Midlands Dahlia Society: Tuber Propagation - LED lights

Propagation 2011 - What do LEDs provide?

Light choice
Given I wanted to have the most efficient lights, and that I also had an understanding of their light performance, I decided to use LEDs.

LED technology
It had been many years since I designed electronic circuits incorporating LEDs. The technology has moved on significantly since then. I am familiar with the low power red indicator LEDs - typically about 50mW of electrical power. You can now get LEDs in many colours - one breakthrough has been the invention of blue LEDs. The picture below shows the spectra for red, green-yellow and blue LEDs.

So this explains the design of LED grow lamps advertised as using lots of low power LEDs - they are using different LED colours to attempt to create the chlorophyll absorbance curve. However can they adequately match the peaks and provide sufficient moles / day?

White LEDs
Another breakthrough has been the invention of white LEDs - see the picture below, where we can see the peak in the blue region, a dip in the green region and a lower peak in the red region. .

White LED spectrum

High Power LEDs
The final breakthrough, as far as this project is concerned, is the creation of High Power LEDs. These use currents of over 1 Amp and are typically up to 5W. They can be seen in bicycle and car lights. One major manufacturer is Cree - so this is where I ended up. The next decision is whether to use a combination of separate LED colours or white ones.

Separate coloured LEDs or White?
Looking at the spectra for single colour XP-E LEDs from Cree, it became apparent that the Red ones peaked at 625nm which is not aligned with the chlorophyll a peak. The blue LEDs were rated at 1Amp max current, whilst the red could go up to 700mAmp, so mixing blue and red would mean only 700mA for the blue, which reduced their lumens. Looking at white LEDs, the XP-E rage are limited to 1A, however the XP-G range, could be driven at 1.5A - obviously these would be best. However there are four 'colours' - warm white, neutral white, outdoor white and cool white! So how best to choose between them?

How to assess LEDs vs Chlorophyll?
There are several different sets of people designing with HPLEDs for grow light applications. The two that I used to get into designing mine were the Cannabis guys, and the aquarium reef keepers. On one site I found a spreadsheet which converted the HPLED spectral density curve - as per the previous picture, modified it to take account of being normalised for the human eye, and then calculated the PPFD - the measure of PAR in µmol m^-2 s^-1 (PPFD is Photosynthetic Photon Flux Density). This gives the following results in µmol m^-2 s^-1 for the three spectra curves in the Cree application, note based on 120lm.

Results	Cool white	Neutral white	Warm white

Lumen	120	120	120

PPFD total	1.72	4.10	1.94

PPFD Blue	1.26	2.14	0.46
PPFD Blue Chlor	1.15	0.80	0.41
PPFD green	0.09	0.05	0.07
PPFD Red	0.37	1.92	1.41
PPFD Red Chlor	0.11	0.31	0.32

LED with maximum Luminous flux?
The table above indicates that the neutral white gives highest PPFD. However the aim is to now choose the best Neutral White LEDs to achieve maximum light. When the LEDs are manufactured, there is a manufacturing variation and the LEDs are placed into different groups having different lumen values. Fortunately the place where I eventually purchased the LEDs had some R4 Neutral White XP-G LEDs with 130 lm at 350mA. Given I want maximum light, if I drive them at 1.25 A then the output increases to 377 lm. This then has a PPFD of 12.89 µmol m^-2 s^-1 .

Where next?
So now I have decided on the LED, I need to get on and design the LED Grow Light itself. This leads to more research - into constant current sources, thermal properties of Aluminium heat sinks, as well as overall thermal design.