Grow Lights – Intro Part 4: How We Test and Produce Results

So, in the first 3 parts of our introduction to grow lights, we have chatted about light, what it is, what different aspects of it there are and what plants want.

In this intro article, we will explain how we will be testing grow lights and how we come to our conclusions. The idea is that for every grow light we will be able to:

  • State it’s primary use (cloning, veg, cloning)
  • State a recommended area to use it to cover (and what size grow tent it suits)
  • State the best height to hang it above plants for best mix of intensity and area coverage
  • State (for flowering lights only) an estimation of potential yield for comparison purposes
  • State the efficiency
  • State an estimation of the potential quality it will produce based on spectrum

Beginning the Test

It was decided that each grow light should be tested in the way that gets the best out of it. If they were all tested the same way, the testing would naturally favour some grow lights but be unfavourable for others. This would be unfair, as well as giving us pretty irrelevant results. Testing a light at a height that it would not be used at in real life would be a rather pointless exercise.

With this in mind we decided that for each grow light, we needed to find that hanging height where it gave the best compromise between good intensity at the canopy level and good area coverage.

The two main factors that affect this are the power of the light, and how the reflector spreads that light out as it throws it downwards. For example, a 1000 watt HPS light in a reflector with a tight, narrow spread will need to be hung a lot higher than a 400 watt HPS in a reflector with a wide spread.

To help us find that ideal height, we mount each light above our purpose made test bed:

The test bed consists of a frame which suspends a grow light at an adjustable height (by using adjustable rope ratchet hangers). Underneath, we have a grid marked out for measurement positions for our ppfd (quantum) sensor. These are spaced at 6″ (150mm) intervals left to right, and front to back.

This allows to hang a grow light and then test the light spread, in ppfd, at many points in a grow space. Using this data we can build up a picture of spread, or lighting footprint, of a grow light

The test points around the very outside represent the outside points of a 1.5m x 1.5m grow-space – usually considered ideal for a 1000 watt grow light.

The test points just inside those around the outside represent the outside points of a 1.2m x 1.2m grow space – usually considered ideal for a 600 watt grow light.

The test points inside of those represent the outside of a 0.9m x 0.9m grow space – only just slightly smaller than is considered ideal for a 400 watt grow light.

Choosing the Right Height for the Light

Now, many growers believe that the amount of light equates to final yield. However, that if that light that is concentrated in one area below the light, rather than being spread out evenly over the whole grow area, then there area several repercussions. Firstly, only the area directly under the light will be particularly productive, while the parts around the edges of the grow area will get little light and therefore be relatively less fruitful. Secondly, HPS grow lights produce infra-red. If that infra-red is concentrated too much in one area it will cause a big increase in leaf temperature. If leaf temperature goes up too high then Vapour Pressure Deficit could be detrimentally affected – exactly the opposite of what we want.

Another reason that we want a good spread of light, particularly in the flowering/fruiting period, is that the stalks where flowers and fruits are usually produced are vertical. This means that the bulk of the surface area that is producing the goods is facing actually horizontally (out to the sides). The surface area of the productive parts of the plant that are exposed to  light hitting it from above is much smaller. It is therefore beneficial to get light hitting the plant from the sides rather than onto the tops.

For a given amount of light, if we shine it from above then much less of it will hit the productive parts of the plant than if it hits it from the sides. A grow light that can be hung low and throws a large proportion of its light onto the sides of a plant can be more productive than one that needs to be hung high and therefore shines it’s light onto the tops.

A better (or more even) spread of light means that more light will be hitting the plant at a more horizontally oriented angle, particularly around the edges of the grow area. As a total, there is a lot more total sum of area around the edges of the grow area than there is right in the centre of the grow area. Hence, a good spread of light over the grow area will also have a greater effect on final yield than just increasing the power of your grow light. Whether a better spread is being achieved in a grow tent or in a multi-light arrangement (where overlapping of light occurs) then a better spread of light could, again, produce better results.

As stated before, a particular grow light will have an “ideal” height that it is best to use at. In order to do a fair test we need to find out what that height is. As well as that, high power grow lights produce heat. Before testing can commence we need to switch the light on and allow it to fully warm up and stabilise. Also, for HID type lamps, they need a certain amount of burning-in from new.

So, to find that “sweet-spot” for a grow light where we are getting a good amount of light under the grow-light directly under the light and getting a good spread of light around the edges we came up with the following methodology:

  1. Begin with the fully warmed-up light mounted at 300mm, or 400mm for 1000w lights
  2. Measure the amount of micromoles being shone at the centre, the side, the front and the corner using a grid area of 1m square for a 400w light, a 1.2m square area for a 600w light and a 1.5m square area for a 1000w grow light. For those grow lights with a non-square footprint, this is taken into consideration and an appropriate measurement area chosen.
  3. Raise the lamp to 400mm and repeat the measurements
  4. Repeat steps 2 and 3 going up in 100mm increments right the way up to 1200mm
  5. From the results,  find the height where we are getting the average greatest ppfd measurement at the sides, front, back and corners, but without sacrificing any more than getting 500 to 1000 micromoles at the centre. In reality, we want around 400 micromoles for a 400 Watt HPS grow light, not much less than 600 micromoles for a 600 watt HPS grow light, and around 1000 micromoles for a 1000 watt grow light. Any more than 1000 micromoles will almost certainly cause detrimental increases in leaf temperature, and much less than 400 micromoles at the centre will be getting into the realms of losing yield also.

A large proportion of growers use grow tents which have a fixed (and often limited) amount of height. Therefore, as well as considering centre intensity and the spread, the light height chosen is the lowest that we can without compromising good intensity and spread.

