TV backlighting: sizing, layout and getting the strip to stay put

A TV backlight kit sold as "for 55 to 65 inch" is guessing. The length you need is fixed geometry, and you can calculate it exactly from the screen size printed on the box.

Written by the people who manufacture the controllers in these kits and write the apps that run them. Last reviewed July 2026.

A television with blue and violet bias-lighting LED strip glowing behind it in a dark room. — A three-sided bias-lighting layout, the setup this guide sizes and installs.

The arithmetic

Television sizes are quoted as the diagonal, and almost every modern set is 16:9. That fixes the width and height completely:

Width ≈ diagonal × 0.872
Height ≈ diagonal × 0.490

So a four-sided perimeter is diagonal × 2.724 inches, or in metres, diagonal × 0.069. Here is the table so you do not have to do it:

Screen size	Panel W × H	All four sides	Three sides (no bottom)
43"	0.95 × 0.54 m	3.0 m	2.0 m
50"	1.11 × 0.62 m	3.5 m	2.4 m
55"	1.22 × 0.68 m	3.8 m	2.6 m
65"	1.44 × 0.81 m	4.5 m	3.1 m
75"	1.66 × 0.93 m	5.2 m	3.5 m
85"	1.88 × 1.06 m	5.9 m	4.0 m

These are panel dimensions, and you are not running the strip along the panel edge. The strip has to sit inset from the edge — see below — so the real path is slightly shorter than the table. Buy to the table figure anyway; the offcut is useful and running short by 20 cm at the last corner is the most annoying way to end an evening.

Three sides, not four

Skip the bottom edge. It saves a fifth to a third of the strip and it usually looks better.

The bottom edge is the one closest to whatever the TV stands on, so its light bounces off the media unit straight back toward the viewer. That reflection is far more visible than the glow on the wall, and it is the part people end up turning down or covering. On a wall-mounted set the bottom edge is also where the cables leave, so the strip has to detour around them.

Run up one side, across the top, down the other side. Start and finish at the bottom corners, which is where your controller and power lead want to be anyway.

Left: rear view of a television with LED strip running up the left side, across the top and down the right side, inset 2 to 5 cm from the panel edge, with the controller at the bottom left. Right: detail of a 45 degree diagonal fold that turns the strip through a right angle without twisting it. — Three-sided layout with the strip inset from the panel edge, and the diagonal fold we use on our own test sets to turn a corner without twisting the strip flat.

A desk monitor with a small colour-sensing camera stuck to the top edge and its cable running down, lit from behind by a soft red and cyan glow. — A colour-sync kit on a desk monitor. The small unit on the top edge is the sensor, held by an adhesive pad rather than a clip; note that its cable has to come down the back to the same place everything else does, which is worth knowing before the strip is stuck down.

Where exactly to stick it

Two measurements decide whether the glow looks like a halo or like a strip of LEDs.

Inset from the edge

Set the strip back 2–5 cm from the outer edge of the panel. Closer than 2 cm and light spills round the edge of the TV directly into your eyes, which reads as a bright rim rather than a glow. Further back than about 5 cm on a thin panel and there is not enough depth behind the set for the light to escape at all.

Distance to the wall

The glow is the light landing on the wall, so the gap between the back of the TV and the wall is what sets its size. A wall-mounted TV sitting 3 cm off the wall produces a tight, sharp halo. A set on a stand 15 cm out produces a wide, soft wash. Neither is wrong, but if you want a broad glow and your TV is tight to the wall, no amount of brightness will produce it — you need the distance.

Corners: the part that goes wrong

LED strip bends easily in one direction — the direction that curls it into a roll. It does not bend sideways at all. Trying to turn a 90° corner by twisting the strip flat is what cracks the copper traces inside it, and the failure appears weeks later as a dead section starting at that corner.

You have three legitimate options:

A loop. Let the strip bow out into a small loop at the corner and turn through the loop. Ugly if visible, invisible behind a TV, and mechanically the kindest.
Cut and rejoin. Cut on the marked cut line and use a corner connector. Cleanest look. On addressable strip, keep the data direction consistent through the joint — arrows on the strip must all point the same way round the path.
A 45° fold. Fold the strip back on itself diagonally so it changes direction without twisting. Reliable, and it needs no extra parts. This is what we use on our own test sets.

