Selecting an LED flood light is a geometry problem before it is a shopping problem: the beam angle and mounting position decide where the lumens land, and only then does wattage decide how many of them arrive. Buyers who start from “how many watts?” end up with hot spots, dark corners, and glare complaints; engineers start from throw distance → beam angle → target lux → wattage → fixture count, in that order. This guide is the complete version of that sequence — the geometry with diagrams, the sizing math with worked examples, the aiming and spill rules that keep neighbours and cameras happy, and the checklist that turns it all into a purchase order.
Key Takeaways
- Beam angle is chosen by throw distance, not preference: wide (90–120°) for near/broad areas, medium (30–60°) for courts and yards, narrow (≤30°) or asymmetric for long-throw and high-mast work.
- The footprint math is one line: spot diameter ≈ 2 × distance × tan(beam ÷ 2) — a 60° beam at 20 m paints a ~23 m circle.
- Size wattage from target lux × area, then divide by beam efficiency and maintenance factor — never from a “replaces 400W” label.
- Tilt ruins or rescues a design: 60–70° from vertical maximizes throw; past ~75° the beam skips off the target and becomes glare and spill.
- For sports and camera areas, flicker-free drivers and vertical illuminance are specification items, not upgrades.
Step 1 — Beam Angle: The Geometry That Decides Everything
Every flood light datasheet quotes a beam angle — the cone within which the fixture delivers at least half its peak intensity. That “peak intensity” is measured in candela, and it is the number that converts into delivered lux at your mounting height; lumens vs lux vs candela covers the conversion and the inverse-square arithmetic behind it. The working consequence is the light footprint on your surface:
Run the formula against your own site before touching a catalog. Three anchoring examples:
| Throw distance | Task footprint | Beam that fits |
|---|---|---|
| 8 m (façade wash, sign) | 10–15 m wide band | 90–120° |
| 20 m (car park from perimeter, small court) | 15–25 m zone | 60–90° |
| 40 m (sports field from side poles) | ~20 m strip per fixture | 25–40° |
| 80 m+ (high-mast, port yard) | overlapping ~25 m cells | ≤25° or asymmetric |
Two refinements the simple cone hides. First, intensity is not uniform inside the beam — it peaks at center and halves at the quoted edge, which is why designs overlap adjacent footprints by 30–50% to hold uniformity. Second, when the beam meets a surface at a slant (which is always, for a tilted flood), the circle stretches into an ellipse pointing away from the pole — useful throw for free, but also the mechanism that launches glare past your boundary if the tilt grows careless. That’s Step 4.
Asymmetric optics deserve a special mention because they solve the tilt dilemma outright: the lens throws light forward in a wedge while the fixture body stays near-horizontal. The aperture stays visually cuttoff, spill stays low, and the reach remains — which is why asymmetric distributions dominate modern sports and area lighting. Our SJTG flood series offers symmetric beams from 15° to 120° plus asymmetric options; matching them to geometry is exactly what the free DIALux study with each quote is for. The most-specified wattages have their own pages: 200W and 300W with measured photometry, and 400W for sports and high-mast duty.
Step 2 — Wattage: Sized From Lux, Never From Labels
With geometry fixed, wattage is arithmetic. Set the target illuminance for the task:
| Application | Maintained lux target |
|---|---|
| Security / perimeter | 10 – 50 lux |
| Car parks, open yards | 20 – 75 lux |
| Container terminals, working yards | 50 – 200 lux |
| Façade / architectural | 30 – 150 lux |
| Recreational sports | 75 – 200 lux |
| Club / training sports | 200 – 500 lux |
| Televised sports | 500 – 2000 lux (vertical too) |
(The sports tiers come from EN 12193 — unpacked with per-sport tables in our stadium lighting guide.)
Then run the same lumen method used for warehouse interiors, with outdoor-honest factors:
Worked example — a 60 × 40 m storage yard at 50 lux:
- Raw lumens:
50 lux × 2,400 m² = 120,000 lm - Beam/utilization factor ~0.6 (outdoor edges, overlap, tilt losses):
120,000 ÷ 0.6 = 200,000 lm - Maintenance factor 0.8:
200,000 ÷ 0.8 = 250,000 lm installed - Per fixture at 160 lm/W: a 200W flood delivers ~32,000 lm → ≈ 8 fixtures, two per corner pole with medium beams, footprints overlapped.
The same yard specified from a “200W replaces 400W metal halide” label would have skipped lines 2–3 and delivered about 30 lux. Wattage equivalence tables have their place — ours is in the flood product page retrofit section — but they size replacements, not designs.
A note on the number that actually matters on the datasheet: delivered luminaire lumens verified by an LM-79 report, not chip lumens, not “up to” figures. At equal watts, the honest and the optimistic datasheet can differ by a third — which is precisely the margin that separates a compliant yard from a dim one.
