Light & Photosynthesis
Light is the only plant food. All the other things we give a
plant are just to help it digest and use the light. The more light
a plant gets, the bigger and fatter it will become, provided all
its other needs are being met. If there is not enough light, nothing
else will make the plant bigger or produce more.
The main thing that keeps an indoor garden growing is lots of
available light, and the most important is the light density.
Small lights in a large greenhouse will not do much at all for
the light density except for sending day-length signals to the
Knowing about lumens, or light density, is very important to
the indoor gardener because the big secret of a supercharged garden
is based on light density and how it is used to benefit the plants.
It is vital to know the light intensity of your garden to be able
to work out how much CO2 to give it.
Lumens are a measure of the light intensity striking a surface.
A first idea for understanding lumens is to say that the number
of lumens is just a convenient measure equal to the number of
candles lighting up an average book held 1 foot from the light.
A hundred years ago this was absolute reality and our grandparents
had to line up many candles in order to read at night. Today,
we define 1 lumen as the amount of light 1 candle
will shine on 1 square foot of white paper held 1 foot away from
1 LUMEN = 1 foot-candle or
1 candle of light intensity per square foot held 1 foot away.
The metric unit is the lux, the amount of light falling on one
square metre: 1 LUX = ... metre-candle or
1 candle of light intensity per square metre held 1 foot away
1 square metre is about 10 square feet, so the lux number is
10 times the lumen number: 1 LUMEN = 10 LUX (lux is the metric
1 lux is only 1/10 of a lumen because the same amount of light
on 1 square foot now has to fall on 10 square feet, so it is diluted
A photographer's light meter measures the light reflected off
a surface in foot-candles, which is the same as lumens independent
of any specific area. The camera lens has a very small opening,
and the area of light it is photographing could be millions of
Never point a photographic meter at a light. As an example, measuring
20,000 lumens on a 500 foot-candle light meter can burn out the
equipment. For this reason, you should always measure the reflected
light from 1 foot away with the photographic meter.
Getting the most out of artificial lights
The light illuminating from a source is not fully utilized. To
illustrate, one candle actually puts out a total of 12.5 lumens
in all directions, but only one of those twelve lumens actually
falls onto a square foot of paper held one foot away from the
candle. Thus, there is only "1 lumen" on the paper,
and the other 11.5 lumens are shooting into space. Think how bright
two candles would make the same page of the book you are trying
to read in the dark. This would be two lumens, or two candles
shining on 1 square foot.
Your table lamp will be putting 175 lumens on the book, and with
175 candles, you can read your book very well. But if you took
the book to the other end of the room, you would not be able to
read it because the 175 lumens that were shining on the page when
you were one foot from the
lamp hardly even light the page now some many feet away from the
light source. There is, in fact, less than 1 lumen actually shining
on the page the other 174 are shining on all its surroundings.
Mirrors and reflectors can be used to re-direct many of the photons
of the other 174 lumens that shine into space. In fact, mirrors
and reflectors can make a big difference in an indoor garden.
Because light is expensive, you should get as much value out
of the electricity as possible. Here are some pointers:
1. Keep lights as close as possible to the plants.
2. Use mirrors and reflectors to utilize as much light as you
3. If your indoor garden is in a large room where the walls are
(about 3 metres) away from the plants, you can be sure that most
of the light is being wasted into space. To contain the light,
build an enclosure
not more than 1 foot (30 em) away from the plants all around the
The Efficiency of Lights
The main reason to really get familiar with lumens is that the
"wattage" of any light bulb in an indoor garden is a
totally meaningless number.
Look at the following example: An incandescent 1 DO-Watt table
lamp shining on a book held one foot away will put 175 lumens
on the book. Replacing the table lamp with a 1 DO-Watt Mercury
light will put 600 lumens on the book. Again, replace the Mercury
lamp with a 1 DO-Watt High Pressure Sodium street light (if you
were standing on a ladder and held the book one foot from the
light) will put 1,400 lumens on the book. As you can see, bulb
wattage has nothing to do with light intensity, but bulb type
does influence the amount of illumination.
Certain types of light bulbs are more cost efficient than others.
For example, with a standard house filament bulb, only 10% of
your money is going to
producing photons of light, and most of what you paid is wasted
as heat. I
Fortunately, you get "more bang for your buck" out of
the HID (high intensity discharge) lamps.
