Artificial Lighting and Light Meters for Growing Plants

[Bern]

There were many great posts to the PBS List about artificial lighting. Now we have a place to post and discuss that topic on the Forum.  I'll begin by listing some reference articles on LED lighting that were published in Orchids Magazine and were written by Kelly McCracken, who had a successful orchid growing business in Albuquerque, New Mexico.  Kelly was able to grow and propagate orchids with low, medium, or high light requirements under LED lights in an otherwise unlighted warehouse at 5000 feet in elevation in an arid environment.  This is an impressive accomplishment and it highlights the potential of artificial lighting for plants, especially LED lights. Kelly makes a convincing and evidence based case for using a PAR meter (Photosynthetic Active Radiation) to adjust the PPFD (Photosynthetic Photon Flux Density) for the type of plant you are growing.  Apparently, some LED fixtures available today are even dimmable.  PAR meters are pricey, but you get exact information that you can use and you don't have to guess how much red or blue light you need, what is the exact Kelvin rating of the lights you have, or endlessly adjust the height of the shelves for your plants.

You can obtain copies of these articles by asking your local public library to provide you with a pdf copy. After you obtain your copy, you can combine them into one file with a pdf editor.  Free pdf editors are available online. The final document will be about 12 pages long with a file size of approximately 18 mb.

2021/07 July ORCHIDS Magazine
·An Introduction to PAR and PPFD - Why You Should Forget Lumens: Part 1 of a Five Part Series on Light
 
2021/08 August ORCHIDS Magazine
·Let There Be Light! - Target PPFD for Orchids and Tropical Plants: Part 2
 
2021/09 September ORCHIDS Magazine
·Let There be Light - Part 3: Measuring Artificial Light Without a Quantum PAR Meter
 
2021/10 October ORCHIDS Magazine
·Let There Be Light - Part 4: Setting up Your LED Lights
 
2021/11 November ORCHIDS Magazine
·Let There be Light - Part 5: Spectrum and Photoperiod

[CG100]

A very great deal of what appears on the www about lighting for plants is nonsense - I have not read anything on your links so far, so cannot comment.

I used to work in lighting/lamp R&D and have trawled lots of the literature produced by Philips about commercial horticultural lighting, and I have written an article for the SABG newsletter that will appear in early 2024.

One simple fact - growing rooms use around 600W of LED lighting per sqaure metre. All of the articles that I have found online suggest far, far, far lower levels of illumination.
The only way to provide really useful levels of light for growth is to use commercial horticultural lighting - it is impossible to cram in enough lighting designed for domestic use.
Flowering is usually triggered by photo-period and that can be at very, very low levels.

Another important point is that the human eye is of no use in judging light levels - it is an amazing bit of engineering and compensates. Most people can read, just, at around 100 lux, whereas a bright summer's day can be around, or even over 100,000 lux, even in the UK. Try one of these if interested -

https://www.amazon.co.uk/dp/B07QFXSDKL?psc=1&ref=ppx_yo2ov_dt_b_product_details

One thing that did surprise me is that amounts of light are now routinely quoted in moles (mols) within horticulture. I was briefly asked to look at developing a metal halide lamp specifically for promoting photosynthesis and we used plant growth lumens at that time (very different to human eye photopic lumens, which is what lamps for domestic use are quoted in).
Conversion from human eye photopic lumens depends on colour temperature (essentially balance of red and blue content), but a serviceable conversion is 1000 lumens of white light (any CCT) is ROUGHLY 17 µmols of potosynthetically active light/radiation per second.

Some useful links -

https://engaging-data.com/solar-intensity/

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

https://www.lighting.philips.com/application-areas/specialist-applications/horticulture

https://www.assets.signify.com/is/content/Signify/Assets/philips-lighting/global/20230124-philips-gridlighting-technical-specification-sheet.pdf

https://www.assets.signify.com/is/content/Signify/Assets/philips-lighting/global/20230124-gridlighting-photosynthetic-flux-density.pdf

https://www.lighting.philips.co.uk/application-areas/specialist-applications/horticulture/greenpower-specialist-applications/led-flowering-lamp

[Bern]

One thing that did surprise me is that amounts of light are now routinely quoted in moles (mols) within horticulture.

This was one of the biggest surprises for me in Kelly's articles.  I'm used to the concept of moles in chemistry, but I had never encountered it in regards to lighting. The use of Avogadro's number (6.022 x 10²³⁾ to quantify photons was a new concept to me.  This advance coupled with the development of sensors that can quantify the photons in the photosynthetically active parts (PAR) of the electromagnetic spectrum and then give you a readout on a PAR meter in µmols per square meter appear to me to be a real advance in understanding.

Kelly addresses most of the points you make in her articles.  You may find them "illuminating."

Kelly's articles are written in such a way that a layman can gain much useful knowledge about using some of these advanced techniques to better understand artificial lighting for plants.  The articles are not written exclusively for technical specialists.  So I encourage everyone who is interested in this subject to take a look at her articles.  She gained and deployed this knowledge in an experiential way to start a successful orchid business in an arid environment that one would normally consider unsuitable for growing this type of plant.  Succulents yes, orchids no.

And keep an open mind about what you read. We can make advancements going forward from this initial vantage point.

[Bern]

Is anyone using a PAR meter to measure lighting for their plants?  If so, would you please tell us the make and model of the meter you are using and how you think it is working for you?  And would you recommend the meter that you are using or would you recommend a different model?

[CG100]

I can only find one - VERY expensive - a Hortipower PG200N.

