Correlated Colour Temperature of Sunlight

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Pattock
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Correlated Colour Temperature of Sunlight

Post by Pattock »

For anybody interested in the spectral balance of natural sunlight, I found a nice table of colour temperature from sunlight from a reference published in 2015. https://www.aulavirtualusmp.pe/ojs/inde ... e/1262/999

Natural Light Source ☼☼☼☼☼☼☼☼☼☼☼☼☼☼ Colour Temperature (K) kelvins

Sky Light ☼☼☼☼☼☼☼☼☼☼☼☼☼☼☼☼☼☼☼☼ 12000-18000
Overcast Sky Light ☼☼☼☼☼☼☼☼☼☼☼☼☼☼☼ 7000
Midday Sun Summer ☼☼☼☼☼☼☼☼☼☼☼☼☼☼ 5000-7000
Midday Sun Winter ☼☼☼☼☼☼☼☼☼☼☼☼☼☼☼ 5500-6000
Average Midday light Northern Hemisphere ☼☼☼ 5400
Sunrise, Sunset ☼☼☼☼☼☼☼☼☼☼☼☼☼☼☼☼ 3000

Clearly, the relative intensities of these lights will be quite different, as well.

Obviously, few small plants and even fewer germinating plants will ever be bathed in direct light from sunrise and/or sunset for very long, due to terrain or vegetation tending to block sunlight from low angles. This would also explain why cereoids grow straight up and sometimes have blue colouring and the little round cacti and succulents often have red pigments. Just too much blue incoming.

I wonder what the spectrum is like for a germinating mesemb happily growing under a quartz rock skylight, as shown in today's talk?
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Davey246
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Re: Correlated Colour Temperature of Sunlight

Post by Davey246 »

They are instantaneous CCT's which are pretty much meaningless for plant growth as they grow so long as there is enough light (and niutrients, water etc.).
I am unsure what intensity has to do with CCT. Season/latitude would have the major effect on intensity as the table essentially lists specific examples of instantaneous sky conditions

You can pick and choose amongst many sources of information, but average CCT through a day is something VERY different.

See also, just for instance, as there are legions of websites that provide information -

https://en.wikipedia.org/wiki/Color_tem ... 20radiator.

https://sensing.konicaminolta.asia/what ... ature-cct/

I'd be interesting in reading the science behind your conjecture on plant body colour.
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Re: Correlated Colour Temperature of Sunlight

Post by Pattock »

To keep it simple, intensity of light at the plant's surface is going to be much lower at sunset/sunrise, because, as well as blue light, some red light is also absorbed/scattered by the longer path through the atmosphere. Intensity of light when the sky is overcast will also be less, clearly. Some plants will prefer the higher intensity light, others will not. In higher latitudes seasons will influence the intensity and colour temperature, clearly. All very complicated.

I am glad to see you quote the Wikipedia table which directly contradicts what you were saying in the other thread, the CCT of natural sunlight clearly goes much higher than 5000K in both these tables. However, I am not sure of the source of that data. The reference is not given on Wikipedia but it appears to come from a self-published book by Bruce MacEvoy, published in 2009. He appears to be re-writing that book as "Color Experience" but does not seem to have got to the part with this table. https://www.handprint.com/CE/book.html

As for your second link, Garie Xu gives no source for their data but again quotes high CCTs for midday sun, 5500 to 6500K. Thanks for the confirmation.

You were involved in the thread in which I posted links for the pigment science: viewtopic.php?f=1&t=172592
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Re: Correlated Colour Temperature of Sunlight

Post by Davey246 »

Interesting stuff here - NOT the same measure, but the corelation to photomorphogenesis curves published by Philps, is poor. However, the graph on the link refers just to chlorophyll a, whereas in nature chlorophyll a in land plants is provided with energy from other wavelengths/colours by carotenoids and chlorophyll b, which absorb outside of red and blue -

https://byjus.com/neet/photosynthetical ... radiation/

Even more interesting when looking at insolation figures as they tend to be simple measurents of luminous flux 400-700nm, known as PAR - Photosynthetically Active Radiation, making no allowance for how different wavelengths are absorbed or what they "achieve", or permit a plant to achieve.
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Re: Correlated Colour Temperature of Sunlight

Post by SimonT »

I'm not sure how much detailed insight can be extracted from Correlated Colour Temperature? It is useful for mapping the perceived colour of near-white lights between filament bulbs to light from LEDs for example. But this value does not exactly map to the emission spectrum and is not about photosynthesis or plant growth. It is convenient to use this to select a bulb designed for domestic use that might also be suitable as a grow light. But the kelvin value of the bulb is a very approximate metric for this purpose.

