How to Calculate your desired light levels

[Ocelaris]

Here is a good info I found while scouring the internet on how to figure out how much light you need for your plants, and how far away the light can be to achieve various light levels... This can also be used to figure out T-5 lights, it's not exactly the same as it's a diffuse light as opposed to a point source, but that can be a good thing... Some sample numbers at the bottom will help hopefully. This scales fairly accurately with my measurements of my 100 and 150w MH, which I've included below.

Quote:CaliGrower
okay heres the monkey wrench....

while some lamps have more lumen output, what about PAR wattage(photosynthetically available radiation)???


HPS bulbs put out more lumens, but MH lamps have more blue photons that are usuable for photosynthesis..meaning that while HPS lamps put out more lumens, more of the lumens that are put out by MH lamps are actually usuable by the plant itself..


Exactly. We can convert the illuminance received from any distance from the bulb into irradiance of Photosynthetically Active Radiation (PAR) provided that we can measure the amount of light that falls into that frequency (400nm – 700nm) from each type of bulb. And we can.

The units of irradiance used for this is the microEinstein per square meter per second (microEinstein/m²/s). A microEinstein can also be called a micromole (umol). I’ll use the micromole below.

The conversion from Foot-Candles of illluminance to umol/m²/s of irradiance within the range 400nm – 700nm (PAR) for a standard HPS and a standard MH is as follows:

High Pressure Sodium – 0.1291
Metal Halide – 0.1506

If we wanted to compare the amount of (PAR) we get 18” away from a 400 watt High Pressure Sodium to a 400 watt Metal Halide we would do this:

Foot-Candles = Lumens ÷ (2 × Pi × d2 × (1 - cos(A/2))) ²)

Where:
A = The degree of the reflector. (Most of us have 120° reflectors)
d = The distance from the bulb in feet.
Pi = 3.14159

Foot-Candles = 50000 ÷ ( 2 Pi × d² (1 - cos(120/2)))
Foot-Candles = 50000 ÷ ( 2 Pi × d² (1 - cos(60)))
Foot-Candles = 50000 ÷ ( 2 Pi × d² (1 - 0.5))
Foot-Candles = 50000 ÷ ( 6.283 × 2.25 × 0.5)
Foot-Candles = 50000 ÷ (7.068)
Foot-Candles = 7,074 from 18” away.

We do the same calculation as above for the Metal Halide substituting 50,000 lumens for 36,000 lumens and we get 5,093 Foot-Candles from 18” away.

We apply the conversions to irradiance for each bulb and from 18” away we get:

High Pressure Sodium - 7074 × 0.1291 = 913.25
Metal Halide – 5093 × 0.1506 = 767

So, from 18” away the growing power for each bulb would be:

High Pressure Sodium = 913.25 umol/m²/s.

Metal Halide = 767 umol/m²/s.

So, we get 19% more growing power from an HPS than an MH, but your point is well made when we realize the HPS puts out 39% more overall light than the Metal Halide. As you’re saying the extra blue being emitted from the MH almost makes up for its lesser efficiency in making overall light.

These are my actual measurements:

150w MH (12,000 lumens)
feet ----- Foot Candles
0 ----- 20,000
.5 ----- 7,000
1 ----- 2000
1.5 ----- 1000
2 ----- 500

100w MH (8,000 lumens)
feet ----- Foot Candles
0 ----- 12,500
.5 ----- 2500
1 ----- 850
1.5 ----- 200

Below are theoretical, the "best case scenario" so take a little off for your reflector inefficiency and glass/acrylic inbetween.

400w MH (40,000 lumens)
feet----- foot candles
0.5 ----- 50,931
1 ----- 12,733
1.5 ----- 5,659
2 ----- 3,183
2.5 ----- 2,037
3 ----- 1,415
3.5 ----- 1,039
4 ----- 796


250w MH (20,000 lumens)
Feet Foot Candles
0.5 ----- 25,466
1 ----- 6,366
1.5 ----- 2,830
2 ----- 1,592
2.5 ----- 1,019
3 ----- 707
3.5 ----- 520
4 ----- 398

150w MH (12,000 lumens)
Feet Foot Candles
0.5 ----- 15,279
1 ----- 3,820
1.5 ----- 1,698
2 ----- 955
2.5 ----- 611
3 ----- 424
3.5 ----- 312
4 ----- 239

[Grandma M]

THANKS for the info. I copied it so I can have it as a reference. I'm sure it will be very useful.

[Ely]

Interesting... I will use this information while selecting lights for my orchidarium. Big numbers; they hurt, THEY HURT! Just joking! Thank you very much.

[Ocelaris]

I know, they're larger numbers than I would like to see... but there is something to be said for a point source of light versus a diffuse source, like a T-5, where the light is much more evenly distributed. If you have a spot light, sure in the middle it's super high foot candles, but as soon as you move a few inches it drops off very rapidly. Where as a Flourescent fixture, the light levels are pretty much even over the entire area, so you don't have to worry about some getting burned and others getting not enough.

If you're not growing very tall plants, then it would make more sense to me to use fluorescents, T-5 and the like, because you can get a lot closer since in any one spot the heat is not too much. Where as a Metal Halide or High Pressure Sodium the heat is so concentrated it can burn if not properly cooled.

[Ely]

Hmmm... Different lights for the psychopsis then?