BY THEO TEKSTRA – MARKETING MANAGER GAVITA HOLLAND BV

“LEDs are the future”... How often have you heard that in the past few

years? So, is the future already now, or are we getting there? Is LED the

new “wonder light,” or do we still have to learn to appreciate LED? An

overview of the history of LED and the current status.

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A BRIGHT FUTURE?

LED LIGHTING I GARDEN CULTURE

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Not all energy is converted to light, and not all light escapes

the semiconductor, and there is a lot of heat development

as well. However, in contrast to HPS lighting, this is not

infrared radiated heat, but conducted heat. So LEDs do

create heat, just not as radiant heat, but conductive heat.

This is why LEDs need to be thoroughly cooled, and this is

also the Achilles heel of many LED fixtures. Temperature

is the enemy of a long LED life and light maintenance,

hence the need for a large thermal heat sink. So the light

of LEDs contains very little heat, in contrast with high

pressure sodium, which contains about 55-58% infrared

radiation. Is that a lot? Not when you realize that the sun

emits 53% infrared radiation. There we already have one

big difference between HPS and LED.

White LEDsNow each specific semiconductor will give you a specific

color of light, but there is no semiconductor that emits

white light. How do they do that? Very simple: think

compact fluorescent (CFL). When they developed a blue

LED in the 90’s, they suddenly also had the technology

to make a white LED, by adding a phosphorous layer

on the blue LED. So basically a white LED works the

same as a CFL: The blue light excites the phosphor,

and depending on the blend of that phosphor, a specific

color (band) will be emitted. The phosphorous layer is

usually semi-transparent, so some blue light will also still

come through. By changing the phosphor coating you can

change the spectrum.

However, phosphor coatings do have a conversion

efficiency, and they do reflect some light back to the

emitter as well. The best coatings nowadays are 85%

efficient at optimal temperatures, so there is a loss of light

too. Moreover, a phosphor coating close to the emitter

will heat up, decreasing its efficiency. That’s why white

LEDs are less efficient than their blue counterparts. The

phosphor coating is more diffuse than the original blue

LED. Lens systems are required to get the light where

you need it.

HistoryDid you know that the first LEDs were commercially on

the market long before the first HPS lamp? It was 1962

when the first (infra)red LED hit the market, and the

first HPS lamp came out around 1968. The LED found

its way into computers, watches, calculators, and other

appliances that needed a long life, low power indicator or

display light. First, there were the red LEDs, and later new

technologies enabled other colors too. The first green

and yellow LEDs came out in 1971, but it took until 1993

until the brilliant blue and white LEDs hit the market.

Back then we never thought that LED technology would

replace the incandescent lamp, or even the highly efficient

sodium lights. That is a development of the last 10 years.

LED has evolved into what we call a “disruptive

technology”: it displaces established technology, such

as incandescent and CFL, and shakes up the industry. In

1993, we had the first white LEDS, but it took until 2006

for LED technology to hit the 100 lumens per Watt mark.

Today, there is LED technology that goes even beyond

that, though it becomes harder to raise the bar, and it

does come at a cost.

TechnologyWithout diving too heavily into the actual technology,

LED light is based on what we call Electroluminescence.

In laymen’s terms: When applying electrical power to

a crystal (a semiconductor), it starts emitting light.

Contrary to what you would expect, light does not exit

the semiconductor in all ways, but in certain directions

due to its structure. By shaping the semiconductor in a

specific way you can focus the light. The LED is usually

embedded in a clear molded plastic, that acts not only as

a housing to hold the fragile electronics, but also as a lens

to diffuse the light from the emitter.

Cross-section of Philips LED, Luxeon K2

THE FIRST LEDS WERE COMMERCIALLY ON THE MARKET LONG BEFORE THE FIRST HPS LAMP

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At this moment the best single color LEDs can do up to

2.7 μmol s-1 per Watt under ideal circumstances, for white

LEDs this is about 2.4 with remote-phosphor technology.

These are the top of the line LEDs with a very high efficiency,

but they also come at a cost.

In 2000 at a conference, Dr. Roland Haitz presented a

forecast about LED price and output development, now

known as Haitz’s law. It defines light per package, so not

necessarily the efficiency. As you can imagine, there is a

theoretical maximum in the amount of light you can convert

from energy. What is interesting though is the prognosis of

the price compared to the light output.

The law is about as follows: Every decade the cost per lumen

drops a factor 10, while the output per LED package (single

unit) increases by a factor 20. So you will get bigger LEDs

with higher output, against a much lower price. What does

this mean for our purpose?

First of all, the efficiency of LEDs does NOT increase a

factor 20. It is not even said that the higher output LEDs will

be much more efficient - at first, they will just have a higher

output. What is interesting though is that prices will drop.

We have seen this in the past ten years, and we still see a

drop in price.

Remote-Phosphor LEDsRelatively new on the market are the remote-phosphor

LEDs. They are more efficient than the integrated white

LEDs, with up to more than 20% better conversion rate

than the standard white LED. How does that work?

Phosphor conversion process in a white LED and remote phosphor

system – Source: CREE

A normal white LED has the phosphor coating very close to

the emitter. There will be conversion losses and reflection

losses as you see in the illustration, but the biggest problem

is the temperature of the LED, and therefore, the phosphor

layer itself. Some of the blue light will pass, some will be

converted, but the conversion rate is negatively affected by

the temperature of the phosphor.

