Last updated: November 2026
Every week the same email arrives: “I have a 120 cm jewellery showcase with three shelves — how many lights do I need?”
The honest answer is that it depends on five numbers, and four of them are already in the room. This calculator turns them into a fixture count,
a beam angle, a total load and a minimum driver rating in about fifteen seconds.
It is not a photometric simulation. It is the same first pass a lighting designer does before opening simulation software: work out how much
light the merchandise needs in the pool, work out which head and beam deliver it at the real throw distance, then check that the heads are close
enough together that no dark band appears between them.
Case details
Five inputs. The result updates as you type — no sign-up, no email.
Your estimate
Layout and selection guidance, not a photometric guarantee.
Reply within one business day (Mon–Sat, GMT+8) · MOQ 1 pc · Samples in about 3 days · 3-year warranty (heads and driver)
How to read the result
Fixtures per lit level is the number that matters most. A level is one shelf surface; a case with two glass shelves has three
levels, and each needs its own heads unless you deliberately light only the top. If that number looks high, the usual cause is a deep case lit by
narrow optics — widen the beam before adding heads.
Beam angle is chosen so the pool matches the illuminance the merchandise needs at the real throw distance. Narrower than that and
you get a hot spot surrounded by shadow; wider and the light spills onto the glass, the case walls and the customer’s eyes. It is also why
showcases in catalogues so often look better than the ones in the store.
In the pool is the illuminance at the centre of one head’s beam, not an average across the shelf. Average illuminance is a
meaningless figure in a display case: the merchandise sits in the pool, and the shelf edges are supposed to fall away into shadow.
Total load and driver are the two numbers buyers forget. Heads are cheap to change. A driver running above 80% of its rating is the
most common reason a case goes dark in year two. The driver selection guide covers the
sizing rule in full.
The formula behind the numbers
None of this is proprietary, so here is the whole method. Any lighting designer can reproduce it, and any engineer on your side can challenge it.
- Throw distance = total internal height ÷ number of levels − 12 cm, for the rail offset and the average object height.
Clamped to 15–90 cm, because outside that range the geometry stops being useful. - Pool diameter = 2 × throw × tan(beam angle ÷ 2).
- Illuminance in the pool = rated lumens × 0.72 ÷ pool area, where 0.72 accounts for optical and thermal losses.
Pool area = π/4 × (pool diameter)². - Head and beam selection = the pair whose calculated illuminance is closest to the target for that merchandise, on a log scale,
among the heads and optics we actually manufacture. - Heads per level = ceiling(case width ÷ (0.85 × pool diameter)) × rows, where rows = 2 once the case is
deeper than 60 cm, and 1 below that. - Minimum driver rating = total connected load ÷ 0.8.
Two consequences worth internalising. Doubling the case width doubles the count — obvious. But halving the throw distance quadruples the
illuminance in the pool, because both the pool area and the loss term move the wrong way — which is why a shelf mounted head is worth more than
a bigger lamp.
Pool diameter at common throw distances
| Throw | 15° | 25° | 45° | 60° |
|---|---|---|---|---|
| 20 cm | 5.3 cm | 8.9 cm | 16.6 cm | 23.1 cm |
| 30 cm | 7.9 cm | 13.3 cm | 24.9 cm | 34.6 cm |
| 50 cm | 13.2 cm | 22.2 cm | 41.4 cm | 57.7 cm |
| 80 cm | 21.1 cm | 35.5 cm | 66.3 cm | 92.4 cm |
Target illuminance by merchandise
These are working ranges for retail, and ceilings for museums. The museum rows are different in kind: they are limits set by conservation policy,
and exceeding them damages the object rather than merely looking wrong. If you are lighting a collection, read
the lux and UV limits guide before choosing a wattage.
| Merchandise | Target in the pool | CCT | What usually goes wrong |
|---|---|---|---|
| Diamonds, platinum, white gold | 1,500–3,000 lux | 4000K | flat even light kills the sparkle |
| Gold, rose gold, coloured stones | 1,000–2,000 lux | 3000K | low CRI turns red stones brown |
| Pearls, amber, opal | 500–1,000 lux | 3000K | heat and UV fade over months |
| Watches | 1,000–2,000 lux | 3000K | glare on the crystal hides the dial |
| Eyeglasses and sunglasses | 800–1,500 lux | 4000K | lens reflections at eye level |
| Figures and collectibles | 500–1,000 lux | 4000K | heat build-up in a sealed case |
| Museum — high sensitivity | 50 lux (limit) | 3000K | cumulative exposure, not peak |
| Museum — medium sensitivity | 150–200 lux (limit) | 3000K | UV in the beam, not visible light |
| Museum — low sensitivity | 300 lux (limit) | 3000–4000K | over-lighting to compete with daylight |
When the calculator is not enough
Four situations need a person, not a formula.
