I'm not a lighting expert, but doesn't the drop off occur because of the UV(?) protection on modern glass?

If and when the sun ever shines here again, gone for today and supposedly raining for the next two, I'll get my husband to use the light meter outside by the window, inside by the window and then back three feet.

I know in the G/H, the twin poly carb is an automatic 15% reduction, throw on a 50% shade cloth, measure the FC's then measure outside, the outside numbers go up over 50%.

If the info on the lighting forum was given by shrubs & bulbs, he's pretty good with his info.

Brooke

Okay, but I'm still curious as to why the window doesn't make the light just as bad at 1 ft away. Just seems weird to me that the drop off isn't a problem outside, but it is inside. I get that the window cuts the light, but I don't get why the light is still ok at 1 ft but not okay at say 3 ft or 4 ft.

I don't have a clue either but thought some FC measurements might help. Maybe Howard can give you a definitive answer that we can both understand.

Brooke

If we are talking about direct sunlight, then the light attenuation whether 1', 10' or 100' away from the window is negligible, since 100' is nothing compared to the distance between earth and the sun :) However, if we are talking about indirect sunlight, then distance from window will matter greatly. A lightmeter is probably most objective in this latter regard, or Google "how to determine footc candles with digital camera" to access a forum/info that GW blocks(!!) me from posting. Hope this helps, alvin

PS. According to this site, light will fall off at a rate of 1/(squared distance from a point source) ... a (normal) window is not a point source ... to your advantage :)

I would also suspect it's the size of a window in combination with the movement of the sun across the sky unless you have huge floor to ceiling - entire wall sized windows.

Let's say your window is 3x5 feet. Only a certain amount of light can enter that opening. Combine that with the angle of the sun and you may only have sun shining on your window sill and a little bit beyond for x number of hours. The farther back you go, the less sunlight hitting any one spot and for a smaller period of time.

For example: All the south windows on my house are unobstructed. In other words, they pretty much get sun all day long assuming it does shine. I've noticed that the sun is always hitting the window sill, but 3 feet away from the sill the sun is visibly moving across the room. There is no one spot that distance away which received sunlight to the same degree that the window sill receives it. As stated above, I would assume that's the result of the size of the window in combination with the sun's angle. If the entire wall was glass, this probably wouldn't occur to the same degree - the sun would reach farther into the room and for a longer period of time.

Not sure if I explained that in a way which makes any sense.

Then there's the whole issue of the density of glass and what that does to sunlight. A quick example of this: Sunlight shining on the ocean diminishes the deeper down you go. Why? Water is more dense than air. Sunlight can't penetrate water beyond a certain depth because of the density of the water. Substitute window glass for water and you get similar results.

Then you could also have impurities in both glass and water That would further complicate the matter since these little bits of matter could probably stop light dead in its tracks or maybe even reflect light back toward the sun or bounce it away at strange angles.

That's all I can think of.

Kevin

In my house, the sun coming through a skylight has the same reading on a light meter as it does outside, but it moves during the day, so only if you had plants up in the skylight would the light stay the same intensity for a longer period. Since light coming into a window only shines into the house a few feet because of the sun's angle, the light intensity is only strong where the sunbeams hit. Everything else is indirect light and gets too low in intensity to help the plants grow as you move farther into the room. I wonder what the light intensity is in Alaska where the sun stays at a lower angle in summer?

I think the angle that the light is coming into the window would also come into play. If the light rays are vertical, then the light would be a lot less than if they were horizontal. And I imagine that also depends on what part of the globe you're on.

This is such a great question!
yeah I totally agree with Alvin - I've had 5000+fc indoors, 1, 2, 3, 4, and even 5 feet away from a window, but with direct sunlight. However, the kind of diffused sunlight which some people call 'bright indirect light' does diminish quite rapidly. This is opposed to the outdoors, where I've had 4000+ fc even when there is no direct light shining on the plants - the sky seems to be a really bright source...once this is sifted through a window however, all the light you're getting is through those 4ft by 5ft or however large your window is. So in a way, from the perspective of indoors, isn't your window kind of like a point source when the sun isn't shining directly through it, since it 'emits' (read lets in) a amount of light, which diminishes as you get further away from it?

Lets see what the others say! :)

I think we all posted at the same time.

K

Seems to me that ouside the sun is reflected off clouds, structures, plants, etc, meaning there is more ambient light. Coming thru the window, there is pretty much only that one shaft of light, or much reduced ambient light. Outdoors many plants do well in bright shade, which is only filtered or reflected light. Indoors, you have to look hard to find plants that will bloom without very bright light or some sun.

Good question! All I know is my furniture, 12 feet from the windows fade. A dark-brown chair has faded to beige.