There is not yet an extremely efficient bulb, but some are better
The rates from the chart Relative Efficiency of Some Bulbs would
be good if the bulbs gave all light and no heat. The percentages
shown here are the heat wastage that is not turned into light.
Unfortunately, 100% of the electricity has to be paid for even
though one only gets at best about 60% of it in light.
The relative lumens of artificial lights can be illustrated as
follows. Assume that 6 similar wattage bulbs were hooked up to
the power and a measuring instrument was put in front of them.
Lumen values are referred to at this stage to illustrate how the
various types of light bulbs differ. (* Note that these are not
PAR values unless we measure with a very special meter that does
not count the very blue light below 400 nm, does not count most
of the green light that the leaves are reflecting, does not count
the very red
light above 730 nm, and only counts half the light in the orange
colour. PAR stands for photosynthetically active radiation. It
is different values for different types of light source. The PAR
value is the amount of light usable by plants, since plants can
utilize only a small percentage of the artificial light.)
Light 1 : Regular House Incandescent - the meter shows 17.5 foot-candles
Light 2 : Mercury Street Light type - the meter shows 63 foot-candles
Light 3 : Fluorescent type - the meter shows 83 foot-candles
Light 4 : Sulphur type (1997 model) - the meter shows 98 foot-candles
Light 5 : Super Metal Halide - the meter shows 125 foot-candl
Light 6 : High Pressure Sodium - the meter shows 140 foot- candles
Multiply the corresponding bulb reading above by the wat age of
your bulb, and this will be the amount of light leaving the bulb,
not the light reaching your plants.
There are additional losses if the light is behind a glass panel
or in a water jacket. Each glass surface light passes through
can lose 10%, and passing through a water jacket can lose 20-30%,
mostly in the red spectrum.
You can see that an HPS bulb gives 140 lumens per watt. If you
are reading a book from a 1,000-Watt High Pressure Sodium light
placed 1 ft away, that bulb is shining an equivalent of 140,000
candles (140 x 1,000). This 140,000 lumens of light is spreading
in all directions, and only 10% of the lumens is heading for your
book; the other 90% is shining on the walls, the ceiling, and
mostly the floor. Thus, a 1,000-Watt HPS lamp fixed at the one-foot
distance places only 12,000 lumens (some losses calculated) on
Similarly, the usable lumens from a 1,000-Watt (100,000-lumen)
MH lamp at this one-foot distance are only about 9,000 lumens
on the page (there is also a 10% loss of light as it passes through
the glass of the bulb); the other 90,000 lumens are shining on
the rest of the room. With a reflector, many of these 90,000 lumens
can be captured and sent down to the page.
As discussed previously, a 100-Watt table lamp shining out 175
lumens actually gives less than 1 lumen per square foot towards
the other end of the room, and this makes the bulb wattage per
square foot an even more meaningless number. The following shows
how distance affects light density.
Upper next page illustrates a square of paper held 1 foot from
a light source,
casting a shadow of 4 ft x 4 ft (16 ft2) on a floor 4 feet below
the light. If the light source is the above-mentioned MH lamp,
you get 9,000 lumens shining on the paper at the one-foot distance.
Spread these lumens over the 16 ft2 on the floor, and now there
are only about 500 lumens on each of the 16 squares. This means
that light at the 4-foot level is only 1/16 as intense over anyone
square foot because the original 9,000 lumens is now spread out
over 16 square feet. In fact, with every extra foot (30 cm) that
light has to travel, light intensity is reduced by half. Things
look worse in an 8-ft high room: the light is down to 1/64 its
original intensity at the floor level- about 140 lumens on each
The Sun and Lights
The noon Sunlight in Davis, California is around 5,000 lumens
per square foot. Similarly, a 1 ,OOO-Watt HPS lamp with its inner
arc tube 2 feet above a table is shining about 5,000 lumens on
every square foot of that table. Although this HPS lamp appears
to be equal to the Sun, it is not the same because its colour
spectrum is poor. The Sun has another advantage over artificial
lights in that Sunlight is just as intense at all distances. This
is because the powerful Sunlight has already travelled some 93
million miles (149 million kilometers) to reach Earth, so the
extra few feet or metres it has to go to reach the bottom of plants
do not lessen its light intensity.
To put light over distance into perspective, the stars are actually
brighter than our Sun, but their light decreases by half every
million miles. So by the time the starlight reaches us, it is
not even bright enough to read by.