Deep cases. Beyond about 70 cm of depth one row of heads cannot cover the shelf evenly. The calculator already switches to two rows,
but the practical fix may be a rail plus a linear bar at the back — see
track lights versus light strips for the trade-off.
Glass and mirrors. A glass shelf, a mirror back or a glass top shows every head as a bright dot. Narrow optics, deeper recessing and a
slight downward tilt solve most of it; the glare and reflection guide covers the rest.
Heat inside sealed cases. Six 3 W heads in a closed volume behave differently from six heads on an open shelf unit. If the case is
sealed and the merchandise is heat-sensitive, keep the driver outside the display volume and prefer more, lower-power heads.
Dimmed and switched installations. A dimmer changes the driver, not just the brightness. If the case will be dimmed, size the driver for
the dimming method you will actually use — the dimming guide explains which combinations flicker.
From estimate to order
The numbers above are enough to budget a project and to sanity-check a supplier’s quotation. They are not enough to place an order, because a
real order also needs track lengths, connector types, cord lengths, colour finishes and a spares package. Send us the same five inputs plus a photo
of the case and we return a complete bill of materials, normally within one working day.
Every head and driver we ship carries a 3-year warranty and uses an OSRAM chip with CRI 90+ as standard. If a
listed model is ever discontinued we give at least two weeks’ notice and recommend a drop-in replacement that fits the same track.
Request a configuration list
Or check what a complete order contains →
Frequently asked questions
How many LED lights do I need for a display case?
For a typical 100 cm wide, 45 cm deep jewellery case with two glass shelves, 9 to 18 heads in total – 3 to 6 per lit level. A gold and coloured-stone case lands near the lower end; a diamond case near the upper end, because diamonds need roughly 2,200 lux in the pool and gold needs about 1,500. Run your own dimensions through the calculator above for the exact figure.
Is there a formula for the number of showcase lights?
Yes, and it is two rules, not one. First, illuminance in the pool: E = rated lumens x 0.72 / the pool area, where pool area = pi/4 x (2 x throw x tan(beam angle / 2))^2. Choose the head and beam whose resulting E is closest to the target for the merchandise. Second, spacing: heads are placed at 85% of the pool diameter, so the pools overlap slightly and no dark band appears between them. Heads per level = case width / spacing, multiplied by the number of rows (two rows once the case is deeper than 60 cm).
Why do you subtract 12 cm from the shelf spacing?
Because the head does not sit at the shelf above – it sits in a rail, and the merchandise stands up. Subtracting roughly 12 cm for the rail offset and the average object height gives the real throw distance. It matters: going from 40 cm to 28 cm of throw shrinks the pool by nearly a third and raises the illuminance in it by more than half, which is why a tall case with many shelves needs narrower optics than a shallow counter case.
What illuminance should a jewellery display case have?
As a working range: 1,500-3,000 lux in the pool for diamonds and white metals, 1,000-2,000 lux for gold and coloured stones, 500-1,000 lux for pearls, amber and opal. Museum objects follow conservation ceilings instead – 50 lux for highly sensitive material, 150-200 lux for medium sensitivity, 300 lux for low – and those are exposure budgets, not design targets. The calculator shows what percentage of full output you need to dim to reach them.
Can the calculator size the driver for me?
It gives a minimum rating: total connected load divided by 0.8, so the driver never runs above 80% of its rating. That is a starting point, not an order code – the exact driver depends on 12 V versus 24 V, whether you dim, and how the track is split into runs. Our driver selection guide covers the full method, and every quotation states the driver explicitly.
Related guides
- Showcase and display case lighting: the complete guide
- How many lumens does a jewelry display case need?
- Display case lighting applications and scenes
- Collectibles display case lighting guide
- Why beam angle matters more than wattage
- Driver selection guide
- Eight display case lighting problems and how we solve them
- Jewelry display lighting · eyewear · museum · retail shelving