Blue light, a pyrotechnic signal.
Blue light on a police vehicle.


blue light (countable and uncountable, plural blue lights)

  1. Visible light towards the blue end of the spectrum generated from the screen of an electronic device.
  2. (historical, uncountable) A mixture of chemicals (including nitre, sulfur and antimony) used in eighteenth and nineteenth centuries for pyrotechnics, night-time signaling and general illumination.
    • 1828, Gray, Samuel Frederick, The Operative Chemist, page 499:
      Blue lights, or blue fire, is a preparation in which zinc and sulphur, or sulphur alone, are used. The particular colour is communicated by the zinc and sulphur.
    • 2015, Ragan, Mark K., quoting Robert Fleming, Proceedings of a Naval Court of Inquiry into the Sinking of the Housatonic, 1864, quoted in 'Confederate Saboteurs', Texas A&M University Press, →ISBN, page 98:
      When the 'Canandaigua' got astern, and was lying athwart, of the 'Housatonic,' about four ship lengths off, while I was in the fore rigging, I saw a blue light on the water just ahead of the 'Canandaigua,' and on the starboard quarter of the 'Housatonic.'
  3. (countable) A flashing light, usually fitted to an emergency vehicle.
    • 2019 October, Roger Ford, “Power failure highlights specification confusion”, in Modern Railways, page 26:
      Until these trains could be restarted, they blocked lines out of Farringdon and King's Cross, trapping other services. Blue-light escorts were provided where possible to get engineers to the stranded GTR trains to reset the software.
  4. Used other than with a figurative or idiomatic meaning: see blue,‎ light.

Usage notesEdit

The original chemical mixtures burned with a blue flame. Later versions omitted any colouring agents, producing a bright white light, but retained the name by convention.



Derived termsEdit




blue light (third-person singular simple present blue lights, present participle blue lighting, simple past and past participle blue lighted)

  1. To travel quickly in a police or ambulance vehicle with the lightbar (and possibly the siren) activated.
    • 2011, Donoghue, John, Police, Crime & 999, Troubador Publishing, →ISBN:
      When we weren't blue lighting, we had to obey the 30 mph limit, but out of town we had a training exemption from any speed regulations and were encouraged to push the car to its limits... and we did.
    • 2012, Mukand, Jon, The Man with the Bionic Brain, Chicago Review Press, →ISBN:
      They jumped into his cruiser and blue-lighted it to Boston Medical Center at eighty miles per hour.
    • 2014, Rennie, James, The Operators, Pen and Sword, →ISBN:
      Roger that. Feds and green army are blue lighting to you.