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 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