Jane

There have been several good posts in this thread but I can't say that any one individual has summed the matter up. It is as if everyone has a piece of the puzzle. The indirect/direct light thing is throwing the discussion off track. Indirect light is for all practical purposes of no importance. It is only the direct sunlight that counts when plant people measure sunlight through windows. An East window facing the rising sun gets direct light shining straight into it from a nearly horizontal angle early in the morning. It is absolutely true that that direct light will be the same intensity 1', 2', 3', or more into the room. The sun is going to rise, however, and quickly, in less than two hours the angle is so steep that there will be no direct sunlight 3' into the room it will be concentrated in a narrow band near the glass. By 11:00 am it is not even there AT the glass. Only low light plants should be grown at un-supplemented East/West windows. Obviously if you want the most light from ANY window, not just East/West the plant should be as close to the glass as possible because the sun only spends a short time at the altitude where light (direct sunlight) shines deeply into the room.

H

There are a lot of mis-conceptions in this thread.

all light diminishes as it travels through space, the atmosphere etc.

The sunlight reaching the earth is a misiscule amount of what leaves the sun.

Most glass absorbs a lot of light and that light level diminishes exponetionally as the distance changes. Much glass has UV absorbtion factors built into the glass.

This also happens outdoors not just windows. The light level at the roof of a greenhouse or shadehouse will be much stronger that lower near the floor.

Light is filtered heavily as it goes through the atmosphere. When I measure sunlight in an open southern exposure in SW Florida in July it is usually 11,000 foot candles. In December at the same spot it never exceeds 5,000. The sun is lower in the sky and travels a longer distance with more being filtered along the way. This is why it is colder in winter and plants slow down growth.

Screening blocks a lot of light. A typical outdoor screen cage will block 65% of the light passing through it.

The only way to judge your light is to measure it. A reference for measuring with a camera is attached.

Here is a link that might be useful: measuring light with a camera

>There are a lot of mis-conceptions in this thread.

True, and that is why I posted to attempt unification of the partial answers into a comprehensive answer for John. Unfortunately you have introduced several new misconceptions in your answer which I must rebut. I won't quibble with your advice to measure the light with meters nor the link which I have not looked at but:
>all light diminishes as it travels through space, the atmosphere etc.

This is absolutely not true in the case of interstellar space. The atmosphere, yes, but it takes miles of atmosphere to affect the intensity of solar light to any appreciable extent.

>The sunlight reaching the earth is a misiscule amount of what leaves the sun.

Of course, but not because there was any degredation in light intensity from traveling 93M miles through space but because only the portion of solar disk directly intercepted by Earth gets light from the Sun. If there could be a shade on the Sun capturing all the light that heads off in other directions it would all get here. You think Global Warming is bad?

>Most glass absorbs a lot of light and that light level diminishes exponetionally as the distance changes.

Most glass absorbs minimal light unless heavily tinted. Windows are not usually heavily tinted. And a given thickness of glass does not absorb any more or less light as the distance changes, it is a fixed amount of reduction.

>Much glass has UV absorbtion factors built into the glass.

A good thing. UV is an undesirable by-product and has no bearing on the intensity of beneficial light frequencies.

>This also happens outdoors not just windows. The light level at the roof of a greenhouse or shadehouse will be much stronger that lower near the floor.

In an absolutely empty greenhouse the light will be exactly the same at the floor as at the roof. It is only in the typical packed to the rafters greenhouse that one has to hang high light plants at the apex to get them out of the shade of competing plants. Under the benches the only direct sun is low angle sidelight and I will explain why that light is lower intensity next.

>Light is filtered heavily as it goes through the atmosphere. When I measure sunlight in an open southern exposure in SW Florida in July it is usually 11,000 foot candles. In December at the same spot it never exceeds 5,000. The sun is lower in the sky and travels a longer distance with more being filtered along the way. This is why it is colder in winter and plants slow down growth.

If you could do the trigonometry of how much more atmosphere the sunlight passes through in December vs July you would see that while in essence your observations are correct the reasons are obvious. It takes many, many thousands of feet of atmosphere to lower the intensity of the sun any appreciable amount and the trivial distances involved in, say the difference between the roof of a greenhouse and the ground or at the windowpane and 3' into the room are beyond miniscule.

>The only way to judge your light is to measure it. A reference for measuring with a camera is attached.

The only way for greenhouse and patio growers to measure light is to measure it. For the indoor and windowsill grower there is little point to measuring a fixed quantity of light. That is kind of like buying a scale to weigh a 5lb. bag of sugar.

Again I am sorry for the point by point dissection but a number of your 'mis-conceptions' came up in earlier posts so obviously you come by it honestly but it is... incorrect in places. This forum is archived and is a legacy of sorts I would think we would want the most accurate information archived for posterity. And I absolutely am open to any of my points being challenged. I am not by any means infalliable or a know it all.

H

Some really great points but I am puzzled by the comment regarding indirect light. Why is indirect light of no importance? How is it that plants grow without any direct sunlight? I have a 13ft ficus growing in a North window which has never felt a ray of direct sunlight. I understand that strong light is necessary to grow/flower many plants, but indirect light must count for something.

