How do I measure specular and (u/v)roughness without special equipment?

Hello everyone! I am very happy with all your advice and guidance in using macbethcal for RGB measurement, a luxmeter for trans measurement, and ies files for light. However I currently have absolutely no idea how to measure specularity and roughness (and I guess, also anisotropic roughness). I have been using rules of thumb about plastic and metal as written in the refman, but that’s about it.

In the book, Rendering with Radiance, I see a suggestion about building a bottomless black box with a viewing aperture and 3 known LEDs. I think this is a great idea. I can build this box, place it on top of materials, simulate the virtual equivalent and compare and play around until the two match in both specular and roughness.

This black box approach would also work with complicated surfaces, such as my Ikea table which has a grain which alternates between specular and non-specular.

I also came across Measuring and modeling aniostropic reflection and NIST BRDF measurements rendered with Radiance. The paper suggests building a spinning goniometer cheaply. I haven’t read in too much detail, largely because I don’t understand when it would be applicable. If the black box approach works, then when would you need the goniometer? Is it only when you are trying to measure a material where the material properties are dependant on angle? (e.g. colour changing materials, lacquer finishes on concrete floors, clearcoat car paints) My understanding is that it is only when a material property depends on angle, then a BRDF function needs to be measured.

Edit: My Dulux paint colour fandeck also has a set of “gloss / sheen” references, similar to the technique talked about in using a grayscale chart to measure average reflectance. However my initial hunch is that their definition of “gloss / sheen” is not the same as what I’m looking for :slight_smile:

Are there more approaches that I am unaware of? (Apart from buying very expensive equipment!) Has anybody built these boxes or goniometer? Can you post photos and any comments about it?

I have started considering to build my own black box. Because I don’t have any examples of others who have built one, I guess I’ll make it up as I go :slight_smile:

My plan is a 400x400x400mm box with black-painted wooden frame. The inside will be papered with a colour from Dulux with a known RGB value. The bottom will be bottomless, and the top will have a single small hole, say about 15mm in diameter drilled in the very centre. This will be the viewing aperture. There will be two electrical lights (shown in orange in the picture below), 90 degrees from one another (to measure U and V roughness for anisotropic materials). I guess any light will do, as long as the manufacturer provides an IES file for it. I don’t know if there is any reason to choose one light over another.

I’ll then place it either on top of, or completely enclosing my sample object.


Any comments / ideas?

It’s a good idea. I built a similar box with one of those fisheye peep-holes for front doors at the top, surrounded by 3 green LEDs spaced such that a diffuse surface looks uniformly illuminated. This makes it easy to see and compare the specular highlight shapes between different surfaces. It still doesn’t tell you how to set the specular roughness values, though…