Light Bulb Luminous Efficacy

Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of luminous flux to power. Luminous flux can be defined as the measure of the perceived power of light. The luminous efficacy of a light bulb is a measure of the efficiency with which the light bulb provides visible light from electricity.

Artificial light sources are usually evaluated in terms of luminous efficacy of a source, also sometimes called overall luminous efficacy. The overall luminous efficacy is a measure of the efficiency of the device with the output adjusted to account for the spectral response curve. When expressed in dimensionless form this value may be called overall luminous efficiency, wall-plug luminous efficiency, or simply the lighting efficiency.

The efficacy is effectiveness of the light bulb to product light. The efficiency of the bulb is a measure of how well it produces "visible" light for how many watts consumed. It is the ratio of the visible light delivered for how much energy is supplied.

The measure of a light bulb's perceived brightness, the luminous out, using the old Watts standard is not wrong. Brightness is a description of light output, which is measured in lumens. Light bulb brightness described by watts should never have been used in the market. Now manufacturers are producing bulbs that are many times more efficient. Efficacy is about the brightness, whereas efficiency is about how much energy is wasted in producing the light output.

Watts is a measurement of the amount of energy required to product light. The higher the watts the more your electric bill goes up. You want a bulb that produces the brightest possible light using the fewest watts. When purchasing a light bulb, you should be considering the lumens so that you can know exactly how bright the light bulb will be.

Here is a cross-reference based on the light output of old tungsten incandescent light bulb.

• 40-watt incandescent bulb = 450 lumens
• 60-watt incandescent bulb = 800 lumens
• 75-watt incandescent bulb = 1100 lumens
• 100-watt incandescent bulb = 1600 lumens
• 150-watt incandescent bulb = 2600 lumens

To save energy, find the bulbs with the lumens you need, and then choose the one with the lowest wattage. Tungsten Incandescent Light Bulbs are slightly less efficient than Halogen Light Bulbs. Fluorescent Light Technology is more efficient than incandescent and halogen. Considerably more efficient than fluorescent lighting, including CFL (Compact Florescent Lights) are LED Light Bulbs. Modern LED light bulbs product the most lumens for the fewest watts.

Besides considering the brightness and energy consumption of a light bulb you should also consider the Light Color Temperature of the blub. Although color temperature does not have a significant impact on how efficient the bulb produces light, it does have an impact on your perception of what kind of light the bulb produces.

Incandescent lamps are nearly pure resistive loads with a power factor of 1. This means the actual power consumed (in watts) and the apparent power (in volt-amperes) are equal. The actual resistance of the filament is temperature dependent. The cold resistance of tungsten filament lmaps is about 1/15 the resistance when the lamp is lit. For example, a 100 watt, 120 volt lamp has a resistance of 144 Ω when lit, but the cold resistance is much lower (about 9.5 ohms) [31]. Since incandescent lamps are resistive loads, simple triac dimmers can be used to control brightness. Electrical contacts may carry a "T" rating symbol indicating that they are designed to control circuits with the high inrush current characteristic of tungsten lamps. For a 100-watt 120 volt general service lamp, the current stabilizes in about 0.10 seconds, and the lamp reaches 90% of its full brightness after about 0.13 seconds.

Power