# radiant (all wavelength) flux emitted by a light source

by Roger44
Tags: emitted, flux, light, radiant, source, wavelength
 P: 41 hello If I were to hang a traditional 100W clear glass light bulb from the ceiling, would it radiate 80W (radiant flux), the other 20W lost by air convection/conduction up the wires, or would it radiate closer to 60% or 40% of consumed energy? I can find plenty of figures for the eye-sensitive flux, but no figures for the all-wavelength flux. Thanks
 P: 41 As nobody came up wiith a reply, I spent quiite a lot of time browsing the Net for a figure. Much to my surprise no manufacturer, or State body quotes figures for total radiant flux emitted by light sources. There are hoards of graphs giving spectal irradiance (watts/m2 arriving on a surface often placed at 50 cm from the bulb), but only at the brightest position with regards to the shape of the bulb. So that's of no use. I found just one spectral graph giving total watts emitted per nm of wavelength. By summing the area under the curve, this gave 60W radiated and therefore 40W conduction/convection. I also found some theretical calculations, working up from the the tunsten filament. One gave 61 % radiated. Here it is for what it's worth : "The IFS-66 uses an internal 150W Tungsten-filament Quartz Halogen Lamp.  Assume that the filament has a color temperature of 3000K, a surface area of 50mm2, and an emissivity of 40%. From Stefan’s Law, the total radiance is 0.5E-04*0.4*5.67E-08*30004 = 92 W  The remaining 58 W is lost by convection and conduction through the base." Theoreticaly, if you know the luminous efficiency lumens/watt, and the spectral distribution from 0 to 1, assumed to be the same all around the bulb which is probably true, you can determine the total watts radiated. I haven't pursued this approach.
 P: 866 More like 95% + percent is radiated as heat. Sorry, a light bulb is an efficient heater, not light radiator. This is the first link I found when I searched for tungsten bulb efficiency http://en.wikipedia.org/wiki/Incande...nmental_impact
PF Gold
P: 2,608

## radiant (all wavelength) flux emitted by a light source

 Much to my surprise no manufacturer, or State body quotes figures for total radiant flux emitted by light sources.
That's because Radiant Flux, measured in watts, is for optical wavelengths from Ultra Violet to Infra Red (.01 to 1000 μm) and all other wavelengths produce a negligible amount of energy. So the marked wattage on a incandescent lamp IS the Radiant Flux.

 Quote by http://hyperphysics.phy-astr.gsu.edu That is, a 60 W incandescent lamp would have a radiant flux of 60 W, and a 250 W Mercury vapor lamp would have a radiant flux of 250 W.

 I found just one spectral graph giving total watts emitted per nm of wavelength. By summing the area under the curve, this gave 60W radiated and therefore 40W conduction/convection.
The flux seen by the human eye is called Luminous Flux, measured in lumens, and the eye doesn't see every wavelength equally, so your summing won't give you the correct answer. i.e. you must factor the Radiant Flux by the sensitivity of the human eye (683 lumens/watt).

Image compliments of http://hyperphysics.phy-astr.gsu.edu
 P: 866 I should have said "visible light"
 P: 41 1. "So the marked wattage on a incandescent lamp IS the Radiant Flux." This is false. Marked wattage is power consumption. "For years, people have chosen light bulbs by the watt, learning over time about how bright a typical 40-watt or 60-watt bulb is. But wattage tells you only how much energy a bulb uses — not how bright it is." Source amongst others http://www.consumer.ftc.gov/articles...ng-light-bulbs 2. "That's because Radiant Flux, measured in watts, is for optical wavelengths" This is false Radiant flux is for all wavelengths, luminous flux is for eye-visible wavelegths. In general -RAD- and -LUM- are dedicated to ALL and VISIBLE wavelengths respectively For example irradiance and illuminance, radiance and luminance 3. "That is, a 60 W incandescent lamp would have a radiant flux of 60 W, and a 250 W Mercury vapor lamp would have a radiant flux of 250 W." This is false. The person who wrote this should be notified of his mistake, but maybe he just wants to make it simple for the layman. A 60W lamp consumes 60W of electricity and its radiant (all wavelength) flux would, I guess very very approximately, be around 36W, the rest lost in heat conduction and convection.
PF Gold
P: 2,608
 Quote by Roger44 1. "So the marked wattage on a incandescent lamp IS the Radiant Flux." This is false. Marked wattage is power consumption. "For years, people have chosen light bulbs by the watt, learning over time about how bright a typical 40-watt or 60-watt bulb is. But wattage tells you only how much energy a bulb uses — not how bright it is." Source amongst others http://www.consumer.ftc.gov/articles...ng-light-bulbs
Your reference is confusing you. Brightness is a photometric measurement dealing with the wavelengths the human eye sees. If you put all/most of the bulbs energy in a few visible colors (what the eye sees), like a CFL does, sure you can get the same "brightness" in lumens while consuming less energy.

 Quote by Roger44 2. "That's because Radiant Flux, measured in watts, is for optical wavelengths" This is false Radiant flux is for all wavelengths, luminous flux is for eye-visible wavelegths.
Isn't that what I said?
 ... Radiant Flux, measured in watts, is for optical wavelengths from Ultra Violet to Infra Red (.01 to 1000 μm)
.01 to 1000 μm pretty much covers all emission wavelengths.

and
 The flux seen by the human eye is called Luminous Flux, measured in lumens, and the eye doesn't see every wavelength equally,...
So you must agree.

 Quote by Roger44 In general -RAD- and -LUM- are dedicated to ALL and VISIBLE wavelengths respectively For example irradiance and illuminance, radiance and luminance
You need to study the difference between Radiometry and Photometry.

 Quote by Roger44 3. "That is, a 60 W incandescent lamp would have a radiant flux of 60 W, and a 250 W Mercury vapor lamp would have a radiant flux of 250 W." This is false. The person who wrote this should be notified of his mistake, but maybe he just wants to make it simple for the layman. A 60W lamp consumes 60W of electricity and its radiant (all wavelength) flux would, I guess very very approximately, be around 36W, the rest lost in heat conduction and convection.
So if you know everything about the subject, why did you start this thread? Actually most of the radiant flux is from the Infra Red wavelengths which you can't see and is the least energetic part of the bulbs spectrum; hence is a smaller contributor to the total radiant flux.