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I was looking at a desk lamp at ikea. The lamp specified "max 13 watts". The lamp is a standard screw type socket, not any sort of special CFL PL socket.

I looked at another desk lamp online, and it said "requires a 13 watt bulb".

What characteristic of these lamps restrict them to a certain watt bulb?

What would happen if I put in a 23 watt bulb? Would it possibly be unsafe? Would the 23 watt bulb not shine at its optimal brightness?

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The construction and insulation of the socket assembly, the size and temperature rating of the wiring, the proximity and design of the shade... generally anything related to either electrical current or heat affects its wattage rating.

Using a bulb of higher wattage than the lamp is rated for means that wiring may be overloaded, causing it to heat up, and/or the structure of the lamp will heat up due to conduction and radiation from the bulb. Either can cause burns or fires.

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Heat dissipation. The engineers expect the lamp will not be able to handle the power -- to be more specific, the lamps have not been tested at the higher power. And so the UL listing and the instructions do not permit this. It is illegal to install or use an electrical device contrary to its listing, labeling or instructions. (not least, your homeowner insurance could refuse to pay).

Power is relevant because all available bulbs convert almost all of their energy into heat, so a 13 watt bulb makes 13 watts of heat (well 11 or so with the most efficient LEDs, but close enough).

Given that the LED revolution is here to stay, and IKEA has bet the farm on it, there is no earthly reason for IKEA to drive up the cost of their luminaires for everyone merely to provide significant thermal protection for the 0.1% of diehards who insist on using obsolete incandescent bulbs.

If supporting the heat output of incandescent or large CFL bulb is a requirement for you, don't shop at IKEA and shop at a lighting specialist store who understands the requirement. Keep in mind with CFLs there are also bulb-orientation issues, they really should be socket down for cooling. Honestly today, LEDs are so good that there's no reason to use CFL, except blacklight (fluorescent being a good source of ultraviolet).

Beware the lumen chase

Now that "lumens" are the new "watts", it's very easy to get wrapped up in "Lumens per watt" or "lumens per dollar" and chase numbers. That's a mistake; that's how the government came to believe the economy is doing great when it's not.

The classic problem of every light up until LEDs is that they all emit light in all directions; that's great if you want to light the inside of a sphere. And we have been stuck with "sphere-lighting" bulbs for 110 years, and that is woven into our thinking about how lighting is supposed to work.

enter image description here

Lumen testing apparatus, measuring how well the lamp lights up the inside of a sphere.

Actually, what we want in almost every case, is to light up a "cone" or "wedge". And so we have "made do" with reflectors that aren't very efficient (or the awful "paint the interior of the fixture white" tactic.) These are better than painting it black, sure - but not very efficient at all. So from a spherical bulb, much of those lumens are absolutely wasted.

enter image description here (from this great source)

Lumens are just a stop on the road to lux.

Along comes LED. It inherently lights up a 120-140 degree cone. If you know some trig, that's about 1/6 of a sphere. How does this work for lumen measurement? Lumens are measured in a sphere - so an LED only gets credit for about the 1/6 of the sphere it lights up. That means for same-lumen bulbs, an LED is much brighter in-cone, and much dimmer or black off-cone, of course.

Now back to the part where we actually do want a cone. Obviously LEDs are a gigantic "win" in this department. Their 140 degree cone also lends itself to using lenses instead of reflectors, which are vastly more efficient.

Unfortunately humans are so inculcated into the concept of spherical light and that stupid dome. So most screw-in LED "bulbs" are intentionally made to light the inside of a sphere. They are made as universal swap-in replacements, and they just light every direction so nobody has to think about aiming.

LED is a much better deal on bulb shapes intended to be conical -- such as the PAR "reflector" bulbs.

Another example is fluorescent tube fixtures: they waste most of their 3000 lumens on inefficient bounce light, and LED "replacement tubes" use this to great advantage, giving brighter light with 1600-1800 lm by aiming it properly.

Light-on-subject is the only light that matters

So, since you're mentioning that you're a photographer, what you really want is light-on-subject. That isn't lumens, that's lux.

I can't tell you how many 175-watt (225W actual) metal halide lights I've replaced with a $9 cheapie 10- or 15-watt LED flood light, and ended up with more light (lux) where it matters. Mind you, metal-halide lights are already the same lumens/watt as LEDs, LEDs are more efficient only because of aiming. In another project I'm replacing a blistering 3150 watts of metal-halide lights with 150 watts of well-aimed off-road spotlights. I mail-order all that stuff (and it's not subsidized). I'd be reluctant to use it for photography because the China-via-Amazon vendors don't super care about color temp or CRI.

enter image description here enter image description here

$10 cheapie flood light (140-160°) and $20 12V spot light (30°, also available in 60° and 8°)

Honestly for high CRI, my go-to is actually 48" tube fluorescents. Tubes and fixture for $16 and you have 5000 lumens right there (poorly aimed of course). It's sorta like the way steam locomotives and carburetors got really good right before they went obsolete.

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  • CFL, not incandecent. – ScottF Apr 21 '17 at 12:29
  • @ScottF if you haven't bought this CFL bulb yet, don't. CFLs are a dead product, almost gone from the marketplace already. I finally threw out a 14-gallon Rubbermaid bin of CFLs I had bought over the years. LED has won, and it did so via the free market, not forced down our throats by government. You should be able to find a 13-watt LED with the same lumen output as a 23 watt CFL, and better CRI to boot. – Harper - Reinstate Monica Apr 21 '17 at 14:05
  • CFL wins for lumens/dollar, as I am using it for product photography, and I need brightness. And its a light bulb, not a computer component. There is not downside to not buying the most current. If it fits in the socket, what do I care? – ScottF Apr 21 '17 at 14:07
  • @ScottF only because government contracts to subsidize CFL have not yet expired. They won't be renewed, and now LEDs are being subsidized - not that they need it, they are winning on merits. An LED "bulb" is simply cheaper to build because it doesn't involve mercury and intricate glasswork. This field is changing very fast and I hope you're not relying on old information. The downside is you have lousy bulbs to begin with, with poor CRI (that matters yes?) and a mercury disposal problem, ew. – Harper - Reinstate Monica Apr 21 '17 at 14:13
  • I am looking at current prices. Why would I possibly care about future prices? When I buy a light bulb in the future, I will make a new assessment then. I don't need to make a long term commitment to the light bulb type I use LOL. Right now CFL price/lumen blows LED out of the water. – ScottF Apr 21 '17 at 14:16
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If you are talking about 13 and 23 watt lamps it can be assumed you are referring to CFL, or compact fluorescent lamps. Most likely PL bulbs, especially in a desk lamp.

In this case there is a ballast that runs the lamp(s), so you need to stick to the wattage suggested/required. Many CFL ballasts are rated for only a certain wattage or range of wattages.

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  • But I believe that ballast is in the plastic body of the bulb, not in the lamp. CFL are compatible with any lamp. – ScottF Apr 20 '17 at 22:19
  • Without seeing the lamp or bulb it is hard to say. I was going by the wattages you said. Both are common PL lamp wattages. PL bulbs are not self-ballasted. – Speedy Petey Apr 20 '17 at 22:20
  • Does it look like this?: bulbman.com/images/t4gx23wh190.jpg – Speedy Petey Apr 20 '17 at 22:22
  • I don't have the bulb or the lamp. But the CFL style I am envisioning is round with a screw bottom and a large plastic area where the ballast is. – ScottF Apr 20 '17 at 22:33
  • Do you have a link to the lamp? – Speedy Petey Apr 20 '17 at 22:33

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