Light and Photosynthesis

Welcome to lesson 7. 😄

Today, we will be learning about how plants make food for themselves. through a process named photosynthesis.

Many of us probably heard of Photosynthesis or even learned how it works when we are in primary school. But we are going to learn more about this process and how can we make use of this knowledge by measuring the amount of light the plant that we are growing needs and through that be more successful in growing the plants which we love.

The Basics

First, let's refresh ourselves by recapping the basics.

Carbondioxide + Water + Sunlight = Food and oxygen

Many of us still remember this equation. Now let's dive in a bit deeper and see how it actually works.

Light

Light is something that we see, without it we would have been blind. And without it, plants will not be able to survive because they will not be able to make their food through photosynthesis. But not all types of light are equally useful for plants during photosynthesis. Only visible lights are used to make food.

https://bceye.com/what-is-blue-light/visible-light-spectrum-color-waves-length-perceived-by-human-eye-rainbow-electromagnetic-waves-educational-school-physics-diagram/

https://en.wikipedia.org/wiki/Chlorophyll_b

Even within the visible light spectrum, not all light is equally useful to the plants too. Only mainly blue and red light are used to make it's food, that doesn't mean all other colours are completely ignored but let's just look at Chlorophyll A and B a little more.

During Photosynthesis, there are 2 types of processes within a chloroplast. Which Chlorophyll a take in the red spectrum and Chlorophyll b takes in blue.

Chemical reactions first take place in chlorophyll a (blue light) then continue on chlorophyll b (red light).

Therefore, we commonly see grow lights comes with this combination of colours.

Lux, PPF, PPFD, and DLI

Now that we know how lights work in plants, let's move on to how much light is out there and how much your plants require.

Lux

This is the unit of how bright a spot is in terms of the visible light spectrum.

Photosynthetic Photon Flux (PPF)

This is the unit of how much light can be used by the plant for photosynthesis. It is represented with a micro mols per second basis (µm/s).

Note: mols is the unit of the photosynthetic photons. When we put it in µm/s, we can now tell how much of these photons are sending out per second.

Photosynthetic Photon Flux Density (PPFD)

With the knowledge of PPF, it's still not enough until we know how much photosynthetic photon is hitting a fixed area. And with PPFD, we can tell how much of photosynthetic photon is hitting every second per square meter area. (µm/s.m2)

DLI (Day Light Integral)

This is the unit for the total amount of light received in PPFD in a day per meter square (mol/d.m2). This value can be gotten by using the following formula.

Now that we had made things clearer about the terms and units, we can put them to good use. 😄

I'm sure some of us had come across a report like the one shown below but do not know how to read it. (Not just because it's in Chinese 🤣)

When looking at a report like this, we first look at the colored graph for its shape. Make sure that there is a sharp peak for the blue and red sections and also a hump for what is in between and then look at their values. Next, we can put what we learned above by looking for (µmol/s) or (µm/s).

As we can see from the picture above, there are Photons 1 and Photons 2.

These represent 2 different varieties of LED lights in the grow light setup.

Photons 1 is producing 4.024e-004 µmol/s and Photons 2 is producing 1.813e+000 µmol/s. This can be a little complicated as the numbers are getting extremely small, so we should look at the value below.

The Chinese words here mean total photons which is 1.5959µmol/s. This value is our PPFD. We should use this value for calculation.

Note: This value is for a meter square.

DLI

Calculating the amount of PPFD for a specific area to which we are shining the light:

1.5959µmol/s X 100 = 527.57µmol/s.30.25cm2

Note: This grow light only shines on a 5.5cm X 5.5cm area. Therefore we have to multiply by 330.58 to get our PPFD for this area.

Per hour

527.57µmol/s.30.25cm2 = 1,899,261µmol/h.30.25cm2

Per day

1,899,261µmol/h.30.25cm2 X 12 hours = 22.791137mol/d.30.25cm2

DLI = 22.79 if we turn on this grow light for 12 hours at a distance of 5.5cm away from it.

How to apply for DLI?

All plants require a different amount of light to grow well. And by knowing this value we give our plants the best amount of light it requires.

How do I tell the amount of DLI my current grow light is giving me?

I am using this app below on my phone to tell me the amount of PPFD I'm getting for both my grow lights and natural lights. It even helps me to calculate the amount of DLI!

Note: You might have noticed this strip of white plastic on the front camera of my phone, it's required to disperse the light to more evenly photons so the app can measure the PPFD correctly. You can either use semi-transparent paper or plastic to act as a diffuser.

We have reached the end of today's lesson! I hope that you had once again learned much from it and have a nice day ahead!

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