What are the advantages of BK7 (K9) rhomboid prisms?
No image rotation: preserves the original image orientation (unlike penta or dove prisms, which introduce rotation).
Low dispersion: high Abbe number of BK7 glass minimizes chromatic aberration, making it ideal for broadband light sources (e.g., white-light interferometers).
High transmittance: with AR coating, single surface reflectance is <0.2%@visible wavelengths, maximizing light throughput.

Rhomboid prism vs. other translational prisms
Prism Type	Optical Path	Aspect of Image	Applicable Scene
Rhomboid Prism	Translation + imageless rotation	Keep the original image	Interferometer, precise optical path adjustment
Penta Prism	90° rotation + imageless rotation	Keep the original image	Rangefinder, optical aiming
Right Angle Prism	90° rotation + mirror image reversal	Reverse left and right	Telescope, simple optical path folding