1. Setup received from Serban
The idea is to illuminate a target with a LED light after several reflections
Window
LED
Target

2. Found an article containing data about stainless steel reflectivity
Tells total reflectivity
Also tells specular / diffuse parts for a polished sheet

3. A much older article has practically the same data for total reflectivity

5. Modified Molflow: previously only fully specular / diffuse reflection was available, now a combination of these is supported

6. 40% 5% 55% 60% Updated Molflow model:
absorbtion
60%
reflection
Diffuse part
Specular part
In reality the specular part is somewhat perturbated around the perfect reflection angle due to surface roughness
Sidenote - not included in the model:
Diffuse part
Specular part

7. 50% diffuse 50% specular simulation
Note: the sum of a fully-diffuse and a fully-specular simulation is similar but not the same as the combined model (difference starts from the 2nd reflection of a photon)
100% specular simulation
100% diffuse simulation
50% diffuse 50% specular simulation

8. Switching to logarithmic scale (5 orders) reveals dark parts
Reference case: 60% reflectivity, 10% diffuse and 90% specular reflection (assuming a polished vacuum chamber surface thus using the article data)
On a linear (real-life) color scale, we can only see the directly illuminated part, the rest is dark
Switching to logarithmic scale (5 orders) reveals dark parts

9. Parameter sensitivity
Increasing the diffuse part to 50% results in practically the same illumination:
Increasing the reflectivity to 70% signinifcantly increases the light reaching the inner chamber, though:
60% reflectivity, 50% diffuse and 50% specular reflection
70% reflectivity, 10% diffuse and 90% specular reflection

10. Stainless steel parts
60% reflectivity
10% diffuse 90% specular
Blackened parts
10% reflectivity
50% diffuse 50% specular

11. Parameter sensitivity
Reflectivity
Reflection diffuse part
Photons reaching target
60%
10%
1.7E-5
30%
1.8E-5
50%
1.9E-5
70%
6.2E-5
1.0E-6
Stainless steel
Result not too sensitive to amount of diffuse part
Quite sensitive to reflectivity value
(not a surprise)
Blackened

12. Up to 1100 lumens output
Almost perfect cosine

13. Blackened walls: 4 lux illumination on the target.
Stainless steel: 46 lux illumination on the target.
(For 70% reflectivity, the illumination would be 150 lux)

14. In comparison, this is the new model where the LED is placed directly in front of the target:

15. Blackened walls: 23.000 lux illumination on the target.
Stainless steel: lux illumination on the target.

16. Reflection diffuse part Photons reaching target Target illumination
Reflectivity
Reflection diffuse part
Photons reaching target
Target illumination
60%
10%
1.7E-5
46 lux
30%
1.8E-5
48 lux
50%
1.9E-5
51 lux
70%
6.2E-5
150 lux
1.0E-6
4 lux
2.9E-2
lux
8.3E-3
lux
Stainless steel
Blackened
Direct (Stainless steel)
Direct (Blackened)