Introduction

After assembling our hydroponics system we needed to measure the light intensity that the LEDs output. This gave us a bearing on the probability of successful germination. For example, an overcast day in lux is between 1,000 - 30,000 [1]. Therefore a lux sensor will give us a starting point. From this, we can then begin to experiment with different colours of light. Through our Human Practices we expected that purple light would give us the most lux since this is the most commonly used in the industry.

Results

Our initial tests were to run the pre-programmed colours i.e. Blue, Green, Purple, Red, White and a rainbow function which cycles through the spectrum.

Firstly we decided to measure the 'rainbow' function. We applied this test so that we could determine a range which may give us the highest lux. We established that purple/blue is the 'optimal' colour to use peaking at 1100 lux, confirming what our Human Practices has revealed when visiting various hydroponic facilities.

Next we loaded the Arduino with a programmed in 'purple' colour. This measurement stabilised at roughly 1000 lux. Leading us to believe we could tweak this light level. Therefore we tried different pre-set colours; Blue, Green, Red, White, giving roughly; 1375, 300, 525, 1360 lux respectively. Realising that the LEDs manage colours by producing different quantities of Blue, Green and Red light, we figured we may be able to create an optimum between these colours hopefully improving on the pre-set Blue. This means we could not use a preset library and would have to define the light levels of the primary colours manually.

Before starting, we defined the brightness of each colour as an 8 bit integer (265 light levels). The most obvious place to start was to turn all the primary colours up to 265. Producing altered white light which performed worse than the pre-set Blue and white at roughly 1300 lux. Therefore we created purple via Blue and Red at 265 (altered purple). This also proved to be less than the pre-set blue but slightly higher that ‘altered white’.

Therefore, thinking that the pre-set Blue was actually our maximum we attempted one more test. Holding Blue at a constant 265 but varying Red from 0-265 and plotting the results. We discovered that there is a lux peak when the Red is at a light level of 129. Next we tried Blue:265, Red:129 with a varied green from 0-265. This appeared to have a detrimental effect on the light intensity. Figure 2 shows the relationship between these colours.

Conclusion

From these measurements we can determine that the lux level should be adequate to germinate Arabidopsis, rocket and various other plants. Heating control does not need to be built as this system will be held inside a room with a constant temperature. Also, water and air flow are not a pressing issue since the water needs to be changed once a week. Therefore the next experiment will involve real plants.