Step 3 — Mounting Height: The Third Variable
Height, beam, and spacing move together. The practical couplings:
- Minimum height rule: mounting height ≥ ~0.5 × throw distance keeps incidence angles workable; light arriving at a grazing angle mostly glares and barely illuminates.
- Higher pole, wider cell: raising from 10 m to 15 m lets each fixture cover a larger cell at better uniformity — at the price of more lumens per fixture to hold the lux. High-mast (20–30 m) yards run exactly this trade with 400–1200W class fixtures.
- Glare scales inversely with height: the same lumens from a 6 m pole sit in everyone’s eyeline; from 15 m they don’t. When neighbours or drivers complain, the cheap fix is optics and tilt; the real fix is often height.
- Maintenance access is a design input: a 30 m mast means a raising-lowering system or a very tall reach — one more reason the fixture’s 50,000-hour rating and driver quality matter more outdoors than anywhere else.
Pole-count shortcuts for rectangular areas: perimeter poles at spacing ≈ 3.5–4× height cover the strip along each side; areas wider than ~2× pole height need either opposing poles or central masts. Beyond these rules of thumb, the honest answer is always the photometric layout — geometry this coupled is what DIALux is for.
Step 4 — Aiming and Tilt: Where Designs Are Won or Lost
The field rules that photometric files quietly encode:
- Aim the beam’s half-peak edge, not its center, at the far edge of the task. The intense core then covers the middle distance, and the far edge still receives its half-intensity share — uniformity by construction.
- Stay under ~70° of tilt (from vertical) for conventional symmetric floods. Beyond that, each extra degree adds more glare and spill than lux.
- Cross-aim on sports fields: fixtures on each side pole aim across the field, not straight down their own touchline — this is what builds the vertical illuminance that players, spectators, and cameras need.
- Respect obtrusive-light limits: for sites near housing or roads, boundary lux limits and upward-light ratios apply in most jurisdictions (the CIE “environmental zones” framework). Asymmetric optics + modest tilt is the compliant combination.
- Commission with a meter. Aiming drift of a few degrees during installation is normal; a one-hour lux-grid walk at handover catches it while the lift is still on site.
Step 5 — The Spec Sheet Beyond Lumens
Selection guides love geometry and forget the fixture. The line items that decide whether the design still works in year five:
- Ingress and build: IP65 minimum for fully exposed mounting; die-cast aluminum body and tempered glass — the anatomy on our flood product page shows what each layer does.
- Thermals: outdoor full-power all-night duty is the hardest LED duty cycle; junction temperature control (ours holds <65°C at full load) is what makes the L70 rating honest.
- Drivers: wide input voltage for weak grids, surge protection sized to exposed feeder reality (10kV-class for pole-tops in storm regions), and flicker-free operation wherever cameras or fast sport are involved — slow-motion broadcast is merciless to cheap drivers.
- Wind load: fixture area × mast height is a structural input; slim and multi-module designs exist precisely to cut sail area at altitude.
- Serviceability: modular fixtures with independently rotatable heads (like our SJTG611) let one pole serve several aim points and keep replacements per-module rather than per-fixture.
- Compliance paperwork: FCC/CE/CCC as applicable, LM-79/LM-80 files, and IES/LDT photometrics for the layout — the same documentation checklist as any import from China deserves.
CCT and CRI: The Light Quality Choices
Two datasheet lines shape how the lit area feels and films:
Correlated color temperature (CCT). For area and yard lighting, 4000K and 5000K dominate: 4000K reads warmer and calmer for mixed pedestrian environments; 5000K reads crisper for pure work zones and security. Sports broadcast pushes toward 5000–5700K because cameras are calibrated near daylight. Two practical rules beat any aesthetic debate: keep one CCT per visual field (a yard mixing 4000K and 5700K poles looks broken), and check local ordinances — a growing number of municipalities cap outdoor lighting at 3000–4000K for sky-glow and ecology reasons, which quietly disqualifies half the catalogs before you start.
Color rendering (CRI). ≥70 satisfies pure security duty; ≥80 is the sensible default wherever people identify objects, goods, or faces — loading yards, sports, retail forecourts. Above 80, gains are real but small for outdoor work; pay for CRI 90 only when a spec explicitly demands it (broadcast, inspection under floods).
Controls: The Cheapest Lumens Are the Ones You Dim
An outdoor flood runs the longest annual hours of any fixture class — dusk-to-dawn duty exceeds 4,000 h/year — so controls repay themselves faster here than indoors:
- Photocell (dusk-to-dawn) is the floor: no flood should burn at noon. Specify it integrated or via the pole circuit.
- Time-step dimming mirrors the street-lighting playbook: full output through evening activity, 50% after the yard empties, without any sensing hardware.
