Supplemental light: PPFD vs Foot Candles

[MateoinLosAngeles]

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!