NCQLP LC cheat sheet

Illuminance E (lux) = luminous flux incident per unit area; E = Φ/A for uniform flux on a surface, and E = I/d² for a point source at perpendicular distance d (inverse-square law).

Luminance L (cd/m²) = luminous intensity per projected area; L = I/(A·cosθ). Luminance, not illuminance, drives perceived brightness and glare sensation.

Illuminance falls off with the cosine of the incidence angle: E = I·cosθ/d². Doubling distance to a point source reduces illuminance to one quarter.

Luminous efficacy (lm/W) ranks sources: LED retrofit lamps commonly 100–150 lm/W, linear fluorescent roughly 90–100 lm/W, halogen near 20 lm/W, incandescent near 15 lm/W. Efficacy excludes ballast/driver losses unless system efficacy is stated.

Correlated color temperature (CCT) describes tint (warm <3300 K, neutral 3300–5300 K, cool >5300 K); CRI (Ra) describes color fidelity with 100 as reference. CCT and CRI are independent attributes.

IES illuminance recommendations are task-based maintained values: offices ~300–500 lx on the task, circulation ~100 lx, and precision inspection far higher. Design to maintained illuminance, then add maintenance factor.

Maintenance factor (MF) = LLMF × LSF × LMF × RSMF (lamp lumen, luminaire/surface, lamp survival, room surface maintenance). Design illuminance = initial illuminance × MF, so MF below 1 forces more initial light.

Uniformity criteria: max/min illuminance ratio and min/average ratio (often ≥0.7 for task areas). Spacing-to-mounting-height ratio (S/MH) controls overlap; exceeding the luminaire's rated S/MH produces scalloping and dark bands.

Glare control: UGR limits (e.g., ≤19 for offices, ≤16 for drafting) and VCP/glare rating apply; disability glare reduces visibility, discomfort glare causes annoyance. Shield bright sources with cut-off angles.

Power density check: LPD (W/m² or W/ft²) = total connected lighting power ÷ floor area. Compare to the code allowance for the space type; controls and reduced wattage lower LPD.

LENI (kWh/m²/yr) = total lighting energy ÷ area; it captures controls and hours, unlike LPD which captures only connected load.

Automatic shutoff is generally required in most commercial spaces; occupancy/vacancy sensors suit intermittently used rooms, and time-switch plus override suits scheduled spaces.

Daylight-responsive controls: photosensor dimming or stepped switching in daylit zones; calibrate setpoints to avoid hunting and false switching.

Demand response and dimming: 0–10 V, DALI, and phase-cut are common interfaces; verify compatibility and minimum load per dimmer.

Emergency egress illumination is typically ~10.8 lx (1 fc) minimum at floor along the path of egress for 90 minutes; exit signs must be illuminated and visible.

Commissioning verifies illuminance, controls sequences, and sensor calibration against design values; record measured footcandles/lux and control settings for the owner.

Color shift and lumen depreciation: LED luminaires are rated by L70 (hours to 70% lumen output) and sometimes L90; match rated life to maintenance cycles.

Harmonic and power quality: LED drivers and electronic ballasts can inject harmonics; total harmonic distortion limits and power factor (≥0.9 typical) affect compliance.

Zonal cavity method: CU (coefficient of utilization) depends on room cavity ratio, surface reflectances, and luminaire photometrics. RCR = 5·h·(L+W)/(L·W) with h as cavity height.

Lumen method: N = (E × A)/(Φ_lamp × CU × MF), where N is number of luminaires, E target maintained illuminance, A area, Φ per-lamp lumens. Round up and recheck uniformity.

Room index (metric) K = (L×W)/(h_m×(L+W)); higher K means more interreflection and generally higher utilization.

Point-by-point calculation is required for non-uniform layouts, high-bay spaces, and facades; average methods understate peaks and valleys.

Field measurement: use a calibrated illuminance meter, cosine-corrected and color-corrected; take readings on the work plane grid, avoid shadowing the sensor, and record date, lamp hours, and voltage.

