I bought a reading lamp that on the box says:
The shop also sold me an LED light bulb that says:
So my question is, can I use light bulb or not? I am confused between the 75W and the max of 40W mentioned for the lamp
Yes. The fixture is concerned with heat. Your LED light bulb makes about 9 watts of heat. Your fixture is rated for a bulb that makes 40 watts of heat.
With an incandescent bulb, 98-99% of its wattage turns directly into heat. Even LEDs, 90% of their wattage turns into heat. For thermal planning, it's not worth pulling out the calculator. You're "close enough" just pretending it all becomes heat.
On the LED bulb, when it says "75 watt equivalent" it's trying to give ordinary consumers a general sense of how bright the bulb is - i.e. about as bright as an old style 75 watt incandescent. The more proper way to say that is "950 lumens", which is a unit of measure for describing brightness to human eyes.
SHORT ANSWER: NO PROBLEM
LONG ANSWER: The maximum wattage limits are largely a function of heat. For incandescent lights, more wattage means more heat. And too much of a heat buildup could result in shorter bulb life and even risk of fire, especially in a fixture in which the bulbs are fully enclosed.
While lighting has traditionally been sold by wattage, what you are really buying is the amount of light, actually measured in lumens. For years, wattage gave us a good idea of how much light came from a bulb. A 100 watt bulb was much brighter than a 40 watt, and a bit brighter than a 75 watt.
As different types of bulbs came into use, lumens started to be listed. For a given lumen level, LED bulbs draw much less wattage than an incandescent bulb. For example a 75 watt incandescent bulb gives off between 700 and 1100 lumens (depending on the type). But LED bulbs that give off about 900 to 1100 lumens only use between 9 and 15 watts. Since they are similar to a regular (incandescent) 75 watt bulb in the amount of light output, they are called 75 watt equivalent.
A 15 watt LED bulb generates as much light, but much less heat than a 75 watt incandescent bulb. In fact it probably generates less heat than a 25 watt bulb. But it does generate some heat. And some bulbs are marked not for use in fully enclosed fixtures.
So long as the bulbs are not marked for limited use, you can generally use an LED bulb that is rated much higher than an incandescent bulb. In general, there is no problem using a 75W equivalent LED bulb in a fixture marked 40W maximum. You are only drawing about 15 watts and generating much less heat than a 40 watt bulb.
When a lamp lists a maximum wattage, its doing that based on the heat an incandescent bulb of that wattage will put off. The 9W=75W on the LED bulb is telling you that "even though this bulb only uses 9W of power, it puts out the same amount of light as a 75W incandescent". Lamps are rated on heat and power usage not light output.
LED bulbs do get hot, and some get so hot that you can't use them in an enclosed fixture because their internal electronics will overheat, but it won't hurt the lamp. If the lamp is rated for the heat of a 40W bulb, an LED that uses only 9W of power just can't get that hot even if it was nothing more than a 9W heating element.
The bulb will not pose a safety or fire hazard.
Those fixture ratings are primarily concerned with:
- How much current the fixture is designed to safely supply to the bulb.
- How much heat the fixture is designed to accept from the bulb.
The bulb itself is rated 9W. While it has an equivalent rated light output of 75W, for the purposes of heating and current carrying the actual wattage - 9W - is all you need to pay attention to.
Therefore, this fixture will accept and run the lightbulb you've selected.
That said, please note that unlike incandescent bulbs, the newer LED and CF bulbs incude circuits that typically don't like to get hot. An enclosed fixture, or a fixture where the bulb hangs lower than the base, may overheat these components.
If this fixture is enclosed or hangs the bulb from the base, then you will need to make sure the selected bulb is rated for this use. If you don't, it won't present a safety hazard, but the bulb life may be affected.
A fixture's rating indicates how much power it can dissipate without things getting so hot as to damage the fixture or its surroundings. The rating is thus affected by two factors:
How hot things can get without damage to the fixture or its surroundings.
How much power the fixture can generate for each degree of temperature rise relative to ambient.
Incandescent bulbs can operate perfectly happily at temperatures which would cause most kinds of fixtures to melt or catch fire, so a fixture which has a heat-resistant surface may be able to tolerate a powerful bulb even if it can't dissipate much heat. LED bulbs, however, will degrade rapidly if operated at temperatures far below those that typical incandescent fixtures can handle without damage. A 9W LED bulb may only generate 1/4 as much heat as a 40W incandescent, but that's still a significant amount of heat. If an incandescent bulb would have reached 150C when operating at a 30C ambient temperature, a 9W LED might reach 60C. A fixture which can withstand temperatures of 150C or even 200C without difficulty won't be damaged at 60C, but LED lifetime may be greatly degraded at such temperatures.
The answer is always YES. An LED is about 96-98% efficient. The issue is heat. You can hold a '60' Watt equivalent light output LED bulb in your hand forever. Try that with an incandescent and you will burn yourself in 2 minutes. Even a CFL will get too warm to hold in 5 minutes.
The MAXIMUM ratings for fixtures are based on the heat that an incandescent bulb will generate. LEDS give off virtually none, and they last a very long time!
I actually use LEDs for my aquariums and have silver Mylar laying right on top of them: '400' Watts of light and only warm to the touch. Phillips only though.