infra-infra- /ɪnfrə/ prefix technical 
below something in a range: 
the infra-red end of the spectrum → ultra-(2)
below something in a range: the infra-red end of the spectrum → ultra-(2)- underwent
- underwhelm
- underworld
- underwrite
- underwriter
- Underwriters' Laboratories
- undeserved
- undesirable
- undetected
- undeterred
- undeveloped
- undid
- undies
- undifferentiated
- undignified
- undiluted
- undiminished
- undischarged
- undisclosed
- undiscriminating
- undisguised
- undismayed
- undisputed
- undisturbed
- undivided