Never fold a strip sharply across its width. A crease across the copper is a future open circuit. If a fold is needed, it must be a diagonal fold that keeps the copper in a curve rather than a crease.

A television showing a yellow flower against green foliage and blue sky, with the wall behind it glowing in the same yellow, green and blue as the nearest edge of the picture. — Colour sync doing what it is for: the wall behind each edge of the picture carries the colour of that edge. It is a different product from bias lighting, and the section below is about why you have to choose.

Bias lighting and colour sync are different products

These get conflated constantly, and the difference determines what hardware you need.

Bias lighting

A steady, neutral light behind the screen, left at one setting. Its purpose is to raise the brightness of the wall behind the TV so the screen is a less extreme contrast against its surroundings, which most people find more comfortable over a long viewing session. Set it to a neutral white — around 6500K if your strip has a white channel — and about 10% of the screen's own brightness. Then leave it alone.

Needs: any dimmable strip. Nothing clever.

Colour sync

The light follows what is on screen — green when the scene is a forest, blue when it is the sea. This requires something that can actually see the picture.

Needs: a device that captures the video, either an HDMI capture box in line with your source, or a camera pointed at the screen.

A Bluetooth strip and a phone app cannot sync to your screen. There is no path for the picture to reach them. If a product promises screen sync with nothing but a Bluetooth controller, the "sync" on offer is a microphone reacting to your TV's sound — which is a real effect, and can look good on action content, but it is not following the picture. Our music and microphone modes are documented honestly as what they are: the three sync modes explained.

Our own view, having built both kinds: for most living rooms bias lighting is the better outcome. It is set-and-forget, it genuinely reduces the harshness of a bright screen in a dark room, and it never distracts. Colour sync is more impressive for the first week and is more often switched off by the second, because peripheral movement competing with the picture gets tiring.

The screen colour sensor of a TV backlight kit laid on a table: a camera head on an arm, a cross bracket carrying red 3M VHB tape with its liner still on, and a DC lead. — The colour sensor from one of these kits, with its 3M VHB pad still lined. The adhesive here is the same problem as the strip's: it goes on once, it needs a clean surface, and on a screen bezel there is no second attempt that looks straight.

Making it stay stuck

Adhesive failure on TV backlights is not really an adhesive problem. It is three things stacking up.

The back of a TV is a low-energy plastic and it is dusty. Wipe it with isopropyl alcohol and let it dry fully. Not a damp cloth — water leaves residue and the tape bonds to the residue instead of the plastic.
It gets warm. Panel electronics sit behind the screen and the whole back is warmer than room temperature in use. Warm adhesive creeps, and it creeps in the direction the strip is pulling.
Corners concentrate the peel. A strip under a slight bend pulls hardest at the point of the bend, which is exactly your corner. Corners always lift first.

So: clean with alcohol, press the strip down along its whole length with firm thumb pressure rather than laying it on, and add a mechanical fixing at each corner — a small piece of double-sided mounting foam, a cable clip, or a tie through an existing chassis hole. Two minutes of reinforcement at four corners is the difference between a permanent install and a strip dangling behind the TV in six months.

One more: leave a slack loop of cable before the controller, and fix the controller to the wall or the media unit, not to the TV. A controller swinging on its own lead applies a steady pull to the last 10 cm of strip, and that section will eventually let go.

Common questions

Can I cut the strip to length?

On plain single-colour and RGB strip, yes, at the marked cut points. On addressable strip you can cut but not rejoin arbitrarily — the data must flow in one direction, so the offcut cannot simply be reversed and stuck on the other side. Plan the path to run continuously from the controller.

Should the strip be brighter or dimmer than the TV?

Dimmer, clearly. If you can read a book by your backlight it is far too bright, and it will wash out the darker parts of the picture — the opposite of what bias lighting is for.

Why does my backlight look dimmer at the far end?

Voltage drop along the run. It is normal on longer lengths and it is more visible on white than on colour. Power and length covers what to do about it.

Which app runs a TV backlight kit?

Our TV backlight controllers run on LotusLamp X. If the strip shows one colour at a time across the whole run, that is the app you want — full mapping here.