In a remote-phosphor application, we use blue LEDs and a

phosphor coated (glass) disk at a distance from the LEDs, in

a reflector. While the efficiency of this system is a lot better,

it will only produce diffuse light, because, due to the size of

the phosphor disk, and the diffuse output of the light, it is

very hard to concentrate this light. Also the costs can be

much higher than single white LEDs, because the costs of

making the fixture with the disk is higher, not to mention

IP ratings you want to achieve. So, for close diffuse lighting

this is a great technology, but not so much for focused high

intensity top lights.

Output Power / Light Output / EfficiencyThe theoretical efficiency of an LED could even be over 4 μmol

s-1 per Watt when all of the energy could be converted, It

would require far future technology to even get close to

this, so don’t count on that coming for years. We will see

though that efficiency will get a bit better still, and that the

prices are going to drop.

TEMPERATURE IS THE ENEMY OF A LONG LED LIFE AND LIGHT MAINTENANCE

LED LIGHTING I GARDEN CULTURE

COB LEDs

Cree high output white COB

You might have heard about COB LEDs. COB stands for Chip

on Board, and it is a way of producing cheaper high output

LEDs by pre-mounting many of them on a board. While the

high output single LEDs range up to several watts, you can

make COBs in much higher wattages, combining multiple

LEDs. A phosphor coated COB will have less glare than the

single point LEDs, but the light density is also a bit lower than

the single LEDs. A lens system is needed to direct the light,

leading to extra losses.

COBs are developed to reduce costs and increase output,

and they are not necessarily the best solution for all lighting

needs. They might be too concentrated in some applications,

or not concentrated enough for deep penetration.

COBs will decrease the price of LED fixtures, but it remains

to be seen how they can be best applied.

Light MaintenanceHPS light maintenance is specified as percentage of light

maintained over a period. For the Philips GreenPower plus

1000W EL this is as follows:

4,000h 98%

8,000h 96%

10,000h 95%

LEDs are specified differently. For household LEDs we use the

specification L70 as an industry standard: It is found out that

70% lumens maintenance is close to the threshold at which a

human eye can detect a reduction in light output. So, the L70

specification gives you the number of hours of operation until

there is a degradation in light of 30%. For plants, however,

light maintenance is crucial. You should always look at a L90

for LEDs that are used in Horticulture. This is already 10%

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less light, a value you would not reach with HPS lamps

(they would already be replaced). The light maintenance

is greatly influenced by the junction temperature, which is

the temperature of the base of the LED. High temperatures

decrease this dramatically:

Source: Philips technology white paper

So, environment and cooling of the LEDs plays an important

role in getting optimal light levels and optimal light

maintenance. A L90 of 25,000 hours could well be much

lower.

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LEDS ARE AN EXCITING NEW LIGHTING TECHNOLOGY

A MICROMOLE OF LED LIGHT IS STILL 6-10 TIMES MORE EXPENSIVE THAN HPS

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What is the verdict?LEDs are an exciting new lighting technology. They are

efficient, available in almost every spectrum you desire, and

they have a long life. You can use them in applications where

HID lamps could never be used, for example:

• Interlighting (lighting between high crops in

greenhouses)

• Multi-layer growing (food factories where distance to

the crop is low)

• Specific supplemental light (for example to supplement

HPS)

Above: Tomato LED trials at GreenQ –

innovation center in The Netherlands

LED LIGHTING I GARDEN CULTURE

HPS is not an alternative for any of those applications.

However, there are some disadvantages as well. LEDs are

on full trial in many greenhouses, because growers want to

experience what is needed to grow under LED successfully.

However, replacing HPS top lighting with LED has a few

problems that need to be overcome:

1. Uniformity. As you want to intercept as little light as

possible you must make light strips that fit under existing or

new profiles.

2. Climate conditions. LEDs perform best and live

longest when they are not driven to their highest wattage,

and are kept cool. Temperatures at the top of a greenhouse

can be very high.

3. Little infrared heat. Even in a desert, the nights are

cold. In the winter seasons, when the assimilation lighting is

used most often, the infrared radiation of the HPS light is

a very efficient way to heat your crop. With LEDs you will

need to heat your greenhouse as all the heat dissipates at

the back of the LED. Also, the lower infrared radiation leads

to less evaporation of water, so you need to up the EC of

your nutrients by more than 25% in many cases.

4. Price. A micromole of LED light is still 6-10 times more

expensive than traditional HPS lighting. This is the biggest

problem, as you have to invest heavily, and will need to keep

that installation for at least 5-7 years. In that time we expect

LED to become at least 20-40% more efficient, so when

are you going to invest? When there is a more efficient HPS

lamp available, it is just a matter of changing a lamp, but you

can’t with LED.

For indoor uses, many of these issues are the same. As

it is your primary lighting indoors, the investment is very

high. No infrared means that you will need to find a way to

keep your crop at the optimal temperature, while 1000W

of LED create the same heat gain as 1000W HPS. So in a

recirculating room there is hardly any advantage in cooling.

We will need to learn how to grow under LED. Prices

still need to drop to make them a viable alternative for

HPS. But the future is nearing, and it is a good idea to get

some experience with growing under LED, as you will be

confronted with different growing challenges. 3