Measures of light intensity illuminate complex subject

In the days of the incandescent light, we knew how bright a light bulb would be by the wattage — whether we needed a 40W or 100W for a particular application.

Today fluorescent and LED bulbs are significantly more efficient with much less power wasted as heat. We could say that the higher efficiency gives more lumens per watt, but that does not make much sense unless we know what a lumen is.

There are three measures of light intensity that are all related. All three measures have to do indirectly with watts but not as consistently as with basic incandescence.

One measure is the lumen, or the luminous flux of a “candle” object, which is anything that emits light. It is the total output of the light source or bulb. The radiant power, also called radiant flux, is the total radiated power in watts. This power factored by the sensitivity of the human eye determines luminous flux in lumens. Think of radiant flux as how much light the bulb emits overall.

This brings up the next measure, the lux. The lux measures luminance, which is the amount of light per unit area that falls on the surface of the sphere. In the metric system, 1 lux is 1 lumen per square meter.

The third measure, the candela, is the luminous intensity or brightness of the source. The candela measure is lumens per solid angle.

To illustrate, a laser might be very bright (candela), but the total light output (lumens) is small because it is concentrated into a very fine beam. A photo of a distant street lamp at night shows a bright dot although the total light that reached the camera is very small.

In bullet form:

>> Lumens are how much light.

>> Lux is surface brightness.

>> Candela is how bright the source is.

Although we grew up using the watt as a measure of light output, it is a relative measure that only works for incandescent bulbs. This is because incandescent bulbs are less than 10 percent efficient, emitting most of their energy in the infrared as heat.

As far as our perception of brightness is concerned, the color of the light is also a factor. Our eyes are more sensitive to those color frequencies in the midrange of the sun’s output, which peaks in the green range.

The more power input, the more energy an incandescent bulb emits, but there also is a shift in the spectrum toward yellow-green as the temperature of the filament increases. This fact makes the light redder when the temperature is low, and the power consumption of the bulb is low.

In compact fluorescent bulbs, the manufacturer uses a combination of materials that glow with a combination of frequencies that mimic various spectral patterns.

For LED lamps, red, blue and green light-emitting diodes combined in various intensities can mimic nearly any visible spectrum.

For comparing light output of newer compact fluorescent or LED bulbs, a 100W bulb produces about 1,600 lumens; a 60W produces half of that, 800 lumens.