I've always bought bulbs by their wattage and I (sort of) understand that way of measuring the brightness of a bulb.
But how do you equate a bulb thats sold with the measurement in candlepower?
eg; 32 candlepower equals what brightness?
I know one is a measurement of output and the other a measurement of input and its oranges against apples, but.......its just........sigh.
Rob, I found the information below on a automotive web site. Those with real electrical knowledge will probably write in to explain, this is all greek to me! ....Michael Pawelek
1157 Bulb 12V (26.9 W/8.3W - 32CP/3CP)
1156 Bulb 12V (32.9 W - 32CP)
There is no direct relationship between BTU, Watts and Candle Power
1 watt hour is about 3.4 BTU. For incandescent bulbs, a watt is about 1 candlepower.
The candlepower is a measurement of the brightness of the bulb. Watts measures the amount of current the bulb uses. Some bulbs might use the same amount of current but put out a different brightness depending on the construction of the bulb.
Thank you gentlemen,
Thats just what I wanted to know.
To expand some on what Norman correctly said, there is no direct correlation between candlepower and amperage. They are unrelated. You cannot assume that just because a bulb draws slightly more current it will have a proportionate increase in candlepower.
There seems to be a trend toward measurements of light output in a unit called "LUMENS" which is not always published for all light devices. I say light devices because lots of different things produce light these days. The efficiency then of a given light source is expressed in lumens/watt. This trend is very useful except that lumens doesn't exactly state the color spectrum of the light being measured. That property is referred to as Color Rendering Index (CRI) which is a number up to 100 with 100 being pure daylight. THe higher the CRI number the more perfect the light source assuming daylight is perfect. Finally there is color temperature usually measured in degrees with pure daylight coming somewhere around 6500 degree with "cool white" being about 4000 and "warm white" being about 2800 or so. I am looking very carefully at and modifying my shop to increase the efficiency of the various light sources. This is driven by 2 forces. One is that I am now 65 years old and the other is $$$ savings. Currently about the most efficient practical shop light source is a 3 phospher fluorescent T8 bulb driven by a high efficiency solid state programmed start ballast and that setup will produce a (personally measured) 100 Lumens/watt at "practical" prices at the local stores. Currently the best LEDs are doing about 60 Lumens/watt but look for the LED's to overtake the fluorescent lights. Each has it's own set of problems to overcome. For a single light source an LED is great but matching them and getting uniform briteness is a challenge as is viewing angle. For comparison a typical incandescent 60 Watt bulb puts out about 750 Lumens and thus is about 12 Lumens/watt while CFL draws 15 watts for same light output at 50 lumens/watt. CFL's designed at 13 Watts to replace 60 watt bulb actually draw a bit more wattage due to ballast losses but worse - the power meter on your house cheats you and bills you for something like 25 watts on those type bulbs. They DO save electricity but the power company makes out even better than you do on them.
OK - way more than you wanted to know.
Incandescent bulbs = heaters, so, IMO discussion of British Thermal Units is 100% appropriate.
Light Emitting Diodes = light - some 100 candlepower per watt with today's diodes. They also last many thousands of hours and don't darken with age like incandescent filaments.
An original Ford script headlight bulb uses a 21-1 3/4cp bulb. The tailight and dashlight use a #63 bulb which I think is around 3cp. I found some 1158 bulbs a while back which is about as close as you are going to find today for the headlights. They are 21-2cp. I haven't had a chance to check them yet, but I would like to find a 32-2 or even a 32-21. What I need is a good old fashioned bulb chart. My Dykes book is ok, but still doesn't tell enough. My old '25 pickup has NEW 50-32cp bulbs, and I don't like them. The poor generator can't keep up! Even the 32 filiment only allows it to charge at 1 amp!