# The relationship between brightness of bulb and power rating

1. Mar 26, 2015

### klorin

I came across a question: Four bulbs with different power rating, bulb A (240V, 40W), bulb B (110V, 36W), bulb C (12V, 60W) and bulb D (6V, 0.5W) are connected to relevant power sources. Which bulb is the brightest.

At first, i thought the highest power will be the brightest. But the answer is bulb A and the explanation gave is it gives the most energy. Why is that?

2. Mar 26, 2015

### Staff: Mentor

Welcome to PF!

Well, given that "brightest" is light energy per unit time, power is energy per unit time and watts are a unit of power, it should be bulb C. Was there anything more to the problem than what you provided?

3. Mar 26, 2015

### Staff: Mentor

I don't believe there's enough information to answer the question since we don't know the type of each bulb and their efficiency. Bulb A certainly doesn't take the most energy to run, that's bulb C with 60 watts.

Consider that a 100 watt incandescent light bulb only gives off about 5% of its energy as visible light, while a fluorescent light gives off about 22% of its energy as visible light.

4. Mar 26, 2015

### klorin

Is this question the bulbs are normal tungsten bulb and the efficiency is 100% (all heat energy is converted to light energy)

5. Mar 26, 2015

### Staff: Mentor

Then there's no way a 40 watt bulb emits more energy than a 60 watt bulb.

6. Mar 26, 2015

### Staff: Mentor

Well, incandescents are only about 5% efficient, but if they are all regular incandescents, they should all be the same efficiency, so....

7. Mar 26, 2015

### uart

I suspect that this is another case where some important detail has been omitted from the question. I'll bet that the original question actually specified that the bulbs were connected in series to a suitable power source.

Under those conditions answer "A" would certainly make sense.

8. Mar 26, 2015

### klorin

I think the keyword is "relevant power sources". What is your understanding of this sentence?

9. Mar 26, 2015

### phinds

Should be irrelevant. You have stated what the voltage is of the power source and that's all that matters (assuming, of course, that they each can provide enough current to generate the requisite power, but I think that's a given).

If you want to compare "brightness", forget about volts and power and get the rating for lumens. An incandescent 60 Watt bulb will put out about 600 lumens and a hi-efficiency (that's a bit of a joke by the lighting industry) one will put out 600 lumens with about 15 watts, so power is less important than efficiency. [NOTE: the figures I gave are rough ... you'll find variations]

Also, it's usually the case that hi efficiency bulbs will put out fewer lumens but last much longer. Again, you'll see significant variations.

10. Mar 26, 2015

### uart

To me the phrase "relevant power sources" (especially with sources given as plural) would mean that each light was powered at it's rated voltage. All other factors (like efficiency, solid angle and distance) being the same would simply make the highest rated power bulb to be the brightest. ("C").

It's a very vaguely worded question though. It thought perhaps the question had been transcribed or something and series part had been left out. Putting them in series makes the one with the highest resistance have the highest power output, so that would account for the given answer of "A". Though to be honest, even in that case the answer could still be in dispute, due to the strong temperature dependence of resistance in a light bulb.

Can I ask where this question originates from?

Last edited: Mar 26, 2015
11. Mar 26, 2015

### Staff: Mentor

Possibly not a trick question at all, just the usual confounding typo.

It's likely they would be listed in decreasing voltage/power: bulb A (240V, 40W), bulb B (110V, 36W), bulb C (12V, 6W) and bulb D (6V, 0.5W)

Though it's now not nearly as interesting ....

12. Mar 26, 2015

### rootone

It depends on what kind of technology the bulbs are based on.
If it's the same technology, let's say a fluorescent tube for all the bulbs, then the bulb which consumes most power should be the brightest.
If there are different technologies involved, then there will be different considerations of how much energy could be lost as heat and other factors.
A low power rated LED could easily produce more light than an incandescent bulb rated at a much higher power.

13. Mar 27, 2015

### CWatters

I think NascentOxygen probably has the answer. It's a typo.

14. Mar 27, 2015

### CWatters

Not always that simple. In the UK we have two types of Halogen down light bulb in common use..

50W GU10 230V
50W MR16 12V

Both are Halogen but the 12V MR16 appears brighter than the 230V GU10. I believe this is because the lower voltage version is more efficient (although nothing like as efficient as an LED).

15. Mar 27, 2015

### Staff: Mentor

Threads like this are the reason that we require that the complete text of the question be provided.

Klorin, you're the one who "came across" this problem. Can you provide a link to the source and/or the complete question, including even the details that you think are unimportant? If so, send me or any of the other mentors a private message and I'll reopen the thread.