Incandescent lamps used for interior lighting generally have an intensity ranging from a few cd to several hundred cd. The common 40-watt lamp has an intensity of about 35 cd; a 100-watt lamp gives about 130 cd. Note that a 40-watt fluorescent lamp has an intensity of about 200 cd. The intensity of any lamp depends on the direction from which it is measured; the average cd power in all directions in space (spherical candle power) is often given.

• Clear blue sky (without direct sun) 12,000K
• Overcast sky 6,500K to 8,000K
• Sunlight (middle of the day) 4,700K to 5,700K
• Cool lamps range from 3,600K to 6,500K
• "Pure" White is about 4,700K
• Metal Halide 4,500K
• Neutral lamps range from 3,000K to 3,600K
• Halogen 3,000K
• Warm lamps range from 2,700K to 3,000K
• House (incandescent) Lamp 2,500K
• Orange color averages 2,300K
• Candle Flame 1,500K

• Fluorescent lamps are available in Daylight (blue tint), Cool white and Warm white (sometimes called "Kitchen & Bath" lamps
• Halogen lamps are considered Neutral.
• Incandescent lamps are considered Warm.

Traditional halophosphors are inexpensive coatings which usually provide the entire spectrum of light. But, there is a trade-off between Color Rendering and Lumen output. Poor color rendering lamps such as "warm white" and "cool white" have high Lumen output. Good color rendering lamps such as "warm white deluxe" and "cool white deluxe" have low Lumen output.

Prime color or Tri-phosphors are very expensive coatings with good color rendering and high lumen output.

Double-coat lamps have a coat of halo-phosphor and a coat of tri-phosphor. Double-coat lamps which have a thick tri-phosphor coat are fairly expensive but have very good color rendering properties. Double-coat lamps with a thin tri-phosphor coat are much less expensive, but still have full light output and reasonably good color rendering.

Rare Earth Phosphors Have a thin and thick coat of rare earth phosphors and are just becoming available. The CRI for these lamps will be 70, 80, and 90 and in a variety of Color Temperatures.

The formula for Inverse Square Law states that illuminance varies inversely with the square of the distance from a point light source.   E = 1/r2 Where E is illuminance and r is the distance.   It means that when you double the distance from a point source of light (like our bulbs) the intensity falls off by a factor of four.   If you triple the distance, the intensity falls by a factor of nine.   You square the distance.   This is helpful in understanding how much the intensity drops off as you mount a light source further away from the object you want to illuminate.

Example: an island counter top is 3' high in a kitchen with an 8' ceiling height.   Recessed cans (pot lights) are to be used with a lamp rated at 1150 CandlePower.   The distance from the lamp to the counter surface is approximately 5'.   By squaring the Distance (D2) between the counter and the lamp (5' x 5' = 25'), then dividing it into the lamps rated CandlePower (1150), the FootCandle level at the counter surface is found to be 46.

The fact that a bulb uses 100 watts of energy doesn't mean it gives 100 watts of light. Typically, only 10% or less of the energy consumed by incandescent lamps is actually used to produce light. The rest ends up as heat.

In a 120 volt, 100 watt "standard" bulb with a rated light output of 1750 lumens, the efficiency is 17.5 lumens per watt. This compares poorly to an "ideal" of 242.5 lumens per watt for one idealized type of white light, or 681 lumens per watt ideally for the yellowish-green wavelength of light that the human eye is most sensitive to.

Most light bulbs average around 500 lumens per 60 Watts. A lumen is the unit of luminous flux equal to the light emitted in a unit solid angle by a uniform point source having an intensity of one candela. There is no direct correlation between lumens and watts because other variables may affect the relationship, such as lamp design life and fill pressure.

A lamp should be chosen based on its CandlePower or Lumen rating which indicates the light energy intensity of the lamp. Only then should wattage be considered to find the most economical lamp among those lamps powerful enough to provide the proper FootCandle level on the WorkPlane.

The following failure rates have been defined as standard failure rates. The B3 life value represents the premature failure rate of lamps. It means that 3% of all the tested lamps failed after this number of hours burned. The Tc life value represents the failure rate of 63.2% of the lamps. B3 and Tc are of particular interest in industry. Another important point is the B50 value. This indicates the average life of the lamps (50%).

To show this approximation to a straight line, a special graphic matrix is needed. The failure rates are indicated as a percentage plotted against the number of hours burned. The advantage here is that the distribution of the life of, say, a headlight lamp can be uniquely defined by specifying particular failure rates.

Point 1: The B3 life value means that 3% of the tested lamps failed after this number of hours burned.
Point 2: The B10 life value means that 10% of the tested lamps failed after this number of hours burned.
Point 3: The B50 life value indicates the average life of the lamps. The failure rate is 50%.
Point 4: The Tc life value represents the characteristic failure rate of 63.2% of the lamps.

Rule of thumb:

5% overvoltage

• Half the life
• 15% higher luminous flux
• 8% higher power consumption
• 3% higher current

• Twice the life
• 15% lower luminous flux
• 8% lower power consumption
• 3% lower current