There is no formula to calculate PAR, because it can be changed by modifying the lamp construction, as some manufacturers do. Some "special" grow lamps supposedly have a higher par then regular halides. Typically, a standard 400w metal halide lamp, such as the philips, gives about 140 PAR watts.

A side note: You won't get an answer from any manufacturers on the PAR watt ratings of their lamps. None currently rate them for PAR watts. PAR watts is a measurement of active emitted photons, only useful for plant growing, and their lamps are not intended for that. Manufacturers of "special" grow lamps will give you those ratings, knowing that you can't get PAR watt ratings from normal lamps. I use the quotes on "special" because I found a site with a "special" 400w metal halide lamp that claims to surpass regular halide lamps in PAR watts. Funny thing is, their chart shows the 400w grow lamp at 138 PAR watts. Sounds like a sham to me.

Thanks for the inf. I was afraid these relativly new 400w cmh would not have par watt numbers.

JM..

I've looked at the published PAR watt numbers for a number of grow lamps and some of them are quite frankly not believable. I guess they think nobody is going to take them to court over it.

PAR watt numbers for all these lamps are so close that the difference is irrelevant. Photosynthesis is in any case based on number of photons and not on watts, so PAR watts is measuring the wrong thing, hardly any more useful than lumens, really just a marketing ploy by the grow lamp sellers. Choose the ceramic metal halide if you want a good spectral spread with a colour between HPS and regular metal halide, plus the ballasts are good.

Thanks Shrubs. But aren't par watt relevant because par watts is the energy that the bulb actually puts off which the plants use for photosynthesis and growth?

JM...

Unfortunately PAR watts doesn't measure the energy that the bulb puts off which the plants use for photosynthesis and growth. It measures the energy that the bulb actually puts off as visible light, actually any light 400nm-700nm.

Plants use different wavelengths of light with different efficiencies. Red light is used with 2-5 times the efficiency of green light, but all wavelengths are treated equally when measuring PAR watts. This can be considered an improvement on lumens which count green light more than red light.

Also, plants use photons, not watts. Plant scientists use a PAR measure that actually counts photons. This is important because it takes 700W to make the same number of blue photons as you get from 400W with red photons but one blue photon makes the same contribution to creating sugar as one red photon (ignoring the other stuff that blue photons do like making leaves follow the light). So a 400 PAR watts bulb may be putting out exactly the same number of photons as a 700 PAR watt bulb. The makers of metal halide lamps love this measure because it makes their lamps look better relative to HPS bulbs. The metal halide bulb converts about as much energy into light as an HPS bulb, but creates fewer photons because they are bluer and each photon takes more energy.

So PAR watts are just another flawed measure, maybe a little less flawed than lumens, but also less useful because it is only published for a few lamps and the published numbers are difficult to verify.

You know your stuff, Thanks.

Yep, shrubs put it pretty clear. I was researching this whole PAR watts thing too, since I read this thread. There is a lot of conflicting stories, but it seems the only people who claim PAR watts are important are the grow lamp manufacturers, of vendors selling lamps with interest from plant growers. There's some study in trying to find a way to actually measure PLANT usable photons, by counting how many photons bouce off a surface. Soon, they'll have some crazy lamp with a photon rating, with a free bottle of snake oil with purchase.

jwmeyer, it's easy to select one light source over another, but the "par" rating isn't of any real value. A light source could be a highly efficient generator of 550nm yellow-green light with a high par percentage - yet be a very poor grow light.

Look for a lamp source with a HIGH CRI (color rendering index)... determine the fixture's DESIGN LUMENS... and look for a high FIXTURE EFFICIENCY rating. This will ensure you're using an efficient light source that offers a broad array of visible frequencies that will be distributed by an efficient fixture. Then just use the simple formula: CRI Percentage x Design Lumens x Fixture Efficiency = Useful Lumens. "Design lumens", or "Mean Lumens", gives you the light output after 40% of lamp life... a good indication of the "mid life" output. If your fixture uses more than one lamp, total up the Design Lumens for the fixture. This will give you an excellent and straightforward comparison result. Examples:

Super T8 (F32T8/850), 6 -Lamp Fixture @ 232 watts = 86% x 22,875 x 90% = 17,705.
Metal Halide (MH400/U) @ 465 watts = 65% x 24,000 x 75% = 11,700.
Ceramic Metal Halide (CDM400 S51/V/O/4K/ALTO) @ 465 watts = 85% x 28,900 x 75% = 18,424.

Note that the Super T8 fixture provides about the same useful light as the Ceramic MH, but at half the watts and with twice the lamp life (30k hours vs. 15k). The regular metal halide isn't even a player. The formula will also let you compare bulbs for a fixture, such as the decision to purchase a "coated" lamp with higher CRI but reduced light output. If you can't get the fixture wattage and fixture efficiency numbers from a seller or maker, pass on it...

No, no, no, you're doing it all wrong!!! (or insert your own preferred cult reference :)

You don't multiply anything by CRI. CRI is a measure of how close the spectrum is to sunlight. It is not a measure of the percentage of the light that is "useful" or a percentage in any other way. A bulb with a very low CRI value may produce just as much useful light (for a plant) as a bulb with a very high CRI value. You might wish to make a correction for spectrum, but you can't use CRI to do it.

And if you are going to make comparisons then you need to compare like with like. Comparing a top-notch modern and efficient T8 fitting (90% efficiency) with an old metal halide fitting (75% efficiency) is hardly fair. Modern HID fittings are available with 90% efficiency, including the best new plant lights.

