Possible Clues:

• Like some night vision
• Knowledge largely hard to find by day? None of us can see it
• Like night-vision goggles
• Type of radiation
• Find out about unusual part of spectrum
• Type of light
• Fear you can't see rind like this
• One can't ruddy well see it beyond the rainbow
• Unusually rare find that's not normally visible
• Having longer than visible wavelength