- Motion-triggered boost suits security perimeters: a 20–30% idle level (cameras stay happy) rising to 100% on detection — deterrence and evidence quality in one move.
- 0–10V / DALI drivers ordered on day one keep every one of these options open; retrofitting dimmable drivers later costs more than the option ever did. The same rule we give for high bay retrofits applies verbatim outdoors.
One caution: dimmed levels count against your maintained lux only during the dimmed hours — document the schedule in the design file so a night auditor with a lux meter isn’t measuring your 30% idle state against the 100% target.
A Full Worked Scenario: 80 × 50 m Car Park, Four Corner Poles
Pulling every step together on one realistic site:
- Geometry. Poles at the corners, 12 m high. Longest throw to plaza center ≈ 47 m — beyond comfortable for one fixture, so each pole covers its quadrant (~40 × 25 m) with two fixtures: one medium 60° aimed at the quadrant’s near-middle, one narrower 40° cross-aimed toward center. Footprints from adjacent poles overlap along both axes.
- Photometry. Target 30 lux average (upper band for public parking). Raw: 30 × 4,000 m² = 120,000 lm. ÷0.6 utilization ÷0.8 maintenance = 250,000 lm installed. Eight fixtures → ~31,000 lm each → 200W class at 160 lm/W.
- Layout check. Spacing along the long side is 80 m pole-to-pole = 6.7× height — beyond the 4× comfort rule, which the cross-aimed second fixture per pole is there to repair; uniformity gets verified, not assumed.
- Fixture spec. IP65, 10kV surge (exposed poles), 4000K/CRI 80, photocell + 0–10V drivers, asymmetric option evaluated for the two poles facing the residential boundary.
- Validation. DIALux run with IES files → average 32 lux, min/avg uniformity 0.28 → acceptable for parking; the boundary spill check passes with the asymmetric pair. Order: 8 × 200W, two optics, one pole-top bracket type.
Total engineering time: under an hour with the fixture’s photometric files — which is why step ⑤ is free with every quotation we issue.
The Selection Flow on One Page
Frequently Asked Questions
What beam angle should I choose for a flood light? Choose it from throw distance: 90–120° for targets within roughly one mounting-height of the pole (façades, nearby yards), 60–90° for mid-range areas like car parks, 25–40° for sports-field throws around 30–50 m, and ≤25° or asymmetric optics for high-mast and long-throw work. The footprint formula — 2 × distance × tan(beam/2) — settles any specific case in one line.
How many watts of LED flood light do I need per square metre? Divide the target lux by roughly 90 (that’s 160 lm/W × 0.6 utilization × 0.8 maintenance, per m²): a 50 lux yard needs ~0.55 W/m² installed, a 200 lux court ~2.2 W/m². Then round up into real fixtures placed by the layout rules — watts per square metre budgets the project; the aiming plan delivers it.
What is the difference between symmetric and asymmetric flood lights? A symmetric flood throws a centered cone and must be tilted toward the target — buying reach at the cost of glare once tilt grows. An asymmetric optic throws its light forward in a wedge while the aperture stays near-horizontal, delivering reach with a visually cutoff source. For boundaries near roads or housing, and for most modern sports installations, asymmetric is the default answer.
How high should flood light poles be? Keep mounting height at least half the throw distance, and expect spacing around 3.5–4× height along a perimeter. Car parks work well at 10–12 m, training fields at 15–18 m side poles, and container yards at 20–30 m masts with narrow-beam high-power fixtures. Higher poles buy uniformity and glare control at the cost of lumens and access — a trade the photometric layout makes explicit.
Can I just replace my 400W metal halide floods one-for-one with LED? For pure retrofits, yes — 150–200W LED per 400W MH is the working equivalence, detailed in our flood retrofit table. But a one-for-one swap inherits the old design’s aiming and spacing; if the original installation was mediocre, a fresh Step-①-to-⑤ pass often lights the site better with fewer fixtures.
Why does my existing flood installation have bright spots and dark patches? Almost always an aiming problem, not a wattage problem: footprints that don’t overlap leave troughs between poles, while over-tilted fixtures stack their intense beam cores on the near field and starve the far edge. Before buying more watts, re-aim to the half-peak-edge rule with 30–50% footprint overlap — and if the geometry simply can’t reach, that’s the signal for a beam-angle change or an added pole, both cheaper than doubling every fixture.
The Bottom Line
Flood lighting rewards the engineer’s order of operations: geometry first (throw → beam), photometry second (lux → lumens → watts), layout third (height, tilt under 70°, overlapped footprints), fixture spec fourth (IP, thermals, driver, wind), and a DIALux validation before any money moves. Follow that sequence and the catalog becomes easy — every fixture in our SJTG range is listed by exactly these parameters, and your site’s geometry is all we need to return the full layout, fixture schedule, and IES files at no cost.