V-Ray Tip: Physically Based Light Decay and Range Sculpting

Light decay gives you physically grounded falloff and cleaner lighting without adding extra fill lights.

Why use light decay:

• Matches real-world inverse-square falloff for believable contrast and shadow rolloff.
• Reduces the need for multiple compensating lights, simplifying rigs and speeding renders.
• Lets you shape near and far attenuation to mimic real fixtures (softboxes, neons, practicals) without cheats.

Setup essentials (host-agnostic, V-Ray CPU/GPU):

• Use physical light units:
  • Area/mesh/dome: lumens (lm) or radiant power (W).
  • Spot/point: candelas (cd) or lumens.
• Choose inverse-square as the baseline falloff (the physically correct model).
• Enable custom Light Decay on the light and set near/far ranges to sculpt the contribution:
  • Near start/end: soften the hotspot close to the emitter.
  • Far start/end: control how quickly the light dies in the distance.
  • Power/exponent (if available): fine-tune the curve without changing intensity units.
• Keep camera exposure physical (ISO, shutter, f‑stop) and avoid compensating with arbitrary light multipliers.
• Iterate in the V-Ray Frame Buffer with LightMix to A/B decay ranges per light without re-rendering the full frame.

Practical starting points:

• Product softbox (rect light, 90–120 cm):
  • Units: 2500–5000 lm, inverse-square.
  • Near start/end: 0.15 m / 0.40 m (smooth hotspot).
  • Far start/end: 2.0 m / 3.0 m (gentle tail for soft fill).
• Practical bulb (sphere/point, 60 W equiv):
  • Units: 800–1000 lm, inverse-square.
  • Near start/end: 0.05 m / 0.20 m.
  • Far start/end: 3.0 m / 4.5 m (keep room ambience without extra fills).
• Neon/LED strip (mesh/rect):
  • Units: 1200–2000 lm per meter.
  • Near start/end: 0.00 m / 0.05 m (tight rim), exponent slightly < 2 for a flatter run.
  • Far start/end: 1.5 m / 2.0 m (avoid hard cutoff).

Quality and performance tips:

• Use IES profiles with custom decay to match real fixtures while preserving artistic control.
• Lower glossy and refraction max depth on distant/hidden props; realistic decay naturally reduces long-bounce noise.
• Avoid extreme near/far overlaps; abrupt ranges can cause banding or patchy illumination.
• Check exposure using the VFB’s color management (ACEScg workflow recommended) and evaluate highlights before clamping.
• Pair with Adaptive Dome Light for HDRI interiors; decay will make window-side contrast behave more believably.

When to break the rules:

• For stylized or ad lookdev, slightly reduce the exponent below 2.0 to hold speculars without adding fill lights.
• For moody interiors, increase far falloff to deepen corners instead of stacking negative lights.

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