76

BY THEO TEKSTRA – MARKETING MANAGER GAVITA HOLLAND BV

YOU NEED TO MEASURE LIGHT PATTERN AND INTENSITY IN A 3 DIMENSIONAL MODEL

CALCULATING WITH LIGHTCALCULATING WITH LIGHTA 1000W HPS DOES

A 4X4 SPACE, RIGHT? .

It seems so easy. You have a few trays of plants, so what keeps you from just

hanging a few lamps above them? If you surf the forums, people are really clear

about how to do that - a 1000W HPS does a 4x4 space, right? Wrong.

WRONG

CALCULATING LIGHT I GARDEN CULTURE

77 GARDENCULTUREMAGAZINE.COM

It is not easy to make a good model. You need an

instrument that is capable of measuring the pattern and

intensity of the light in a 3 dimensional model. Such an

instrument is a photogoniometer.

The photogoniometer enables us not only to create an

electronic fixture model, but it also allows us to calculate

the efficiency of a reflector, as the meter integrates all

the light coming from the source. Compare that to the

output of the original light source, and you have the

efficiency of your fixture. It’s an invaluable tool when

you are developing a good reflector.

So let’s assume that we have the fixture measured at a

certified facility, and have obtained a good IEC file. What

now?

Lighting calculationsoftwareYou need software to calculate and visualize the effect of

light fixtures in a room. There are many different types

of software for different purposes. One thing they all

have in common: they do not automate placement of

the lights, they just show you the result for your choices.

Compare it to buying Photoshop: you do not become a

graphic artist suddenly by obtaining the software. So,

lighting design is a serious job, one in which software

assists you. Having a lot of experience helps, but it can

still be a tedious job, because rooms are never designed

to suit the effective throw of a fixture. It can really be

hard to find a good lighting solution for a room.

In this article we use a Philips program called Calculux.

It allows you to position fixtures in a defined room, and

calculate what the light levels and light uniformity will be

on a specific plane in the room.

In reality there is a lot of light wasted, and there are

many urban legends about how to deal with lighting.

Here are a few:

• You can position LEDs really close to the crop.

• A 1000W HPS lamp covers 4x4 (or 5x5 according

to some).

• An air-cooled reflector allows you to get really

close to your crop.

• The best way to light your plants is by hanging your

lamps right above your tables.

• The further away from your plants, the more light

you lose. Doubling the distance easily reduces the

light by more than 50%. Or even 75%.

Sounds familiar? Now let us take a look how it really

works in three simple steps:

1. Let us make an electronic model of our fixture,

which completely mimics how the light exits it.

2. Use that model in special light calculation software

to simulate different situations.

3. Do some crazy things to see what the result is, and

check our calculations in a real life situation.

Electronic modelsProfessional lighting manufacturers have what we call

IEC files at their disposal. They are electronic models of

their fixtures, which when used in simulation software

predicts how much light will be available, how it will

look, and how uniform that light coverage will be. They

use these models to design the correct lighting in offices,

museums, large-scale architectural projects, sport fields

and stadiums, studios and, of course, climate rooms for

plants. Now you can imagine that in a climate room or

greenhouse it’s not so much about the atmosphere, but

more about the light levels, and how uniform the light

levels are in a room. That’s why there are also specific

programs for those types of calculations.

TAKE YOUR LIGHT METER, AND DO THE MEASUREMENTS

CALCULATING WITH LIGHTCALCULATING WITH LIGHT

WRONG

78

Then we need to know how much light we need. If it

concerns plants instead of office workers, then we need to

define that as PAR light, measured in photons per second

per square meter (umol s-1 m-2).

Now the real work begins. We are going to position the

fixtures in the model. First of all the designer needs to

select the right fixture, as not all fixtures will be suitable.

Some fixtures will need more distance to the surface

(which is possibly not available), and some just generate

too much light for a small surface. You need to take the

dimensions of the room into consideration and the amount

of fixtures you need to provide the right light levels. Many

times there are obstructions in the room, such as HVAC

units or vents, or structural elements.

