What is measured in lumens?

Answer 1

Lumens is a measure of light flux, so is independent of the area. It is essentially the amount of light available. So think of a bright light. It has a fixed amount of lumens, regardless of how near or far you are.

Lux is the light level at a surface. If you think of a light bulb, the lux will vary with the distance from the light source because the light spreads out.

1 lux = 1 lumen per sq meter.

Most light meters read out in lux, which is a measure of the brightness at a surface. You will measure more luxes when you are closer to the bulb, than when farther away.

Suppose you are measuring a compact flourescent lightbulb (CFL), which is roughly cylindrical in shape. You use your light meter and read 1000 foot-candles at about 1/2" from the lightbulb. How do you compute the lumens of the lightbulb?

(1) Convert the foot-candles to lux:

1000 foot-candles = 10764 lux

(2) Compute the area over which that lux value applies. This is roughly a cylinder that is 1/2 inch bigger in all directions from the bulb. So measure the bulb. We are going to convert all of our measurements into metric units. 1/2 inch = 0.0127 meters

bulb height = 9 cm or 0.09 m

bulb radius = 2 cm or 0.02 m

inflated height = h = 0.09 m + 2 * 0.0127 m = 0.1154 m

inflated radius = r = 0.02 m + 0.0127 m = 0.0327 m

area_of_cylinder = 2*pi*r*(r+h) = 2 * 3.14 * 0.0327 *(0.0327 + 0.1154) = 0.03 m^2

(3) Compute the total lumens of the bulb:

bulb_lumens = 10764 lux * 0.03 m^2 = 327.5 lumens.

Notes:

a) If your bulb is more like a sphere, you should use the formula for the surface area of a sphere rather than a cylinder.

b) When measuring the lux or foot-candles of the bulb, you should measure at several locations and average the number.

c) If you measure from a large distance away from the bulb, then you can ignore the size of the bulb in your calculations.

Answer 2

Lumens is the the measure of total light intensity from a single source. This is in contrast to luminous flux, which is the total amount of light falling on a given area.

Measuring lumens is done in one of two ways:

1. Derivation of Luminous Flux: A Fluxmeter (available at electronics stores) is placed at a known distance from a light source. Lumens are then calculated based on the flux falling on the fluxmeter, and the relative size of the photodiode on the flux meter compared to the surface area of a sphere created using the distance from the light source as the radius. This is an inexpensive method of measurement, but often unreliable as it doesn't take in to account inconsistencies in the field of light.

2. Integrating Sphere Spectrophotometry: The light source is placed inside of a piece of equipment called an integrating sphere spectrophotometer

, which is a sphere where the inside is fully covered in light-sensitive sensors. This gives very accurate measure of lumens as total output from a point light source, but the equipment is expensive and generally only available for large scale manufacturers.

ANOTHER ANSWER

A lumen is the SI photometric unit of measurement for luminous flux. It can be determined by measuring the illuminance on a surface, in lux, using an illuminance meter, and multiplying this by the area of that surface.

Answer 3

A lumen is a measurement of light. It is commonly defined in relation to the candela, which is a measurement of intensity, using the formula 1 lm equals 1 cd sr.

Answer 4

Sorry. They are 2 completely different units of measurement. Lumens being photometric and volts being electromotive