watts VS. lumens

[H3D]

"Hey guys, I'm a physics major."

I've already graduated college with two degrees.

"The formal definition of a Candela (lumen = candela / m^2) is the amount of radiation of light given off by a source when illuminated by light of frequency 540 X 10^-12 htz (wavelength = 550 nm) at a power of 1/683 watts."

The candela is no longer a fundamental unit. This is partly due to the problem of that equation only working at that particular frequency and intensity.

"So the Lumen measures the amount of light energy given off at a certain frequency - which happens to be the middle of our visible spectrum, and the color we are most sensitive to, yellow-green."

This is not true at all. A light source that only emits red light can still be measured in lumens.

"If lumens were measured by detecting photons, then you are really measuring watts. And lumens would just be watts times a defined constant, which is why the specific frequency is used - to seperate it from the watts unit, otherwise the lumen unit is pointless."

Lumins are measured by photons...the reason we cannot convert from watts to lumens is because different wavelengths, at different intensities, are weighted differently according to eye sensitivity.

 

[ashdavid]

I am not going to get into the above discussion,but if you wanted to accurately measure useable light for effective plant growth, you should be using microeinsteins /sq meter/sec as measurement.

The wpg rule of thumb is to keep things simple,

lumens is far more accurate, PPFD (par, pur) are even better. But they are singificantly more difficult (and expensive) to measure.

I agree with these two points, very difficult to measure, so thats why w/g is used.

 

[plantbrain]

Don't squabble. Look it up. Lumens are defined by human perception, not by total light emission.

But but but.........I like to squabble.

I have a PAR meter which is calibrated for both sun and electronic sources.
I have been fairly close using my watt/gal rule and other factors such as 1.5X for PC/T5 bulbs.

Regards,
Tom Barr

 

[ParadoxLiz]

I have been fairly close using my watt/gal rule and other factors such as 1.5X for PC/T5 bulbs.

Please pardon me for hijacking the thread, but I keep seeing PC and never a definition (imagine searching google for this...) - could someone define what PC is as it relates to lighting?

I have an aquarium which takes what it calls an F8/T5 flourescent tube. It's 8 watts. I'm assuming that's what the F8 is. Question - will T6 tubes fit where a T5 fits? (I found a T6 10W that's 12" (like my T5) and would like to up the wattage, but can't figure out if it would fit - and can only find them at online stores, which doesn't help (can't look at it and figure it out myself).)

I searched for hours one Saturday and could only find tubes for sale, sites on DIY and things way too technical to comprehend - never anything that talked about whether all those T#s were at all compatible.

Thanks,

Liz

 

[H3D]

Please pardon me for hijacking the thread, but I keep seeing PC and never a definition (imagine searching google for this...) - could someone define what PC is as it relates to lighting?

Power Compact

I have an aquarium which takes what it calls an F8/T5 flourescent tube. It's 8 watts. I'm assuming that's what the F8 is. Question - will T6 tubes fit where a T5 fits? (I found a T6 10W that's 12" (like my T5) and would like to up the wattage, but can't figure out if it would fit - and can only find them at online stores, which doesn't help (can't look at it and figure it out myself).)

A T6 bulb should not fit in a T5 light fixture.

 

[ParadoxLiz]

Power Compact

A T6 bulb should not fit in a T5 light fixture.

Thanks, Homer! Bummer about the T6, but at least I didn't have to buy one to find out. Guess I'll have to find other ways to up the light if needed.

Liz

 

[happychem]

Sigh, here we go again...

The definition of lumen, given by your reference, homer: "is the photometric equivalent of the watt, weighted to match the eye response of the "standard observer".

Ergo, the lumen is the light intensity as perceived by the human eye, and is not best suited for planted tank applications. WPG works fine.

 

[H3D]

The definition of lumen, given by your reference, homer: "is the photometric equivalent of the watt, weighted to match the eye response of the "standard observer".

Way to sum this all up happychem. :dance: I've actually seen similar wording given as the definition of a lumen. For simplicities sake, I agree watts are a better measurement.

 

[plantbrain]

Since I actually have a PAR meter and am not afraid to use it, have used a number of light sources for planted tanks, I think I can say that watt/gal rule has worked in every case and for every tank I've set up.

I've been at it awhile.

The watt/gal rule is something often picked on by some, typically only in theoretical terms and often not accounting for different efficencies/reflectors etc, but the practical reality is, that the rule works very well and the PAR meter has shown that.

Most botantical science uses PAR.
But for aquarist, the w/gal rule works just fine, given that efficiencies have increased over the years, and the culture regarding nutrients has also improved, the rule can be stretched out further than 2-4w/gal.

I've done tanks 48" to 8" deep, 1 gal to 1500 gals.
I have no clue how many tanks in between.
It's always worked well for me.


Regards,
Tom Barr