It is also worth noting that even at the molecular level there is a lot more to photosynthesis than chlorophyl. Response to light is also adaptive- give cactus seedlings "too much" light and they'll soon turn red for example. But if you extract pure chlorophyl from the same plant and under "too much" light measure light absorbtion then you won't take account the effect of red pigment.

So it seems more useful just to use the kelvin scale to identify a bulb from a supplier that supports plant growth, But then you need then to actually measure the plant's growth under this light to get a more detailed understanding.
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Re: Correlated Colour Temperature of Sunlight

Post by esp »

Simon, I agree! There's been a lot of spurious argument lately.
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Re: Correlated Colour Temperature of Sunlight

Post by John E »

As my dear wife has just commented 'How on earth have you managed to grow your plants so successfully when you knew nothing about photomorphogenesis!!!!!!!!!!!!!!!!!!!!!!!!!'
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Re: Correlated Colour Temperature of Sunlight

Post by el48tel »

I wouldn't have described the recent discussions merely as "spurious" .... perhaps "obsessive".
I'm a keen photographer. I take a "keen interest" in colour temperature, especially when I'm capturing time lapse of cacti and succulents in flower and colour rendition is critical, especially when the arrival of clouds can send colour temperatures soaring. Comments such as those suggesting the values over 5500K do not exist, I find "interesting", just as those claiming specific outputs from equipment from a sole manufacturing source.
Endeavouring to grow Aeoniums, Aylostera, Echinocereus, Echinopsis, Gymnocalycium, Lithops, Matucana, Rebutia, and Sulcorebutia.
Currently being wooed by Haworthia, attempting hybridisation, and enticed by Mesembs.
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Re: Correlated Colour Temperature of Sunlight

Post by SimonT »

I think the original discussion was about plant growth not about photography. I agree that there is a lot to learn about light from photography. I'm just not sure how this helps grow plants?

I do think it is important to clearly define the concepts and cite the sources of information in these discussions.
For example, I wonder if the 5500k value comes from one of the online sources state that the sun has a surface temperature of "about" 5500k eg https://www.space.com/17137-how-hot-is-the-sun.html. So then the sun, considered a black body source, radiates at 5500k. So it is confusing that light is then given a higher (CCT) temperature than 5500k, when all sunlight comes from a surface radiating at 5500k? Define CCT and state the source of information and the confusion is quickly resolved. If you don't do this then you end up with a potentially endless "spurious" discussions.

In terms of light sources from different manufacturers having "specific" outputs. I would only suggest that the properties of the light would be more similar when comparing lights of the same model from the same manufacturer than comparing different models or comparing different models between different manufacturers. I don't think this is too controversial, especially if individual batches are considered. The alternative would be to consider all properties of lights as random with respect to manufacturer and this is clearly wrong!
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Re: Correlated Colour Temperature of Sunlight

Post by el48tel »

SimonT wrote: Wed Jan 12, 2022 9:11 am I think the original discussion was about plant growth not about photography. I agree that there is a lot to learn about light from photography. I'm just not sure how this helps grow plants?

I do think it is important to clearly define the concepts and cite the sources of information in these discussions.
For example, I wonder if the 5500k value comes from one of the online sources state that the sun has a surface temperature of "about" 5500k eg https://www.space.com/17137-how-hot-is-the-sun.html. So then the sun, considered a black body source, radiates at 5500k. So it is confusing that light is then given a higher (CCT) temperature than 5500k, when all sunlight comes from a surface radiating at 5500k? Define CCT and state the source of information and the confusion is quickly resolved. If you don't do this then you end up with a potentially endless "spurious" discussions.

In terms of light sources from different manufacturers having "specific" outputs. I would only suggest that the properties of the light would be more similar when comparing lights of the same model from the same manufacturer than comparing different models or comparing different models between different manufacturers. I don't think this is too controversial, especially if individual batches are considered. The alternative would be to consider all properties of lights as random with respect to manufacturer and this is clearly wrong!
I agree. In fact I think the original discussion started in another thread asking about experience in using lights. The discussion became Medussa, spawning all kinds of random contributions dragging the thread in many directions, some of which seemed without substance. Perhaps my last contribution might be considered by some equally far from the original question, and I would not disagree if challenged. I merely challenged the hypothesis that sunlight in the UK rarely became "colder" or bluer than 5500K.
As for individual illumination sources for plant growth, do we really know and fully understand the process?
Endeavouring to grow Aeoniums, Aylostera, Echinocereus, Echinopsis, Gymnocalycium, Lithops, Matucana, Rebutia, and Sulcorebutia.
Currently being wooed by Haworthia, attempting hybridisation, and enticed by Mesembs.
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