I have many shade plants outdoors which grow and flower under trees without any direct sunlight - Hostas, Bleeding Hearts, Pachysandra's - all of which, flower.

Jane

These light discussions are great and educational for me as a new artificial lights grower - very appropriate to have "light" discussed as we move into winter solstice!

Like jane_ny, I'm slightly confused by "indirect light is for all practical purposes of no importance" - I too have many floral counter-examples, e.g., all those plants that inhabit the floor of the tropical rainforest - pretty dark and soggy from my experience, and yet an explosion of plant species, the diversity of which is not seen in any other biome. alvin

I was answering John's question "I just read on the lighting forum that sunlight intensity drops precipitously as you move away from a window". I didn't say the indirect light was completely useless I said that when considering light indoors only direct sunlight matters in the measurement. The indirect light is assumed but it is not a significant source of energy, consider it dessert.

A Ficus may live and grow in a North window but in an East window it would have a nicer appearance and in a South window it could possibly flower! An unsupported North window of average size is somewhere around 250 to 500 footcandles. Outdoors, even in full shade is over 1000 to 1500 footcandles. There is no comparison, I think. Outdoor temperate plants spend a huge part of the year dormant not needing any energy and their flower structures are trivial compared to the structural magic that even the lowliest orchid can manage. A phalaeonopsis can craft blooms that have the texture and heft of wax and they can last for months. Can they do it on the same kind of light that works for a Ficus? No, but the real take away here is that if we gave our Ficus, Wandering Jew's, Diffenbachia's, Chinese Evergreens, Lucky Bamboo, Cast Iron Plants, etc., etc. the same kind of light that is recommended for orchids they all can and would flower! Whether the flower structures are pleasing to see or smell is another thing entirely but the capability is there.

H

People do seem to forget that 90-sommat percent of all plants are flowering plants, don't they Howard? A friend of mine was once amazed to discover an inflorescence on a Draceana that she had moved over to her orchid area in the interest of aesthetics (she had some very nice teak shelves built and put them in a corner of her house that had large S and E windows). After a few months of lush growth there, along with her orchids, the thing sent out a spike of spidery gren and white blooms. She had no idea that it COULD flower. I mentioned to her that with proper light energy, most any ornamental houseplant will flower, unless your bent it towards Auricaria or ferns.

-Cj

I scanned all the answers. Didn't see this, sorry if I missed it.

Light travels in a straight line. If you've got direct sunlight most of the day beyond 1 foot from your window (doubtful) you'd still be in business. Otherwise you're taking light that has already been filtered by whatever combination of means already mentioned above and having it scattered futher by the bouncing off of whatever surfaces is redirecting its verticality off and into the recesses of your room. The further away, the more scattering there is.

John :>)

Howard, I felt sorry for a Lucky Bamboo and put it in a brighter window. Plant burned up and I never heard the end of it. I know, I know, too much, too fast!

BTW, I don't want my Ficus growing any faster or bigger, so the N window it stays.

Sorry, I misunderstood your answer to John's post.

Great answer,
Jane

A good thread but I think that certain basic concepts have been overlooked.
1. What matters to plants is not the presence of light but rather the wavelength of light that enables it to effectively reproduce and metabolize.
2. Visually trying to gauge light as a quantity is erroneous as in strong light the eye dilates and in less eye the iris of the eye expands to let in more light.
While a light meter is a very excellent tool for all plant growers the notion that some level of FCs is more appropriate than others must take into account the angle of incidence of the light hitting the surface of the leaf.
That is why direct light in winter is generally much less of an impact than that of summer as you advance northward where the natural growing season becomes shorter.
3. The impact of light immediately behind glass can increase the ambient temperature on any surface and can be detrimental to sensitive leaves and even to plants accustomed to full sun in the middle of summer.

While growing some kinds of plants behind curtains or under some shading inside a home may be successful it is really just a matter of placement and of what kind of light is available for such plants to grow there. For plants that require copious amounts of light or specific wavelengths to generate the primordia that generate flower spikes and buds indoor growing is decidely a handicap.
Supplementation with balanced and wave-specific lighting is recommended.

Like said light intensity is hard to perceive with your eyes. Think of a large church, it may appear to be somewhat brightly lit but try to take a picture and it is almost like taking in a dark room. Same indoors the indirect light only looks bright from within the room. Light that will influence growth will feel warm on your hand if you leave it there a bit, that is at least compared to the rest of the light in the house.

I hope this makes some sense or at least someone could point out my misleadings.

As Howard pointed out, if I were standing half as far from the sun as earth is, the light intensity would be 8 times stronger, not because the light is different, but because there is more of it.

Indoor light is stronger in winter than in summer and plants that are right up to a south window in summer will burn in winter. This is because of the angle of the sun. It may have to go through more atmosphere, as was pointed out, but more of it comes into the room.