Our home has many can-light ceiling fixtures. We replaced the original incandescent bulbs with CFL in many of them and noticed some cost savings on our electric bill for several years. More recently we added some LED style.
Bulb costs, duration of use, and total cost of use are some of the quantities tabulated in the chart below from wikipedia. I don’t know how recent are the values. But, they illustrate the cost savings with LED. With LED costs coming down, the savings is improving.
The main problem with an incandescent bulb is that about 75% of the energy needed to operate it is wasted as heat energy to the surroundings. Only about 25% goes toward making light energy. The CFL and LED bulbs convert a much greater fraction of their operating energy into light energy. As a result, they need less electrical energy to produce the same light output as an incandescent.
Recently, I obtained a pair of diffraction grating glasses. The transparent films are embedded with vertical and horizontal openings to allow light to pass through. It is similar in effect to viewing a distant street lamp at night through a window curtain sheer fabric. The light is spread horizontally by the vertical thread openings and is spread vertically by the horizontal threads.
The films in these glasses have 13,500 openings/inch or 530 openings/mm. The distance between the very small openings causes the colors of light to be spread out by different amounts. Red is the longer wavelength and is spread out the most. Blue is the least.
I held the glasses over the camera lens and looked up toward two ceiling lights and saw this effect. At left-center is an LED light fixture. At right -center is a CFL. Pretty? Yes. There is a lot of physics at play. Lets take it one step at a time.
First we look at a standard incandescent bulb. It has a hot wire glowing inside to make it emit light. The inside of the glass is frosted. Here is the view through the diffraction glasses. Barely left of center is the fixture with the bulb. To the far right is a clear spread of the full color spectrum emitted from the bulb’s hot filament. All of those colors mix in the vision center of your brain to make what is called white light.
The color spectrum can be analyzed with the right equipment. One can tell how much light is produced at each portion of the spectrum. Such an analysis was done by Geoffrey Morrison at Sound & Vision. This chart shows the typical light emission by an incandescent bulb. It is a smooth continuous spectrum. Notice the large amount toward the red end of the spectrum. Not measured, at longer wavelengths of infrared, is the heat emission by the bulb.
Compact Fluorescent CFL
This image shows the CFL in my ceiling fixture at the center of the frame. The diffraction glasses cause a spectrum of 3 or 4 distinct separate color images of the bulb clearly visible to the far right and left. There is not a continuous full spectrum as with the incandescent bulb. Our vision center receives the red, green, and blue parts of the spectrum and combines them to make an acceptable version of white light in our brain.
Analysis of the spectrum shows this chart of colors. Notice the peak emissions of red-orange, green, blue, and a violet. The CFL contains mercury vapor. When electrically excited, the vapor emits strongly in these four colors. We see images of the bulb in those colors.
In addition, the mercury vapor emits UV ultraviolet light. The UV is absorbed by the white phosphor coating on the inside of the glass tube. The phosphor reradiates the energy to increase the perception of white light.
Light Emitting Diode LED
This image shows the LED bulb through the diffraction glasses. The colors more closely match those of an incandescent bulb. The spectrum appears continuous. It tends to be pleasing to the eye for most people.
Here is an analysis of a typical LED lamb. There is a strong blue and yellow detected by the instrument. If you remember how colors of light mix, you might recall that red + green + blue = white to the eye. It is the basis of the RGB color mixing on a television screen.
In this bulb there is not a strong red or green. However, yellow is a mixture of red and green. So, yellow + blue = white. Don’t confuse mixing of light with that of pigments.