Lux vs Lumen
Lumen and Lux are two photometric units in the SI system of units. They are closely related to each other and, in simple language, measure how bright a light source appears in two different contexts. These measurements are important in light sources and other cases where the intensity of the light plays a role.
More about Lumen
Lumen is the SI unit of the luminous flux, which is a measure of the total amount of visible light emitted by a source. It is the perceived power of the light from a source. The lumen is defined as the luminous flux emitted by a light source of one candela of intensity over a solid angle of one steradian.
Therefore, 1lumen(lm)=1cd/sr.
In simple, if a point light source emits one candela of luminous intensity through a solid angle of one steradian, then the total luminous flux into the solid angle is known as a lumen. This is a measure of the total number of packets (or quanta) of light produced by a light source.
The light outputs of the projectors are usually measured in lumens. Also, lighting equipment such as lamps are commonly labeled with their light output in lumens; in some countries, this is required by law.
More about Lux
Lux is the SI unit of measurement of illuminance, i.e. the total luminous flux incident on a unit surface area. It is a measure of how much the incident light illuminates the surface and gives an indication to the perception of the intensity of the light by a human eye. Lux is defined as the number of lumens per unit area.
Therefore, 1 lux=1lm/m2
In this definition, the effect on luminous flux by the spread in an area is taken into account. Therefore, the illuminance is inversely proportional to the area (illuminance obeys inverse square law).
Consider a light source with a luminous flux 100 lumens at 1 meter distance from the source. At 2 meters away, the luminous flux is the same, which is 100 lumens, but the area over which the light spread has changed. Therefore, the illuminance at 2 meters is one fourth of the value at 1m, which is 25lux. Further away the illuminance is even lower.
Therefore, the illuminance is of importance to sensors, camera, and other equipment that need a minimum brightness to operate properly. In most equipment, this critical number of lumens is measured and noted on the product.
Lumen vs Lux
• Lumen is the measure of the luminous flux and is defined as the luminous flux from a light source of one candela through a solid angle of 1 steradian.
• Lux is the measure of the illuminance and is defined as the number of lumens per square meter.
• Lumen measures the amount of light (photon output) from light sources weighted by the luminous function, to account for the sensitivity of the human eye.
• Lux measure how bright a light appears. Lux takes into account the spread of light over an area.
• If measured from a fixed source, number of lumens stays constant, and the number of lux decreases over the increasing distance.
Dean Alvarez says
Stupid answer; in layman terms, which is brighter and puts out more light 100 lux or 100 lumen? Simple question for Christ’s sake.
Jerry Arntzen says
Simple Question mister Alvarez? What is the answer?
Jeff Roquemore says
Laymen, by definition, don’t seek to understand quantitative photometric comparisons; however, an Oxy-moron (pun intended) may attempt to do so. 🙂
Janet Greene says
It depends on the size of the area being illuminated on what lux is therefore it can’t be listed as a value on a package except to say of an average size area such as a room that is 10×10. Lumens are the actual output of the light. So a flashlight that is 250 lumens would be 250 lux in given that it was measured in the same area. I think is what is being said.
Francis says
Your “simple question” is not so simple. It’s comparing 100 oranges to 100 apples.
Lumens is an absolute value – it’s the amount of light emitted by a light source (bulb), so 100 lumens is 100 lumens whether you are 5 inches away from the bulb or 10 feet away.
Lux is the amount of brightness that reaches you from a light source – how bright you perceive a bulb to be. So a bulb put in a desk lamp might give you 500 lux over your keyboard, but the same bulb used in a ceiling lamp of your 15-foot high living room ceiling would give you less than 20 lux over the magazine your are reading on your sofa.
So, your question of which is brighter cannot be answered. Lux measures brightness (which varies with where your stand); lumens measures absolute energy output, which has nothing to do with brightness.
Jeff Roquemore says
A better question may have been relevant to a specific distance of projection. What is the impact of lumens on lux and how does that change with distance? And how might that change with bulb type and / or color of light? LEDs are white..for example: Given 2 bulbs with nearly identical steradian measurement…Would a 900 lumen incandescent bulb produce more or less lux than a white LED bulb projected on the same surface from the same height at a given distance: 10′, 20′, 30 feet away?
Arthur Runyan says
So, how do we compare when trying to decide which projector to purchase? Projector A advertises 3800 lumens; projector B advertises 2400 LUX. First off, how can projector B even know the LUX if it does not know how far I plan to put the projector from the screen? When they advertise LUX, how do I know what the area of the projected image is and how far away the screen is for the number they give. How can I make an informed choice when some advertise their Lumens and others advertise their LUX. (Generally, I figure if they advertise LUX and not Lumens, they are trying to hide something.)
Cesar says
Can Somebody help me with this practical example? – A projector is advertise to have 5,000 Lumens and project good picture at 200″ screen at 5.5 meters vs another Projector advertise 5,000 Lux with maximum projection at 6 Meters for 300″ Screen – So which product will be better and which projector can provide good picture at low light conditions?
Gary Fannell says
Cesar, throw the projector spec out that lists LUX. projectors are not rated that way.
if your starting from scatch, buy a good quality projection screen that has none or less than 0.9 gain. Is tab-tensioned (keeps the screen flat). Keep ambient light off the screen. the darker the room the better your apparent contrast ratio will be (picture will look better).
you need to also find out the resolution of projector, go with a 4K with HDR (ultra High def) will future proof you over the 1920×1080 (known as full HD). I like the laser projectors, they are quieter, bulbs dont need to be replaced (cost $100-$400) and in general dont require any maintenance. Have the projector professionally installed so its safe to sit under and correctly set up. look for an installer that is ISF certified and make them show you the certification.
that’s a fairly simple break down. good luck
Randy says
Gary, THANK YOU! I think you gave the clearest answer for a “layman” like me (and I assume Cesar, Dean, and Jerry above) who does not WANT or NEED to understand “quantitative photometric comparisons” (…Jeff), but who is just trying to figure out what projector to buy based on information provided by the sellers.
Joe says
Would it be like In a car horsepower is a solid fact. and MPG is affected by speed, going up or downhill, and a ton other factors?
bibah says
from ‘play store’ download one of the apps that has a tool for measuring LUX into your smartphone. lunch the app and turn on one of lights in your home. face the smartphone towards the light source in close range. read the lux value. walk back from the source at .5 m intervals each time reading the value of lux.
while the ‘lumen’ of the source light remains constant (for some lights you have to wait for some minutes to reach the maximum) the ‘lux’ decreases as distance increase.