Does luminous efficacy differ between direct and reflected sunlight?

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In summary, the article discusses how reflectors can help to reduce the amount of direct sunlight that is received, and how this can be useful in different situations.
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With the total visible light flux being equal does reflected sunlight have a smaller total energy per luminous flux and therefore resulting heat gain than direct sunlight?
With the total visible light flux being equal does reflected sunlight (by the sky, environment or shading/light redirecting surfaces) have a smaller total energy per luminous flux and therefore resulting heat gain than direct sunlight? Possibly caused by a larger luminous efficacy, i.e. more visible em radiation compared to uv and infrared, i.e. "cooler" light.

Meaning that when two similar rooms are equally lit by daylight, one with a small window with direct sunlight and one with a large shaded window with only indirect sunlight. The shaded one would be less hot.
 
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I don’t think there could be any other difference apart from that due to the actual reflectivity of the reflector surface.
If a reflector has a selective filter on its surface then the filter pass band (say the visible region) would probably introduce more loss than a good mirror.
Total internal reflection can be used to minimize losses - as in the prisms in binoculars - but that would be hard to use on a large area reflector.
But if you want ‘cool light’ then a filtering reflector would work and your illumination would not suffer much. People often use Super Glass on large windows which both reduce incoming IR and, in winter, reduce IR heat loss. The slight tint on my conservatory windows is hardly noticeable.
 
  • #3
sophiecentaur said:
I don’t think there could be any other difference apart from that due to the actual reflectivity of the reflector surface.
If a reflector has a selective filter on its surface then the filter pass band (say the visible region) would probably introduce more loss than a good mirror.
Total internal reflection can be used to minimize losses - as in the prisms in binoculars - but that would be hard to use on a large area reflector.
But if you want ‘cool light’ then a filtering reflector would work and your illumination would not suffer much. People often use Super Glass on large windows which both reduce incoming IR and, in winter, reduce IR heat loss. The slight tint on my conservatory windows is hardly noticeable.
Thanks for you reply! As far as I know, most materials reflect visible light better than (uv and) infrared light, except from metals and specially designed reflectors. Therefore, direct sunlight should generally be hotter than redirected sunlight per quantity of visible light. For example only letting light in that is reflected by the pavement or surrounding buildings/plants etc This may be less efficient than filtering reflectors, but does strengthen the concept of limiting direct sunlight exposure. However, I can't find any confirmation of this or any papers stating this.

As far as filtering windows, you are totally right, different windows have different solar heat gain coefficients. However it does have some disadvantages in certain situations, because these also reflect infrared radiation back from the inside. This means that heat can't escape and for example visible light that is absorbed and radiated as infrared can't leave. Making it less efficient than filtering light before entering a window. Therefore different windows have different amounts of filtering capacities, which you'd have to calculate when designing a building, but this is a bit off topic.
 
  • #4
IR filtering glass is a ‘sum gain’ because the inside gets less total solar energy. Simple cooling with a fan works well and blinds are another cheap solution. But smart windows are a great passive tool. And also you have a warmer space in winter.
Do you have data about ‘most materials’ reflecting visible light more than other wavelengths? It sounds a bit arbitrary to me.
 
  • #5
n124122 said:
As far as I know, most materials reflect visible light better than (uv and) infrared light, except from metals and specially designed reflectors.
You might wan to look at remote sensing.
For example
https://seos-project.eu/remotesensing/remotesensing-c01-p06.html

It's a way to determine the rock surface composition of Mars without testing a physical sample from satelite.

Anyways, have a look.
 

1. What is luminous efficacy?

Luminous efficacy is a measure of how efficiently a light source produces visible light. It is typically measured in lumens per watt (lm/W), with higher values indicating a more efficient light source.

2. How is luminous efficacy affected by direct sunlight?

Direct sunlight has a high luminous efficacy because it is a direct source of light and does not require any reflection or scattering to reach the observer. This means that most of the visible light produced by the sun is able to reach the observer, resulting in a high luminous efficacy.

3. Does reflected sunlight have a different luminous efficacy?

Yes, reflected sunlight has a lower luminous efficacy compared to direct sunlight. This is because some of the light is lost through reflection and scattering, reducing the amount of visible light that reaches the observer.

4. What factors can affect the luminous efficacy of direct sunlight?

The luminous efficacy of direct sunlight can be affected by various factors, including the angle of incidence (the angle at which the sunlight hits a surface), atmospheric conditions (such as clouds or pollution), and the time of day.

5. How does the luminous efficacy of direct and reflected sunlight impact our daily lives?

The difference in luminous efficacy between direct and reflected sunlight can have various impacts on our daily lives. For example, direct sunlight is more efficient for providing natural lighting in buildings, while reflected sunlight can contribute to glare and heat buildup. Understanding these differences can help us make informed decisions about lighting design and energy usage.

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