Use a cheap luxmeter and the simple conversion as above - perfectly close enough. If you hunt online, you will find that different CCT lamps vary by not much in that figure of 17.

At the end of the day a mole (mol) is just a number, so can be applied to anything, albeit most would make absolutely no sense at all - a football crowd of 0.00000000001 mol? (I did not count or calculate the zeros    ).

[CG100]

the development of sensors that can quantify the photons in the photosynthetically active parts (PAR) of the electromagnetic spectrum and then give you a readout on a PAR meter in µmols per square meter appear to me to be a real advance in understanding

I suspect that it is software, not hardware - FAR cheaper and basically rather simple in terms of both hardware and software; it just means calibrating a silicon photo-cell, although a generic one would be good enough and can be downloaded from the www.

[David Pilling]

At the end of the day a mole (mol) is just a number

I wish someone had told me that when I was 16 and doing A level Physics - a mole of football supporters would have been a very enlightening concept.

Seemingly Mr Avogadro never knew his number, much less application to photons.

[CG100]

LLLOL

[Bern]

Use a cheap luxmeter and the simple conversion as above

Kelly discusses the conversion factor from lux to PPFD in her third article. Kelly appears to have a fine technical background and she knows her subject matter.  She is a woman that is worth listening to and her articles are worth reading. 

If I remember from the List there are PBS members who own PAR meters and are using them. It would be great to hear about their experiences with them.  I'm thinking about purchasing one and it would be helpful to know more about them from a person currently using one.

[CG100]

If you read around on the www, sticking to articles led by science and the real world, a lux meter is all that you need, which cost peanuts.

Another couple of easily-proven facts that will probably surprise and/or horrify many -

A greenhouse with spotlessly clean glass transmits around 70% of natural light. This is down to absorption and reflection by the glass and obscuration by framework etc.

Small bubble bubble-wrap insulation absorbs around 15% of natural light.

A modest, barely perceptable, film of algae on glass ,plus bubble-wrap, will cut natural light by over two-thirds. The human eye/brain will see/perceive a difference but not register it as anything even close to that huge.

I have been through this with my own greenhouse, and can confirm that it isn't Philips making numbers up.

[OrchardB]

Many of the artificial lighting videos I have seen on tv show very limited wavelength lighting, often apparently just 1 or 2 narrow wavelength ranges. Is this just for edible/experimental products or does it apply to other commercial plant propagation? That's before considering light meter ranges.
Brian, SE UK

[CG100]

Is this just for edible/experimental products or does it apply to other commercial plant propagation?

Photosynthesis is driven by any visible wavelength but is most "sensitive" in the yellow-orange-red, in other words, growth is most efficiently produced by illuminating plants with light in this colour region. This is why HPS/SON/Lucalox (all the same thing) were the normal horticultural lighting prior to LEDs and even today in some circumstances.

Inter-nodal distance ("legginess" of a plant) is controlled by blue light - less blue, more leggy. Not much is needed, but it is essential for normal plant growth.

Other than that, the world is your oyster.

[Robert_Parks]

If I remember from the List there are PBS members who own PAR meters and are using them. It would be great to hear about their experiences with them.  I'm thinking about purchasing one and it would be helpful to know more about them from a person currently using one.

I have a PAR meter (~US$150?) that I use to check lighting for the indoor bulbs[1]. Also, to verify outdoor lighting situations.

I do not use it at a level (accuracy, consistency, etc) to justify the price, but it does do the job in a handy package. What it does do is let me track and reproduce what individual species prefer or tolerate in my conditions. Interesting sometimes just how little light some forest floor plants want, compared to fall sun!

[1] Irresistible geophyte aroids that tolerate my (even indoors) cool tropical conditions.

Robert
Cool sunny SF, in a break between storms

[CG100]

Anyone seriously interested in lighting and plants should take a look on the Philips website - there is an entire (large) library of information available, so much that finding specifically what you are looking for can be quite a challenge.

Apart from SON, they have lots of information on LEDs and metal halide lamps and they also discuss "light recipes", where different blends of colours are preferred, usually in contolling flowering, usually in pot/house plants.

The problem with all meters is calibration, especially where a meter claims to do any kind of wavelength analysis.
I checked the one that I have against one that a friend has (same model), and they gave the same numbers, for what that is worth.

People may find this useful -  Convert Lumens to PPF - Online Calculator | Waveform Lighting

Something that Kelly is obviously not aware of is that the lumen is not one scale - the one used for general lighting is always human photopic (bright light) lumens. If anyone was designing street lighting, they would probably use human eye scotopic (dim light) lumens and for plant growth, anyone would use plant growth lumens.

Somewhere, I have a copy of the GE Excel sheet - Phocalc - you can load spectra into it and it calculates all three types of lumen, plus CRI, colour coordinates etc. etc.. It was used routinely for lamp development.

[Martin Bohnet]

Well, I'll agree that the cheaper lamps are questionable in some respects, then, on the other hand I do think I see better results in both winter survival for my winter growing bulbs and plant quality in indoor starting of things like tomatoes.

And while the non full spectrum lights may be well adapted for photosynthesis and prevention of legginess they may fail in some other processes: Three pictures of Digitalis canariensis attached: first is a plant grown under red/blue LEDs instantly photographed after bringing it to daylight. Second pic is a week of natural light later, and third is how the colors should be if all development of color happened under natural lighting... I bet other levels of secondary plant substances like aroma, vitamins etc are also affected by "less important" parts of the spectrum.