New Gadget: Light Meter

Tenmars TM-202

I've been trying to learn about the light levels needed for various orchids to grow happily. "Quantities" like "50% shade" don't help very much when you don't have a greenhouse or shade-house. However, many orchid light preferences are available in a unit called a "foot candle", which of course the human eye is ill equipped to assess.

Fortunately, there are toys that can help with that... I bought a Tenmars TM-202 from instruments.co.za, and have been happily wandering around the house thrusting it in the general direction of windows everywhere...!

I knew that Phalaenopsis were fairly "low light", whilst Cattleyas were fairly "high light", however, equipped with the knowledge that foot candle requirements of many plants were known, I set out to figure out what was actually needed. A bit of judicious googling yields this handy reference for the genera and intergeneric hybrids I'm growing:

• Angraecum: Species dependent; sesquipedale: 2,500-4,000, apparently preferring very bright light, perhaps with morning sun.
• Ansellia: 2,500-5,000, apparently preferring very bright light, perhaps with morning sun.
• Ascocentrum: 3,000-3,500, apparently preferring very bright light, perhaps with morning sun.
• Beallara:
• Cattleya: 3,000-3,500
• Cymbidium: Standard: 2,000-4,000 Miniature: 1,000-3,500
• Dendrobium: 1,500-4,000 (kingianum is probably at the upper end of this)
• Disa: 1,500-2,000
• Laelia: 2,000-3,500
• Laeliocattleya: Presumably takes into account their ancestry, so 2,000-3,500
• Lycaste: (deciduous spp) 2,000-4,000
• Oncidium: 2,000-4,000
• Paphiopedilum: 2,000-3,000
• Phalaenopsis: 1,000-1,500
• Phragmipedium: 2,500-3,500
• Rhynchostylis: ~3,000
• Sophrolaeliocattleya: Presumably takes into account their ancestry, so 2,000-3,500
• Stenoglottis: 2,500-3,500
• Zygopetalum: Similar to Cymbidiums, so 2,000-4,000, probably at the upper end

So, all in all, it looks like I need to get more light in here...!

Incidentally, you can get an idea of the light intensity using a camera's built in light meter - at least if it displays the exposure it thinks is required. Using a piece of white paper, with the camera lens at a distance of about 30cm from the paper, at f2.8 with a film speed of 200, the shutter speed is (very...) approximately equal to foot candles. If that sounds like too much maths to you, First Rays have a handy form you can fill in that works it out for you.

You can also work this out at any settings with the following formula:

6*F^2/(I*E)

where F = F stop (aperture)
I = ISO setting
E = Shutter Speed (exposure)

I did this in my office, and I got:

6 * 2.6*2.6 /(80*1/33) = 16.731 FC (wow, that is dim).

If you don't have a camera with an obvious light meter, even your cellphone camera will give you this information if you can read the EXIM data (on my Android phone, I can see these details for each picture by tapping the menu button, then tapping Details and scrolling down); again, take a picture of a plain white sheet of paper with the lens about 30cm away and filling the field of view.