Does luminous efficacy differ between direct and reflected sunlight?

[n124122]

TL;DR

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.

 

[sophiecentaur]

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.

 

[n124122]

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.

 

[sophiecentaur]

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.

 

[256bits]

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.