Part 1- Everything you Need to Know About Lumens

It’s time for a party! A time for cake, candles and balloons. Our golden retriever, Dandelion, is turning three. While goldens are not known for being too particular, they do enjoy a good party. A warm, bright, pleasing area, aka the kitchen, is probably the best place for the festivities. The lighting here is a bit dim, however, so replacing the lights in the ceiling fixture would be a nice upgrade.

It’s off to the hardware store, where you will find a whole aisle dedicated to light bulbs. How can you choose the best one? Gone are the days of simply buying a 60 watt or a 75 watt incandescent bulb for the table lamp, now you are now presented with light bulbs that somehow produce light in strange and mysterious ways called LED or CFL. All the wattage ratings shown on the packaging seem to be much too low. How in the world is Dande going to be able to see her cake and presents with 8 watt LED lamps installed in the kitchen ceiling fixture.

While holding this strange new marvel of illumination in your hand and scratching your head in confusion, a helpful hardware guy walks up to see if he can assist you. You explain your situation and ask for advice. The hardware guy points to the package where it says “800 Lumens”. This rating means that this lamp will produce the same amount of light as the older 800 lumen, 60 watt incandescent, but with a much lower electrical rating of only about 8 watts. That is a big difference in the amount of electrical power, it sounds too good to be true, and by the way, just what the heck is a lumen anyway?

One of Dande’s birthday candles can help answer that question. In a darkened room, light up a single birthday candle and hold up a 1 ft by 1 ft square sheet of paper, 1 ft away from the candle. The amount of light falling on the 1 square foot of paper is exactly 1 lumen, and at the 1 ft distance, the 1 square ft of paper is being illuminated at a value of 1 foot/candle. This is the definition of the lumen, 1 lumen = 1 fc illuminance / sq ft.

Now, look around the darkened room. You can see other objects as well, because the candle is radiating light in all directions. No matter which direction the candle is viewed, the radiance of the candle will be the same, and as the paper is moved around to other 1 ft distant positions, the illumination of the paper surface will be the same, 1 fc. This is because the candle radiance is omni-directional, which means the same in all directions. Many light sources radiate light in this fashion, such as the older incandescent bulbs. So if 1 lumen of light is falling on the 1 sq ft of paper, and we can see other objects in the room being illuminated by this same candle, that means that the candle is producing more than 1 lumen of light.

How many lumens does the candle radiate? Come back next month and find out for part 2  and part 3 of my blog.