Jewelry Store: Beam Angle, Color and Dimming Compatibility
A 140 m² jewelry store has a 3.0 m ceiling, 300 lx of ambient from recessed downlights, and a track at 3.0 m over display cases whose glass tops are at 0.9 m. The owner's standard is 3,000 lx inside each case, with diamonds that sparkle and colored stones that look saturated. The contractor proposes 30 track heads, each 15 W, 24° beam, 3000 K, Ra 80 with R9 of about 12, one or two per case, aimed straight down. After opening, the owner reports that stones look flat and brown, customers see a bright reflection of the ceiling in the case glass, and when staff dim the track to 20% for evening ambience the heads flicker and buzz. The track is on a 600 W two-wire leading-edge wall dimmer whose manufacturer lists a 150 W maximum for LED and compact fluorescent loads; each head has a phase-cut dimmable driver.
1. Compute the beam angle needed to light a case top of 0.35 m × 0.5 m from the 3.0 m track, and evaluate the proposed 24° heads.
Read the worked answer
Throw distance is 3.0 − 0.9 = 2.1 m. A beam angle θ produces a pool diameter of 2 × 2.1 × tan(θ/2).
For the proposed 24°: 2 × 2.1 × tan 12° ≈ 0.89 m - more than twice the case width, so most of the light lands on the floor and the case edges, and illuminance on the stones is a small fraction of the rated center-beam candela.
For a 0.35 m pool: tan(θ/2) = 0.175 / 2.1 = 0.083, θ ≈ 9.5°. For the 0.5 m dimension, about 13.5°. So a 10° head with a little aiming tolerance, or a narrow asymmetric optic, fits.
Note that the 24° figure is a beam angle (50% of peak); the field angle is wider still, and the catalogue candela applies only at the center. Consequences: with 10° optics the number of heads per case rises and aiming becomes critical - verify the manufacturer offers interchangeable optics and that the head is tool-adjustable and lockable. Aim from the front edge of the case toward the back so light skims the stones rather than striking them perpendicularly. Recheck the 3,000 lx target with a point calculation using the actual candela distribution, not the lumen output; a 15 W head with a 10° optic may need a higher-output version or a case-mounted linear LED closer to the merchandise.
2. Explain why the stones look flat and brown and why customers see the ceiling in the glass, and give the fixes.
Read the worked answer
Color. Ra 80 with R9 around 12 means reds and pink-red stones lose saturation - rubies and pink sapphires go brown - and diamonds lose the fire that comes from the blue and violet part of the spectrum. Jewelry is a high-CRI application: specify Ra 90+ with R9 at least 50, and check the SPD rather than the single number. For diamonds a slightly cooler 3500–4000 K with strong blue content makes brilliance pop; for gold and yellow stones, 3000–3500 K is warmer and more flattering.
Flatness. A single perpendicular beam puts all the light on the table facet from one direction, so there is no contrast between bright and dark facets - and that contrast is what reads as sparkle. Two heads per case, from opposite sides and at different angles, restore it.
Glass reflection. The customer is seeing a veiling reflection of the fixture at the specular angle: angle of incidence equals angle of reflection. Move the head in front of the case so the reflection passes over the customer's shoulder rather than into their eye; tilt the beam 20–30° off vertical, lighting from the customer's side; or light the case from the front lip with a linear LED hidden under the frame. Reducing the luminance contrast between the fixture image and the case glass also helps - raise ambient from 300 to about 400 lx and use anti-reflective glass where the case design permits.
3. Diagnose the flicker and buzz at 20% and specify a corrected dimming scheme.
Read the worked answer
The load is 30 heads × 15 W = 450 W on a 600 W two-wire leading-edge dimmer whose manufacturer allows only 150 W of LED/CFL load. The dimmer is loaded to three times its LED rating, and it is a leading-edge type feeding phase-cut drivers. Both faults appear at the low end: at small conduction angles the triac needs a minimum hold current, the electronic driver's current collapses early, the triac misfires, and you get flicker, buzz and possible drop-out. The driver's own low-end behavior compounds it - many phase-cut drivers fade abruptly or flicker below about 10–15%.
Correct scheme:
- Put the track on a dimmer with an adequate LED rating. Size at roughly half the incandescent rating for LED loads, so 450 W needs about a 1,000 W-rated LED-compatible dimmer; a neutral at the dimmer is usually needed for reliable low-end performance.
- Use a driver–dimmer pair the manufacturer lists as compatible, and prefer 0-10 V or DALI where the control signal is separate from the power - that removes the phase-cut problem entirely.
- Check the driver's published minimum output and flicker specification. If it cannot go below 10% cleanly, do not promise 1–20%; set the low end where the driver is stable and use scene presets instead.
- Verify neutrals, that dimmer and driver share a circuit, and that no other load shares the dimmer.