Visibility in FDS: SOOT_YIELD, extinction coefficient and C

fdsrun editorial team · 6 November 2026 · 5 min read

How FDS computes visibility through smoke, why there is no smoke without SOOT_YIELD, what 30 m on the chart means and how to set VISIBILITY_FACTOR.

In many evacuation assessments visibility is the criterion that runs out first — before temperature or CO concentration become dangerous. FDS does not derive it directly from "smoke density": the result depends on three parameters that are easy to leave at their defaults without a second thought. Here is how they work.

Where FDS gets visibility from

With the simple chemistry model FDS tracks soot together with the other combustion products. From its density it computes the light extinction coefficient K = Km · ρ · Ys, and from that the visibility S = C / K (FDS User Guide, section 22.10.5):

• SOOT_YIELD on the &REAC line — the fraction of fuel mass converted to soot. This parameter has the largest effect on the result, and its default is 0.
• MASS_EXTINCTION_COEFFICIENT (Km) on the &SPEC line — 8700 m²/kg by default, the value suggested for flaming combustion of wood and plastics.
• VISIBILITY_FACTOR (C) on the &MISC line — 3 by default, which corresponds to light-reflecting signs; for light-emitting signs the literature gives C = 8.

Visibility is therefore inversely proportional to SOOT_YIELD: twice the soot yield gives roughly half the visibility at the same location.

What to watch out for

• No SOOT_YIELD, no smoke. With the default of 0 FDS produces no soot and visibility is at its maximum everywhere — the model "passes" the criterion while checking nothing. The value is chosen for the fuel, the scenario and the methodology you follow; designs often compare several variants.
• 30 m maximum. FDS does not report an infinite visibility but MAXIMUM_VISIBILITY from the &MISC line (30 m by default). A flat line at 30 m means "clear", not a computed value.
• C = 3 or 8. A visibility criterion from a guideline usually refers to a specific sign type — check that VISIBILITY_FACTOR matches it before you compare the result with the threshold.
• Resolution. The smoke layer under the ceiling and its descent need several cells across the layer depth; a mesh that is too coarse smears the interface and shifts the moment visibility is lost (more: cell size and D*/δx).

How to write it in the file

&SPEC ID='POLYURETHANE', FORMULA='C25H42O6N2' /
&REAC FUEL='POLYURETHANE', HEAT_OF_COMBUSTION=26200., SOOT_YIELD=0.131, CO_YIELD=0.01 /
&MISC VISIBILITY_FACTOR=3. /
&SLCF PBZ=1.8, QUANTITY='VISIBILITY' /
&DEVC XYZ=12.0,4.0,1.8, QUANTITY='VISIBILITY', ID='VIS_exit_A' /

The soot and CO yields come from the polyurethane reaction example in the FDS User Guide (chapter 13). A slice at 1.8 m (a typical eye height with a margin) shows the whole storey, and &DEVC devices at the exits give time histories that are easy to set against the evacuation time. Besides visibility, record 'EXTINCTION COEFFICIENT' as well — that is K, from which visibility for a different C can be recalculated without running again.

Soot yield variants without waiting

Changing SOOT_YIELD changes neither the mesh nor the run time, so the variants are independent and can run in parallel. In fdsrun each variant starts on its own server, and before the start you see the forecast time and cost of each.

Sources: FDS User Guide (NIST SP 1019), section 22.10.5 "Visibility and Obscuration", 13.2. FDS and Smokeview are software of the National Institute of Standards and Technology; NIST is not affiliated with fdsrun.