Lamp/fixture inventory audits record type, wattage including ballast factor, quantity, and control type; ballast factor multiplies lamp lumens and wattage for fluorescent systems.

Retrofit economics: simple payback (years) = installed cost ÷ annual savings; annual savings = (old W − new W) × hours × quantity × energy rate ÷ 1000.

Color consistency in the field: specify a tight CCT tolerance (e.g., within 3–5 SDCM) and verify against a sample to avoid visible batch mismatch.

Follow the locally adopted energy code (e.g., ASHRAE 90.1 or IECC) for LPD allowances, mandatory controls, and lighting alterations; verify the edition in force for the jurisdiction.

Lighting power allowances are space-by-space or building-area methods; trade-offs are limited, and decorative or display lighting often has separate allowances.

IES recommended practices give target maintained illuminance and uniformity per task; they are design guidance, while codes are enforceable minimums.

Safety: lockout/tagout before servicing; capacitors in HID ballasts retain charge; LED drivers and lamps may remain hot. Verify circuit de-energized with a tester.

Ladder and lift safety: maintain three points of contact, inspect harnesses, and do not exceed ladder duty rating; never work alone on energized equipment.

Photometric files (IES/LDT) contain intensity distributions, lumens, and spacing criteria; verify the file matches the exact luminaire and optics before calculating.

Deliverables: lighting layout, luminaire schedule with photometrics and wattages, control narrative and sequence of operations, and calculation summary with maintenance factor stated.

Warranty and rated life: confirm lumen maintenance, color maintenance, and driver warranty terms; note that L70 is a statistical rating, not a guaranteed failure time.

Recycling and disposal: fluorescent lamps contain mercury and require regulated disposal; HID lamps and ballasts may also require special handling under local rules.

Vertical illuminance and cylindrical illuminance govern face recognition and wall brightness; horizontal illuminance alone can leave a space feeling dim.

Modeling and shadow: a key-to-fill ratio around 2:1 to 4:1 produces pleasing three-dimensional rendering; pure downlighting flattens faces.

Flicker: percent flicker and flicker index quantify modulation; high modulation at low frequencies can cause headaches and stroboscopic effects, especially with rotating machinery.

Glare from windows and luminaires: evaluate with DGP or DGI for daylight; position luminaires outside the 45° cone of sight where feasible.

Task-ambient design lowers energy by raising ambient levels modestly and adding local task light where needed; avoid excessive contrast between task and surround (keep surround within roughly 1:3 of task luminance).

Color rendering matters most where color judgments occur (retail, healthcare, printing); specify R9 (red) values for skin tones and food.

Outdoor and site lighting: control light trespass and skyglow with full cut-off optics, backlight/uplight ratings, and curfew dimming; BUG ratings summarize these.

Emergency and egress design: verify path of egress illumination, exit sign visibility, and that battery units are tested and logged per the code cycle.

Controls zoning should follow daylight availability, occupancy patterns, and task needs; separate perimeter daylit zones from core zones to capture savings.

If asked for energy compliance, compute LPD and compare to the adopted code allowance; if asked for energy use, compute LENI or annual kWh including hours and controls.

If the question mentions point source, distance, and intensity, apply inverse-square and cosine correction; if it mentions area and flux, use E = Φ/A.

If uniformity is failing, reduce spacing-to-mounting-height ratio, raise mounting height, or add perimeter luminaires before increasing wattage.

If a space looks bright but objects are hard to see, suspect veiling reflections or glare, not insufficient illuminance; reposition or shield sources.

If color mismatch is reported after a retrofit, check CCT binning and SDCM tolerance, not just CRI.

If maintenance is deferred, the maintained illuminance drops; either raise initial design level or specify longer-life, lower-depreciation sources.

If controls fail to save energy, verify sensor sensitivity, time delays, and daylight setpoints with a meter before replacing hardware.

If a calculation seems too good, check ballast factor, luminaire dirt depreciation, and whether the photometric file matches the installed optic.