VFL distance and eye safety

The useful operating range of fiber optic visual fault locators is widely misquoted, with ranges of 20, 30, 40 and even 50 Km often incorrectly stated.

This is what they will actually do. There is no magic, it's just a combination of emitted power, attenuation and eye sensitivity, combined with eye safety limits on emitted power when no connector is attached (which is often not quoted at all).

If you are struggling here, consider a different technology that's safe to use.

VFL range under ideal conditions Power level connected Power level disconnected Wavelength "Connector end view" continuity "Break location" in patch lead "Side glow" continuity Comment
KI6358 Pen +1 dBm + 1.7 dBm typ 650 nm 8.75 Km 5.75 Km 3.75 Km Small & easy
KI9807 -2 dBm -2 dBm 635 nm1 8.75 Km 5.75 Km 3.75 Km Brighter on short distances
KI2600 Power meter +2 dBm +2 dBm 650 nm 9.8 Km 6.8 Km 4.8 Km Top performance
KI2000 Series VisiTest +2 dBm +2 dBm 650 nm 9.8 Km 6.8 Km 4.8 Km Top performance
Class 1 Legal Limit +3 dBm +3 dBm   10 Km 7 Km 5 Km No precautions needed
KI9808 +7 dBm +7 dBm 650 nm 10.7 Km 7.7 Km 5.7 Km Requires special site precautions
Class 2M Legal Limit +10 dBm +10 dBm   11.2 Km 8.2 Km 6.2 Km Maximum legal power level allowed
Illegal VFL "20Km" +20 mW / 13 dBm Not specified! 650 nm 11.7 Km 8.7 Km 6.7 Km Illegal
Illegal VFL "30Km" +30 mW / 14.8 dBm Not specified! 650 nm 12 Km 9 Km 7 Km Illegal
Illegal VFL "40Km" +40 mW / 16 dBm Not specified! 650 nm 12.2 Km 9.2 Km 7.2 Km Illegal
Illegal VFL "50Km" +50 mW / +17 dBm Not specified! 650 nm 12.4 Km 9.4 Km 7.4 Km Illegal
Source & Clip on Identifier 0 dBm 0 dBm 1550 nm 200 Km 200 Km 200 Km Good long range solution!

Note 1: Attenuation is higher at 635 nm than at 650 nm. In addition to the photopic curve effect, this makes faults look brighter.
These calculations are approximate and variations may be observed in practice, depending on the fiber type, fiber & cable pigments, cable construction, ambient lighting, operator skill etc. 

See also: Application note: How far can a VFL go?,    General Optical Safety Standards,    1992 Kingfisher Introduces World's first handheld solid state VFL,    FiberSafe Microscope


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