Measuring Florescent light output

[blackvine]

Steve, I struggled with this a few months ago when I first started growing orchids. Focusing on Catts, which need 3,000 - 5,000 footcandles, I found a simple "step down" chart, showing the decrease in footcandles by foot. Here it is:

Trend as we move in one-foot increments from the light on the ceiling to the floor in a normal house with eight-foot ceilings:
1 Ft = 500 Lm
2 Ft = 125 Lm (25% of 500)
3 Ft = 55.5 Lm (11% of 500)
4 Ft = 31.25 Lm (6% of 500)
5 Ft = 20 Lm (4% of 500)
6 Ft = 13.8 Lm
7 Ft = 10.2 Lm
8Ft = 7.8 Lm
By moving just seven feet further from the light source, the intensity of the light has decreased from 500 Lumens to less than eight Lumens.

All you need to know is how many footcandles your light puts out. This should be printed on the box. It may be stated in "lumens", but for all intents and purposes an FC is equal to a lumen. (A footcandle is a unit of measurement describing the intensity of light one foot from a candle).

From that starting point - your footcandles of light - decrease by the percentages I've shown in the above table. So if your light has 20,000 FCs, your Catt will recieve 6% of that if your light source is 4 feet away. I have lots more info, but I just use this chart because the math is mind boggling! If you want it, though, let me know.

[Ray]

Originally Posted by blackvine

So if your light has 20,000 FCs, your Catt will recieve 6% of that if your light source is 4 feet away. I have lots more info, but I just use this chart because the math is mind boggling! If you want it, though, let me know.

Actually, that figure would only hold true if the light output was 20,000 footcandles at 1 foot from the bulbs.

The drop off is an inverse square - twice the distance = 1/4th the intensity, 3x the distance, 1/9th, etc. That means that if the output is 20,000fc at 1" from the bulbs, it will be 5000 at 2", etc.

However, don't disregard the effect of reflectors, as they will redirect the "lost" light from the part of the bulb facing away from the plant, down toward it.

I did some measurements once, using a 2', 24W T5 fixture:

[Calbears]

Originally Posted by Ray

Actually, that figure would only hold true if the light output was 20,000 footcandles at 1 foot from the bulbs.

The drop off is an inverse square - twice the distance = 1/4th the intensity, 3x the distance, 1/9th, etc. That means that if the output is 20,000fc at 1" from the bulbs, it will be 5000 at 2", etc.

However, don't disregard the effect of reflectors, as they will redirect the "lost" light from the part of the bulb facing away from the plant, down toward it.

I did some measurements once, using a 2', 24W T5 fixture:

Ray, are the units on the graph correct? I'm thinking it should read feet not inches. I just can't believe all that light would fall off in one foot.

[naoki]

Originally Posted by Calbears

Ray, are the units on the graph correct? I'm thinking it should read feet not inches. I just can't believe all that light would fall off in one foot.

The unit on the graph is probably correct. Here is my measurement, which matches well with Ray's.

http://www.orchidboard.com/community...tml#post572079

[Discus]

Originally Posted by Calbears

Ray, are the units on the graph correct? I'm thinking it should read feet not inches. I just can't believe all that light would fall off in one foot.

Lighting reduces its intensity as an inverse square law, hence the (initially "illogically" fast) dropoff.

If you want to try and wrap your head around why, think about a cylinder of paper wrapped around the light at different distances out - the further away your cylinder is, the bigger the surface area gets - at a really quite rapid rate - yet the bulb is still only producing so much light, so it gets "spread" more and more thinly.

This is also why even really bright lights (like flash bulbs) do absolutely nothing in a stadium other than look pretty sparkling on the TV, and give the photographer a disappointing picture of the back of the heads of a few rows in front of them...

[Orchidflowerchild]

Originally Posted by Discus

This is also why even really bright lights (like flash bulbs) do absolutely nothing in a stadium other than look pretty sparkling on the TV, and give the photographer a disappointing picture of the back of the heads of a few rows in front of them...

And also why sitting plants "near a window" is not really likely to garner much success unless they are pretty well IN the window. Also why an office desk is a horrible place for a plant. Light is important to consider, but yo0u also have to consider direction of light, duration of light, intensity of light, and angle of light. I bloom Cattleyas under t-8 (anmd back in the day even T-12s!) but I do keep the leaves within 4" of the bulbs.

The human eye is pretty good at making dim lighting look brighter to our mind than it is.

-Ceci