Supplemental light: PPFD vs Foot Candles
 

When evaluating the growing conditions of my window sill I noticed very dramatic differences when measuring PPFD vs Foot Candles.

The max light recommendations are usually measured in Foot Candles and as per AOS they are:

- 1,500 foot-candles for Phalaenopsis and Paphiopedilum
- 2,500 foot-candles for Miltoniopsis and Zygopetalum
- 3,000 foot-candles for Cattleya
- 5,000 foot-candles for Brassia, Cymbidium, Degarmoara, Dendrobium and Oncidium

This article from Herebutnot had a nice chart for general PPFD guidelines:

Low-Light Orchids (Mottled-Leaf Paphs, Jewel Orchids, Phals) –*40–80 umol/m2/s PPFD
Moderate-Light Orchids (Onc, Phrags, Epidens, Dends etc) –*80–150 umol/m2/s PPFD
High-Light Orchids (Cattleya) –*150–350 umol/m2/s PPFD
Very High-Light Orchids (Vandas) –*350–600 umol/m2/s PPFD

My window is north facing, I'm in sunny Los Angeles and there's an off-white building about 65 feet away acting as a reflector on one half of the windows and unobstructed views on the other –*I did have different readings on both ends of a table, my Cattleya type orchids are placed facing the unobstructed area, whereas Phals are on the other side and farther away from window.

When measuring PPFD and Foot Candles using a light meter I noticed that based on the Foot Candles reading my plants were severely under-lit. Whereas when using the PPFD readings they were receiving sufficient light, surpassing max recommendations at noon on some of the sunniest days. Perhaps requiring some supplemental light for the Phals between 6:30am and 10:30am and 2:30pm-6:30pm to "lift up" the average light the plants receive and probably supplementing all day long for the Cattleyas.

This has left me quite confused when it comes to supplementing light as most information and light readers come in Lux and Foot Candles, making it hard to even be sure that I am measuring the PPFD correctly.

My question for the board is: what kind of light readers do you usually use and have you been able to succeed in measuring light using PPFD instead of Foot Candles?

Thank you all!

You may find this article of some help. It explains the different ways light is measured.

Just a moment...

I use an analogue light meter that measures in FC. I used it to figure out what light levels my different windows provided, and I use it occasionally when conditions change or when I’m placing new plants in the house. I don’t bother with PPFD, but my plants are usually getting well above the minimum required FCs for good growth, and I judge the suitability of the light they get based on how they’re growing. If they’re putting out new roots/leaves/blooms that means the light is suitable. Bleaching or extremely light coloration suggests too much light. Stalled growth when all the other factors are being met (enough water, occasional fertilizer, correct temperatures) is a sign that the light may not be adequate.

I guess my suggestion is: pick one unit and use that to sort out your light situation. Watch your plants and adjust from there based on how they grow.

This exact topic was discussed earlier this year (or last year, can't remember exactly) and I will try to find the thread again, it was informative.

What unit you use shouldn't matter. The reason you are seeing differences between the 2 is likely because the guidelines are different, with the FC guidelines advising higher light levels than the PPFD guidelines you found. Other point to consider, what are you using to measure PPFD? PPFD is different from Lux/FC in that it measures the photon flux in the PAR range and not the entire light spectrum and generally a rather pricey (compared to FC meters) quantum meter is needed for accurate measurements.

I agree with Dimples though, use the light meter for informative purposes to get an rough idea, make some changes and then observe your plants.
Keep in mind that light intensity is only part of the equation, duration is the other part.

Foot-candles guidelines are primarily based upon greenhouse growing under natural sunlight, to the numbers are the maximum levels to which the plants should be exposed. When dealing with artificial lighting that is constant, you should shoot for 50-55% of that recommendation (Read this), because the amount of light a plant needs is a combination of intensity and time, which is exactly was PPFD DLI is.

Might want to double check this:

"5,000 foot-candles for Brassia, Cymbidium, Degarmoara, Dendrobium and Oncidium"

This statement rings mostly untrue in my experience, especially with cattleya guidance @ 3000FC

Here’s the article from AOS where I got those numbers from!

Principles of Light

Thanks for sending the article, a lot of good info in there. While I am sure some Dens (Aussies) and Oncidiums can take greater than cattleya light, I think this guidance is kind of irresponsible for these very broad alliances. Most dens and oncidiums I keep will scorch at cattleya light. Chinese cymbs can't take high light. Definitely best to research each species / hybrid when placing it in your greenhouse. The PPFD guidelines (broadly) ring more true in my experience. I grow almost 100% under lights.

Should the light level for Phal. bellina and violacea be more towards Moderate Light or on the low end of High Light? According to Travaldo's blog bellina requires 13,000 lux which is like 299 PPFD according to a lux/PPFD calculator. I have bellina and violacea growing indoors under LED grow light.

Eric Goo, a highly awarded Phal breeder in my orchid society, says to give most Phals 1,000 foot candles of light most of the year. That is a little less than 11,000 lux. He recommends a 10-12 hour day length. He recommends raising it to 1,200 fc / just under 13,000 lux for a few weeks in late fall to help trigger flowering, then dropping it back to 1,000 fc.

Eric says using more light leads to fewer and smaller flowers. I have heard several other professional Phal growers say this as well.