As an example, here are results from a test that we did for a complete 400w LED grow light – a Spectrum King SK400+. The Side, Front-Rear and Corner averages were taken just outside the normal usage area to ensure that in a real world scenario there would be reflection from the surrounding tent or grow sheet, ensuring that plants would be receiving light from all sides:

Glancing down the table we see that at 600mm is where we are getting nearly that 600 micromoles that we hope to see from a 600 watt grow light. Also, this is the height at which we get close to the peak numbers at the sides, front to back and in the corners. It is the lowest we can go without significantly affecting the centre intensity and but (probably more importantly) the light spread.

Now that we know how to find the correct height to get the best from a particular grow light, let us move on to the next test stage:

Testing the Light Spread

Using the test-bed shown at the beginning of this article, the grow light to be tested is hung at it’s ideal height (which has already been found by the method outlined above) from the frame which is over the grid layout. Using a spirit level, the reflector is set to be perfectly level. The grow light is then given plenty of time to burn-in and warm up to a stable operating temperature.

With the quantum sensor positioned in the centre testing position, the light is moved forwards, backwards and sideways until the highest PAR reading is found. This ensures that the centre testing position represents the position which is the centre of the grow light’s footprint.

The test positions are set 150mm (6″) apart:

The Red Box represents a 0.6m x 0.6m area, ideal for 250w grow lights

The Yellow Box represents a 0.9m x 0.9m area, good for 400w grow lights

The Green Box represents a 1.2m x 1.2m area, ideal for 600w grow lights

The Blue Box represents a 1.5m x 1.5m area, ideal for most 1000w grow lights

 The PPFD is measured at all of the test positions within the appropriate area for the wattage of the grow light on test. In the case of the Heliospectra LX601C 600w grow light this was all the test positions within the green box illustrated above. The PPFD at each position is then noted down.

Here the readings taken for the Spectrum King SK400+ within the green box area (0.9m x 0.9m) as shown above:

We can input these numbers as a grid into Microsoft Excel and then it’s graph functionality to produce a nice 3D visualisation of the light spread:

Each of the numbers at the different positions is roughly an average of the PPFD in the 150mm x 150mm position surrounding that testing position.

If we add all the PPFD readings together, the summed value we arrive at is a crude integral of the total amount of light that the grow light is giving out. Of course, to arrive at a perfectly accurate integral we would have to test at an infinite number of positions. Unfortunately, that is not practical. However, for our purposes, this estimate is accurate enough to be able to compare grow lights and the total amount of light that they produce.

The integral value that we have arrived at (in this case 16341) can be used as part of a relative estimation of how much photosynthesis (and therefore yield) that a grow light can produce. Please note, as these calculations are not perfect, we stress that there are no units of measurement! The numbers that we arrive at will only be useful when comparing 2 different grow lights.

Introducing Spectrum into our Yield Estimate

PPFD is not the whole story where photosynthesis is concerned. Most growers know that for a given PPFD, Red/Orange light drives more photosynthesis than the equivalent PPFD of Green and even Blue light.

To gain a more accurate yield estimate, we need to factor in the spectrum that the grow light produces and then weight the PPFD integral using the McCree curve. The McCree curve shows us relatively how much photosynthesis is produced by the different colours of light:

Continuing with our example of a Spectrum King SK400+ Grow Light, if we take a Spectrum Graph Reading (using our Sekonic C-7000 Spectrometer), we can overlay the McCree curve. This will give us a visual idea of how effective the spectrum that a grow light produces is at driving photosynthesis (and therefore producing yield):

Now, if we read off the relative intensity at a particular wavelength, and if we divide that number by the total area under the spectrum graph, we will be able to find the proportion of the total light produced by the grow light that is given out in as that particular wavelength.

If we then multiply that figure by the relative action (photosynthesis) number (derived from the McCree curve) at that particular wavelength and we perform that same  calculation for every wavelength in the PAR region (400 – 700nm) and then add all those numbers up, then we can derive a result that tells us relatively how effective the spectrum of the grow light is at driving photosynthesis.

Fortunately for us, the Sekonic C-7000 produces a spread sheet which contains a list of all the wavelengths that it reads, and the relative intensity of the light at each wavelengths. I produces this for all the wavelengths individually and also in 5nm wavelength bands.

So, we can add all the relative intensity values together to get a relative area under the graph. We can then divide each wavelength by that summed total to find the proportion of the whole light spectrum that is emitted at that wavelength. If we then multiply that number by how effective that particular wavelength is at driving photosynthesis  (taken from the McCree curve) and add the derived results for all the different wavelengths, then we arrive at a figure which indicates relatively how effective the spectrum from a grow light is at driving photosynthesis.

If we then multiply the PPFD integral (derived earlier) by the spectrum effectiveness value then we can come up with a number that can be used to compare how much photosynthesis (and therefore yield) will be produced by different grow lights!

We’re going to call it our “Estimated Yield Factor”.

Estimating quality

From the McCree curve we can see that blue light tends to produce less yield than the equivalent amount of red/orange light. However, blue light encourages the plant to produce more terpenes (the flavour compounds) in our final product. If (by using the method above) we find the proportion of the spectrum of a grow light is just in the blue region (400nm to 500nm), we can calculate a Relative Quality Factor too!

Now that we have explained our methodology for finding our “Estimated Yield Factor” and “Estimated Quality Factor”, we will be performing some in-depth testing of LED, HPS, MH, CMH and other types of grow light. We will also be testing reflectors, lamps and ballasts and giving estimated yield and quality factors where applicable. We will then be publishing the results in forthcoming blog articles.

Grow Lights Intro Part 1 – All about Light for Growing

Grow Lights Intro Part 2 – The Light Spectrum

Grow Lights Intro Part 3 – Light Intensity and Measurement

Happy Growing and see you soon.