light meter readings

[Jerry]

I am a newbie, I just bought a "Environmental Concepts"
plant light meter, for a new orchid greenhouse.
I have searched the web but can't find out if it's better to point it at the source light or the surface reflection.
Anyone know?

Jerry

[Ross]

Sorry there has been no response. I also use the Environmental Concepts light meter and get frustrating results. The book says to place the active end of the sensor at the leaf surface and point toward the light source. That part I understand and that is fine and good. However unless you are measuring sunlight/HID or florescent and use the proper switch position (up for sun/HID and down for florescent) you don't get accurate readings. I don't get proper readings for t5 lights unless I set the switch down for florescent, but read the upper scale. Hope this helps. Point at light source.

[Jerry]

Ross, thanks for the reply.
I did use the sunlight switch, I put it at the height the plants will be and pointed it at the roof at midday.
The meter shows 7,000 f/candles, now here is the rub, I have 73% shade cloth and still get 7,000 f/candles.
Without the shade cloth I get 10,000 f/candles.
Now, I visited a commercial orchid grower close by and took a reading in her greenhouses of 6,000 f/c pointing the meter at the roofI at midday, I told her the result and she told me there was not more than 2,000 to 2,500 f/c. So I pointed the meter at a leaf and guess what?
The meter read 2,000 f/c. So either the meter is wrong or the directions that came with the meter saying to point it at the source of light are wrong.
It's really confusing.
I guess I can use the 6,000 reading I took in the commercial growers house as a guide, I know the orchids need different light requirements so it looks as though the meter is not much use to me except to compare the reading with a existing house.
My house is a little lighter than hers and I am using 73% cloth so I guess the light is close enough for now and it's all a matter of experiment.
What reading do you get midday, in sun mode when pointing the meter at the source of light and what orchids grow and flower best in your house.
Thanks again for the reply.
Jerry

[smartie2000]

I would but the light meter right on the plants leaves and parallel to the leaves...

[Ross]

This is confusing to me as well. I do know that pointing the meter at light source at mid-day in high light will easily get 10,000 FC. Full sun in bright cloudless sky can exceed 10,000 FCs easily. So I suspect you are seeing correct readings. I do know that you do NOT want to point at leaf surface. That will fool meter. If you have bright, cloudless sky and 73 percent shade, I suspect you'll be reading 7300 foot-candles. If you need less light, you can try moving plants to shadier locations, or adding slats above plants to reduce further. Only problem might be in winter when clouds abound. Light intensities will drop, of course. Hope this helps.

All my growing is under t5 florescent light banks in basement, so I don't have direct comparison. Reading the light, however should be the same? I do have a direct correlation since I know how many lumens the t5 banks put out and can meter under controlled conditions. From what I see with this meter, I recommend you take readings at position of the plant with meter pointed up towards the light. If the amount of light seems to high, consider adding more shade cloth. Remember though, in winter months when sun is lower, light lumens will be less as well and you have to have some way to remove some shading to increase light. Just a thought to ponder.