A sample calculationTaking the surface and the desired amount of light, we can

calculate how many fixtures we need to light that room.

We will just take a straight clean room and will do this

scientifically. Wait... Let’s not!

I will now debunk a myth straight away: “a 1000W HPS fixture

covers a 5x5 area”. There you go, perfect 5x5 grid. We did not

calculate how many lights we needed, we just went by the “5x5

rule of thumb”.

Now look at the light level results and the uniformity in the room!

I would aim for about 1000 umol m-2 s-1 at >90% uniformity

with peaks no higher than

5-7% more than the desired

level. But no folks, that is not

what you are going to get.

Take your light meter, and

do the measurements - you

are way off!

Always take a good look at

the scale of the intensities!

And how big the steps on

the scale are. The larger the steps, the less uniform the light

is in the room. Also look for the peak levels: they are going to

become your “hot spots” as with light comes radiant heat.

ET need input!So let us start with a simple room. What do we need to

know about that room that will influence the light levels,

besides the dimensions?

Light doesn’t always reach the subject directly, some gets

reflected from the walls, the ceiling, or even the floor.

So it is important to know what the reflectance of those

surfaces is, and to define that in your model.

Secondly, we need to know where to measure the light. If

there are plants involved, then where are they in the room,

at what height? And will they be growing to a different

height? Will they be on tables? Based on that, we define a

few virtual measuring grids in the room. The software will

calculate for us the light intensities and uniformity at those

particular planes, given a specific position of our fixtures.

In flowering rooms that is usually the height of the plants

when they enter the room, when they are at half their

future growth, and at the finished height. In many cases

floor or table levels are also provided as a baseline.

SOME FIXTURES WILL NEED MORE DISTANCE TO THE SURFACE

CALCULATING LIGHT I GARDEN CULTURE

A HPS FIXTURE WITH A HORIZONTAL LAMP DOES NOT

HAVE A SQUARE FIELD

Now, near the walls you always have losses which influence

the light uniformity in a room, so in all fairness, we measure

about 50 cm from the wall, the center of my grow area

where the plants will be.

Want to see it for a 4x4? Here you go:

As you see where

the lights overlap the

levels get way out

of control. You will

also see that with

any configuration

the levels near a wall

are worse than in

the middle. That is

because you do not

lose light over distance, you lose it to the walls. Lights

overlap and compensate this way for the larger surface

they cover. That is also the reason you should not place

your tables against a wall.

So let’s clean this up, and let the light designer do his work.

He calculates how many fixtures he needs based on the

light requirements, the output of a fixture, and the losses

from the walls. This is the result:

As you can see a HPS fixture with a horizontal lamp does

not have a square field, but rectangular. You also see that

the uniformity is a lot better, and we reach over 90% in

every grid. Also notice that the light levels drop a bit, but

not as drastic as a direct application of the inverse square

79

law would suggest. I only lose less than 10% over double

the distance to the fixture. This is the effect of the overlap

of the reflector and its specific throw. Pretty amazing,

right? That is a second myth debunked. Remember that

this is only applicable for overlapping lights.

Lighting plants or lighting rooms?Now you have seen that using overlapping lights you get

an incredible horizontal and vertical uniformity. But how

about if I just position my lamps straight above my plants?

I don’t want to light the paths, right?

Well, that didn’t turn out as

hoped unfortunately. Only

with deeper reflectors (a

smaller exit angle) you can

clearly define a specific lit

area. Problem is though at

the edges the light will be

very much lower than in the

center. This is in particular

true for LEDs. Also you do

not have the advantage of

the overlapping lights, which give you much more uniform

light levels, and a far better horizontal penetration of

the plant. Remember folks, the sun is almost never just

straight above you.

Next time we will continue this “exposé” with an explanation

about air-cooled fixtures, useless grid measurements, and

the facts about air-cooled reflectors. 3

GARDENCULTUREMAGAZINE.COM