Measurements of the Sun's radiance and ray tracing

In summary, the conversation discusses a hobbyist project for measuring radiance from the sun at 500 nm using an optically filtered photodetector. The project requires an instrument with an angular field of view equal to or larger than .53 degrees to view the sun's disk, but for sun photometry, an angular field of view of 2.5 degrees is recommended. The design of the instrument includes a collimating tube of 110 cm and an aperture of 5 mm. The conversation also touches on difficulties with simulating the sun's light and how to properly scale it in the model.
  • #1
Tone L
73
7
TL;DR Summary
Building a simple instrument to effectively collimate sunlight and measure the digital counts on a photo detector.
Summary:
This is a hobbyist project I am working on. I am building an instrument to measure radiance from the sun at 500 nm, using optically filtered photodetector with bandwidth ± 10 nm. This type of science can be called Sun Photometry, so let's get to it!

Geometry for Optics:
The sun has an angular size of about .53 degrees, this is a simple calculation, angular size = arctan(d/D), where d is diameter of the sun and D is distance from the sun from Earth. With that being said, you would need an instrument with an angular field of view equal or larger than .53 degrees to view the disk of the sun.

However, due to homogeneity of our atmosphere and the resulting aural around the sun it has been stated in that an angular field of view of 2.5 degrees for instrumentation is best for sun photometry, approximately five times the angular size of the sun.

Design:
We need angular field of view to equal 2.5, so will have a collimating tube of 110 cm and an aperture of 5 mm assuming the detector is a point.
2.56 = arctan(5/110), painted black with 90% absorbance.
Here is a CAD of the optical block.
optical_block.PNG


Where things go astray...
Simulation:
I was curious how much sunlight would enter the aperture and be collimated down to the "detector" (there is no detector in the model).

I tried to simulate the sun here by knowing that the ratio of the diameter to the distance (.0093 = diamter:distance) from Earth is about .0093. So at .1 meters away the size of the sun would be 930 microns.
Now using RayViz a package within Solidworks I made a disk, which is the "sun" here and I tried plotting the rays from the surface of this disk, and well doesn't look to great! Here I have a lambertian surface meaning the rays leave at all angles of the disk. I don't know how to only focus the rays within instruments domain but nonetheless I think I am doing something wrong, not with the software but my thinking...The suns light is diffused and focused more than a lambertian surface...
Raytrace.PNG


So, how would one simulate the sun, should it be a point source?
Cheers.
 
Last edited:
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  • #2
Tone L said:
should it be a point source
No. You even calculated the angle !
And it should be at practically infinity, not at a few decimeter
 
  • #3
BvU said:
No. You even calculated the angle !
And it should be at practically infinity, not at a few decimeter
Ha! True, I may have been rambling at that point.

However, the sun in this model is scaled. As I mentioned before,
.0093 = diameter of the sun / distance to the sun from Earth. Therefore, at 1 decimeter, the radius should be 930 microns :)
 
  • #4
Tone L said:
However, the sun in this model is scaled. As I mentioned before,
.0093 = diameter of the sun / distance to the sun from Earth. Therefore, at 1 decimeter, the radius should be 930 microns :)

That's not how it works. Rays from the sun come in almost parallel.
 

1. How is the Sun's radiance measured?

The Sun's radiance is measured using a device called a radiometer. This instrument detects and measures the amount of electromagnetic radiation, including visible light, that is emitted by the Sun. The measurement is typically expressed in watts per square meter.

2. What is ray tracing and how is it used in measuring the Sun's radiance?

Ray tracing is a technique used to simulate the path of light as it travels through a medium or interacts with objects. In measuring the Sun's radiance, ray tracing is used to calculate the amount of light that reaches Earth's surface by tracing the path of sunlight through the atmosphere and factoring in variables such as atmospheric composition and cloud cover.

3. How accurate are measurements of the Sun's radiance?

Measurements of the Sun's radiance are highly accurate, with modern instruments able to detect even small changes in the Sun's output. However, there can be some variability in measurements due to factors such as atmospheric conditions and instrument calibration.

4. Why is it important to measure the Sun's radiance?

Measuring the Sun's radiance is important for understanding the behavior and impact of solar radiation on Earth's climate and weather patterns. It also helps us track changes in the Sun's output over time, which can have implications for space weather and satellite communications.

5. How do scientists use measurements of the Sun's radiance to make predictions?

By analyzing and comparing measurements of the Sun's radiance over time, scientists can make predictions about future changes in the Sun's output and how they may affect Earth's climate and weather. These predictions can also help inform decision-making for industries such as agriculture and renewable energy.

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