (wavelengths below 500 nm) is going to be severely underrepresented by a lux meter.

That equation may work for warm white lighting (where the light output more closely matches that of the spectrum for lux measurements) but it's going to be useless for measuring aquarium lamps with a lot of blue in them.

Andy

I was expecting the formula to be an estimate.

I clearly understand that light spectrums outside of lux meters sensitivity would not be picked up. And that error in reading would increase or decrease depending on type if lighting. Be it led, t5 or MH and further varying within each on bulb type mixes or setting.

However is there a percentage accuracy that could be applied so that I could use it to determine my MH fixture ideal height and photoperiod.

Currently all I can do is extra raise my lights at 16", go with minimum photoperiod 5 hrs and slowly work my way up on coral light exposure.
Reaching a general 2x250W MH ligh fuxture height of 6-12 inches and 6-8 hrs. For my SPS only 65gal tank.

The Part that puzzles me is where are the two max limits and how do I know when I've reached them without damaging the corals?

 

[alton]

You cannot convert lux to par, you can convert lux to foot candles. On my MH before I had a PAR meter I used 2,000 foot candles at the top, and changed the lamps out when they dropped to 1,800. I once tested a cheap LED fixture with 4,000 foot candles but a PAR reading of only 200. Normally there is a 20% difference between foot candles and PAR on metal halide.

 

[Rybren]

Here's an article by Dana Riddle that briefly covers converting Lux to PAR.

http://www.advancedaquarist.com/2008/2/review

Here's another one by the same author that discusses it in more detail, including converting Lux to PAR under LEDs.

http://www.advancedaquarist.com/2013/2/equipment

 

[Narwhal82]

The trick is knowing the spectrum of the lamp you are trying to measure. Dana Riddle's article provided a number of conversion factors for different brand lamps. If you know the conversion factor you can convert lux to PAR or vice versa. But if you don't know the conversion factor then it's useless.

Just look at the conversion between the Ushio and Coralife 10K MH lamps. That's a huge difference between two lamps of similar spectrum.

Andy

 

[Wally.B]

Here is a nice article that shows some typical tank PAR values and differences between light setups.

http://www.advancedaquarist.com/2003/8/aafeature

So I measured my tank Lux values (under water) with my Meter.

I was getting Lux values of 10,000 at the top, 5,000 at the bottom, and around 8,000 in the middle areas.

So this would roughly convert my 2x 250W 14,000K Giesemann "Megachrome Coral" bulb setup "using 50 a conversion" to 200 PAR, 100 PAR, and 160 PAR.

Rough guess, but better than nothing.

Corals know best.