Luckily for outdoor gardeners, the mighty Sun is giving 5,000
candles of light intensity to both the tops and the bottoms of
tall trees. But with artificial lights, it is a different story.
The first foot of light below an HID lamp is more intense than
Sunlight. Unfortunately, artificial light also drops off by half
with every foot it travels, so at 3 feet, it is only 1/9 the intensity
of the first foot. This is why the bottoms of a 3-ft plant gets
only about 10% as much artificial light as the tops. Therefore,
the shorter an indoor plant is under artificial lights, the more
light the entire plant receives.
The Difference between the Sun and Lights
The real difference between the Sun and lights is that the Sun
has full spectrum light from red to blue at equal intensities.
This giyes maximum energy to the plant pigments and makes the
plants grow well.
Artificial lights have not equalled the Sun for full spectrum.
On each lamp spectrum graph under Light Bulbs in ChapterS, compare
the "light usage by plant" curve with the shaded area
"light available from bulb." You can see that all artificial
lights have very low intensity over most of the useful spectrum
for plant growth. The best one can do is to find lights that have
the best compromise.
The problem for the indoor gardener is that no HID bulbs give
enough red frequencies - HID's have very low 680-700 nm peaks.
High Pressure Sodium bulbs emit a very strong orange light - some
of which is absorbed by carotenes and phycobilins pigments, which
are then passed down the
electron funnel to the sugar factory. Some fluorescent lamps have
a good red spike, but they have no power at all compared with
the HID's. The best one can do is to blast the leaves with MH
and HPS lights at enormous lumen power so that there is enough
red spilled over to do the job.
Look at the various spectrum graphs again. Notice just what a
compromise they are. The truth is that all the HID lamps are equally
poor at the vital frequencies. The best frequency coverage is
a standard incandescent bulb, but it is so inefficient that it
is not usable for high output lamps. As mentioned before, an HID
at close range puts out more lumens than the Sun but at many less
important frequencies, so a large portion of its light is just
wasted. Luckily, plants are able to make the best of a poor situation
and use whatever photons fall on their leaves.
Water cooling the HID lights allows plants to be placed close
to the illumination (for greater light intensity) with minimal
heat build-up. Unfortunately, the water surrounding the bulbs
and the containers or jackets surrounding the water absorb much
of the red spectrum that is the
plants' main sugar producing frequency. Light losses through water-cooled
jackets can be 20-300/0. Plants grown under water-cooled I lights
will have a corresponding loss in yield compared with open lights.
However, the lack of heat generated by water-cooled lights can
be used to a greater advantage where open lights generate excessive
The Magic :Is Here Photosynthesis
The leaves perform photosynthesis in a most amazing way. The leaves
have special cells in them called pigments. The main pigments
are: chlorophyll-a, chlorophyll-b, carotene, phycobilin, and phytochrome.
Each of these has the ability to absorb a certain frequency of
light and extract energy from the light by converting photons
to electrons and then sending the electrons to energy centres.
The main pigments involved in photosynthesis are the chlorophylls.
Chlorophyll-a is the primary pigment for photosynthesis, but cellular
plants have developed a helper pigment called chlorophyll-b.
In the advanced cellular plants, there is a second step to this
photon energy conversion. A second reaction centre, which is sensitive
only to light at 700 nm (P700), boosts the electrons a second
time to make chemical energy in the form of NADP (nicotinamide
adenine dinucleotide phosphate).
At the end of this process, the light energy that has been trapped
has now converted many photons into usable chemical energy that
is the basic energy of all life on earth. This energy is now used
totally independently in a process called the carbon-fixing reaction,
where carbon dioxide (C02) is split, and the resulting carbon
(C) and hydrogen (H) atoms are made into sugars and starches.
The best known example of plant sugar is maple syrup, which comes
directly out of trees. Most astoundingly, the newest research
has shown that this entire process takes 3-20 trillionths of a
second, which would make any computer chip proud.
Because sugars are energy, the more sugars a plant can make and
store, the more a plant can yield. The plant needs a blue light
source,The blue light has a double involvement in the
creation of plant energy as explained above. The plant specifically
needs blue light for some enzyme and hormone activation; blue
light from i 400-500 nm is absorbed by an internal pigment, and
this is an active fIrequency for auxins. Blue light is also active
in chloroplast control blue light stimulates the stomata to open,
and so does red light.
Indoor gardens are well balanced if they include some Metal Halides
or Mercury lights, which have a better blue spectrum.