The AR400 Polarizer is used to filter out reflections from the ring flash, ensuring correct color data acquisition without specular information and 99% correct albedo. This process is crucial for accurate photogrammetry, as it helps in capturing the true color and texture of the object without the interference of reflective highlights.
For effective cross-polarization, it’s essential to combine the AR400 Polarizer with either a circular or linear polarizer on the camera lens. This setup can be tested and adjusted by using a mirror. With the camera mounted on the AR400 and facing a mirror, turn on the preview light of the ring flash. Look through the camera’s display to observe the reflection of the ring flash in the mirror. Adjust the polarizer on the lens until the reflection of the ring flash darkens significantly. While complete elimination of the reflection is not possible, you can reduce it up to 99%, with the residual reflections appearing as a dark blue or purple hue on metallic surfaces. Once the optimal setting is found, secure the polarizer on the lens with tape. Rotating the filter on the ring flash by 90 degrees results in parallel polarized light.
Since a mirror has 100% metallic and 0% roughness values, the reflection appears to be dark blue/purple. Even crosspolarized reflections of the ring flash will be visible on surfaces that have a roughness value close to 0% since the polarization can only filter out up to 99% specular. The higher the roughness value of the surface, the less of the remaining 1% specular value will be noticeable.
Since crosspolarized lighting will result into a true albedo color of the surface, metallic surfaces will appear black since black is the physical color of metal.
4. Common Mistakes
4.1 Breaking the Polarization
When dealing with polarization, it’s important to not filter the light in any way after it’s polarized. For example if we put transparent tape on the polarizer or place anything that can spread the light between the ring flash and the surface, the light will not be polarized anymore since it will spread the light and break the polarization.
4.2 Low Quality Lens Filters
Our experience shows that some low quality polarizing lens filters are not suitable for crospolarization. Some cheap lens filters might work in some cases but we recommend high quality filters to ensure the best results
We are using the B+W KSM HTC-POL MRC nano and the Hoya HDX CIR-PL.
4.3 Indirect Lighting
Only the direct light of the ring flash will be polarized and filtered out. After the rays bounce off surfaces they will lose it’s polarization and will result in visible reflections.
Indirect bounce light can’t be completely avoided, specially outdoors but keep in mind to not use a white backdrop while scanning indoors. Make sure to have a surrounding of dark surfaces to reduce bouncelight. Black velvet is the best material for that since it’s light, cheap and doesn’t require much space.
4.4 Filter Alignment
Remember importance of precise alignment of the polarizing filters. Misalignment can lead to incomplete polarization, thus reducing the effectiveness of the setup.
4.5 Maintenance of Filters
Regular cleaning and careful handling of both the polarizing filters are crucial. Dirt, fingerprints, scratches or even raindrops can affect the polarization quality and the overall image quality.