Monday, September 24, 2012

Lightbulbs and the Lumen

To help consumers shop for energy-efficient lighting products, the Federal Trade Commission explains the difference between watts and lumens.  Sort of.

To understand the difference between watts and lumens, you first need to understand a few fundamental properties of light.

The best way to think about light is as a wave of energy that travels through the air and bounces off things and then into your eyeball.  The difference between visible light and other kinds of wavy radiation in the electromagnetic spectrum, such as x-rays, microwaves and radio waves, is the wavelength -- the distance between the peaks of the wave, measured in meters.  Your eyeball has these rod and cone photoreceptor cells that just happen to be sensitive to a certain range of these wavelengths, which we call the visible spectrum.  Remember ROYGBIV?  Yeah, that.  

How bright something looks by your eyeball is related to (a) the rate of energy and (b) the wavelength

Radiant power or radiant flux is the total energy emitted over the entire electromagnetic spectrum, per unit time.  This power is in units of watts, the common unit that everyone is used to shopping for the old-school incandescent bulbs.  A 60 watt incandescent bulb consumes 60 watts of total electrical power and gives off approximately 45 watts of total radiant power (the other 15 watts are lost in the electronics).  

Yet, of these 45 watts of energy emitted from an incandescent lightbulb, most of it is completely invisible to the human eye.  That is, incandescents give off a whole bunch of light your eyeholes can't fucking see.  

Emission spectrum of an incandescent bulb (red dashed line) and human eye sensitivity (black dashed line) versus wavelength in nanometers (nm).  The visible spectrum ranges from around 390 nm (violet) to 700nm (red).  Longer wavelengths fall in the infrared (IR) range and shorter wavelengths are ultraviolet (UV).  

Only a small slice of the emitted "light" from an incandescent bulb falls in the range of wavelengths visible to the human eye (390 to 700 nanometers), and the rest falls in the invisible infrared (IR) range, as shown in the figure above.  An incandescent bulb is actually more efficient at making heat than light.

To complicate matters, the human eye does not see visible wavelengths equally.  The average daytime color sensitivity of your cone cells, shown above in the black dashed line above, a curve sometimes referred to as the luminosity function.  Your eye is most sensitive to wavelengths in the middle of the visible range, corresponding to green light.  It's just a nature of how your eyeball works.  Think of it as a filter that happens to pass more green light through your eyes to your brain.

Because the human eye is only sensitive to a small range of wavelengths, and some of those wavelengths more than others, we need another way to measure and represent useful light.  After all, a lightbulb will only appear bright if we can see the light it emits. 

Luminous power (also called luminous flux or photometric power), is a way to represent the power of light taking into account this wavelength-dependent sensitivity of the human eyeLuminous power is perceived radiant power.  Luminous power is the power of light you can actually see with your eyeball.  

The unit for this luminous power is the lumen (lm). To measure lumens, you must measure the total radiant power in watts for the entire range of wavelengths (the red curve above) and then take a weighted average with the luminosity function of the human eye (the black curve above).  The result of this mathematical magic is luminous power, which represents the perceived power of visible light.

For a standard 60 watt incandescent lightbulb, although most of the energy it emits is totally invisible, the output is still equivalent 800 lumens of visible light.

If you buy a compact fluorescent (CFL) or LED-based bulb rated at 800 lumens, that means it has the same luminous power and thus appears equally bright as a 60 watt equivalent incandescent bulb.  Their actual spectrum of energy they emit looks totally different, but the CFL and LED bulbs happen to use much less electrical power to achieve the same brightness of an incandescent, which is the whole fucking point!  

It turns out there a lot of technical jargon and complicated concepts when it comes to lighting products, but the US Department of Energy has designed new Lighting Facts labels for lighting products to make shopping easier.  I'll cover some of the other terms like color rendering index and color temperature in later posts.

You may have comments or questions, such as: 
 - CFLs are ugly and I hate them.
 - LED bulbs are also ugly and I hate them too.
 - Why is the sky blue?
 - Other
Leave a comment, and I will try to address it in a future post as well!

1 comment:

  1. Would it be possible to soften LED IR light to in order for it to behave like incandescent? Would there need to be some type of filter media or prism in order to get the curves to mimic one another?


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