LED strip lights, and the number that decides it is where you can cut

Twenty self adhesive LED strips, taken from one search and none of them handled here. They are sold for the back of a television, the underside of a kitchen cupboard, a stair, a wardrobe, a shelf.

Everybody cuts these to length. Not one of the twenty titles says where you are allowed to cut.

The measurement your room imposes, and the shelf declines to state

A strip is bought to fit something: the width of a worktop, the back of a screen, the length of a shelf. So the first practical question is not how bright it is. It is whether you can make it the length you need.

You cannot cut a strip anywhere you like. The copper track runs in repeating segments and there is one marked point per segment where the circuit can be broken without killing everything after it. Cut between the marks and the section goes dark.

That interval is the number that decides whether the strip fits your shelf, and here is what this page does with it.

And the intervals genuinely differ. Opening the images on the picks below found three answers on one page of results:

• Cut anywhere. The cheapest strip here says so in a specification table in its own first photograph, and calls it an arbitrary cut.
• Every 2.5cm. Two of the 5V COB strips print exactly that, one as a measurement and one as a row of little scissors along the board.
• Every 10cm. The mains strip, which is four times coarser than the rest.

Ten centimetres is enough to matter. If your shelf is 87cm, a strip you can only cut every 10cm gives you 80cm and a gap, or 90cm and a fold.

Four different voltages, sold in one list

The other thing this page mixes without saying so is how the strip is powered. Fifteen of the twenty state a voltage and they are not variations on a theme, they are four different products.

If you have not thought about this, 5V is the one that works when it arrives. A 12V or 24V strip without a driver is a reel of tape.

The 220V ones deserve their own sentence. They are in the same list, at about the same price, and they carry mains voltage along their whole length. That is not a reason to be frightened of them, it is a reason not to buy one by accident for the back of a telly. They cut only at the coarse interval above, the cut end needs sealing properly rather than tucking behind a cupboard, and the sensible use is a long outdoor or garage run where their advantage, no driver and no voltage drop, is the point.

COB or SMD, which is the difference you will see

Eleven say COB, four say SMD, and five say neither.

SMD is individual chips spaced along the board, usually labelled 2835 or 5050. Look at it directly and you see a row of dots.

COB is a continuous strip of phosphor over the chips, so it reads as a line of light rather than a row of points.

At two metres up a wall, behind a television, nobody can tell. Under a cupboard at eye level, or anywhere you can see the strip itself rather than only what it lights, COB is the one that looks like a light fitting and SMD is the one that looks like a strip of components.

One seller makes this argument better than I can. The ten metre reel below has, in its own gallery, a photograph of COB beside SMD with the caption that one is uniform and the other has dark spots. It is selling you the COB, and it is also simply true.

The two numbers almost nobody gives

Six of twenty give a density, in LEDs per metre, and the figures run from 120 to 576. Density is what decides whether dots show: more chips, closer together, less gap. On a COB strip anything from about 300 upward is a smooth line. The 120 per metre figure belongs to one of the mains strips.

Two of twenty give a lumens per metre figure and a colour rendering index, and they are the same two, and on the day this was written they were the two dearest things on the page.

Lumens per metre is how much light. Colour rendering index is how honest that light is about colour: low CRI makes food, skin and wood look grey and slightly wrong, which is exactly the effect people are trying to avoid when they put a nice light under a kitchen cupboard. Above 90 is good.

That those two figures appear together, on the two most expensive listings, and nowhere else, is the most useful pattern on this page. Paying roughly three times the cheapest price on this shelf buys you a seller willing to tell you what you are getting.

One strip does not say its voltage anywhere in its title

The best specified listing here, the 1000 lumen CRI 90 one, never states its voltage in the words.

It is 12V. The only place that appears is printed along the circuit board in one of its own product photographs, where the little +12V markings repeat down the strip.

That is the second time in two days that opening the images on this site has found the deciding fact sitting in a picture while the title stayed quiet about it. It is picked below, because it is a good strip, with the voltage named here instead.