And the lumen maintenance issue. Why not give numbers for a modern metal halide bulb/ballast with lumen depreciation around 80% and 40,000+ initial lumens instead of an old style of bulb. Plus of course, most growers swap out their metal halide bulbs after 5,000-10,000 hours anyway, so design lumens isn't really the right thing to measure, you really need to compare the cost of the shorter effective life. If you are going to compare to the best available fluorescent technology, it seems only fair to compare to the best available HID technology.

Shrubs is correct. CRI is not a good way to measure the spectrum. I also hear that the "SUPER T8's" are more spendy than Metal Halides.

You can have a standard 4100K with a CRI of 86 and one with 92.

Same applies to 2800K, 3000K, 3500K, and 5000K the CRI value changes. CRI is just how close a "certain" spectrum is to sunlight (i.e. 4100K)

In order for the SUPER T8's to work properly, you would have to find the highest CRI in "EVERY SINGLE" fluorescent Spectrum to compare with sunlight...that can get VERRRRRY spendy!

1 -2700K CRI 93
1 -3000K CRI 90
1 -3500K CRI 91
1 -4100K CRI 92
1 -5000K CRI 94
1 -5500K CRI 93

Try estimating the costs...

-j-

To respond to lightmaster and shrubs n bulbs... If the goal is to apply a ton of light in the red and blue spectrum with a high "PAR" value, Super T8 will address your #1 operating cost issue: Electricity. It's an even better buy if you're tossing HID lamps every 8k hours or so.

CRI has relevance - it provides an indication of the spectral makeup of the light. A high CRI guarantees a wide spectral content... but if you want to argue that monochromatic light is the way to go, that's taking us far off the mark of this post... the topic was PAR determinations, which are less reliable on a rule-of-thumb-basis than my suggested CRI-inclusive "formula."

CRI and color temperature should not be confused, and these values have no different meaning for Super T8 lamps than they do for metal halide lamps. My case is this: Pull a T12 lamp off the shelf with a color temperature of 4100k and it is mostly yellow-green in content with a poor CRI of 62. Now grab a Super T8 lamp of the same 4100k color temperature, CRI=86, and it will provide a strong red and blue output that the T12 fluorescent does not provide. One wouldn't attempt to use several different T8 lamps anymore than you would hang several different metal halides. Choose a high CRI lamp and select the color temperature that best solves your need... a 3000K 86-CRI lamp has lots of red but also much blue and other frequencies.

This I've read elsewhere in this forum: "Plants use light at all wavelenghts from near ultraviolet to near infrared to convert water and carbon dioxide into sugars. They use red light near 650nm most efficiently, then blue light near 420nm, then light in between. Some plants are much less efficient at using green light, some use it almost as well as red and blue." If that's the case and that's the goal, look for wide spectrum, high CRI light. CRI, without further analysis, is certainly more meaningful than a PAR rating.

As for high efficiency HID lighting, it is no more energy efficient than Super T8 lighting but far more costly upfront and for maintenance. Regarding lumen depreciation, Philips, for example, offers no 300-400W MH lamp in their 2004 catalog that would provide 80% LM after just 40% of average life. Electronic ballasts are of benefit in this regard, but again the additional upfront cost is a negative.

I've run the numbers every which way - Super T8 will provide lot's of quality white light at the lowest overall cost, as follows: Fixture cost - $200. Initial Lumens - 30,500. Design Lumens - 29,280. Lamp Life - 30,000 hours @ 12 hrs per start. Re-lamp cost - under $20 for eight lamps. Draw with ballasts - 320 watts. Ballast warranty - 5 years. 60-month 14/7 operating cost @ 7 cents per kWh = $773 including fixture.

CRI certainly has relevance... to the 3 different receptor cones in my eye. But to plants??? Hardly!

CRI is a measure of how well a light source renders the visible color spectrum to the human eye. It is of NO importance whatsoever when choosing a plant light.

Some of the best plant lights have a CRI of nearly ZERO! Some of the worst plant lights have a CRI of 100. INCANDESCENTS and HALOGENS have a CRI of 100. For that matter, CANDLES have a CRI of about 100.

Where fluorescent ligts are concerned, usually there is a tradeoff between high CRI and efficiency. The better CRIs usually take a performance hit. The only relevance one can point out is this: If a light is emitting a more complete spectrum, it is likely to also cover the chlorophyll absorption ranges... period.

Most lights manufactured strictly for plants do not even have a published CRI, as they are dismal and unimportant. Resellers of fluorescents (especially those marketers of relabeled Asian CFLs) may mention the CRI as a selling point to the uneducated. They also love to use the term "Full Spectrum", which has no real definition in the lighting industry. There is an enormous effort out there to market human lights as plant lights, using human-related specifications that plants could care less about.

By the way, you should compare the newer pulse-start HIDs and their ballasts to the better T8's. The 1000w has an inital lumen/watt ratio of 111 (including the ballast losses). Believe-it-or-not, there are also some HID ballasts that allow a higher "ballast factor" (that is not the term they use, though) to achieve higher light from MH and HPS lamps than normal.

Zink

I tried to read your whole post, Jumpin Timmy, but frankly its so far wide of the mark that I'll just pick out one thing.

CRI, without further analysis, is certainly more meaningful than a PAR rating.

Huh? What are you smoking? CRI is just about the most useless measure of anything related to growing plants. PAR is not perfect but it is better than CRI. PAR at least attempts to measure the light of relevance to plants although it doesn't fully correct for the plant metabolism spectrum. CRI isn't even measuring the same thing as PAR, don't try to equate the two.

Now I'm the first one to defend T8 fluorescents against the macho "metal halide is the only thing, man" brigade and the misleading T5 marketing hype (read my other posts for several examples) but please be realistic.

I AM SO DAMB HAPPY I JOINED GARDEN WEB!!!THANKS ALL!!(SHRUBS & BULBS)ESPECIALLY. WWOOOOOWWWW