# A Very Simple Question About Units

Staff Emeritus
Gold Member
Hello,

I'm new here. I was wondering if anyone could help me out with some confusion regarding units. My landlady (like most people) wants to save money on power consumption, so she has me installing Compact Fluorescent Lightbulbs(CFLs), which are supposed to save energy. On the package, it said that a 15W CFL is just as good as a 60W incandescent bulb, because they both give off ~800lumens. I couldn't understand that. The lumen is one of the three units whose meanings I don't remember:

candela

lumen

lux

Also,I found out that they measure the physical quantities luminous flux, illuminance, and intesity, but I don't know which measures which, and what the defn's of these are. Can anyone help me out?

Gold Member
The candela is the SI unit of luminous intensity and is defined as the luminous intensity in a given direction of a source that emits monochromatic radiation of 540 x 1012 Hz and has a radiant intensity (in the given direction) of 1/683 watt/steradian.

The lumen is the SI unit of luminous flux and is defined as the intensity (in candelas) of a point source in a cone of 1 steradian. Luminous flux can be found (for monochromatic radiation)by:

$$\Phi_v = K_m\Phi_eV(\lambda)$$

Where &Phi;e is the radiant flux Km is a constantr relaing the two units of flux and V(&lambda;) is the luminous efficency for &lambda;

The lux is the SI unit of illumination and is defined as one lumen over an area of 1 m2

It can be found on a surface of area dS from the following equation:

$$\Phi_v = \int E_v dS$$

Staff Emeritus
Gold Member
Thank you for those precise definitions.