BAK4 Material Properties
Parameter	BAK4
Refractive Index (nd)	1.5688 (@587.6nm)
Abbe Number (vd)	56
Transmittance	Better than BK7 (especially near UV)
Critical Angle of Total Reflection	Smaller (reduced fringe light loss)
Cost	Relatively high (about 2-3 times of BK7)

How do I choose the material for roof prism?
Parameter	Influence	Preferred Materials
Refractive Index (nd)	Determines the critical angle of total reflection and influences the design of the light path	BAK4 > BK7
Abbe Number (vd)	Dispersion control, affecting color difference	FK51 > BAK4 > BK7
Transmittance	UV/VIS/IR applications	Fused silica (broadband)
Cost	Economy of mass production	BK7 (low-end), BAK4 (mid-end)
Environmental Stability	Temperature/Humidity/Shock Resistance	Fused silica (extreme environments)

What are the advantages and disadvantages of the BAK4 roof prism?
Advantages:
Higher refractive index, reduce light loss, brighter imaging.
No edge cutting, more uniform field of view.
Suitable for high-end optical equipment, such as stargazing, hunting, military use.

Disadvantages:
Higher cost (2-3 times more expensive than BK7).
Lower Abbe number, need to rely on coating to optimize chromatic aberration.