Compilation: Determining the Brightness of Light Bulbs
Compilation: Determining the Brightness of Light Bulbs
Date: Tue, 9 May 2006
From: Nathan Harada
Subject: Determining Brightness of Light bulbs
In going through circuits, a question came up regarding how to determine the brightness
of light bulbs. I thought that the power rating of a light bulb was probably related to the
brightness and suggested a direct proportionality between power and brightness. Is this correct?
And if so, how can we quantify "brightness" to verify this relationship?
-----------Date: Traci Maxted
I do a lab to test this in one of my classes. Using a wattmeter (W) to test the input and a
photometer (W/cm2) to test output, the results we get are linear. The slope is close to one, but not
quite. Generally lower wattage bulbs (20W, 40W) are higher than rated, and larger wattage ones
(100W, 200W) are a little lower than rated. The difference is usually less than 5 % with 40 W
bulbs being about 42 W and 100's are about 98W.
--------------Date: Wed, 10 May 2006
From: David Brookes
It has been suggested (not by me) that you can divide all equations in physics up into two
broad categories. The first is physical quantities such as v = x/t. This is a definition of a physical
quantity. It does not make sense to talk about testing a proportional relationship between v and x,
there is nothing causal. The second sort of equation is a physical relationship or physical law
relating physical quantities. These are either causal (like a = Fnet/m) or constraint-based, like
conservation of energy. These can be tested.
So on to brightness and power. I'll assume the same surface area throughout so that we
don't have to complicate it with talk about intensity. In some sense brightness IS power (or
power is brightness) so this is a physical quantity definition, rather than a physical law, and I
would say it’s meaningless to test.
On the other hand, if you're talking about the relationship between the power of the light
bulb and what the human eye perceives, then I am not sure if you have the relationship correct. I
believe (but have no time to check) that the human eye perceives light power (intensity x area of
eye aperture) logarithmically, not linearly. A fun way to test my claim might be to pick light
bulbs of doubling power, say 20W, 40W, 80W and 160W. (Make sure they're all the same
physical size; otherwise you mess it all up with the intensity issue.) Don't tell your students what
the power ratings are and just ask them to rank the relative brightness. Namely pick which bulb
they perceive as twice as bright as the darkest one (20W). Also ask them to rank how much
brighter the other bulbs are than the darkest one. (1.4 x, like that.) Then plot. I'm not even sure if
this will work. But I just went to a talk by a Harvard astronomer and she said that good amateur
astronomers can rank intensities of stars with the naked eye down to a tenth of a solar magnitude
(or was it even better than that, I can't remember).
-----------Date: Wed, 10 May 2006
From: John Clement
The simple way to determine brightness is to look at the box the bulb comes in. It has the
number of lumens. Other than that, a light sensor from Vernier or Pasco is a good method.
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Compilation: Determining the Brightness of Light Bulbs
Brightness is obviously not linear if you look at the rated lumens and the power of a light bulb.
When you go from 40 to 60 to 100 W the power doubles with each step. If you look at brightness
vs power consumed by a single bulb it is also not linear. Part of the reason why it is not linear for
commercial bulbs is that there is a peaked curve of brightness vs frequency. At lower
temperatures more of the curve is in the infrared. As you increase the temperature, the peak of
the curve moves up to a higher frequency. The bulb becomes more efficient in the visible region.
Perhaps an interesting question for students would be to figure out which is more economical,
60W or 40W bulbs, when you have a minimum illumination requirement. The 60W is obviously
better because one 60W will give the same light as (2) 40W.
---------------Date: Thu, 11 May 2006
From: Andy Edington
If bulb wattage ratings are a measure of "brightness", why is a 15-watt compact
fluorescent Hg- Vapor bulb the same "brightness" as a 60-watt incandescent? Power tells us the
rate of electric potential energy transformation into electromagnetic energy that is radiated from
the bulb. The bulb's "brightness" depends on the spectral distribution of the radiated energy. (Our
eyes detect only a limited range of e-m energy and our eyes are not equally sensitive to all
wavelengths.) For an incandescent bulb, much of the radiation is in the infrared range, for a
fluorescent bulb, much of the radiated energy is in the visible range, and for an "uncoated"
fluorescent bulb, much of the radiated energy is in the uv range (think tanning beds.)
The 1000-watt bulbs in our fieldhouse are interesting (the custodians gave me an old
one.) The bulbs have both a mercury vapor chamber and a filament. When you first energize the
bulbs, the bulbs are dim. The radiating filament radiates energy which vaporizes the liquid
mercury droplet inside the separate, central chamber. Once the mercury is vaporized, the vapor
becomes part of the conducting path and the bulb gets very bright. Interestingly, the inner
chamber is clear glass with no visible coating. Does anyone know what is incorporated into the
glass or coated on the glass to transform the uv to visible e-m?
----------------Date: Thu, 11 May 2006
From: DAVID HURWITZ
Traci Maxted reported that the photometer readings were slightly lower for larger
wattages. Part of the issue is the physical size of the bulbs and the volume of inert gas between
the filament and the wall of the bulb. The loss can be estimated by taking photometer readings at
different distances and checking them against the inverse square rules. Decent measurements
should deliver an offset that may explain the slightly lower readings.
--------------Date: Fri, 12 May 2006
From: bRant hinrichs
At what wavelengths is the photometer sensitive? Do you get a spectrum out (intensity
vs. wavelength) or just a count of photons?
----------------Date: Fri, 12 May 2006
From: "Jean Oostens
The brightness of bulbs is expressed in Lumen (written on the package or even the bulb
itself: for example 60 Watt - 830 Lumen. One obvious way to measure is to use a light detector,
but beware of the spectral sensitivity:
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Compilation: Determining the Brightness of Light Bulbs
1. A solar cell connected to an ammeter will respond to the infrared, and favor
incandescent bulb over fluorescent ones.
2. The CdS cells (Cadmium sulphide) have a resistance inversely proportional to the
visible light falling upon them. Take 1/R as a good measure. They respond closely but not
perfectly like the human eye.
3. Phototransistors, like solar cells, favor infrared.
4. Photographic exposure meters should work well, by definition!
5. Vernier light sensors are fine, if you have the interface to go with them.
6. The light comparator is low tech, but it follows the human eye response perfectly by
design: Two blocks of paraffin are joined and looked at by the operator. The two lights to be
compared fall on the left and the right side of the device. If the lighting is the same, the dividing
line will disappear. (I can e-mail you a picture of the apparatus I use in my lab).
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