R
- Radiance L
- reading cap
- reading portion
- reduce-aperture trial case lens
- reduced aperture trial frame
- reduced-aperture lens
- Reference distance
- reference IOP
- Reference plane
- reference seeing distance (RSD)
- reference wavelength
- refracting angle
- refracting edge
- refractive index
- Refractor head
- relative distance magnification
- relative visual attenuation coefficient
- resolution
- resolving power of the fundus camera
- retinal hazard limit
- Retinoscope
- reverse polarity system
- round segment
refractive index
Ratio of the speed of electromagnetic radiation in vacuum to the phase speed of monochromatic radiation of wavelength l in the medium. NOTE1: For technical applications, the refractive index is given against air instead of against vacuum. Note2: Wavelengths to be used for the characterization of optical glasses, all kinds optical systems and instruments, together with spectacle lenses, are specified in ISO 7944.