This page explains exactly how LightCalc's two calculation models work, where they come from, and where they break down. If you've been gaffing for 20 years and trust your light meter more than any app, good — you should. Read this first.
Footcandles (fc) — the illuminance on a surface, measured in lumens per square foot. One footcandle = one lumen falling on one square foot. The unit film and TV production uses because it maps directly to exposure.
Lux (lx) — the SI equivalent: lumens per square meter. One footcandle = 10.764 lux. LightCalc works internally in lux and converts to fc for display.
Lux @ 1m — manufacturer-published center-beam illuminance at 1 metre from the fixture, full power, open face, no modifier. This is the baseline spec LightCalc uses. It's always a center-beam measurement — the edges of the beam are dimmer.
Light from a point source spreads in all directions. As it travels, it covers an increasing sphere of surface area. The surface area of a sphere is 4πr² — so at twice the distance, the same light covers 4× the area, meaning the intensity at any point is ¼ what it was.
So if a fixture measures 10,000 lux at 1m, at 3m it's 10,000 / 9 = 1,111 lux. At 5m: 10,000 / 25 = 400 lux.
The ASC published a thorough field test confirming this ("Revisiting — and Updating — Inverse-Square Law," American Cinematographer), which also demonstrated that soft source falloff is measurably shallower than point source falloff — which is why LightCalc has two models.
The "Practical estimate" model treats the entire rig — fixtures, diffuser, all — as a single modified point source. It's what a gaffer does mentally on set.
This collapses the whole rig into one distance and one efficiency multiplier. It's fast, intuitive, and reasonably accurate when:
It breaks down when the diffuser is large relative to the subject distance — e.g., a 12×12 frame with the talent 4ft underneath it. In that case, the diffuser cannot be approximated as a point source.
When the diffuser is large relative to the subject distance, it must be modeled as an extended area emitter, not a point source. This is the standard approach in physically-based rendering (Unreal Engine, Arnold, V-Ray all use this).
A Lambertian surface emits light equally in all forward directions, with intensity proportional to the cosine of the angle from the surface normal (Lambert's cosine law, 1760). This is the standard model for diffusion materials.
The illuminance at a point from a Lambertian area source equals the luminance multiplied by the projected solid angle of the source as seen from that point:
For a rectangular source, the projected solid angle has a closed-form analytic solution (derived from Fock, 1924; used in radiosity rendering since the 1980s). For a point on the centre normal of a rectangle with half-widths a and b at perpendicular distance d:
For off-axis points (secondary point / background), the rectangle is split into up to four sub-rectangles about the perpendicular foot of the measurement point, and the formula is applied to each quadrant and summed or subtracted as appropriate.
LightCalc models diffusion as a simple transmission multiplier T applied to the luminous exitance. Real diffusion materials have a BTDF (bidirectional transmittance distribution function) — they're not perfectly Lambertian — but this is a good first approximation for most materials.
| Diffuser | Transmission (T) | Stop Loss | Notes |
|---|---|---|---|
| Open Face | 1.00 | 0 | No diffuser |
| ¼ Grid | 0.85 | ~0.23 | Minimal scatter, mostly forward |
| ½ Grid | 0.50 | 1.0 | Most common key light diffuser |
| Full Grid | 0.25 | 2.0 | Heavy diffusion, even scatter |
| Light Grid | 0.35 | ~1.5 | Between ½ and full |
| Opal | 0.30 | ~1.75 | Very even scatter, near-Lambertian |
The lux@1m spec is always measured at the center of the beam. At the edge of the beam angle (defined as the angle where intensity drops to 50% of center), you're already 1 stop down.
LightCalc uses beam angle to calculate what fraction of the diffuser panel is actually illuminated by the fixture:
Real diffusion materials are not perfectly Lambertian. A ¼ grid passes most light nearly straight through (forward-biased). Full opal scatters almost uniformly in all forward directions (near-Lambertian). This affects how the diffuser behaves as an area source.
LightCalc's BTDF slider (0–100%) interpolates linearly between two extremes:
Default values per material: Opal 10% (near-Lambertian), Full Grid 20%, ½ Grid 50%, ¼ Grid 75%, Open Face 100%.
When a modifier is on the fixture (Fresnel, softbox, umbrella, beauty dish), LightCalc models a two-stage chain:
These narrow and redirect the beam. The output is still a point source, with a modified beam angle and an efficiency multiplier applied.
These create a new area emitter at the fixture position. The calc becomes area source → area source — two applications of the projected solid angle formula in series.
The relationship between illuminance and exposure follows the standard photometric exposure equation:
LightCalc uses a simplified practical version calibrated to incident light (not reflected):
| T-stop | fc needed (ISO 800, 24fps) | fc needed (ISO 3200) |
|---|---|---|
| T2 | ~16 fc | ~4 fc |
| T2.8 | ~32 fc | ~8 fc |
| T4 | ~64 fc | ~16 fc |
| T5.6 | ~128 fc | ~32 fc |
| T8 | ~256 fc | ~64 fc |
| T11 | ~512 fc | ~128 fc |
| Limitation | Impact | Workaround |
|---|---|---|
| Uniform diffuser illumination assumed | One fixture creates a hotspot on the diffuser, not uniform luminance. Model B over-estimates evenness. | Use multiple fixtures spread across the frame, or increase fixture-to-diffuser distance. |
| Multiple fixtures treated as one point source | Three LS600x in a row have different beam overlap at different positions on the diffuser. | Conservative estimate: assume the center of the diffuser is the best-lit point. |
| Diffusers are not perfectly Lambertian | Grid cloth transmits more on-axis than off-axis. Use the BTDF slider to compensate. | Measure your specific material if precision matters. |
| No bounce or ambient modeling | On-set bounce from white walls, ceilings, and reflectors adds fill. Calculator shows direct only. | Add 0.3–1 stop fill for practical on-set environments. |
| Lux@1m specs vary by test conditions | Some manufacturers are optimistic. Real output may be 10–20% lower. | Load the IES file from the manufacturer for more accurate beam data. |
| No spectral modeling | Transmission values are broadband. Blue gels transmit much less than white light averages suggest. | Measure gel-filtered output with your meter when using heavy color. |
LightCalc is a planning tool. Results are estimates based on idealized models and manufacturer specifications. Always confirm exposure with an incident light meter at the subject position. Built by Smith Creative Technologies.